Processing fruits: science and technology

Processing Fruits

S e c o n d E d i t i o n

S c i e n c e a n d Te c h n o l o g y

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Processing Fruits

S e c o n d E d i t i o n

S c i e n c e a n d Te c h n o l o g y E d i t e d

b y

Diane M. Barrett Laszlo Somogyi Hosahalli Ramaswamy

CRC PR E S S Boca Raton London New York Washington, D.C.

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Library of Congress Cataloging-in-Publication Data Processing fruits.—2nd ed. / edited by Diane M. Barrett, Laszlo Somogyi, Hosahalli Ramaswamy. p. cm. Includes bibliographical references and index. ISBN 0-8493-1478-X (alk. paper) 1. Fruit—Processing. I. Barrett, Diane M. II. Somogyi, Laszlo P. III. Ramaswamy, Hosahalli S. TP440.P77 2004 664'.8—dc22

2004049679

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $1.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1478-X/05/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2005 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1478-X Library of Congress Card Number 2004049679 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

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Preface Fruits are botanically similar plant organs in that they are all composed of seeds surrounded by a juicy, colorful, and aromatic ovary which we humans consume as food. Some things we traditionally consider to be “vegetables”, such as tomatoes, cucumbers, corn, and squash, are actually fruits from the botanical point of view. However, fruits vary widely in their shape, size, color, texture, flavor, nutritional properties, potential for extended shelf-life, and ability to withstand different types of processing. While fruits are delicious, nutritious and therefore desirable components of our diet, they suffer from being extremely perishable. For this reason, it is often advantageous to preserve them for longer shelf-life and easier transport to locations distant to the site of production. Processing also transforms the raw material into new, and perhaps improved, product. This book endeavors to serve as a single source of information about the biology of fruit and a description of the various methods used to preserve fruit. The book covers both traditional methods of preservation, such as canning, freezing, and drying, and looks to the future of novel processes such as high pressure, pulsed electric fields, and ohmic processing. The second half of the book focuses on the major processed fruit products and describes the diverse methods that may be used to preserve them. This book is the second edition of a two-volume book series published in 1996. The first edition separated the biology, principles, and applications into volume one and the major processed products in volume two. When we were asked to consider the editorship of a second edition, we readily agreed because it was clear that an even more detailed and current publication could result given new research and technology. In this second edition, we have introduced new technologies that appear to show promise for the preservation of fruit, such as the production of fresh-cut fruit. There are two brand new chapters addressing these topics. In many cases, we have gone back to the first edition authors and asked for their assistance in updating their chapters. We have added new authors to some chapters, in order to broaden the scope of what was written previously. For this second edition, we decided to put both of the original volumes into one complete package, allowing the reader to consult one succinct resource for anything he or she wants to know about fruit processing. This book is unique in comparison to others currently available because it covers a greater breadth of topics, and includes detailed descriptions of the processing of over 20 different major fruits. No other book published in the last 25 years (with the exception perhaps of the first edition of this series) comes close to providing the same degree of information on methods for the preservation of fruit. We begin the book with a description of different fruit classes, and the principles of preserving fruits in their most fresh-like state, that are stored as whole fruit (under either refrigerated or controlled atmosphere conditions) or lightly processed as fresh-cut fruit. Then the chapters progress through methods that involve increasing degrees of process severity. For example, the production of fruit juice involves mild pasteurization treatments in combination with refrigerated storage, and drying may be carried out in the sun. Freezing typically involves a mild blanching step, while ionizing radiation and canning involve treatments that are relatively more intense. A new chapter on novel processing technologies has been added. The last chapters in Part I of the book describe other topics relevant to the preservation of fruit, e.g., microbiology, food additives, quality assurance, packaging, grades and standards, and residue management. Part II covers more than 22 major processed fruits, including apples, peaches and apricots, sweet and sour cherries, plums, prunes, strawberries, raspberries, cranberries, grapes, oranges and tangerines, grapefruit, lemons and limes, bananas, tropical fruit, coconut, avocadoes, olives, and nuts.

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This book will fill a critical need for students and professors, industry personnel, government agents and others working on the preservation of fruit. It is intended as a resource for both individuals familiar with fruit processing who want to expand their horizons, and as an introduction to the diversity of preservation methods for the inexperienced individual. It has been a joy, and a continuing learning experience, for us to be involved in its creation. Diane M. Barrett Laszlo P. Somogyi Hosahalli Ramaswamy

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Acknowledgments This book is dedicated to the students, scientists, and food processing industry professionals who strive to produce attractive, nutritious processed fruit products for the international consumer. Diane Barrett is indebted to Dr. Laszlo Somogyi for his initiation of this project, and to her husband Pieter and daughter Jodie for their continuing love, support, and understanding.

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Editors Diane M. Barrett graduated with a B.S. in Food Science & Technology from the University of California, Davis, where she is currently on faculty. She received her M.S. in Food Chemistry from the University of Wisconsin, Madison and her Ph.D. in Food Biochemistry from Cornell University, Ithaca, NY. After acquiring her M.S., Dr. Barrett spent four years doing food science research and education in Indonesia as a consultant with the World Bank and U.S. Agency for International Development. At U.C. Davis, Dr. Barrett conducts extension courses for the fruit and vegetable processing industry and carries out applied research. Extension courses include the Better Process Control School, Freezing Technology Workshop, Juice Processing Course, Tomato Processing School, Fresh-Cut Products Workshop and Aseptic Processing & Packaging Workshop. Her research focuses on the effects of raw materials and processing conditions on the quality of fruit and vegetable products. She is particularly interested in the relationship between endogenous enzymes and the color, texture, flavor, and nutritional quality of fruits and vegetables. When she is not harvesting tomatoes or processing fruit, Dr. Barrett enjoys swimming, kayaking, and traveling with her family. Laszlo P. Somogyi specializes in the technical, regulatory, and nutritional aspects of food processing operations, techno-economic studies of food ingredients, and post-harvest handling of fruits. During his 40 years experience in working for and consultation to food industry firms he worked at HuntWesson Foods, Vacu-Dry Company, Finn-Cal Products, and SRI International. Dr. Somogyi has published over 40 technical papers and has contributed to a number of textbooks on subjects of food ingredients, additives, food and beverage processing technologies, fruit quality, and food irradiation, and has been the lead author for SRI’s Chemical Economics Handbook and Specialty Chemical Economics Update, multiclient reports dealing with food additives and flavors and fragrances. Since his retirement from SRI in 1998 he has been participating in U.S. Agency for International Development programs in Egypt and Lebanon and consulting for several food ingredient manufacturers. Dr. Somogyi received a B.S. degree from the University of Agricultural Sciences in Budapest, Hungary, holds M.S. and Ph.D. degrees from Rutgers University; and was a post-doctoral fellow at the University of California, Davis. He was elected as Fellow of the Institute of Food Technologists, and is a member of the American Association of Cereal Chemists, and the American Oil Chemist’s Society. Dr. Hosahalli Ramaswamy is a Professor in the Department of Food Science at McGill University in Montreal, Canada with teaching and research responsibilities in the areas of Food Processing and Post Harvest Technology. He obtained his M.Sc. and Ph.D. in Food Science from the University of British Columbia, Vancouver. Dr. Ramaswamy’s primary research area is thermal and nonthermal processing. In his research activities, he has explored the use of conventional thermal processing as well as thin profile, rotational, microwave, RF, ohmic and aseptic processing. In the area of non-thermal processing, his research focus has been application of high pressure processing for food systems. He has also carried out extensive research in the area of computer modeling, rheology and use of artificial neural networks for process characterization and optimization. Dr. Ramaswamy is a professional member of several organizations such as IFT, CIFST, IFTPS, ASAE and CSAE. He has served as Chair of the Heat Penetration Committee of the Institute for Thermal Processing Specialists, Chair of the Radiation Committee of ASAE, and has been serving as an

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associate editor of Transactions of ASAE (ASAE) and Journal of Food Science. He is also on the editorial board of Food Research International, Food Science and Technology Journal, and Journal of Food Process Engineering. He has published over 175 refereed scientific papers and supervised over 30 graduate students. He has received the 1999 W.J. Eva Award by the Canadian Institute of Food Science and Technology, the 2002 John Clark Award of the Canadian Society of Agricultural Engineering, and is a 2002 Fellow of the Association of Food Scientists and Technologists (India).

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Contributors P. Alvo (Deceased) McGill University St. Anne Bellevue, Quebec, Canada

Yvan Gariépy McGill University St. Anne de Bellevue, Quebec, Canada

Robert A. Baker USDA-ARS Winter Haven, Florida

Albrecht Höhn AB Enzymes GmbH Darmstadt, Germany

Diane M. Barrett University of California Davis, California

Y.H. Hui American Food & Nutrition Center West Sacramento, California

Norman Berry AVP Baker Ltd. Crawley, West Sussex, U.K.

Charles Huxsoll USDA-WRRC Albany, California

Robert Braddock University of Florida Lake Alfred, Florida

Adel A. Kader University of California Davis, California

Cuiren Chen McGill University–MacDonald Campus St. Anne de Bellevue, Quebec, Canada

Dan Kimball Kimball Consulting Lindsay, California

Sonia Y. de Leon University of the Philippines Quezon City, Philippines

Azriel Kurlaender Rak Consulting Ventura, California

Louise Deschênes Agriculture & Agri-Food Canada St. Hyacinthe, Quebec, Canada Charlotte L. Deuel Medical Ambassadors International Modesto, California

Bor S. Luh Formerly of the University of California Davis, California Jatal D. Mannapperuma University of California Davis, California

Milagros I. Dolores Quezon City, Philippines

Michele Marcotte Agriculture and Agri-Food Canada St. Hyacinthe, Quebec, Canada

Elisabeth Garcia University of California Davis, California

N.R. Markarian CRDH, Agriculture & Agri-Food Canada St. Jean-Sur-Richelieu, Quebec, Canada

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Marvin H. Martin Madera, California Mark R. McLellan Texas A&M University College Station, Texas Justin R. Morris University of Arkansas Fayetteville, Arkansas Arun S. Mujumdar National University of Singapore Singapore Brendan A. Niemira USDA-ARS Wyndmoor, Pennsylvania Francois Nolle Valley Research, Inc. South Bend, Indiana Olga I. Padilla-Zakour Cornell University Geneva, New York Mickey E. Parish University of Florida Lake Alfred, Florida Thomas J. Payne San Mateo, California

Hosahalli S. Ramaswamy McGill University St. Anne de Bellevue, Quebec, Canada Cristina Ratti Laval University St. Foy, Quebec, Canada David S. Reid University of California–Davis Davis, California William H. Root Pacific International Technology Walnut Creek, California Ralph Scorza USDA-ARS Kearneysville, West Virginia James P. Smith McGill University St. Anne de Bellevue, Quebec, Canada Pedro Solé Management Consultant Hayward, California Laszlo P. Somogyi Kensington, California Don F. Splittstoesser (Deceased) Formerly of Cornell University Geneva, New York

Anne D. Perera Crop & Food Research Institute of New Zealand Palmerston North, New Zealand

Ken Stewart GusmerCellulo Hillsboro, Oregon

Conrad O. Perera National University of Singapore Singapore

R. Keith Striegler University of Arkansas Fayetteville, Arkansas

Anne Plotto USDA-ARS Winter Haven, Florida

Daqing Sun Valley Research Inc. South Bend, Indiana

G.S.V. Raghavan McGill University St. Anne de Bellevue, Quebec, Canada

Clement Vigneault CRDH, Agriculture & Agri-Food Canada St. Jean-Sur-Richelieu, Quebec, Canada

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Yu-Ping Wei Chung-Hua University Hsinchu, Taiwan

Ming-Chang Wu National Pingtung University Pingtung, Taiwan

Randy W. Worobo Cornell University Geneva, New York

Devon Zagory Davis Fresh Technologies, LLC Davis, California

James Swi-Bea Wu National Taiwan University Taipei, Taiwan

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Contents PART I Biology, Principles, and Applications Chapter 1

Classification, Composition of Fruits, and Postharvest Maintenance of Quality .......3 Adel A. Kader and Diane M. Barrett

Chapter 2

Refrigerated and Controlled/Modified Atmosphere Storage .....................................23 G.S.V. Raghavan, Clement Vigneault, Yvan Gariépy, N.R. Markarian, and P. Alvo

Chapter 3

Fresh-Cut Fruits..........................................................................................................53 Elisabeth Garcia and Diane M. Barrett

Chapter 4

Juice Processing..........................................................................................................73 Mark R. McLellan and Olga I. Padilla-Zakour

Chapter 5

Enzymes in the Fruit Juice and Wine Industry .........................................................97 Alberecht Höhn, Daqing Sun, and Francois Nolle

Chapter 6

Fruit Preserves and Jams..........................................................................................113 Robert A. Baker, Norman Berry, Y.H. Hui, and Diane M. Barrett

Chapter 7

Drying of Fruits........................................................................................................127 Cristina Ratti and Arun S. Mujumdar

Chapter 8

Fruit Freezing ...........................................................................................................161 David S. Reid and Diane M. Barrett

Chapter 9

Thermal Processing of Fruits ...................................................................................173 Hosahalli S. Ramaswamy

Chapter 10 Novel Processing Technologies for Food Preservation ...........................................201 Hosahalli S. Ramaswamy, Cuiren Chen, and Michele Marcotte Chapter 11 Ionizing Radiation Processing of Fruits and Fruit Products ...................................221 Brendan A. Niemira and Louise Deschênes

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Chapter 12 Microbiology of Fruit Products ...............................................................................261 Randy W. Worobo and Don F. Splittstoesser Chapter 13 Direct Food Additives in Fruit Processing...............................................................285 Laszlo P. Somogyi Chapter 14 Quality Assurance, Quality Control, Inspection, and Sanitation.............................339 Conrad O. Perera and Anne D. Perera Chapter 15 Packaging of Fruits and Vegetables .........................................................................355 James P. Smith, Devon Zagory, and Hosahalli S. Ramaswamy Chapter 16 Grades, Standards, and Food Labeling ....................................................................397 Y.H. Hui and Charles Huxsoll Chapter 17 Residual Management in Fruit Processing Plants ...................................................423 Jatal D. Mannapperuma

PART II Major Processed Products Chapter 18 Apples and Apple Processing...................................................................................455 William H. Root and Diane M. Barrett Chapter 19 Peach and Apricot ....................................................................................................481 Ralph Scorza Chapter 20 Sweet Cherry and Sour Cherry Processing .............................................................497 Mark R. McLellan and Olga I. Padilla-Zakour Chapter 21 Plums and Prunes .....................................................................................................513 Laszlo P. Somogyi Chapter 22 Strawberries and Raspberries ...................................................................................531 Charlotte L. Deuel and Anne Plotto Chapter 23 Processing of Cranberry, Blueberry, Currant, and Gooseberry ...............................563 Ken Stewart

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Chapter 24 Grape Juice: Factors That Influence Quality, Processing Technology, and Economics..........................................................................................................585 Justin R. Morris and R. Keith Striegler Chapter 25 Oranges and Tangerines ...........................................................................................617 Dan Kimball, Mickey E. Parish, and Robert Braddock Chapter 26 Grapefruits, Lemons, and Limes ..............................................................................639 Dan Kimball, Robert Braddock, and Mickey Parish Chapter 27 Bananas (Processed).................................................................................................657 Pedro Solé Chapter 28 Tropical Fruits ..........................................................................................................679 James Swi-Bea Wu, Ming-Chang Wu, and Yu-Ping Wei Chapter 29 Coconut.....................................................................................................................707 Sonia Y. de Leon and Milagros I. Delores Chapter 30 Avocados...................................................................................................................739 Azriel Kurlaender Chapter 31 Olives ........................................................................................................................751 Elisabeth L. Garcia, Bor S. Luh, and Marvin H. Martin Chapter 32 Nuts...........................................................................................................................765 Thomas J. Payne Index ..............................................................................................................................................811

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Part I Biology, Principles, and Applications

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Composition of 1 Classification, Fruits, and Postharvest Maintenance of Quality Adel A. Kader and Diane M. Barrett CONTENTS 1.1

1.2

1.3

1.4 1.5

1.6

Classification of Fruits ...........................................................................................................5 1.1.1 Temperate-Zone Fruits.............................................................................................5 1.1.2 Subtropical Fruits ....................................................................................................5 1.1.3 Tropical Fruits..........................................................................................................5 Contribution of Fruits to Human Nutrition ...........................................................................5 1.2.1 Energy (Calories).....................................................................................................5 1.2.2 Vitamins ...................................................................................................................5 1.2.3 Minerals ...................................................................................................................6 1.2.4 Dietary Fiber............................................................................................................6 1.2.5 Antioxidants.............................................................................................................6 Factors Influencing Composition and Quality of Fruits........................................................6 1.3.1 Preharvest Factors....................................................................................................6 1.3.2 Maturity at Harvest and Harvesting Method ..........................................................7 1.3.3 Postharvest Factors ..................................................................................................7 Fruit Maturity, Ripening, and Quality Relationships ............................................................7 Composition and Compositional Changes ............................................................................8 1.5.1 Carbohydrates ..........................................................................................................8 1.5.2 Proteins ....................................................................................................................9 1.5.3 Lipids .......................................................................................................................9 1.5.4 Organic Acids ..........................................................................................................9 1.5.5 Pigments.................................................................................................................10 1.5.6 Phenolic Compounds .............................................................................................10 1.5.7 Volatiles..................................................................................................................12 1.5.8 Vitamins .................................................................................................................12 1.5.9 Minerals .................................................................................................................12 Biological Factors Involved in Postharvest Deterioration of Fruits ...................................13 1.6.1 Respiration .............................................................................................................13 1.6.2 Ethylene Production...............................................................................................13 1.6.3 Transpiration or Water Loss ..................................................................................14 1.6.4 Physiological Disorders .........................................................................................14 1.6.5 Physical Damage ...................................................................................................15 1.6.6 Pathological Breakdown ........................................................................................15

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Processing Fruits: Science and Technology, Second Edition

1.7

Environmental Factors Influencing Deterioration of Fruits ................................................16 1.7.1 Temperature ...........................................................................................................16 1.7.2 Relative Humidity..................................................................................................16 1.7.3 Air Movement........................................................................................................16 1.7.4 Atmospheric Composition .....................................................................................16 1.7.5 Ethylene .................................................................................................................16 1.8 Harvesting Procedures .........................................................................................................17 1.9 Postharvest Handling Procedures ........................................................................................17 1.9.1 Dumping ................................................................................................................17 1.9.2 Washing..................................................................................................................17 1.9.3 Sorting....................................................................................................................17 1.9.4 Sizing .....................................................................................................................17 1.9.5 Ripening.................................................................................................................18 1.9.6 Inhibiting Ethylene Action ....................................................................................18 1.9.7 Cooling...................................................................................................................18 1.9.8 Storage ...................................................................................................................18 1.9.9 Food Safety Guidelines .........................................................................................19 1.9.10 Food Security Guidelines ......................................................................................19 1.10 Summary: Keys to Successful Handling of Fresh Fruits....................................................19 1.10.1 Maturity and Quality .............................................................................................19 1.10.2 Temperature and Humidity Management Procedures...........................................19 1.10.3 Physical Damage ...................................................................................................20 1.10.4 Sanitation Procedures ............................................................................................20 1.10.5 Expedited Handling ...............................................................................................20 References ........................................................................................................................................20 Internet Sites ....................................................................................................................................21

The quality of processed fruit products depends on their quality at the start of processing; therefore, it is essential to understand how maturity at harvest, harvesting methods, and postharvest handling procedures influence quality and its maintenance in fresh fruits between harvest and process initiation. Using such information, an appropriate system for harvesting and handling each kind of fruit can be selected and used in conjunction with an effective quality control program to ensure the best quality possible for fresh fruits when processed. Quality attributes of fresh fruits include appearance, texture, flavor, and nutritive value. Appearance factors include size, shape, color, and freedom from defects and decay. Texture factors include firmness, crispness, and juiciness. Flavor components incorporate sweetness, sourness (acidity), astringency, bitterness, aroma, and off-flavors. Nutritional quality is determined by a fruit’s content of vitamins (A and C are the most important in fruits), minerals, dietary fiber, carbohydrates, proteins, and antioxidant phytochemicals (carotenoids, flavonoids, and other phenolic compounds). Safety factors that may influence the quality of fresh fruits include residues of pesticides, presence of heavy metals, mycotoxins produced by certain species of fungi, and microbial contamination. Losses in fresh fruits between harvest and processing may be quantitative (e.g., water loss, physical injuries, physiological breakdown, and decay) or qualitative (e.g., loss of acidity, flavor, color, and nutritive value). Many factors influence fruit quality and the extent of postharvest losses that can occur in the orchard, during transportation, and throughout the handling system (sorting, sizing, ripening, cooling, and storage). The total time between harvesting and processing may also be an important factor in maintaining the quality and freshness of fruit. Minimizing the delays throughout the postharvest handling system greatly reduces quality loss, especially in highly perishable fruits such as strawberries, raspberries, blackberries, apricots, and cherries.

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Classification, Composition of Fruits, and Postharvest Maintenance of Quality

5

1.1 CLASSIFICATION OF FRUITS Fruit are commonly classified by growing region as follows: temperate-zone, subtropical, and tropical. Growing region and environmental conditions specific to each region significantly affect fruit quality. Examples of fruit grown in each region are listed below.

1.1.1 TEMPERATE-ZONE FRUITS 1. Pome fruits: apple, Asian pear (nashi), European pear, quince 2. Stone fruits: apricot, cherry, nectarine, peach, plum 3. Small fruits and berries: grape (European and American types), strawberry, raspberry, blueberry, blackberry, cranberry

1.1.2 SUBTROPICAL FRUITS 1. Citrus fruits: grapefruit, lemon, lime, orange, pummelo, tangerine, and mandarin 2. Noncitrus fruits: avocado, cherimoya, fig, kiwifruit, olive, pomegranate

1.1.3 TROPICAL FRUITS 1. Major tropical fruits: banana, mango, papaya, pineapple 2. Minor tropical fruits: carambola, cashew apple, durian, guava, longan, lychee, mangosteen, passion fruit, rambutan, sapota, tamarind

1.2 CONTRIBUTION OF FRUITS TO HUMAN NUTRITION Fruits are not only colorful and flavorful components of our diet, but they also serve as a source of energy, vitamins, minerals, and dietary fiber. The U.S. Department of Agriculture Dietary Guidelines encourage consumers to enjoy “five a day,” i.e., eat at least two servings of fruit and three servings of vegetables each day and to choose fresh, frozen, dried, or canned forms of a variety of colors and kinds of fruits and vegetables. In some countries, consumers are encouraged to eat up to 10 servings of fruits and vegetables per day. For more information access one or more of the following Websites: www.nutrition.gov, www.5aday.gov, and www.5aday.org.

1.2.1 ENERGY (CALORIES) 1. Carbohydrates: banana, breadfruit, jackfruit, plantain, dates, prunes, raisin 2. Proteins and amino acids: nuts, dried apricot, fig 3. Fats: avocado, olive, nuts

1.2.2 VITAMINS 1. Fresh fruits and vegetables contribute about 91% of vitamin C, 48% of vitamin A, 27% of vitamin B6, 17% of thiamin, and 15% of niacin to the U.S. diet. 2. The following fruits are important contributors (based on their vitamin content and the amount consumed) to the supply of indicated vitamins in the U.S. diet: Vitamin A: apricot, peach, cherry, orange, mango, papaya, persimmon, pineapple, cantaloupe, watermelon Vitamin C: strawberry, orange, grapefruit, kiwifruit, pineapple, banana, apple, cantaloupe Niacin: peach, banana, orange, apricot Riboflavin: banana, peach, orange, apple, avocado Thiamin: orange, banana, grapefruit, apple

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Processing Fruits: Science and Technology, Second Edition

1.2.3 MINERALS 1. Fresh fruits and vegetables contribute about 26% of the magnesium and 19% of the iron to the U.S. diet. 2. The following fruits are important contributors to the supply of indicated minerals in the U.S. diet: Potassium: banana, peach, orange, apple, dried fruits such as apricot and prune Phosphorus: banana, orange, peach, fig, raisin Calcium: tangerine, grapefruit, orange Iron: strawberry, banana, apple, orange

1.2.4 DIETARY FIBER 1. All fruits and nuts contribute to dietary fiber. Dietary fiber consists of cellulose, hemicellulose, lignin, and pectic substances, which are derived primarily from fruit cell walls and skin. 2. The dietary fiber content of fruits ranges from 0.5 to 1.5% (fresh weight basis). 3. Dietary fiber plays an important role in relieving constipation by increasing water-holding capacity of feces. Its consumption is also linked to decreased incidence of cardiovascular disease, diverticulosis, and colon cancer.

1.2.5 ANTIOXIDANTS 1. Fruits, nuts, and vegetables in the daily diet have been strongly associated with reduced risk for some forms of cancer, heart disease, stroke, and other chronic diseases. This is attributed, in part, to their content of antioxidant phytochemicals. 2. Red, blue, and purple fruits (such as apple, blackberry, blueberry, blood orange, cranberry, grape, nectarine, peach, plum, prune, pomegranate, raspberry, and strawberry) are good sources of flavonoids and other phenolic compounds that are positively correlated with antioxidant capacity of the fruit. 3. Orange-flesh fruits (such as apricot, cantaloupe, mango, nectarine, orange, papaya, peach, persimmon, and pineapple) and some red-flesh fruits (such as tomato, watermelon, and pink grapefruit) are good sources of carotenoids. Availability of lycopene to humans is increased during tomato processing.

1.3 FACTORS INFLUENCING COMPOSITION AND QUALITY OF FRUITS 1.3.1 PREHARVEST FACTORS 1. Genetic: selection of cultivars, rootstocks. Cultivar and rootstock selection are important because there are often differences in raw fruit composition, postharvest-life potential, and response to processing. In many cases, fruit cultivars grown for fresh market sale are not optimal cultivars for processing. 2. Climatic: temperature, light, wind. Climatic factors may have a strong influence on nutritional quality of fruits. Light intensity significantly affects vitamin concentration, and temperature influences transpiration rate, which will affect mineral uptake and metabolism. 3. Cultural practices: soil type, soil nutrient and water supply, pruning, thinning, pest control. Fertilizer addition may significantly affect the mineral content of fruit, while other cultural practices such as pruning and thinning may influence nutritional composition by changing fruit crop load and size.

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Classification, Composition of Fruits, and Postharvest Maintenance of Quality

1.3.2 MATURITY

AT

HARVEST

AND

7

HARVESTING METHOD

Maturity at harvest is one of the primary factors affecting fruit composition, quality, and storage life. Although most fruits reach peak eating quality when harvested fully ripe, they are usually picked mature, but not ripe to decrease mechanical damage during postharvest handling. Harvesting may also mechanically damage fruit; therefore, choice of harvest method should allow for maintenance of quality.

1.3.3 POSTHARVEST FACTORS 1. Environmental: temperature, relative humidity, atmospheric composition. Temperature management is the most important tool for extension of shelf life and maintenance of the quality of fresh fruit. Relative humidity influences water loss, decay development, incidence of some physiological disorders, and uniformity of fruit ripening. Optimal relative humidity for storage of fruits is 85 to 90%. Finally, atmospheric composition (O2, CO2, and C2H4, in particular) can greatly affect respiration rate and storage life. 2. Handling methods: Postharvest handling systems involve the channels through which harvested fruit reaches the processing facility or consumer. Handling methods should be chosen such that they maintain fruit quality and avoid delays. 3. Time period between harvesting and consumption: Delays between harvesting and cooling or processing may result in direct losses (due to water loss and decay) and indirect losses (decrease in flavor and nutritional quality).

1.4 FRUIT MATURITY, RIPENING, AND QUALITY RELATIONSHIPS Maturity at harvest is the most important factor that determines storage life and final fruit quality. Immature fruits are more subject to shriveling and mechanical damage, and are of inferior quality when ripened. Overripe fruits are likely to become soft and mealy with insipid flavor soon after harvest. Fruits picked either too early or too late in the season are more susceptible to physiological disorders and have a shorter storage life than those picked at mid-season. With very few exceptions (e.g., pears, avocados, and bananas), all fruits reach their best eating quality when allowed to ripen on the tree or plant. In general, fruits become sweeter, more colorful, and softer as they mature. However, some fruits are usually picked mature but unripe so that they can withstand the postharvest handling system when shipped long distances. Most currently used maturity indices are based on a compromise between those indices that would ensure the best eating quality to the consumer and those that provide the needed flexibility in transportation and marketing. Fruits can be divided into two groups: (1) nonclimacteric fruits that are not capable of continuing their ripening process once removed from the plant, and (2) climacteric fruits that can be harvested mature and ripened off the plant. Following are examples of each group: Group 1: Berries (e.g., blackberry, cranberry, raspberry, strawberry), grape, cherry, citrus (grapefruit, lemon, lime, orange, mandarin, tangerine), pineapple, pomegranate, lychee, tamarillo, loquat. Group 2: Apples, pear, quince, persimmon, apricot, nectarine, peach, plum, kiwifruit, avocado, banana, plantain, mango, papaya, cherimoya, sapodilla, sapota, guava, passion fruit. Fruits of the first group (nonclimacteric) produce very small quantities of ethylene and do not respond to ethylene treatment except in terms of degreening (degradation of chlorophyll) in citrus fruits and pineapples. Fruits in Group 2 (climacteric) produce much larger quantities of ethylene in association with their ripening and exposure to ethylene treatment will result in faster and more uniform ripening.

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Maturity indices used vary among fruits and often among cultivars within a specific fruit, but generally include one or several (combination indices) of the following: fruit size and shape, overall color, ground color of the skin, flesh color, flesh firmness, soluble solids content, starch content, acidity, and internal ethylene concentration. Listed below are some examples of maturity indices that can be used for selected fruits: Index Elapsed days from full bloom to harvest Size Shape (fullness of fruit shoulders and suture) Dry Weight Color, external Color, internal Firmness Compositional factors Starch content Soluble solids content Acid content Sugar/acid ratio Tannin content

Fruits Apple, pear Most fruits Stone fruits Avocado, kiwifruit All fruits Stone fruits, mango Pome and stone fruits, berries Apple, pear Pome and stone fruits, grapes, kiwifruit Pomegranate Citrus fruits, grape Persimmon

1.5 COMPOSITION AND COMPOSITIONAL CHANGES The flesh of young developing fruits contains very little sugar, and the large amounts of starch, acid, and tannins make them inedible. As the fruits approach maturity, flesh cells enlarge considerably, and sugar content increases while starch, acid, and tannin contents decrease. In addition, certain volatile compounds develop, giving the fruit its characteristic aroma. Chlorophyll degradation (loss of green color) and synthesis of carotenoids (yellow and orange colors) and anthocyanins (red and blue colors) takes place both in the skin and the flesh with fruit ripening. All fruits soften as they ripen due to changes in cell wall composition and structure. In this section, we present an overview of fruit constituents in relation to quality and changes after harvest.

1.5.1 CARBOHYDRATES Carbohydrates are the most abundant and widely distributed food component derived from plants. Fresh fruits vary greatly in their carbohydrate content, with the general range between 10 to 25%. The structural framework, texture, taste, and food value of a fresh fruit is related to its carbohydrate content. Sucrose, glucose, and fructose are the primary sugars found in fruits (Table 1.1), and their relative importance varies among commodities. Sugars are found primarily in the cytoplasm and range from about 0.9% in limes to 16% in fresh figs. Sucrose content ranges from a trace in cherries, grapes, and pomegranate to more than 8% in ripe bananas and pineapple. Such variation influences taste since fructose is sweeter than sucrose, and sucrose is sweeter than glucose. Starch occurs as small granules within the cells of immature fruits. Starch is converted to sugar as the fruit matures and ripens. Other polysaccharides present in fruits include cellulose, hemicellulose, pectin, and lignin, which are found mainly (up to 50%) in cell walls and vary greatly among commodities. These large molecules are broken down into simpler and more soluble compounds as a result of fruit softening. The transformation of insoluble pectins into soluble pectins is controlled, for the most part, by the enzymes pectinesterase and polygalacturonase. Reduced activities of these two enzymes have been associated with reduced juiciness and poor texture in peaches that were ripened after storage at 1∞C for more than 3 weeks.

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TABLE 1.1 Sugar Composition of Selected Fruits Sugar (g/100 ml of Juice) Fruit Apple Cherry Grape Nectarine Peach Pear Plum Kiwifruit Strawberry

Sucrose 0.82 0.08 0.29 8.38 5.68 0.55 0.51 1.81 0.17

± ± ± ± ± ± ± ± ±

0.13 0.02 0.08 0.73 0.52 0.12 0.36 0.72 0.06

Glucose 2.14 7.50 9.59 0.85 0.67 1.68 4.28 6.94 1.80

± ± ± ± ± ± ± ± ±

0.43 0.81 1.03 0.04 0.06 0.36 1.18 2.85 0.16

Fructose 5.31 6.83 10.53 0.59 0.49 8.12 4.86 8.24 2.18

± ± ± ± ± ± ± ± ±

0.94 0.74 1.04 0.02 0.01 1.56 1.30 3.43 0.19

Sorbitol 0.20 ± 0.04 2.95 ± 0.33 ND 0.27 ± 0.04 0.09 ± 0.02 4.08 ± 0.79 6.29 ± 1.97 ND ND

Note: ND = not detected (less than 0.05 g/100 ml). Source: Van Gorsel, H., C. Li, E.L. Kerbel, M. Smits, and A.A. Kader. 1992. Compositional characterization of prune juice. J. Agric. Food Chem., 40: 784–789.

1.5.2 PROTEINS Fruits contain less than 1% protein (vs. 9 to 20% protein in nuts such as almond, pistachio, and walnut). Changes in the level and activity of proteins resulting from permeability changes in cell membranes may be involved in chilling injury. Enzymes, which catalyze metabolic processes in fruits, are proteins that are important in the reactions involved in fruit ripening and senescence. Some of the enzymes important to fruit quality include the following: Enzyme Polyphenoloxidase Polygalacturonase Pectinesterase Lipoxygenase Ascorbic acid oxidase Chlorophyllase

Action Catalyzes oxidation of phenolics, resulting in formation of brown polymers Catalyzes hydrolysis of glycosidic bonds between adjacent polygalacturonic acid residues in pectin; results in tissue softening Catalyzes deesterification of galacturonans in pectin; may result in tissue firming Catalyzes oxidation of lipids; results in off-odor and off-flavor production Catalyzes oxidation of ascorbic acid; results in loss of nutritional quality Catalyzes removal of phytol ring from chlorophyll; results in loss of green color

1.5.3 LIPIDS Lipids constitute only 0.1 to 0.2% of most fresh fruits, except for avocados, olives, and nuts. However, lipids are very important because they make up the surface wax that contributes to fruit appearance and the cuticle that protects the fruit against water loss and pathogens. Lipids are also important constituents of cell membranes. The degree of fatty acid saturation establishes membrane flexibility, with greater saturation resulting in less flexibility. Desaturation of fatty acids can occur upon chilling in chilling-sensitive fruits; in which case membranes undergo a phase change (liquid crystalline to solid gel) at chilling temperatures resulting in disruption of normal metabolism.

1.5.4 ORGANIC ACIDS Organic acids are important intermediate products of metabolism. The Krebs (TCA) cycle is the main channel for the oxidation of organic acids in living cells and it provides the energy required

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TABLE 1.2 Organic Acids of Selected Fruits Organic Acid (mg/100 ml of Juice) Fruit

Citric

Ascorbic

Apple Cherry Grape Kiwifruit Nectarine Peach Pear Plum Strawberry

ND ND tr 730 ± 92 140 ± 39 109 ± 16 ND ND 207 ± 35

tr tr tr 114 ± 6 tr tr tr tr 56 ± 4

Malic 518 727 285 501 383 358 371 294 199

± ± ± ± ± ± ± ± ±

32 20 58 42 67 72 16 24 26

Quinic

Tartaric

ND ND ND 774 ± 57 136 ± 28 121 ± 11 220 ± 2 214 ± 68 ND

ND ND 162 ± 24 tr ND tr ND ND ND

Note: ND = not detected, tr = trace (<10 mg/100 ml). Source: Van Gorsel, H., C. Li, E.L. Kerbel, M. Smits, and A.A. Kader. 1992. Compositional characterization of prune juice. J. Agric. Food Chem., 40: 784–789.

for maintenance of cell integrity. Organic acids are metabolized into many constituents, including amino acids, which are the building blocks of proteins. Most fresh fruits are acidic with a pH range of 3 to 5. Some fruits, such as lemons and limes, contain as much as 2 to 3% of their total fresh weight as acid. Total titratable acidity, specific organic acids present and their relative quantities, and other factors influence the buffering system and affect pH. Acid content usually decreases during ripening due to the utilization of organic acids during respiration or their conversion to sugars. Malic and citric acids are the most abundant in fruits (Table 1.2), except grapes (tartaric acid is the most important in most cultivars) and kiwifruits (quinic acid is the most abundant).

1.5.5 PIGMENTS Pigments, which are the chemicals responsible for skin and flesh colors, undergo many changes during the maturation and ripening of fruits. These include the following: 1. Loss of chlorophyll (green color), which is influenced by pH changes, oxidative conditions, and chlorophyllase activity 2. Synthesis and revelation of carotenoids (yellow and orange colors) 3. Development of anthocyanins (red, blue, and purple colors), which are fruit specific (Table 1.3) Beta-carotene is a precursor to vitamin A and thus is important in terms of nutritional quality. Carotenoids are very stable and remain intact in fruit tissues even when extensive senescence has occurred. Anthocyanins occur as glycosides in the cell sap. They are water soluble, unstable, and are readily hydrolyzed by enzymes to free anthocyanins, which may be oxidized by phenoloxidases to give brown oxidation products.

1.5.6 PHENOLIC COMPOUNDS Total phenolic content is higher in immature than in mature fruits and typically ranges between 0.1 and 2 g/100 g fresh weight. Fruit phenolics include chlorogenic acid, catechin, epicatechin,

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TABLE 1.3 Anthocyanins of Selected Fruits Anthocyanin Fruit

Identification

Apple

Cherry

Grape

Nectarine Peach Plum Strawberry

Cyanidin 3-arabinoside Cyanidin 7-arabinoside or cyanidin 3-glucoside Cyanidin 3-galactoside Cyanidin 3-glucoside Cyanidin 3-rutinoside Peonidin 3-rutinoside Cyanidin 3-glucoside Delphinidin 3-glucoside Malvidin 3-glucoside Peonidin 3-glucoside Cyanidin 3-glucoside Cyanidin 3-glucoside Cyanidin 3-glucoside Cyanidin 3-glucoside Pelargonidin 3-glucoside Pelargonidin 3-glycoside

Peak Area 22 27 260 189 1320 47 121 586 2157 478 322 180 42 70 1302 78

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

6 15 69 40 109 36 33 110 375 92 51 43 5 18 29 9

Source: Van Gorsel, H., C. Li, E.L. Kerbel, M. Smits, and A.A. Kader. 1992. Compositional characterization of prune juice. J. Agric. Food Chem., 40: 784–789.

leucoanthocyanidins, flavonols, cinnamic acid derivatives, and simple phenols. Chlorogenic acid (ester of caffeic acid) occurs widely in fruits (Figure 1.1) and is the main substrate involved in enzymatic browning of cut or otherwise damaged fruit tissues when exposed to air. Enzymatic browning occurs due to the oxidation of phenolic compounds and is mediated, in the presence of O2, by the enzyme polyphenoloxidase (PPO). The initial product of oxidation is usually O-quinone, which is highly unstable and undergoes polymerization to yield brown pigments of higher molecular weight. Polyphenoloxidase catalyzes the following two reactions: Monophenol + O2 æppO æÆ 2 0-quinone + H 2O 2 0-diphenol + O2 æppO æÆ 2 0-quinone + H 2O Normally phenolic compounds are separated from the PPO enzyme in the intact cells of plant tissue. Once the tissue is damaged, PPO and the phenolic compounds that it acts on are decompartmentalized and the above-mentioned reactions occur, leading to tissue browning. The extent of brown discoloration depends upon the total amount of phenolic compounds in the tissue and the level of PPO activity. Differences among cultivars of a given species in terms of browning potential in response to mechanical injury are related to the variation in the total phenolic content and PPO activity of the cultivar. Differences in phenylalanine ammonia lyase (PAL) enzyme activity influence phenolic content. Astringency is directly related to phenolic content, and it usually decreases with fruit ripening because of conversion of astringent phenolic compounds from the soluble to the insoluble nonastringent form. Loss of astringency occurs via (1) binding or polymerization of phenolics, (2) change in molecular size of phenolics, and (3) change in the hydroxylation pattern of phenolic compounds. There is a strong positive relationship between phenolic content and antioxidant capacity of fruits and their products.

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chlorogenic acid

(−)-epicatechin

2

caffeic acid

0

strawberry

plum

prune

pear

peach

kiwi

nectarine

apple

cherry

plum

prune

pear

ferulic acid

strawberry

4

peach

0

kiwi

2

0

nectarine

4

2 apple

6

4

cherry

8

6

prune

10

8

strawberry

12

10

pear

14

12

plum

16

14

peach

18

16

kiwi

20

18

nectarine

22

20

apple

24

22

cherry

Phenol concentration (mg per 100 ml juice)

(+)-catechin 24

phloridzin

4 2 0

FIGURE 1.1 Phenolic compounds of selected fruits.

1.5.7 VOLATILES Volatiles are responsible for the characteristic aroma of fruits. They are present in extremely small quantities (<100 mg/g fresh wt.). The total amount of carbon involved in the synthesis of volatiles is <1% of that expelled as CO2. The major volatile formed in climacteric fruits is ethylene (50 to 75% of the total carbon content of all volatiles). Ethylene does not have a strong aroma and does not contribute to typical fruit aromas. Volatile compounds are largely esters, alcohols, acids, aldehydes and ketones (low-molecularweight compounds). Very large numbers of volatile compounds have been identified in fruits and more are identified as advances in separation and detection techniques and gas chromatographic methods are made. However, only a few key volatiles are important for the particular aroma of a given fruit. Their relative importance depends upon threshold concentration (which can be as low as 1 ppb), potency, and interaction with other compounds.

1.5.8 VITAMINS The water-soluble vitamins include vitamins C, thiamin, riboflavin, niacin, vitamin B6, folacin, vitamin B12, biotin, and pantothenic acid. Fat-soluble vitamins include vitamins A, D, E, and K. Fat soluble vitamins are less susceptible to postharvest losses. Ascorbic acid is most sensitive to destruction when the commodity is subjected to adverse handling and storage conditions. Losses are enhanced by extended storage, higher temperatures, low relative humidity (which may cause wilting), physical damage, and chilling injury. Postharvest losses in vitamins A and B are usually much smaller than losses in vitamin C. They are, however, susceptible to degradation at high temperatures in the presence of oxygen.

1.5.9 MINERALS Important fruit minerals include base-forming elements (Ca, Mg, Na, and K) and acid-forming elements (P, Cl, S). Minerals present in microquantities include Fe, Cu, Co, Mn, Zn, I, and Mo. High nitrogen content is often associated with reduced soluble solids content, lower acidity, and increased susceptibility to physiological disorders in fruits.

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TABLE 1.4 Classification of Fruits According to Their Respiration Rates and Degree of Perishability under Optimum Conditions Relative Respiration Rate and Perishability

Fruits

Very low Low Moderate High Very high

Nuts, dates, dried fruits Apple, pear, kiwifruit, pomegranate, Chinese date (jujube) Citrus fruits, banana, cherry, nectarine, peach, plum, avocado (unripe), persimmon Apricot, fig (fresh), avocado (ripe), cherimoya, papaya Strawberry, blackberry, raspberry

Potassium is the most abundant mineral found in fruits. It most often occurs in combination with organic acids. High potassium content is often associated with increased acidity and improved color of fruits. Calcium is the second most important mineral constituent and is associated primarily with the cell wall. High calcium contents reduce CO2 and C2H4 production rates, delay ripening, reduce the incidence of physiological disorders, and extend the storage life of apples and other fruits. Calcium deficiency has been associated with several physiological disorders such as bitter pit of apples. Magnesium is a component of the chlorophyll molecule, which is responsible for the intensity of green color in fresh produce. Phosphorus is a constituent of cytoplasmic and nuclear proteins and plays a major role in carbohydrate metabolism and energy transfer. High phosphorus content may result in decreased acidity in some fruits.

1.6 BIOLOGICAL FACTORS INVOLVED IN POSTHARVEST DETERIORATION OF FRUITS 1.6.1 RESPIRATION Respiration is the process by which stored organic materials (carbohydrates, proteins, and fats) are broken down into simple end products with a release of energy. Oxygen (O2) is used in this process, and carbon dioxide (CO2) is produced. The loss of stored food reserves in the commodity during respiration hastens senescence as the reserves that provide energy to maintain the commodity’s living status are exhausted. For the consumer, food value (energy value) is gone, flavor quality is reduced, and sweetness, especially, is lost. Salable dry weight is also lost (especially important for commodities destined for dehydration). The energy released as heat, which is known as vital heat, affects postharvest technology considerations such as estimations of refrigeration and ventilation requirements. The rate of deterioration (degree of perishability) of fruits is generally proportional to their respiration rate (Table 1.4).

1.6.2 ETHYLENE PRODUCTION Ethylene, the simplest of the organic compounds affecting the physiological processes of plants, is a natural product of plant metabolism and is produced by all tissues of higher plants and by some microorganisms. As a plant hormone, ethylene regulates many aspects of growth, development, and senescence and is physiologically active in trace amounts (less than 0.1 ppm). Ethylene biosynthesis starts with the amino acid methionine, which is energized by ATP to produce S-adenosyl methionine (SAM). The key enzyme in the pathway, ACC synthase, converts SAM to 1-aminocyclopropane-1-carboxylic acid (ACC), which is converted to ethylene by the action of ACC oxidase.

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TABLE 1.5 Classification of Fruits According to Their Ethylene Production Ethylene Production Rate

Fruits

Very low Low Moderate High Very high

Cherry, citrus fruits, grape, jujube, strawberry, pomegranate Blueberry, cranberry, olive, persimmon, pineapple, raspberry, tamarillo Banana, fig, guava, mango, plantain Apple, apricot, avocado (ripe), nectarine, papaya, peach, pear, plum Cherimoya, passion fruit

Ethylene production rates, which depend on the fruit (Table 1.5), generally increase with maturity at harvest, physical injuries, disease incidence, increased temperatures up to 30∞C, and water stress. On the other hand, ethylene production rates by fresh fruits are reduced by storage at low temperature and by reduced O2 (less than 8%) and elevated CO2 (above 1%) levels in the storage environment around the commodity.

1.6.3 TRANSPIRATION

OR

WATER LOSS

Water loss is the main cause of deterioration because it results not only in indirect quantitative losses (loss of salable weight) but also in losses in appearance (wilting and shriveling), textural quality (softening, flaccidity, limpness, and loss of crispness and juiciness), and nutritional quality. The dermal system (outer protective coverings) governs the regulation of water loss by the commodity. This system includes the cuticle, epidermal cells, stomata, lenticels, and trichomes (hairs). The cuticle is composed of surface waxes, cutin embedded in wax, and a layer of mixtures of cutin, wax, and carbohydrate polymers. The thickness, structure, and chemical composition of the cuticle vary greatly among commodities and among developmental stages of a given commodity. Transpiration rate is influenced by internal or commodity factors (morphological and anatomical characteristics, surface-to-volume ratio, surface injuries, and maturity stage) and external or environmental factors (temperature, relative humidity, air movement, and atmospheric pressure). Transpiration (evaporation of water from the plant tissues) is a physical process that can be controlled by applying treatments to the commodity (e.g., waxes and other surface coatings or wrapping with plastic films) or by manipulation of the environment (e.g., maintenance of high relative humidity and control of air circulation).

1.6.4 PHYSIOLOGICAL DISORDERS The following physiological disorders occur in fruits: 1. Freezing injury when fruits are held below their freezing temperatures. The disruption caused by freezing usually results in immediate collapse of the tissues and total loss. 2. Chilling injury when fruits (mainly those of tropical and subtropical origin) are held at temperatures above their freezing point and below 5 to 15∞C depending on the commodity. This physiological injury is manifested in a variety of symptoms, which include surface and internal discoloration, pitting, water-soaked areas, uneven ripening or failure to ripen, off-flavor development, and accelerated incidence of surface molds and decay. A classification of fruits according to their sensitivity to chilling injury is shown in Figure 1.2.

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GROUP I

GROUP II

NON-CHILLING SENSITIVE COMMODITIES

HIGH TEMP. INJURY

°F 122

°C 50

113

45

104

40

95 86 77

35 30 25

68

20

59

15

50

10

41

5

32

0

23

−5

OPTIMUM RIPENING TEMP. RANGE FOR FRUITS

IDEAL TEMP. RANGE FOR TRANSIT AND STORAGE FREEZING INJURY

Apples* Apricots Bush berries Cherries Figs Grapes Kiwifruits

15

Nectarines Peaches Pears Persimmons* Plums Prunes Strawberries

CHILLING SENSITIVE COMMODITIES

HIGH TEMP. INJURY

OPTIMUM RIPENING TEMP. RANGE FOR FRUITS IDEAL TEMP. RANGE FOR TRANSIT AND STORAGE CHILLING INJURY FREEZING INJURY

Avocados Bananas Cherimoya Citrus Feijoa Guava Jujubes

Mangoes Olives Papayas Passion fruit Pineapples Plantain Pomegranates Sapota

*Some varieties are chilling sensitive.

FIGURE 1.2 Classification of fruits according to their sensitivity to chilling injury.

3. Heat injury results from exposure to direct sunlight or to excessively high temperatures. Symptoms include surface scalding, uneven ripening, and excessive softening and desiccation. 4. Certain types of physiological disorders originate from preharvest nutritional imbalances such as calcium deficiency, causing bitter pit of apples. 5. Very low (<1%) oxygen and elevated (>20%) carbon dioxide concentrations can result in physiological breakdown of all fruits.

1.6.5 PHYSICAL DAMAGE Various types of physical damage (surface injuries, impact bruising, and vibration bruising, etc.) are major contributors to deterioration. Mechanical injuries are not only unsightly but also accelerate water loss, stimulate higher respiration and ethylene production rates, and favor decay incidence.

1.6.6 PATHOLOGICAL BREAKDOWN Decay is one of the most common or apparent causes of deterioration. However, attack by many microorganisms usually follows mechanical injury or physiological breakdown of the commodity, which allow entry to the microorganism. In a few cases, pathogens can infect healthy tissues and become the primary cause of deterioration.

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TABLE 1.6 Current and Potential Use of Controlled Atmospheres to Maintain Quality during Transport and Storage of Fruits Range of Postharvest Life (Months)

Fruits

Up to 1 1 to 3 3 to 6 6 to 12 >12

Banana, mango, papaya, cherry, grape, fig, blackberry, blueberry, raspberry, strawberry Avocado, olive, persimmon, pomegranate, some peach, nectarine, and plum cultivars, Kiwifruit, some cultivars of Asian pears Some cultivars of apples and European pears Almond, filbert, macadamia, pecan, pistachio, walnut, dried fruits

1.7 ENVIRONMENTAL FACTORS INFLUENCING DETERIORATION OF FRUITS 1.7.1 TEMPERATURE Temperature is the most important environmental factor that influences the deterioration rate of harvested fruits. For each increase of 10∞C (18∞F) above the optimum temperature, the rate of deterioration increases by two- or threefold. Exposure to undesirable temperatures results in many physiological disorders as mentioned above. Temperature also influences how ethylene, reduced oxygen, and elevated carbon dioxide levels affect the commodity. The growth rate of pathogens is greatly influenced by temperature and some pathogens, such as Rhizopus rot, are sensitive to low temperatures. Thus, cooling of commodities below 5∞C immediately after harvest can greatly reduce Rhizopus rot incidence.

1.7.2 RELATIVE HUMIDITY The rate of water loss from fruits depends upon the vapor pressure deficit between the commodity and the surrounding ambient air, which is influenced by temperature and relative humidity.

1.7.3 AIR MOVEMENT Air circulation rate and velocity can influence the uniformity of temperature and relative humidity in a given environment and consequently rate of the water loss from the commodity.

1.7.4 ATMOSPHERIC COMPOSITION Reduction of oxygen and elevation of carbon dioxide, whether intentional (modified or controlled atmosphere storage) or unintentional, can have a beneficial or harmful effect on deterioration. The magnitude of these effects depends upon commodity, cultivar, physiological age, O2 and CO2 level, temperature, and duration of storage. A summary of current and potential use of controlled atmospheres to maintain the quality of fruits during transport and storage is given in Table 1.6.

1.7.5 ETHYLENE Ethylene is a natural plant hormone and its effect on harvested fruits can be desirable (degreening and ripening) or undesirable (abbreviated storage and softening). Ethylene effects are cumulative throughout the postharvest life of the fruit, and the magnitude of ethylene effects depends upon temperature, exposure time, and ethylene concentration. A concentration as low as 50 ppb ethylene

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enhances kiwifruit softening at 0∞C. Avocados and “Fuyu” persimmons are also very sensitive to ethylene action; exposure to 1 ppm (or higher) ethylene increases chilling injury symptoms. Use of ethylene to degreen citrus fruits can accelerate their senescence and increase their susceptibility to decay-causing pathogens.

1.8 HARVESTING PROCEDURES Harvesting methods, especially those involving a once-over procedure, may determine uniformity of maturity at harvest, which in turn influences quality of the fruit. Maturity also influences susceptibility of the fruit to water loss and mechanical injury. The harvesting system used and its management have a direct effect on incidence and severity of mechanical injuries. Such injuries can result in tissue browning, accelerated water loss, higher respiration and ethylene production rates, and increased decay incidence. Physical injuries may also induce some undesirable compositional changes, such as loss of ascorbic acid content and development of off-flavors. Management of the harvesting operation, whether manual or mechanical, can have a major impact on quality of the harvested fruits. Proper management procedures include selection of optimum time to harvest in relation to fruit maturity and climatic conditions, training and supervision of workers, and effective quality control.

1.9 POSTHARVEST HANDLING PROCEDURES 1.9.1 DUMPING Fresh fruits should be handled with care throughout the postharvest handling system in order to minimize mechanical injuries. Dumping in water or in floatation tanks should be used for fruits that withstand wetting. If dry dumping systems are used, they should be well padded to reduce impact bruising. Also, a bin cover may be used to permit inverting the bin and to regulate the flow of fruits out of the bin.

1.9.2 WASHING To clean fruit, water alone or with added cleaning agents and/or chlorine (typically 100 to 150 ppm) may be used. The final rinse should be made with fresh, clean water. Following washing, removal of excess surface water may be necessary and this can be done by blotting rollers or by air flow over the fruits.

1.9.3 SORTING Manual sorting is usually carried out to eliminate fruit exhibiting defects or decay. For some fruits, it may also be necessary to sort the fruit into two or more classes of maturity or ripeness (according to their color and firmness) before ripening or processing. Mechanical sorters, which operate on the basis of color, soluble solids, moisture, or fat content, are being implemented and may greatly reduce time and labor requirements.

1.9.4 SIZING In some cases, sizing the fruits into two or more size categories may be required before processing. Sizing can be done mechanically on the basis of fruit dimension or by weight. Mechanical sizing can be a major source of physical damage to the fruit if the machines were not adequately padded and adjusted to the minimum possible fruit drop heights.

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1.9.5 RIPENING Ripening before processing may be required for certain fruits (e.g., avocado, banana, kiwifruit, mango, nectarine, papaya, peach, pear, persimmon, plum, and melons) that are picked immature. Ethylene treatment can be used to obtain faster and more uniform ripening. The optimum temperature range for ripening is 15 to 25∞C and within this range, the higher the temperature the faster the ripening. Relative humidity should be maintained between 90 and 95% during ripening. Although 10 ppm ethylene is sufficient to initiate ripening, a concentration of 20 to 100 ppm for at least 2 d is recommended for commercial application. Adequate air circulation within the room is important to ensure uniform distribution of ethylene. One method to achieve this is by forcing the ethylene-containing air through the fruit containers (forced-air ripening or pressure ripening). It is also important to avoid accumulation of carbon dioxide (produced by the commodity through respiration) above 1% in the ripening room since carbon dioxide counteracts ethylene effects. This can be accomplished by periodic air exchange (introduction of fresh air into the ripening room) or by using hydrated lime to absorb carbon dioxide.

1.9.6 INHIBITING ETHYLENE ACTION Since July 2002, the ethylene-action inhibitor, 1-methylcyclopropene gas (1-MCP) under the trade name “SmartFresh” at concentrations up to 1 ppm was approved by the U.S. Environmental Protection Agency for use on apples, apricots, avocados, kiwifruit, mangoes, nectarines, papayas, peaches, pears, persimmons, plums, and tomatoes. The first commercial application has been on apples to retard their softening and scald development and extend their postharvest life. As more research is completed, the use of 1-MCP will no doubt be extended to several other commodities in the future.

1.9.7 COOLING Cooling is utilized to remove field heat and lower the fresh fruit’s temperature to near its optimum storage temperature. Cooling can be done using cold water (hydrocooling) or cold air (forced-air cooling or pressure cooling). Highly perishable fruits, such as strawberries, bush berries, and apricots should be cooled to near 0∞C (32∞F) within 6 h of harvest. Other fruits should be cooled to their optimum temperature within 12 h of harvest.

1.9.8 STORAGE Short-term or long-term storage of fresh fruits may be needed before processing to regulate the product flow and extend the processing season. A classification of fresh fruits according to their optimum storage temperature and potential storage life is shown in Table 1.7. In all cases, the relative humidity in the storage facility should be kept between 90 and 95%. To reduce decay, elevated CO2 (15 to 20%) may be added to the atmosphere within pallet covers for strawberries, bush berries, and cherries; sulfur dioxide (200 ppm) fumigation may be used on table grapes. There is a continuing trend toward increased precision in temperature and relative humidity management to provide the optimum environment for fresh produce during cooling, storage, and transport. Precision temperature management tools, including time–temperature monitors, are becoming more common in cooling and storage facilities. Several manufacturers have developed self-contained temperature and RH monitors and recorders, which are small and can be packed in a box with the product. Data are read by connecting these units to a personal computer with the appropriate software made by the manufacturer. Infrared thermometers are used to measure surface temperature of products from a distance in various locations within storage facilities. Electronic thermometers (with very thin, strong probes for fast response) are used for measuring product temperature during cooling, storage, and transport operations.

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TABLE 1.7 Classification of Fruits According to Their Optimum Storage Temperatures and Potential Storage Life Optimum Storage Temperatures

Potential Storage Life (Weeks) <2 2 to 4 4 to 6 >6

0 to 2∞C Apricot, bush berries, strawberry, fig Cherry, nectarine, peach, plum Grape, tamarillo Apple (nonchilling sensitive cultivars), pear, cranberry, kiwifruit, “Hachiya” persimmon, date

3 to 5∞C

12 to 14∞C

Cantaloupe, ripe avocado

Cherimoya, guava, pineapple

Tangerine and mandarin, carambola Orange, pomegranate, kumquat Apple (chilling sensitive cultivars)

Avocado, banana, mango, papaya, honeydew melon Grapefruit, lime, pummelo Lemon, “Fuyu” persimmon

1.9.9 FOOD SAFETY GUIDELINES Over the past few years, food safety has become and continues to be the number one concern of the fresh produce industry. A training manual for trainers titled Improving the Safety and Quality of Fresh Fruits and Vegetable was published by the USFDA in November 2002 to provide uniform, broad-based scientific and practical information on the safe production, handling, storage, and transport of fresh produce. It is available electronically (in English and Spanish) at the following Internet site: http://www.jifsan.umd.edu/gaps.html.

1.9.10 FOOD SECURITY GUIDELINES On March 19, 2003, The U.S. Food and Drug Administration (FDA) released food security guidance documents for food producers, processors, and transporters. These documents are available electronically at http://www.cfsan.fda.gov/~dms/secguid6.html and are voluntary recommendations from the FDA and not mandatory regulations. The goal is to help operators of food handling facilities identify preventive measures to minimize the security risks to their products.

1.10 SUMMARY: KEYS TO SUCCESSFUL HANDLING OF FRESH FRUITS 1.10.1 MATURITY

AND

QUALITY

1. Harvest at the proper maturity stage that will result in the best eating quality. 2. Eliminate fruits with defects in the orchard or soon after delivery to the processing plant.

1.10.2 TEMPERATURE

AND

HUMIDITY MANAGEMENT PROCEDURES

1. Harvest during the cool part of the day. 2. Keep in the shade while accumulating fruits in the orchard. 3. Transport fruits to the processing plant as soon as possible after harvest and use refrigerated transport vehicles for distances that require more than a few hours. 4. Avoid delays at the processing plant. If delays cannot be avoided, cool and hold fruits at or near their optimum storage temperature until processed. 5. Maintain proper temperature and relative humidity during ripening of fruits requiring such treatment (with or without added ethylene).

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1.10.3 PHYSICAL DAMAGE 1. Handle fruits with care during harvesting, hauling to the processing plant, and during handling operations within the plant. 2. Avoid drops, impacts, vibrations, and surface injuries of fruits throughout the handling system. 3. Use containers that would provide adequate protection of the commodity from physical injuries when stacked during temporary storage.

1.10.4 SANITATION PROCEDURES 1. Sort out and properly discard decayed fruits. 2. Clean harvest containers, processing plant machinery, cooling and storage facilities, and transit vehicles periodically with water, soap, and disinfectants.

1.10.5 EXPEDITED HANDLING 1. Reduce the time between harvest and cooling the fruits. 2. Avoid exceeding the fruit’s storage life, based on flavor quality, before processing.

REFERENCES Abeles, F.B., P.W. Morgan, and M.E. Saltveit, Jr. 1992. Ethylene in Plant Biology. 2nd ed. Academic Press, San Diego, CA. Biale, J.B. and R.E. Young. 1981. Respiration and ripening in fruits — retrospect and prospect. In: J. Friend and M.J.C. Rhodes (Eds.). Recent Advances in the Biochemistry of Fruits and Vegetables. Academic Press, New York. p. 1–39. Blankenship, S.M. and J.M. Dole. 2003. 1-Methylcyclopropene: a review. Postharvest Biol. Technol., 28:1–25. Brady, C.J. 1987. Fruit ripening. Annu. Rev. Plant Physiol., 38:155–178. Calderon, M. and R. Barkai-Golan (Eds.). 1990. Food Preservation by Modified Atmospheres. CRC Press, Boca Raton, FL. Champ, B.R., E. Highley, and G.I. Johnson (Eds.). 1994. Postharvest handling of tropical fruits. International Conference Proceedings. The Australian Centre for International Agricultural Research, Canberra, Australia. Eskin, N.A.M. (Ed.). 1991. Quality and Preservation of Fruits. CRC Press, Boca Raton, FL. Gross, J. 1987. Pigments in Fruits. Academic Press, Orlando, FL. Gross, K., C-Y. Wang, and M.E. Saltveit (Eds.). 2002. The commercial storage of fruits, vegetables, and florist and nursery stocks. U.S. Department of Agriculture, Agricultural Handbook, 66 (http://www.ba.ars.usda.gov/hb66/index.html). Hardenburg, R.E., A.E. Watada, and C-Y. Wang. 1986. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. U.S. Department of Agriculture, Agricultural Handbook, 66. Hulme, A.C. (Ed.). 1971. The Biochemistry of Fruits and Their Products, Vol. 1 (620 p.) and Vol. 2. (788 p.) Academic Press, New York. Kader, A.A. 1986a. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 40(5): 99–100, 102–104. Kader, A.A. 1986b. Potential applications of ionizing radiation in postharvest handling of fresh fruits and vegetables. Food Technol. 40(6): 117–121. Kader, A.A. 1999. Fruit maturity, ripening, and quality relationships. Acta Hortic., 485: 203–208. Kader, A.A. (Ed.). 2002. Postharvest Technology of Horticultural Crops. 3rd ed. University of California, Division of Agriculture and Natural Resources, Publication No. 3311. Kader, A.A. 2003. A summary of CA requirements and recommendation for fruits other than pome fruits. Acta Hortic., 600: 737–740. Kader, A.A., D. Zagory, and E.L. Kerbel. 1989. Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr., 28(1): 1–30.

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Kalt, W. 2002. Health functional phytochemicals of fruits. Hortic. Rev., 27: 269–315. Knee, M. (Ed.). 2002. Fruit Quality and Its Biological Basis. Sheffield Academic Press, U.K. Lee, C.Y. and J.R. Whitaker (Eds.). 1995. Enzymatic Browning and Its Prevention. ACS Symposium Series, Vol. 600. American Chemical Society, Washington, D.C. Lee, S.K. and A.A. Kader. 2000. Preharvest and postharvest factors influencing vitamin C content of horticultural crops. Postharvest Biol. Technol., 20: 207–220. Macheix, J., A. Fleuriet, and J. Billot. 1990. Fruit Phenolics. CRC Press, Boca Raton, FL. Mitra, S. (Ed.). 1997. Postharvest Physiology and Storage of Tropical and Subtropical Fruits. CAB International, Wallingford, U.K. Morton, I.D. and A.M. MacLeod (Eds.). 1990. Part C. The flavour of fruits. In Food Flavours. Elsevier Science Publishers, Amsterdam. Nagy, S. and P.E. Shaw (Eds.). 1980. Tropical and Subtropical Fruits: Composition, Properties, and Uses. AVI Publishing, Westport, CT. Nagy, S., P.E. Shaw, and W.F. Wardowski (Eds.). 1990. Fruits of Tropical and Subtropical Origin: Composition, Properties, and Uses. Florida Science Source, Lake Alfred, FL. O’Brien, M., B.F. Cargill, and R.B. Fridley. 1983. Principles and Practices for Harvesting and Handling of Fruits and Nuts. AVI Publishing, Westport, CT. Pattee, H.E. (Ed.). 1985. Evaluation of Quality of Fruits and Vegetables. AVI Publishing, Westport, CT. Paull, R.E. 1990. Postharvest heat treatments and fruit ripening. Postharvest News Inform. 1(5): 355–363. Paull, R.E. and J.W. Armstrong (Eds.). 1994. Insect Pests and Fresh Horticultural Products: Treatments and Responses. CAB International, Wallingford, U.K. Prior, R.L. and G. Cao. 2000. Antioxidant phytochemicals in fruits and vegetables: diet and health implications. HortScience, 35: 588–592. Ryall, A.L. and W.T. Pentzer. 1982. Handling, Transportation, and Storage of Fruits and Vegetables. Vol. 2. Fruits and tree nuts. AVI Publishing, Westport, CT. Salunkhe, D.K. and B.B. Desai. 1984. Postharvest Biotechnology of Fruits. Vol. 1 (168 p.) and Vol. 2 (147 p.). CRC Press, Boca Raton, FL. Salunkhe, D.K. and S.S. Kadam (Eds.). Handbook of Fruit Science and Technology: Production, Composition, Storage, and Processing. Marcel Dekker, New York. Seymour, G.B., J.E. Taylor, and G.A. Tucker (Eds.). 1993. Biochemistry of Fruit Ripening. Chapman & Hall, London. Shaw, P.E., H.T. Chan, Jr., and S. Nagy (Eds.). 1998. Tropical and Subtropical Fruits. AgScience, Auburndale, FL. Shewfelt, R.L. and S.E. Prussia (Eds.). 1993. Postharvest Handling: A Systems Approach. Academic Press, San Diego, CA. Snowdon, A.L. 1990. A Color Atlas of Postharvest Diseases and Disorders of Fruits and Vegetables. Volume 1: General Introduction and Fruits. CRC Press, Boca Raton, FL. Tomas-Barberan, F.A. and R.J. Robins (Eds.). 1997. Phytochemistry of Fruits and Vegetables. Oxford Science, Oxford, U.K. USDA. 2003. Composition of fruits and fruit juices, raw, processed, prepared. U.S. Department of Agriculture, Nutrient Data Laboratory (http://www.nal.Usda.gov/fnic/foodcomp). Vamos-Vigyazo, L. 1981. Polyphenol oxidase and peroxidase in fruits and vegetables. Crit. Rev. Food Sci. Nutr. 15: 49–92. Van Gorsel, H., C. Li, E.L. Kerbel, M. Smits, and A.A. Kader. 1992. Compositional characterization of prune juice. J. Agric. Food Chem., 40: 784–789. Wang, C-Y. (Ed.). 1990. Chilling Injury of Horticultural Crops. CRC Press, Boca Raton, FL. Yang, S.F. and N.E. Hoffman. 1984. Ethylene biosynthesis and its regulation in higher plants. Annu. Rev. Plant. Physiol., 35: 155–189.

INTERNET SITES http://postharvest.ucdavis.edu: University of California Postharvest Research and Information Center. http://www.uckac.edu/postharv: University of California Kearney Agricultural Center. http://postharvest.ifas.ufl.edu: University of Florida Postharvest Group.

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http://www.fdocitrus.com: Florida Department of Citrus postharvest information. http://www.postharvest.tfrec.wsu.edu: Washington State University postharvest information. http://www.ams.usda.gov: U.S. Department of Agriculture, Agricultural Marketing Service information on quality standards, transportation, and marketing. http://www.ams.usda.gov/nop/: National Organic Program Standards. http://www.nutrition.gov: Gateway to U.S. government information on human nutrition and nutritive value of foods. http://www.nal.usda.gov/fnic/foodcomp: Composition of foods.

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and 2 Refrigerated Controlled/Modified Atmosphere Storage G.S.V. Raghavan, Clement Vigneault, Yvan Gariépy, N.R. Markarian, and P. Alvo This chapter is dedicated to Peter Alvo

CONTENTS 2.1 2.2

2.3

Introduction ..........................................................................................................................24 2.1.1 What Are Controlled Storage and Modified Atmosphere Storage? .....................25 Modification of Respiration in Low O2/ High CO2 Atmospheres ......................................26 2.2.1 Respiratory Quotient..............................................................................................29 2.2.2 Factors Influencing Storability ..............................................................................30 Creating Optimal Storage Conditions .................................................................................30 2.3.1 Modified Atmospheres (Passive Establishment) ...................................................30 2.3.1.1 Design Objective: Approximating Steady State..................................32 2.3.1.2 Equations for Calculating Membrane Area Required.........................34 2.3.1.3 Factors Affecting Time to Stabilization ..............................................35 2.3.1.4 Relative Stacking Volume ....................................................................35 2.3.1.5 Design Gas Levels ...............................................................................35 2.3.1.6 Standard Free Volume..........................................................................36 2.3.1.7 Infiltration Rate ....................................................................................36 2.3.1.8 Silicone Membranes.............................................................................36 2.3.2 Commercial Modified Atmosphere Systems.........................................................36 2.3.2.1 The Pallet Package System..................................................................36 2.3.2.2 The Marcellin System..........................................................................37 2.3.2.3 The Atmolysair System .......................................................................38 2.3.3 Controlled Atmospheres (Active Establishment) ..................................................38 2.3.3.1 Oxygen Control Systems .....................................................................39 2.3.3.2 CO2 Control Systems...........................................................................41 2.3.3.3 Ethylene Control Systems ...................................................................42 2.3.4 Automated CA Control Systems...........................................................................43 2.3.4.1 Control Strategy ...................................................................................44 2.3.4.2 Control Devices ...................................................................................44 2.3.4.3 PC-Based Systems ...............................................................................44 2.3.4.4 Commercial Systems ...........................................................................45 2.3.5 Instrumentation ......................................................................................................45 2.3.5.1 Temperature..........................................................................................45 2.3.5.2 Relative Humidity ................................................................................45 2.3.5.3 Oxygen .................................................................................................45

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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2.3.5.4 Carbon Dioxide....................................................................................45 Precooling.............................................................................................................................46 2.4.1 Room Cooling........................................................................................................46 2.4.2 Forced-Air Cooling................................................................................................47 2.4.3 Hydrocooling .........................................................................................................48 2.5 Economic Considerations ....................................................................................................49 Acknowledgments ............................................................................................................................49 References ........................................................................................................................................50 2.4

2.1 INTRODUCTION One can readily imagine that research and development have greatly influenced human society. The opposite view that society has influenced research, is perhaps less common, but is nonetheless equally imaginable. For example, one can entertain the notion that the advent of methods of food preservation and storage had an important influence on the stability and geographical expansion of human civilization. Today, it should seem evident that society’s instinct for survival has given rise to research aimed, directly or indirectly, at eliminating negative anthropogenic impacts on the environment and human contact with chemicals that ultimately damage the body. Society’s concerns have thus created the reactionary movement on the rise in research and development that we are presently witnessing. To what extent this movement will reach its goal of a “clean” planet will depend on the development of technologies that cater to both “sound business” and environmental protection in its broadest sense. Over the last decade, agriculture has been pinpointed as an industry having a significant negative impact on the environment and, in some areas, an equally negative impact on human health. The main environmental impact is the pollution of water resources, including sources of drinking water, by nitrates and pesticides. Effects on health are due to direct contact with toxic chemicals in the preparation and application of pesticides and to ingestion of contaminated water, as well as ingestion of food containing pesticides and their residues. Agriculture has also been implicated as a major contributor to the greenhouse effect because of the partial conversion of nitrogen fertilizer to nitric and nitrous oxide, rapid oxidation of organic matter to carbon dioxide, and the formation of methane from animal manures. Agricultural research must respond by developing technologies that simultaneously minimize or eliminate these problems and permit agricultural producers to survive economically. It is clear that most of agriculture’s negative impact is connected with the production phase — fertilization, tillage, and chemical methods of crop protection. However, there is also substantial use of chemicals as well as energy consumption in the postharvest phase, particularly to protect the harvest from insects and bacterial and fungal pathogens, and to induce or inhibit ripening of fruits and vegetables. One can add to this the coating of seeds with fungicides to protect seedlings at the start of the next production phase. Postharvest energy uses include energy for refrigeration, the huge amounts of fossil fuel spent in grain drying, and energy use in transportation. The energy costs per unit of production are all the more substantial if the postharvest losses (in storage, in transport, at the retail outlet, and after consumer purchase) are taken into consideration. Postharvest engineering research has responded by concentrating on technologies aimed at reducing postharvest losses while eliminating the use of chemicals and reducing the use of combustible energy sources. This chapter deals with a specific area of postharvest research — storage technology. The focus will be on storage of fruit in controlled (CA) and modified (MA) atmospheres. When it pertains to fruit (and vegetables), the term storage is understood to imply “keeping produce in fresh condition.” This leads immediately to a differentiation between the term storage and the term preservation. Although the modern objectives of both storage and preservation are similar — that is, to ensure a continuous supply of food and facilitate marketing and distribution — there are subtle differences in how products that have been processed are perceived in relation to those that

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are fresh. Even though dried, canned, pickled, and frozen foods like fruit, vegetables, meat, and fish may have an equivalent or even better nutritional quality than their fresh counterparts, the perception of most people is that the fresh product tastes better, looks better, and has higher nutritional quality. In today’s increasingly nutritionally and environmentally conscious society, many people even perceive the fresh product to be more environmentally “friendly,” in the sense that it takes fewer resources to produce and less packaging material for eventual disposal. In this sense, storage technology can fit nicely into the ecological, environmentally-friendly ideology. CA storage and MA storage, which are the main topics of this chapter, not only cater to this revised outlook on food production but also represent alternative solutions to the problem of seasonal availability of certain products (which at one time [ca. 1965 to 1980] were believed to be found in year-round production in controlled growth environments, i.e., greenhouses). While CA and MA storage can provide an efficient business advantage to those members of the agrifood industry that adopt them, these technologies also do their part in reducing the need for chemical preservatives and pesticides, as well as the energy requirements for refrigeration.

2.1.1 WHAT ARE CONTROLLED STORAGE AND MODIFIED ATMOSPHERE STORAGE? Fruit storage technology deals essentially with the inhibition of natural, physical, pest-induced, and pathogen-induced decay and damage without going to extremes such as drying or freezing. The object is to maintain fresh quality as long as possible or as long as necessary, depending on market conditions (supply, demand, influx of imports, etc.). The two broad categories of obstacles in achieving these objectives are (1) the biochemical and physiological activities that proceed within the fruit itself after harvest; and, (2) the introduction and proliferation of microbial and fungal pathogens, and insects in the storage environment. It is noted that these two categories are not necessarily independent in action and certainly not with respect to the concept of an optimal storage atmosphere, since one or the other may be the limiting factor for storability. CA storage and MA storage are two sides of the same coin. The main purpose of both is to reduce as much as possible the respiration rate of the stored produce. In a sense, one seeks to place stored fruit or vegetables in a state of suspended animation; that is, keep it alive without letting it age. Clearly, this cannot be done for an indefinite period, but it has been shown that one can significantly extend the time over which fruit can be kept fresh relative to refrigerated storage alone. Both CA and MA imply reduction of the oxygen (O2) content of the storage atmosphere to levels well below atmospheric (i.e., about 21%), and both methods usually imply an increase of the carbon dioxide (CO2) content from the order of parts per million to the order of a few percent. The general equation for aerobic respiration is presented in the next section and should immediately clarify why CA and MA are based on the modification of the concentrations of these two gases. The difference between CA and MA is that in the first case, the appropriate atmosphere for preserving freshness is achieved by active technological control, while in the second case it is achieved passively. Besides reducing the respiration rate, CA and MA often lead to reduced transpiration rates (reduced moisture loss), preservation of chlorophyll levels, retardation of pectic hydrolytic changes, reduction of acidity losses, and reduction of bacterial and pathogenic infections.1,2 They can also control storage pests.3 Fruit may be stored in many environments. However, the nature of the environment determines the storage duration and the end quality of the product. The three major descriptors of a storage environment are: (1) the temperature, (2) the relative humidity (RH), and (3) the composition of the storage atmosphere. Cooling to temperatures between about 0 and 14∞C is important in slowing down the biological, biochemical, and physiological processes involved in ripening, senescence, and the development and proliferation of pathogens. Commodities such as leafy vegetables, which are susceptible to moisture loss, tend to fare best in high relative humidity conditions (HRH). Commodities that do not easily lose moisture (e.g., bananas, and avocados) fare better in less humid environments. Until about 1960, cooling was the only widespread method of preserving fruit in

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fresh condition for any significant length of time, even though Franklin Kidd and Cyril West initiated research on the influence of reduced O2 and high CO2 atmospheres on fruit storage in the 1920s in England, and CA technological developments were brought about by R.M. Smock at Cornell University in the 1940s. In the late 1960s, the French scientist P. Marcellin recognized the possibility of applying membrane technology to the storage of fruits and vegetables. The general idea was to inhibit aerobic respiration and prevent moisture loss simultaneously by creating a storage atmosphere rich in CO2 and poor in O2. The main characteristic of the membrane he used was more permeability to CO2 than to O2. Thus, while the stored product respires, the level of O2 in the storage enclosure drops at a rate higher than the level diffusing through the membrane from the surroundings, and the respiration rate itself drops accordingly. The differential permeability leads eventually to a quasisteady-state in which the CO2 level in the enclosure is higher than ambient and the O2 level is lower than ambient. The reduction in respiration rate slows moisture loss and the breakdown of organic constituents. There are two other advantages to note: (1) reduced refrigeration load due to both lower respiration rate and higher storage temperature (since CA and MA stored products are generally more susceptible to freezing and chilling injuries) and (2) inhibition of the growth of many pathogenic species due to unfavorable atmospheric composition. It is generally accepted that this approach improves the relative storage life by 100% at room temperature storage (20 to 25∞C) and by about 33% at optimal refrigerating temperatures. In principle, the CA/MA concept is clear. The engineering calculations needed to design a membrane enclosure for long-term storage of a commodity are not complex.4 However, there are limitations in the atmospheric compositions that may be achieved for a given commodity’s respiratory activity and a given membrane selectivity. The required physical data (size of enclosure, membrane permeability, amount of stored produce, etc.) are readily available. One of the problems addressed by research is to determine the respiration rate of the produce and the optimum atmospheric composition for storage of a given commodity in terms of percentages of CO2 and O2. The problem is circular, in that it is at once necessary to know the optimum storage atmosphere composition and the respiration rate at that composition. This problem has so far been addressed by either trial and error methods or by experiments aimed at estimating the optimum composition using mathematical analysis. It should be mentioned that nature does hold many surprises. Notwithstanding the basic ideas on respiration mentioned above, some species fare best at ambient O2 concentration combined with enriched CO2. Although other gases such as carbon monoxide (CO) and sulfur dioxide (SO2) are being studied for CA storage applications, the systems presently used are all based on the control of CO2, O2, and ethylene (C2H4).

2.2 MODIFICATION OF RESPIRATION IN LOW O2/ HIGH CO2 ATMOSPHERES Respiration is the process of internal consumption of organic substances. Respiration may proceed aerobically according to the following general equation: C 6 H12 O 6 + 6O 2 Æ 6CO 2 + 6H 2 O + 2870 kJ ≠

(2.1)

Note: The equation (in moles) indicates the oxidation of glucose to CO2 and water with the liberation of heat. In fact, it represents a larger set of reactions including enzymatic breakdown of more complex substances to glucose. It is understood that the rate at which any chemical reaction proceeds can be reduced by lowering the kinetic energy of the reactants, that is, by lowering the temperature of the reaction environment. Cooling is the traditional method of prolonging the

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postharvest life of horticultural goods. MA and CA (combined with refrigeration unless otherwise indicated) go one step further. As seen in Equation (2.1), the respiration reaction depends on the supply of O2. Thus, if the rate of diffusion of O2 from the atmosphere through the outer layers of the fruit is reduced, the number of glucose molecules oxidized per unit time can be reduced. Similarly, increasing the CO2 level in the storage atmosphere will inhibit CO2 produced by respiration from leaving the fruit and will, therefore, interfere with O2 diffusion inward. Kader5 points out that CO2 suppression of respiration does not occur below concentrations of 10 to 20%. In general, the effects of CO2 and O2 on respiration rate are considered to be additive. A useful guide, although a generality, is that at 0∞C, the respiration rate at 3% O2 is 10 to 46% of that in air, while at 10 to 20∞C it is 20 to 60% of that in air. However, high CO2 concentrations can cause physiological damage; if O2 concentration is below a threshold of about 2%, anaerobic respiration processes may be initiated, resulting in rapid spoilage. It should be noted here that the reduction of the rate of this respiration reaction is an oversimplification of reality. In fact, a great deal of research has been concerned with determining the precise stages of the overall respiratory process that may be altered by reduced O2 and CO2 diffusion; however, such details are beyond the scope of this chapter. Anaerobic respiration is more commonly known as fermentation and is represented by the following general equation: C 6 H12 O 6 Æ 2C 2 H 5OH + 2CO 2 + 92 kJ ≠

(2.2)

This equation indicates the breakdown of hexose sugars to ethanol and CO2 with the liberation of heat. The general situation one has to deal with is illustrated in Figure 2.1. The indication here is that there is a lower limit to the reduction in biological respiration activity, and, thus, a finite fresh storage life even under optimal conditions. Again, this is an oversimplification but suffices to convey the general principles behind the effects of altered atmospheric compositions on stored commodities.

B

RELATIVE CARBON DIOXIDE RELEASE

A

A. ANAEROBIC ENERGY RELEASE SYSTEM B. AEROBIC ENERGY RELEASE SYSTEM

5

10

15

20

25

AVAILABLE OXYGEN, %

FIGURE 2.1 Influence of atmospheric O2 concentration upon the relative CO2 released of a produce.

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TABLE 2.1 Recommended Storage Conditions for Various Fruits Fruit

Temperature Range (EC)

O2 Concentration (%)

Apple Apricot Avocado Banana Blackberry Blueberry Cherry Cranberry Fig Grape Grapefruit Kiwifruit Lemon and lime Mango Nectarine Olive Orange Papaya Peach Pear Pineapple Plum Raspberry Strawberry

0–5 0–5 5–13 12–15 0–5 0–5 0–5 2–5 0–5 0–5 10–15 0–5 10–15 10–15 0–5 5–10 5–10 10–15 0–5 0–5 10–15 0–5 0–5 0–5

2–3 2–3 2–5 2–5 5–10 5–10 3–10 1–2 5–10 2–5 3–10 2 5–10 5 1–2 2–5 5–10 5 1–2 2–3 5 1–2 5–10 10

CO2 Concentration (%) 1–2 1–2 3–10 2–5 15–20 15–20 10–12 0–5 15–20 1–3 5–10 5 0–10 5 3–5 5–10 0–5 10 5 0–1 10 0–5 15–20 15–20

Source: Kader, A.A. An overview of the physiological and biochemical basis of CA effects on fresh horticultural crops. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. p. 1–9. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985.

Although the respiration rate is intimately connected to senescence and eventual loss of quality, and although the general principles of respiration are well understood, there is as yet no way to predict which storage atmosphere will most prolong the life of a given species or even of a given cultivar. Fruits come in many shapes and sizes and have important biochemical, anatomical, and physiological differences that have so far prevented the development of a general equation from which to determine the optimal composition and temperature of a storage atmosphere. For example, a ripe banana deteriorates in the refrigerator at the same rate as a strawberry at room temperature. With respect to composition, Yong et al.6 showed that while the respiration rate of avocados is reduced by the addition of CO2, bananas respond only slightly, and the respiration rate of lemons increases significantly. Table 2.17 illustrates the diversity of optimal storage atmospheres, where the recommended concentrations of O2 and CO2 are listed alongside acceptable storage temperature ranges. Table 2.1 not only indicates optimal ranges for temperature along with O2 and CO2 concentrations for various fruits but also implies that there are problems in chilling associated with temperatures and concentrations outside the given ranges. In the case of temperature, higher temperatures lead to high respiration rates. In certain cases (banana, grapefruit, mango, papaya, and pineapple), it is clear that lowering to between zero and the lower optimal temperature cannot result in freezing but is

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associated with problems such as discoloration, enzymatic browning, surface pitting, off-flavors, and rot for certain commodities. With respect to O2, lower concentrations lead to anaerobic conditions while higher concentrations lead to higher respiration. In the case of CO2, concentrations below about 10% do little in driving the respiration reactions backward while higher concentrations lead to various physiological damages and can induce local anaerobic processes. Again, the critical levels depend on the type of produce. In particular, pears are very intolerant to CO2, whereas strawberries, blackberries, raspberries, blueberries and figs respond very well to CO2 at concentrations significantly higher. While the concept of reducing the respiration rate to prolong storage is valid, it is important to maintain a broad perspective with respect to the relationship between atmospheric compositions that reduce commodity respiration and the real reason why a particular atmosphere works. For many species — berries, in particular — the shelf-life is limited primarily by fungal spoilage. High CO2 atmospheres have, therefore, been found optimal primarily due to inhibition of fungal growth (e.g., blueberries: Day et al.8; strawberries: Lépine9). For example, consider the strawberry. This is a fruit which has a high respiration rate (20 to 40 mg CO2 evolved·h–1·kg–1). El Kazzaz et al.10 performed storage experiments on strawberries under the following conditions: (1) atmospheric composition, (2) 2.3% O2, 5.0% CO2, and (3) 17.5% O2, 15% CO2. The storage temperature was 3.3∞C in all cases, and the storage period was 21 days. The best performance was obtained in case 3 where the percentage of decayed strawberries was 0.2%, compared to 14.0% in case 2 and 19.0% in case 1, even though the respiration rate was lowest in case 2. According to studies on proliferation of Botrytis cinerea,9 the better storability obtained with a high level of CO2 and an intermediate concentration of O2 (at 20∞C) was probably due to the inhibition of this pathogen. This finding complicates any expectation of reducing respiration to a minimum since the optimization procedure must take into account the constraints imposed by the conditions inhibiting pathogens of various degrees of virulence. The effects of MA and CA on storage pathogens are reviewed by Farber11 who also raises issues regarding the possible human health dangers associated with favoring the development of anaerobic pathogens. The subject of pathogen proliferation inhibition has been approached also by the investigation of atmospheres containing rather noxious gases such as carbon monoxide. For example, El Kazzaz et al.10 also found that an atmosphere of 10% CO, 2.3% O2, 5.0% CO2 and 82.7% N2 inhibited fungal growth to the extent that only 0.15% of strawberries were decayed after 21 days’ storage, the temperature being 0.6∞C (i.e., equivalent to a 15% CO2 atmosphere without CO). While such studies can lead to the development of performant CA technologies under laboratory conditions without residual contamination of the product, the additional safety considerations for warehousescale storage facilities will probably delay the application of such ideas until fully robotized handling systems are developed. Some authors attribute the major effect of low O2 and high CO2 atmospheres to their influence on ethylene (C2H4), which is involved in ripening and consequently leads to softening and eventual deterioration.12 O2 is involved in the synthesis and activity of C2H4 and inhibits its production at levels of about 8% and below.5,13 CO2 is a competitive inhibitor of C2H4.

2.2.1 RESPIRATORY QUOTIENT The respiratory quotient (RQ) is a useful quantity to consider in the monitoring of biological activity within a storage enclosure and is a useful parameter in the design of MA and CA systems. The RQ is the volumetric ratio of CO2 evolved to O2 consumed by a commodity. Equation (2.1) makes it clear that if a hexose sugar such as glucose is the terminal substrate for respiration, RQ = 1. If RQ > 1, an oxygenated (organic acid) substrate is being used in respiration. If RQ < 1, then (1) the substrate has an O2 to CO2 ratio less than one, (2) oxidation is not complete, or (3) the CO2 evolved is being used in a synthetic process.14 If the RQ for normal postharvest respiration of a given commodity is known, a significant change in this quantity should be a better indicator of the

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onset of an abnormal deterioration process than is the respiration rate, since the respiration rate could be influenced by benign factors such as a rise in storage temperature or an air leak leading to higher O2 concentrations.

2.2.2 FACTORS INFLUENCING STORABILITY Aside from the storage conditions (temperature, relative humidity, and gas composition), there are a number of other factors that determine the storage life of a fruit. The base quality of the fruit depends on the interaction of many factors including climate, soil fertility, pest and pathogen control, harvesting time, quality of harvest (level of physical damage such as bruising, cutting, etc.), and prestorage treatments. Damaged fruit, infected fruit, and fruit exhibiting nutrient deficiencies cannot be expected to remain in fresh condition very long. The physiological age of the fruit at harvest is also important. If the climacteric (characteristic of the pome fruits, for example) has begun prior to harvest, one cannot expect the fruit to have a long storage life because the action of C2H4 is autocatalytic; i.e., the onset of C2H4 production in a single fruit may stimulate the others to ripen. Rapid cooling, rapid O2 pull-down, and a high CO2 treatment for a short period may be helpful in inhibiting the ripening process; however, it is recommended that harvest be done prior to the onset of ethylene biosynthesis. Rapid in-the-field precooling has also been shown to improve the storage performance of certain fruits, particularly berries, since the high surface-to-volume ratio combined with high respiration rates tends to lead to rapid water loss and sugar breakdown upon harvest. Without immediate precooling, the hour or more between picking and storage can lead to a significant reduction in fresh storage time. Precooling technology will be covered at the end of this chapter.

2.3 CREATING OPTIMAL STORAGE CONDITIONS In considering the creation of optimal storage conditions, the most appealing situation with respect to operating and maintenance costs is dealt with first. This is the establishment of optimal atmospheric composition by passive means. Passively established atmospheres of composition other than atmospheric result from the combined processes of product metabolism and selective gas diffusion across specially designed membranes. Such membranes may be polymeric films of various kinds or even wax and edible coatings applied to individual fruits. In this section the focus will be on polymer membranes only.

2.3.1 MODIFIED ATMOSPHERES (PASSIVE ESTABLISHMENT) Consider a mass of harvested fruit in a storage enclosure with a given membrane window area (Figure 2.2). The gas balance is a function of the product respiration and the permeability characteristics of the membrane. The general idea is that after the fruit is placed in the chamber and the chamber is closed, the respiration process consumes O2 and releases CO2. Since the membrane is less permeable to O2 than to CO2, CO2 accumulates and O2 is depleted. The following equations describe the gas balance under such conditions, the assumption being that the evolution of other gases such as C2H4, or volatiles produced by the interaction of pathogens and the stored commodity may be neglected:15 dp CO2 dt

= K( A m D CO2 ( p CO2 - p CO2 ) + MR CO2 ( p CO2 , p O2 , t ) + CO*2 L( P ))

(2.3)

o

dp O2 dt

= K( A m D O2 ( p O2 - p O2 ) + MR O2 ( p CO2 , p O2 , t ) + O*2 L( P )) o

(2.4)

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STORAGE ENCLOSURE

SILICONE MEMBRANE O2 CO2 N2 GAS FLOW

FIGURE 2.2 Gas exchange between the inside and the outside of a fruit storage room equipped with a semipermeable membrane system.

dp N2 dt

= K( A m D N2 ( p N2 - p N2 ) + N*2 L( P ))

(2.5)

o

where Am = membrane surface area (m2) K = volume of gas to pressure proportionality constant at the storage temperature (kPa·mole–1) D CO2 = membrane permeability to CO2 (mol·h–1·m–2·kPa–1) D O2 = membrane permeability to O2 (mol·h–1·m–2·kPa–1) D N2 = membrane permeability to N2 (mol·h–1·m–2·kPa–1) M = mass of stored produce (kg) L(P) = leakage rate as a function of the pressure difference between the inside and the outside of the storage enclosure (mole·h–1) p CO2 = CO2 partial pressure (kPa) p O2 = O2 partial pressure (kPa) p N2 = N2 partial pressure (kPa) P = total pressure in the chamber (kPa) RCO2 = CO2 generation rate of produce (mol·h–1·kg–1) RO2 = O2 consumption rate of produce (mol·h–1·kg–1) t = time (h) CO2*, O2*, and N2* are either ( - p CO2 / P, - p O2 / P, - p N2 / P when P > PO ) or ( + p CO2 / P, + p O2 / P, + p N2 / P when P < PO ) Subscript “o” indicates the initial conditions that are the ambient conditions outside the chamber.

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If the storage enclosure is perfectly airtight such that the only transfer of gas occurs through the membrane window, then the terms including L(P) may be dropped from the equations. In the limit, if the enclosure is not completely airtight, ambient conditions will prevail regardless of the characteristics of the membrane. Based on the pressure differential between the inside and the outside of the storage enclosure, the leakage rate L(P) can be expressed as: L( P ) = CA c ( Po - P ) n

(2.6)

where L(P) = Ac = C= n=

leakage rate (mol·h–1) wall and ceiling surface area of the storage chamber (m2) leakage flow coefficient (mol·h–1·m–2·kPa–n) leakage flow exponent

The main difficulty in solving this set of equations is that the respiratory functions RCO2 and RO2 are both functions of the gas composition, which evolves with time, and there is really no adequate description of these in a dynamic setting. 2.3.1.1 Design Objective: Approximating Steady State Mathematically, steady-state conditions in a leaking system are defined by Equations 2.3, 2.4, and 2.5 with the left-hand derivatives set at zero. The primary design objective is to achieve and maintain equilibrium at the optimal atmospheric composition for the commodity; however, experimental work has shown that temporal changes in produce metabolism, external pressure fluctuations, and possible changes in airtightness (deterioration of seals, etc.) result in trends and fluctuations in the time histories of gas composition and, consequently, in respiratory activity. The state of the system at equilibrium is, nevertheless, of interest for design purposes. One way to quantify the degree to which the equilibrium concentration of a gas is approximated at a given time is by defining a relative gas difference (RGD): È A( t ) - A av ˘ RGD = ln Í ˙ Î ( A o - A av ) ˚

(2.7)

where A(t) = instantaneous gas concentration at time t (kPa) Aav = average gas concentration (kPa) Ao = initial gas concentration (kPa) If the actual partial pressure of a gas is such that RGD is equal to or less than a preset value of RGD, the gas is said to have stabilized after the initial adjustment phase has passed. The parameters to be considered in design are: 1. 2. 3. 4. 5.

Product response to gas composition Design gas concentrations Membrane area Membrane permeability to the storage gases Leakage rate

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Of these, the parameters that are considered fixed are the product response to O2 and CO2 and the stabilized (design) gas concentrations. These must be determined through experimentation since no universal models of respiration presently exist. The leakage rate is a function of the physical characteristics of the enclosure, but can be considered adjustable in the sense that vents could be installed in the storage enclosure. The subject of determining the size of perforations (minivents) in semipermeable film packages necessary to produce an effective permeability has been studied by Emond et al.16 An alternative technique is the manipulation of gas fluxes by active control of partial pressure gradients at the membrane interface. This is done by pressurizing the ambient air before allowing it to contact the exterior surface of the membrane, thereby altering the ratios of the partial pressure gradients of O2 and CO2. The membrane area used as a window to the external atmosphere is adjustable and calculation of the required area has been discussed by Gariépy et al.17 The major drawback has been that for a specific set of membrane permeabilities and respiratory responses by a commodity, one can design either for a specified O2 level or for a specified CO2 level but not necessarily for both. Mathematically, if the leakage rate is also fixed, the situation is one of three equations in one unknown. If the leakage rate is considered unknown, then there are three equations in two unknowns. In this case, the constraint that CO2* + O2* + N2* = 1 can be used to find a unique solution in terms of L(P) and Am; however, this implies the manipulation of L(P), which requires some form of active control. For completely passive establishment and maintenance of the desired atmospheric composition, it is important to consider critical levels of each gas relative to the commodity in question and to determine the relative degree of response of the commodity to each in order to choose a design gas concentration. Clearly, there must be a compromise between the degree of active control and the required precision. Since, in general, the response of respiratory activity to O2 levels is more significant if the corresponding attainable CO2 level is not in or near a critical range, calculation of membrane area should be based on stabilizing O2 levels. In the simplest case of perfect airtightness and total compensation of the net gas deficit (resulting from RQs < 1) by N2, stable conditions occur when the following equations are simultaneously satisfied: A m D CO2 ( p CO2 - p CO2 ) dt = R CO2 Mdt

(2.8)

A m DO2 (pO2 - pO2d ) dt = RO2 Mdt

(2.9)

d

where the subscript “d” signifies the design or target gas level. Dividing these two equations results in the condition: D CO2 D O2

=

R CO2 ( p O2 - PO2 ) o

d

R O2 ( p CO2 - p CO2 ) o

(2.10)

d

or

E CO2 ,O2 = RQ

( p O2 - p O2 ) o

d

( p CO2 - p CO2 ) o

(2.11)

d

where, ECO2,O2 is selectivity of the membrane to CO2 over O2. In designing for stability, one should, therefore, know the desired composition and the RQ of the produce at that composition in order to select a membrane with selectivity ECO2,O2 , or given a fixed selectivity (i.e., no other choice of

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membrane), adjust the external pressure so as to change the ratio of partial pressure differentials. For example, if the produce RQ is 0.58 and the design gas levels are 2.5 kPa CO2 and 3.8 kPa O2, ECO2,O2 should be 4.4 for joint stability to occur at the desired composition. 2.3.1.2 Equations for Calculating Membrane Area Required The following equations can be used to calculate the membrane area required to obtain the design O2 concentrations in an airtight and a leaking system, respectively: R O2 M

Am =

(2.12)

D O2 ( p O2 - p O2 ) o

Am =

d

R O2 M - O*2 L( P )

(2.13)

D O2 ( p O2 - p O2 ) o

d

In order to design for CO2, the appropriate substitutions may be made for the terms involving O2 in the above equations. In either case, the steady-state level of the other gas may be computed by using the appropriate form of the above equations, using the membrane area (Am) calculated for the design gas. The possible atmospheric regimes obtained by varying the membrane area for a leaking or airtight system for different RQs are shown graphically in Figure 2.3. In the figure, the membrane area increases from left to right following Equation (2.11) and Equation (2.12) which show that the membrane area increases as the difference between the initial O2 level and the targeted level pO2 d

7

AIRTIGHT ENCLOSURE

CO2 PARTIAL PRESSURE (kPa)

LEAKING ENCLOSURE 6

5

RQ 1.4

4 1.0 3 0.6 2

1

0 0

5

10 15 O2 PARTIAL PRESSURE (kPa)

20

FIGURE 2.3 Possible atmospheric regimes obtainable by varying the membrane area for an airtight and a leaking enclosure for different produce RQs.

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RSV 0 0.1

LN

− Y(t) −Y − Yo − Y

−1 0.3

−2 −3

0.5

−4 −5 −6 −7

0

10

20

30

40

50

TIME (DAYS)

FIGURE 2.4 Effects of the relative stacking volume (RSV) upon the period necessary for O2 to reach steadystate in an airtight enclosure (RCO2 = 3.5 mg·h–1·kg–1, RQ = 1 and O 2 = 5 kPa). d

decreases. For a given CO2 level, the O2 level is higher in a leaking than in an airtight system, the difference diminishing as membrane area increases. The influence of a higher RQ on the possible gas combinations is to increase the CO2 level corresponding to a design O2 level. In this figure, R CO and RQ were constant over the range of O2 levels. 2

2.3.1.3 Factors Affecting Time to Stabilization The time taken for stabilization of gas composition to design levels is important to consider in a passive system and may lead to a decision to use a rapid O2 pull-down technique, particularly for a fruit species with a short storage life. Experiments have shown that the time taken to reach stable gas levels is of the order of days to weeks, depending on commodity respiration, designed gas composition, relative stacking volume (RSV), and airtightness. Clearly, nonoptimal gas levels are not desirable for long periods. Rapid O2 pull-down is the term used for techniques that drop the O2 concentration of the storage atmosphere to optimal immediately after sealing the chamber. It can be most simply achieved by nitrogen flushing. 2.3.1.4 Relative Stacking Volume The RSV, or volume of produce as a ratio of total storage room volume, is essentially a measure of the initial buffering of the membrane–produce interaction by the free air in the storage room. The effect of RSV on time to stabilization of O2 is shown in Figure 2.4. The time to stabilization decreases with RSV; thus; it is important to properly plan the stacking geometry of the storage room. It is important to remember, however, that the air circulation pattern will change according to the geometry and that the ventilation requirements may increase in order to prevent condensation. 2.3.1.5 Design Gas Levels The closer the design gas levels are to ambient, the more rapidly will stabilization occur. In general, the degree of airtightness affects the O2 stabilization time more than the CO2 stabilization time. The following relations derived from a model presented by Deily and Rizvi18 describe the time “t” to stabilization assuming a constant respiration rate and RQ:

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t CO2 =

t O2 =

- p CO2 RGD CO2 d

R CO2 MK

(O 2o - O 2d )RGD O2 R O2 MK

(2.14)

(2.15)

Given that the respiration rate of a commodity cannot be expected to remain constant, it is clear that such equations are most useful for comparative purposes rather than for absolute evaluations for a given commodity. 2.3.1.6 Standard Free Volume The standard free volume (SFV) provides similar information and is the ratio of air per unit volume of commodity. The SFV is typically 1.5 to 3 for warehouse commodity stacking arrangements and depends on room geometry, commodity shape and density, and whether the commodity is packed in bulk or crated. 2.3.1.7 Infiltration Rate The infiltration rate reflects the degree of airtightness of the storage room and is usually defined as the number of complete air changes per day when the room is empty. Air infiltration is due mainly to barometric pressure fluctuations, wind and temperature gradients between the storage room and the environment, and the cyclical operation of the refrigeration system, as well as the room construction and warehouse management strategy. The standard technique for determining the infiltration characteristics of a room consists of pressurizing the interior to 250 kPa and determining the time taken for the pressure to drop to the standard of 125 kPa. If this time is 10 min for example, the room is referred to as a “10-minute room,” which is roughly equivalent to 0.0333 air changes per day. 2.3.1.8 Silicone Membranes Silicone membranes, such as that used by Marcellin19,20 in his work on MA storage of various fruits and vegetables, consist of a fine nylon fabric covered with a thin layer of silicone rubber. Silicone rubbers are high-molecular-weight polymers whose backbone consists of alternate atoms of silicone and O2, to which various organic side groups may be attached to modify the characteristics of the rubbers. The particular polymer used by Marcellin was dimethyl-polysiloxane whose side groups are CH3. The thermal stability of such rubbers is excellent; they have fine water-repellency, good electrical-insulating characteristics, and good chemical resistance but are not strong. At a pressure of 1 atm, the permeability to CO2 is 1750 L·d–1·m–2. The selectivity (ratio of permeabilities) with respect to O2 is 5.5 and to ethylene is 2.5. Further details on these polymers are given by Ash and Ash,21 Roff et al.,22 and Yescombe.23 Commercial packaging systems such as Atmolysair, the Pallet Package, and the Marcellin System have all used this membrane for gas exchange, although in different configurations as will be described in the next section.

2.3.2 COMMERCIAL MODIFIED ATMOSPHERE SYSTEMS 2.3.2.1 The Pallet Package System The first commercial application of the silicone membrane for long-term storage was the Pallet Package System developed by Marcellin.24 This system consists of a pallet box wrapped in heavy

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WRAP

AIRTIGHT POLYETHYLENE ENVELOPE

CREATES DIFFUSION MEMBRANE CALIBRATED ORIFICE

PALLET

FIGURE 2.5 Pallet package system.

gauge polyethylene (80 to 150 µm thick) with a silicone membrane window installed for regulation of gas exchange and a calibrated hole for pressure regulation (Figure 2.5). The advantages of the system are ease of manipulation, optimal atmospheric composition without the building modifications and equipment needed for CA storage, and the possibility to market intermediate quantities of produce without disturbing the atmospheric composition of the rest of the produce. However, the disadvantages are the extra space required to provide adequate ventilation to all pallet packages and the time involved in wrapping and installing the proper area of membrane, as well as the extra care needed to avoid damaging the packages. This system was originally developed for the storage of apples and pears, and could accommodate other perishables. 2.3.2.2 The Marcellin System The Marcellin System (shown schematically in Figure 2.6) regulates the atmospheric composition in a storage room via a series of rectangular bags of silicone rubber connected in parallel.25 The

COOLING UNIT

DIFFUSION UNIT O2 CO2 N2

CA STORAGE ROOM

FAN SEAL

FIGURE 2.6 Marcellin system.

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PLASTIC TUBING COOLING UNIT

ATMOPILE AIRTIGHT METAL CASING

OPENINGS DIFFUSION PANELS

CA STORAGE ROOM

OUTSIDE AIR INLET

OUTSIDE AIR OUTLET

BLOWER AND FILTER UNIT

FIGURE 2.7 Atmolysair system.

number of bags depends on the capacity of the storage room, the storage temperature, and the respiration properties of the stored produce. The composition of the headspace is analyzed to determine whether or not more bags should be used. Typical recommendations are 50 m2 of membrane surface area for each 100 t of produce at a bulk density of 200 to 250 kg·m–3 (standard free volume of 2.9 to 3.6) to maintain an atmosphere of roughly 3% O2 and 3% CO2. The units can be installed inside or outside the cold room. 2.3.2.3 The Atmolysair System The Atmolysair System (Figure 2.7) consists of a set of gas diffusion panels enclosed in an airtight container with two separate airflow paths and a control panel.26 The panels are arranged side by side so that the exterior air and the modified air from the storage chamber flow on opposite sides of the membrane without direct mixing. Two centrifugal blowers operated by a timer maintain air circulation. The gas composition is analyzed periodically such that the time of blower operation may be adjusted. The advantages of this system over the Marcellin System are ease of operation, better gas exchange due to control of both air streams from the airflow paths, and the potential for complete automation. The surface area of the membrane required is computed as previously outlined. This system has been successfully implemented by several Canadian cabbage producers.25

2.3.3 CONTROLLED ATMOSPHERES (ACTIVE ESTABLISHMENT) There are several ways to actively control the atmospheric composition inside a storage chamber. Each has its merits and demerits. Furthermore, most of the active methods have higher operation and maintenance costs than the passive and semipassive (partial control) technologies. One may divide active systems into O2-control and CO2-control systems. The O2 systems are: external gas generators, liquid nitrogen atmospheric generators, gas separators, and hypobaric storage. The CO2 control systems are lime scrubbers, water scrubbers, activated charcoal scrubbers, and molecularsieve scrubbers. C2H4 control systems should also be mentioned since these can be important in application to fruit such as apples. Typically, these store well enough under CA to accommodate the marketing period of small and intermediate orchards without resorting to MAs, but which are susceptible to problems of quick ripening if C2H4 is not removed.

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COOLING UNIT HEAT BURNER

CA STORAGE ROOM

FUEL CONDENSER SCRUBBER

AIR INLET

FIGURE 2.8 External gas generator.

2.3.3.1 Oxygen Control Systems 2.3.3.1.1 External Gas Generators External gas generators operate either on the open-flame (Figure 2.8) or catalytic burner principle to remove O2 from the incoming air. Heat may be recovered or the exhaust air may be water-spray cooled before being blown into the storage room. The disadvantages of these types of system are that fuel is required and a CO2 scrubber must also be used to remove excess CO2. The advantages of these systems are the flexibility of operation and the rapidity with which O2 is depleted in the initial stages of storage, compared to a membrane-only system. The catalytic system is preferable in ensuring better combustion and fewer plant-toxic or plant-activating combustion products. External burners are recommended for rapid O2 pull-down. Catalytic burners are more expensive to install when used in recirculating configuration but have lower operating costs. However, flushing systems are easier to maintain since they do not require the frequent adjustments characteristic of catalytic burners. If the storage room is well-sealed, relative to the standard free volume and commodity respiration activity, it is often only necessary to operate the burners during the initial O2 reduction stage. Thereafter, infiltrating air combined with commodity respiration and operation of the CO2 scrubber can maintain close to optimal conditions. 2.3.3.1.2 Liquid Nitrogen Atmospheric Generators Flushing the storage room with sprayed liquid nitrogen is an excellent method for establishing the CA condition rapidly. The spraying heads that pulverize the liquid N2 are best placed in front of evaporator blowers. An O2 sensor is used to determine whether further spraying is needed to maintain the recommended O2 level. Lime bags are used to absorb excess CO2. 2.3.3.1.3 Gas Separator Systems The two commercially available gas separator systems are the pressure-swing adsorption system (PSA) and the hollow-fiber membrane system (HFM). Both systems are used primarily for rapid O2 pull-down and can be used in flushing or recirculating modes to control the CO2 level during storage. PSA systems work on the following principle (Figure 2.9): A stream of air is compressed, filtered, and circulated through a molecular sieve that selectively absorbs O2. The airstream leaving the unit consists mainly of N2. O2 is then released from the sieve by depressurizing the vessel and is vented to the outside. Typically, two units are mounted in parallel such that absorption and

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PARTICULATE FILTER

SURGE O2 ANALYZER VESSEL

COOLING UNIT MOLECULAR SIEVE ABSORPTION VESSEL

VENT

VENT

CA STORAGE ROOM

AIR COMPRESSOR AIR INLET

COALESCING FILTER

REFRIGERATION DRYER

FIGURE 2.9 Pressure-swing absorption system.

desorption may be carried out without affecting the flow of N2-rich air. The opening and closing sequences of the valves are microprocessor-controlled. Depending on the level of pressurization (usually 700 kPa) and the temperature, the purity of the N2 gas is between 90 and 99.9%. According to Bartsch and Blanpied,27 the energy consumption of the PSA in apple storage applications is 1 kW·h·m–3 of apples. HFM systems are based on hollow fibers made of semipermeable membranes. Air compressed to about 700 kPa is heated to 38∞C, filtered, and forced into the HFM unit. Since the membranes are more permeable to O2 and CO2, these gases pass to the external-flow stream and vent to the atmosphere, while the N2-rich stream continues to the storage room and purges O2 (Figure 2.10). The energy consumption in CA storage of apples is about 1.5 kW·h·m–3 per m3 of apples.27

GAS SEPARATOR COALESCING FILTERS

N2 HEATER OXYGEN STREAM ENRICHED

COMPRESSOR

COOLING UNIT

CA STORAGE ROOM AIR INLET

FIGURE 2.10 Hollow-fiber membrane system.

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2.3.3.1.4 Hypobaric Storage Low-pressure storage by means of a vacuum pump has the advantages of ease of manipulation of the effective O2 concentration and RH level of the storage enclosure. CO2, C2H4, and other volatiles of metabolism are removed. This offers also the advantage of permitting the storage of noncompatible commodities. However, due to the low required pressures (1.3 to 13 kPa), there are significant costs associated with storage room structure and airtightness quality needed for efficient pump operation. Moreover, commodities requiring high CO2 levels cannot be accommodated, and some quality characteristics can be affected (e.g., flavor). 2.3.3.2 CO2 Control Systems CO2 control is affected by scrubbing systems. The five reagents used are: caustic soda, water, hydrated lime, activated charcoal, and molecular sieves. Monitoring of the atmospheric composition is necessary to control the scrubbing system. 2.3.3.2.1 Scrubbing CO2 with Caustic Soda Caustic soda (NaOH) was one of the first reagents used commercially for CA storage applications and was used in mixture with water. The solution was circulated in open tubes and thus absorbed CO2, the residence time being controlled according to the required removal rate. Its use was discontinued due to its corrosiveness and the potential danger in handling. However, dry caustic material has been proposed as a reasonable alternative.28 2.3.3.2.2 Scrubbing CO2 with Hydrated Lime Perhaps the simplest method of controlling CO2 levels is through hydrated lime scrubbers (Ca(OH)2). The scrubber is an insulated and sealed plywood box, usually containing between half to the total amount of lime required to remove the CO2 produced during the entire storage period. The box is connected to the CA room and airflow to it may be left to natural convection or controlled by forced-airflow using blowers and dampers. The choice between natural convection and forced flow depends on the commodity respiration characteristics, the room airtightness quality, and the RSV or SFV. The efficiency of the lime bag drops with time as it becomes saturated with CO2. The level of efficiency is thus easily determined by simply weighing the bag. A 22 kg bag of hydrated lime weighs 30 kg at its maximum CO2 holding capacity. It is recommended that bags no heavier than 25 kg be used, since crusting of the outer layer may occur in larger bags, thus reducing the efficiency of CO2 absorption. 2.3.3.2.3 Scrubbing CO2 with Water There are two types of water scrubbers for CO2. One type is the brine scrubber in which brine is pumped over evaporator coils, solubilizes CO2, and is pumped to an external reservoir (Figure 2.11) and then to an aerator that causes the CO2 to escape to the atmosphere. Dry evaporator coils are used to prevent corrosion. Smock et al.29 modified the system (Figure 2.12), using two aerators, one internal and one external. Both systems regulate CO2 levels efficiently, the controls needed being water flow-rate adjustment through the aerators and operating time. It is recommended that hydrated lime be used with this system to help regulate CO2 at the beginning of storage when CO2 evolution is high, thus eliminating the need for a higher capacity scrubber. The scrubber should be designed on the basis of the quantity, respiration rate, and CO2 level required by the commodity. In application to apple storage, Pflug30 recommended a circulation rate of 100 l·h–1 per ton of apple stored, based on a recommended atmosphere of 5% CO2 and 2% O2, and a temperature of 1∞C. The scrubbing capacity is 0.02 m3 of CO2 removed per m3 of water circulating through the scrubber. This is four times lower than the theoretical scrubbing capacity of water under these conditions: that is, 0.086 m3 CO2 per m3 of water with a CO2 differential of 5%.

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FAN BRINE

AERATOR

COOLING UNIT

CA STORAGE ROOM

LIME PUMP BRINE RESERVOIR

FIGURE 2.11 Brine-water CO2-scrubber.

COOLING UNIT WATER

AERATOR

CA STORAGE AERATOR ROOM

LIME PUMP

WATER RESERVOIR

FIGURE 2.12 Modified brine-water CO2-scrubber equipped with two aerators.

2.3.3.2.4 Scrubbing CO2 with Activated Charcoal and Other Molecular Sieves Activated charcoal and molecular sieve scrubbers are units filled with the respective absorbers, two blowers, and four timer-controlled valves (Figure 2.13). Air from the CA room is circulated to the scrubber, where CO2 is absorbed. The CO2-depleted air is returned to the storage room. When the absorbent is saturated, outside air is circulated through the scrubber and back to the outside to remove the CO2. If a molecular sieve is used, it is also heated to speed up the reactivation process. These systems have very low operating costs and the absorbent lifetime is about 5 years. 2.3.3.3 Ethylene Control Systems Ethylene may be removed from the storage atmosphere by a heated catalyst system such as that developed by Blanpied,31 and tested in a MacIntosh apple storage facility. The unit is shown in Figure 2.14. Storage air is blown through two electrically-heated ceramic banks in which the ethylene is oxidized to yield CO2 and water vapor. The air leaving the unit is cooled before being returned to the storage room. It was found that the scrubber removed 87% of the C2H4 produced by the stored apples, but required an airflow of 142 l·sec–1. The ceramic packing configuration could be modified to facilitate airflow and reduce the required fan power (which was 7.5 kW).

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SOLENOID VALVE COOLING UNIT

ACTIVATED CHARCOAL CONTAINER

CONTROL UNIT

CA STORAGE ROOM

SOLENOID VALVE

FIGURE 2.13 Activated charcoal CO2-scrubber. ELECTRIC HEATER INSULATION

FORWARD

CATALYSIS CERAMIC PACKING

REVERSE

COOLING UNIT

DAMPER MOTOR

FLOW REVERSING VALVE

BLOWER FAN

CA STORAGE ROOM

FIGURE 2.14 Heated catalyst system for C2H4 scrubbing.

An alternative to the heated catalyst system is the use of an absorbent bead scrubber that contains aluminum silicate spheres mixed with potassium permanganate (KMnO4), which is used as an absorbency indicator. The beads are held in a container through which the storage air is circulated. As the beads become saturated, the KMnO4 turns the beads from purple to brown, thus simplifying maintenance. The number of units required depends on the size of the storage room and the expected C2H4 production from the stored commodity. However, more data are needed to determine the C2H4 production rates in a system with C2H4 removal. It is likely that C2H4 production rates diminish with removal since autocatalycism is reduced. About 280 kg of beads are recommended in two units (each equipped with a 280 l·sec–1 fan) for a 360 m3 storage room for MacIntosh apples.

2.3.4 AUTOMATED CA CONTROL SYSTEMS In recent years, better quality requirements and advances in computer technology have led modern, large CA storage facilities to adopt automated control systems.32 Automation of a CA storage

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facility provides many advantages. They have become essential not only to control the process but also to ensure optimum product quality and efficiency.33 The difficulty in effectively controlling the environment in a CA storage room is due to its dynamic behavior. Physiological response of the stored produce, CO2 and O2 regulating equipment, building airtightness and integrity, cyclical operation of the refrigeration system, and changes in weather are all major elements affecting the gas balance. For these reasons, the process must be carefully studied before an attempt is made to automate the control process. 2.3.4.1 Control Strategy Aside from temperature and humidity control, the major challenge resides in achieving and stabilizing the O2 and CO2 levels to the desired concentrations (setpoints). At regular time intervals, the O2 and CO2 levels are measured and compared to their respective setpoints. The control system attempts to reduce the difference between setpoints and the process variables and decides whether or not to operate the gas regulation equipment. The ability of the control system to effectively decide on the type and magnitude of the interventions is based on the control strategy. A properly designed control system is robust and stable, with adequate response time and proper damping. 2.3.4.2 Control Devices Typical devices used to control the operation of the gas regulating equipment in CA control systems include on/off switches, and proportional (P), proportional-integral (PI), or proportional-integralderivative (PID) controllers. PID controllers are by far the most popular feedback controllers used in controlling automated industrial processes, and a good review can be found in vanDoren.34 More advanced controllers such as adaptive, fuzzy logic, knowledge-based, and artificial intelligence controllers can also be used in CA control systems. Advanced controllers are generally more intelligent and can dynamically optimize the process by a deduction process. These self-learning and self-tuning controllers can monitor the controlled system and adjust controller parameters automatically.35 Usually designed to provide dedicated control over one parameter, they are sturdy, reliable and commercially available in a multitude of configurations. One drawback is their limited ability to exchange information. This could be of concern in CA storage facilities equipped with certain types of regulating equipment. Morimoto and Hashimoto36 propose a new breed of intelligent control systems for the automation of storage processes based on a “speaking fruit” approach (SFA). This approach takes into account the physiological status of the stored commodity to optimize the storage environment. It consists of a fuzzy logic controller and two optimizers. Neural networks identify and describe fruit responses, and genetic algorithms determine the optimal setpoints. These setpoints are then passed to the fuzzy controller to control the environment. The concept was applied to the control of the RH level in an orange storage facility and was found superior to a basic on/off control. 2.3.4.3 PC-Based Systems Personal computer (PC)-based systems can also be used to provide direct on-line control of the gas levels in CA storage facilities. A PC-based system generally consists of a computer, control software, a data acquisition system (DAQ), communication ports, and switching devices. PC-based systems offer great flexibility since in addition to controlling the gas levels, the software can be programmed to compute on-line parameters, to manage interaction between process variables, and automatically log valuable information for later retrieval and analysis. It is important to plan for contingencies in cases of power or equipment failure or emergencies in order to minimize down time and avoid costly spoilage. Also, the software should allow for manual control of all equipment. Alarms should be installed to warn of any critical changes in gas levels in the storage room. Implementation of automatic emergency paging features are highly recommended.

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2.3.4.4 Commercial Systems Dedicated systems for monitoring and control of the environment in CA storage facilities are now available commercially. Most suppliers provide technical support to assist the storage owner in specifying and installing the most suitable equipment for the particular application. The equipment usually consists of a microprocessor-based controller, a CO2/O2 gas analyzer, temperature sensors, and devices to control the operation of the gas regulating equipment. Most of these systems can be interfaced with a PC to allow the storage operator to enter the required control parameters and monitor the process.

2.3.5 INSTRUMENTATION Automation of CA storage facilities requires the utilization of on-line instruments to measure the quality of the environment surrounding the stored produce. Variables that are routinely measured include temperature, relative humidity, and gas concentrations. Vibrations, dust, moisture, and temperature fluctuations affect sensor performance. Therefore, verification of the state and accuracy of the instruments should be made in accordance with manufacturers’ specifications or on an annual basis, if not specified. 2.3.5.1 Temperature Temperature is the easiest parameter to measure in the storage environment. The most common sensors are: the RTD, the thermocouple, the thermistor, and the integrated circuit (IC). All will perform well under CA storage conditions, and the selection of the appropriate sensor should be based on the desired accuracy, precision, and output signal, as well as its temperature range, the installation location, and cost.37,38 2.3.5.2 Relative Humidity Measurements of relative humidity (RH) can be accomplished by several means including sling spychrometers (wet bulb and dry bulb temperatures), synthetic hair hygrometers (changes in length of a moisture sensitive fiber), dew point hygrometers (temperature at which the air becomes saturated and begins to condense), thin film capacitive sensors (changes in capacitance of a polymer film), resistive sensors (changes in resistance of a polymer), and new micromachined RH sensor chip (organic or polymer sensing film with a Wheatstone Bridge piezoresistor circuit).39,40,41 These instruments vary widely in performance and cost. Electronic RH sensors are most commonly used in CA storage facilities. It should be remembered that monitoring and controlling RH above 95% in an environment maintained at a temperature close to the freezing point is extremely difficult. 2.3.5.3 Oxygen The level of oxygen in a CA storage environment can be monitored with either a paramagnetic oxygen analyzer or with an electrochemical fuel cell. With the paramagnetic oxygen analyzer, the level of oxygen is determined by measuring the magnetic properties of oxygen in the air sample.42 Although high in cost, this instrument can provide an accuracy of up to 0.1% after calibration. With the electrochemical cell, oxygen is converted by the fuel cell into a current proportional to the concentration of oxygen in the air sample.43 Recent technological improvements have increased the accuracy of these cells tremendously, and they are now frequently used in CA storage facilities. 2.3.5.4 Carbon Dioxide Measurement of the CO2 level is normally done by infrared absorption. This spectroscopic method is based on the absorption of nondispersive infrared radiation. The reduction in the radiation is a

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function of the wavelength and provides a measure of the concentration of carbon dioxide in the air sample.43,44

2.4 PRECOOLING There are many methods of cooling horticultural products before storage or loading for shipment: room cooling, forced-air cooling, vacuum cooling, hydrocooling, package icing, and top icing.45 Some commodities can be cooled by several methods, but most commodities respond best to one cooling method. The precooling requirements of a given commodity, and therefore the method used, are largely determined by product physiology in relation to harvest maturity and ambient temperature at harvest time.46 Highly perishable commodities must be precooled as soon and as rapidly as possible after harvest. In general, such commodities are those harvested in late spring, summer, and early fall. Precooling is not as important for late-season crops (such as winter apple varieties) or low-respiration commodities. In this section, only cooling methods that apply to fruits will be discussed.

2.4.1

ROOM COOLING

Room cooling is probably the most widely used refrigeration technique due to its versatility and low cost. However, room-cooled produce must be tolerant of slow heat removal since much of the cooling is by heat conduction through the container walls. This technique is not considered to be a true precooling method. It involves placing field containers in a room cooled by a refrigeration system. The ventilation system must provide good air circulation through and around the containers (Figure 2.15). Cold air from the evaporator enters the room near the ceiling, moves horizontally under the ceiling, and then sweeps past the produce containers in returning to the evaporator. For adequate heat removal, the air-flow velocity must be 1 to 2 m·sec–1, with the moving cold air in good contact with all container surfaces.45 Container disposition is, therefore, very important in the handling sequence. Room cooling is best applied to produce harvested at the end of the season45 or with a relatively long storage life. It should also have a low respiration rate and a low harvest temperature. The main advantage of room cooling is that produce can be cooled and stored in the same room without being transferred. However, it is too slow for most commodities and can result in excessive water loss. Rehandling may be needed for better use of the storage space after cooling is achieved.47

FIGURE 2.15 Schematic of the air circulation pattern during air cooling.

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FIGURE 2.16 Forced-air tunnel cooling.

Furthermore, the addition of warm produce into a cold room may result in condensation on already cooled produce.48

2.4.2 FORCED-AIR COOLING Forced-air cooling consists of using fans to pull refrigerated air through the container vents by vacuum so that warm air is swept away by the cold airflow.49 Two basic designs are used: the forced-air tunnel and the cold wall. Mitchell et al.48 observed that forced-air pressure cooling is four to ten times faster than conventional room cooling, but two to three times slower than hydrocooling or vacuum cooling. The product cools as a result of the convective action of refrigerated air sweeping away warm air. Pulling rather than blowing air through the containers is a better option since “short-circuiting” (i.e., refrigerated air that, when the containers are under vacuum, flows directly to the fan without first going around the product) is reduced. Ideally, the ventilation system must deliver between 0.5 and 3 L·sec–1·kg–1 of warm product.49 The forced-air tunnel system (Figure 2.16) consists of a row of palletized containers or bins placed on either side of an exhaust fan, leaving an aisle between the rows. The aisle and the open end are then covered to create an air plenum tunnel. The exhaust fan creates negative air pressure within the tunnel. Cold air from the room then moves through the openings in and between containers toward the low-pressure zone, thus sweeping heat from the product.45 The investment for this method is minimal: a ventilation unit and a covering linen. However, the method also returns warm air into the room, which may condense on the produce. It is possible to eliminate the use of a fan and the condensation problem if one end of the tunnel is put directly on the air intake of the cooling system. In this case, the produce containers must be rearranged in order to prevent dehydration after precooling is achieved. The cold wall cooling system (Figure 2.17) uses a permanent air plenum, formed by the construction of a “dummy wall” equipped with exhaust fans. It is often located at one end or side of a cold room, with the exhaust fans designed to move air over the cooling system. Openings are located along the room side of the plenum against which stacks or pallet loads of containers can be placed. Dampers ensure that airflow is blocked except when a pallet is in place. Each pallet starts cooling as soon as it is in place, thus eliminating the need to await deliveries to complete a tunnel.47 This is a good system for operations in which the produce arrives at variable times. It is

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COOLING SYSTEM

FIGURE 2.17 Cross-sectional view of a cold-wall forced-air cooling unit.

also versatile, in that the treatment time can be adjusted for each product; however, rehandling of the containers is also necessary as soon as cooling is achieved.

2.4.3 HYDROCOOLING Cold water is an old and effective method for quickly precooling a wide range of fruits and vegetables in bins or in bulk. Hydrocoolers can be based on immersion or showering.45 In a typical shower-type hydrocooler (Figure 2.18), cold water is pumped from a bottom reservoir to an overhead perforated pan. The water showers over the commodity, which may be in bins or boxes or loose on a conveyer belt passing beneath. Water leaving the product may be filtered to remove debris, then passed over refrigeration coils or ice, where it is recooled.45 WATER PERFORATED PAN

PRODUCE PUMP BOTTOM RESERVOIR

COOLING SYSTEM

FIGURE 2.18 Schematic view of a typical hydrocooler.

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The evaporator of the refrigeration system recools used water. It is usually located in one of the two reservoirs of the precooler. When the evaporator is on the top reservoir, it frees the bottom reservoir, thus facilitating cleaning which must be done regularly due to the amount of dirt washed off the produce. Since the top reservoir is empty when the pump is off, it cannot be used as an ice reservoir. If the evaporator is located in the bottom reservoir, it can accumulate ice between cooling periods and, thus, considerably increases the refrigeration capacity for a given compressor capacity; it is a great advantage when the produce deliveries vary considerably. Ice may also be used if it can be added to water when the cooling capacity of the system is not enough to keep water temperature near 0∞C.45 Cooling speed is generally greater with a water system than with an air system, and there is no product dehydration. The produce and the packing and packaging materials must be tolerant to wetting, resistant to water impact, and tolerant of chlorine or other chemicals that are used to sanitize the hydrocooling.45 The produce must also be able to tolerate prolonged exposures to a temperature of 0∞C. Hydrocooling operations can require rehandling of the produce before packing or storage, thus increasing labor costs.

2.5 ECONOMIC CONSIDERATIONS CA and MA technologies prolong the storage time of many kinds of fruits and vegetables, and optimal atmospheres often reduce the proliferation of pathogens. However, the economic viability of investing in such technology depends on local, regional, and national socioeconomic conditions, as well as on national and international trade policy. Moreover, the economics of such an endeavor depends on the scale of production and the organizational structure of producer, wholesaler, and retailing groups. In some regions, producers work independently while in others they are organized in cooperatives. Wholesale buying may also be done on an individual or cooperative basis, as with retailing. From region to region and country to country, there are important differences in supply, demand, and seasonality, thus making it nearly impossible to present a reasonable economic analysis of the wisdom of investing in such technology. If the storage goals mentioned in the introduction to this chapter are taken to be worthwhile incentives — that is, they should reduce the use of chemicals in storage, extend the period of availability of a nutritionally superior product, and create the possibility of making quality products available to a wider range of income groups — then the investment is worthwhile. If, however, the decision is to be made on purely profit-making grounds, the analysis is complex. Insofar as CA and MA technology may offer a marketing advantage to one or more interest groups for certain products and in certain regions, widespread adoption of the technology will tend to remove the advantage and make the technology less attractive on those grounds alone. A recent example of this notion is the almost negligible price differential between conventional horticultural produce and so-called certified organic produce. In the early 1980s, the organically-certified product was worth about double the conventional product. Today, market conditions have resulted in a negligible advantage in implementing “organic” techniques.

ACKNOWLEDGMENTS The authors are grateful to NSERC, FCAR, and CORPAQ for their financial assistance, and to Valérie Orsat and Bernard Goyette for their help in preparing this chapter.

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REFERENCES 1. Bohling, H. and Hansen, H. Storage of white cabbage (Brassica oleacea var. capitata) in controlled atmosphere. Acta Hortic., 62: 49–54. 1977. 2. Smock, R.M. Controlled atmosphere storage of fruits. Hortic. Rev., 1: 301–336. 1979. 3. Soderstrom, E.L. and Brandl, D.G. Controlled atmospheres to reduce postharvest insect damage to horticultural crops. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. pp. 207–212. Proceedings of the Fourth National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 4. Gariépy, Y., Raghavan, G.S.V., and Thériault, R. Controlled atmosphere storage of celery with the silicone membrane system. Int. J. Refrig., 9: 234–240. 1986. 5. Kader, A.A. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 40(5): 99–104. 1986. 6. Yong, R.E., Romani, R.J., and Biale, J.B. Carbon dioxide effects on fruit respiration. II. Response of avocados, bananas and lemons. Plant Physiol., 37: 416–422. 1961. 7. Kader, A.A. 1985. A summary of CA requirements and recommendations for fruits other than pome fruits. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. p. 445. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 8. Day, N.B., Skura, B.J., and Powrie, W.D. Modified atmosphere packaging of blueberries: microbiological changes. Can. Inst. Food Sci. Techn. J., 23(1): 59–65. 1990. 9. Lépine, Y. Strawberry handling in Québec. Thesis. Agricultural Engineering Department, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue (QC). 1989. 10. El Kazzaz, M.K., Summer, N.F., and Kader, A.A. Ehtylene effects on in vitro and in vivo growth of certain postharvest fruit-infecting fungi Citrus sinensis, Valencia oranges, Fragaria chiloensis var. ananassa, strawberries. Phytopathol., 998–1001. 1983. 11. Farber, J.M. Microbiological aspects of modified atmosphere packaging technology — A review. J. Food Protect., 54(1): 58–70. 1991. 12. Skrzynski, J., Fica, J., and Dilley, D.R. The effect of ethylene removal during controlled atmosphere storage of Red Delicious, Empire, Idared and Law Rome apples. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. p. 115–126. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 13. Kader, A.A. An overview of the physiological and biochemical basis of CA effects on fresh horticultural crops. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. pp. 1–9. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 14. Ulrich, R. Controlled atmosphere storage, Part 2. In Postharvest Physiology, Handling and Utilization of Tropical and Subtropical Fruit and Vegetables. AVI Publishing, Westport, CT. 1975. 15. Plasse, R. Vegetable storage, respiration, and design criteria in a membrane storage system. Thesis. Agricultural Engineering Department, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue (QC), 215 pp. 1987. 16. Emond, J.P., Castaigne, F., Toupin, C.J., and Desilets, D. Mathematical modelling of gas exchange in modified atmosphere packaging. Trans. ASAE, 34(1): 239–245. 1991. 17. Gariépy, Y., Raghavan, G.S.V., Thériault, R., and Munroe, J.A. Design procedure for the silicone membrane system used for controlled atmosphere storage of leeks and celery. Can. Agric. Eng., 30(2): 231–236. 1988. 18. Deily, K.R. and Rizvi, S.S.H. Packaging of fresh peaches in polymeric films. J. Food Process. Eng., 5: 23–41. 1981. 19. Marcellin, P. Emballages de matière plastique pour la conservation des fruits en atmosphère controlée. Rev. Prat. Froid Condition. Air., 10: 23–29. 1972. 20. Marcellin, P. Application des membranes de polymères à la conservation des fruits et légumes en atmosphère controlée. Proceeding of the 4th International Congress of Food Science and Technology. Valencia, Spain. pp. 91–99. 1974. 21. Ash, M. and Ash, I. Encyclopedia of Plastics, Polymers, and Resins. Vol. 3. Chemical Publishing Company, New York. 1983.

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22. Roff, W.J., Scott, J.R., and Pacitti, J. Handbook of Common Polymers, Fibers, Films, Plastics and Rubbers. CRC Press, Cleveland, OH. 1971. 23. Yescombe, E.R. Plastics and Rubbers: World Source of Information. Applied Science Publishing, London. 1976. 24. Marcellin, P. Principe et réalisation des atmosphères controlées. In Perspectives Nouvelles dans la Conservation des Fruits et Légumes. CRESALA, Montreal (QC). pp. 88–97. 1978. 25. Marcellin, P. and Leteinturier, J. Premières applications des membranes de caoutchouc de silicone à l’entreposage des pommes en atmosphère controlée. Int. Inst. Refrig., Paris. pp. 1–9. 1967. 26. Raghavan, G.S.V., Gariépy, Y., Thériault, R., Phan, C.T., and Lanson, A. System for controlled atmosphere long-term cabbage storage. Int. J. Refrig., 7(1): 66–71. 1984. 27. Bartsch, J.A. and Blanpied, G.D. Air separator technology for controlled atmosphere storage. A workshop sponsored by Cornell Cooperative Extension. Cornell University. Ithaca, NY, 1988. 28. Bartsch, J.A. and Blanpied, G.D. Refrigeration and controlled atmosphere storage for horticultural crops. Northeast Regional Agricultural Engineering Service, Cornell University, Ithaca, NY. NRAES J. Paper No. 22. 1990. 29. Smock, R.M., Creasy, L.L., and Blanpied, G.D. Water scrubbing in CA rooms. Cornell University, Ithaca, NY, Paper No. s-508. 1960. 30. Pflug, I.J. Oxygen reduction in CA storages: A comparison of water vs. caustic soda absorbers. Mich. Agr. Expt. Stat. Q. Bull., 43(2): 455–466. 1960. 31. Blanpied, G.D. Handbook for Low Ethylene CA Storage of McIntosh and Empire Apples. Pomology Department, Cornell University, Ithaca, NY. 1985. 32. Bishop, D.J. Application of new techniques in CA storage. Symposium on New Application of Refrigeration to Fruit and Vegetable Processing. June 8–9, Istanbul, Turkey. 323–331. 1994. 33. Mittal, G.S. Computer-based instrumentation: sensors for in-line measurements. In Computerized Control Systems in the Food Industry. Marcel Dekker, New York. pp. 13–53. 1997. 34. vanDoren, V.J. Basics of proportional-integral-derivative control. Control Engineering. 45(3): 135–142. 1998. 35. Seborg, D.E., Shah, S.L., and Edgar, T.F. Adaptive control strategies for process control: a survey. AIChE paper presented at the AIChE Diamond Jubilee Meeting, Washington, D.C. November 1983. 36. Morimoto, T. and Hashimoto, Y. An intelligent control for greenhouse automation, oriented by the concepts of SPA and SFA — an application to a postharvest process. Comput. Electron. Agri., 29(1–2): 3–20. 2000. 37. Simpson, J.B. and Pettibone, C.A. Temperature. In Instrumentation and Measurement for Environmental Sciences. Bailey, W.M., Ed. ASAE Special Publication 82–13. St. Joseph MI, 6–01 to 6–16. 1983. Chap. 6. 38. Desmarais, R. and Breuer, J. How to select and use the right temperature sensor. Sensors Online (www.sensorsmag.com) 18(1): 11. 2001. 39. White, G.M. and Ross, I.J. Humidity. In Instrumentation and Measurement for Environmental Sciences. Bailey, W.M., Ed. ASAE Special Publication 82–13. St. Joseph MI, 8–01 to 8–12. 1983. Chap. 8. 40. Fenner, R., Kleefstra, M., and Zdankiewicz, E. New micro-machined water vapor sensor for home appliance applications. Paper presented at the Appliance Manufacturer Conference and Expo 2000. Cincinnati, OH, Sept. 11–13, 2000. 2000. 41. Roveti, D.K. Choosing a humidity sensor: A review of three technologies. Sensors Online (www.sensorsmag.com) 18(7): 9. 2001. 42. Anonymous. Application of Oxygen Analyzers. Analytical Specialties (www.analyzer.com) Houston TX. 77058. 43. Anonymous. Gas Analyzer for IR-absorbing Gases and Oxygen 7MB233. Siemens Ultramat23 Instruction Manual. Karlsruhe, Germany 3–5. 1997. 44. Anonymous. Infrared analysis. Analytical Specialties (www.analyzer.com) Houston TX, USA 77058. 45. Kader, A.A. Postharvest Technology of Horticultural Crops. 2nd ed. Cooperative Extension University of California, Davis, CA. Publication No. 3311. 1992. 46. ASHRAE. Methods of precooling fruits, vegetables and ornamentals. In: Refrigeration Systems and Applications Handbook. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. Chap. 11. 1986.

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Processing Fruits: Science and Technology, Second Edition 47. Mitchell, F.G. Cooling methods. In: Postharvest Technology of Horticultural Crops. Kader, A.A. Ed., 2nd ed. Cooperative Extension University of California, Davis, CA. Publication No. 3311. pp. 56–63. 1992. 48. Mitchell, F.G., Guillou, R., and Parsons, R.A. Commercial Cooling of Fruits and Vegetables. Manual 43. University of California. Division of Agricultural Sciences. 1972. 49. Fraser, H.W. Forced-Air Rapid Cooling of Fresh Ontario Fruits and Vegetables. Ministry of Agriculture and Food. Toronto, ON. AGDEX 202–736. 1991.

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3 Fresh-Cut Fruits Elisabeth Garcia and Diane M. Barrett CONTENTS 3.1 3.2 3.3

Introduction ..........................................................................................................................53 Effects of Fresh-Cut Processing on Produce Physiology ...................................................54 Unit Operations in Fresh-Cut Fruit Preparation..................................................................55 3.3.1 Receiving, Inspection, and Storage of Raw Material ...........................................56 3.3.2 Cleaning and Disinfection .....................................................................................56 3.3.3 Peeling, Deseeding, Trimming, Coring, and Cutting ...........................................56 3.3.4 Washing and Cooling ............................................................................................56 3.3.5 Dewatering .............................................................................................................57 3.3.6 Packaging and Distribution ...................................................................................57 3.4 Quality Aspects of Fresh-Cut Fruits....................................................................................57 3.4.1 Importance of Cultivar Selection ..........................................................................57 3.4.2 Optimum Degree of Ripeness for Processing.......................................................59 3.4.3 Microbial Spoilage ................................................................................................60 3.4.4 Browning Control ..................................................................................................61 3.4.5 Prevention of Textural Losses ...............................................................................63 3.4.6 Appearance and Sensory Quality ..........................................................................64 3.4.7 Nutritional Aspects ................................................................................................65 3.5 Shelf Life Extension of Fresh-Cut Fruit .............................................................................65 3.5.1 Temperature Management .....................................................................................66 3.5.2 Modified Atmosphere Packaging ..........................................................................66 3.5.3 Humidity ................................................................................................................68 3.5.4 Edible Coatings......................................................................................................68 References ........................................................................................................................................69

3.1 INTRODUCTION Over the past 25 years the consumption of fresh fruit in the U.S. was reported to have grown by about 26%, according to the Economic Research Service-U.S. Department of Agriculture (Pollack, 2001). However, this does not reflect all fruit because the report does not include melon consumption, which was part of the vegetable category statistics. Over the same period, the U.S. consumption of fresh vegetables and melon has increased by 33%. In addition to awareness of the health benefits of a diet rich in fruits and vegetables, several factors are attributed to these changes, such as rising income, increased production, greater diversity and availability (of domestic and imported produce, on and off season), product convenience, storage, and transportation, among others (Pollack, 2001). Nevertheless, according to a USDA survey (1994 to 1996), the recommended number of at least two daily fruit servings is currently met by only 23% of the American population (USDA, 1998). Frequently, convenience is an important factor in a consumer’s selection of produce, and fruit are no exception. From 1997 to 1999, over 40% of the total fruit intake was consumed as juice, 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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63% of which was solely orange juice (accounting for 86% of orange consumption). Another trend observed since the mid-1970s is that Americans are eating less canned produce, and more fresh and frozen items. Although over the past two decades the consumption of frozen fruit increased 36%, this constitutes a small fraction of all fruit consumed in the U.S. During the last two decades, another category of fruit product has been introduced in the market: fresh-cut fruit. Fresh-cut products have also been referred to as “lightly processed,” “minimally processed,” “fresh-processed,” “partially processed,” or “preprepared” products. According to the International Fresh-Cut Produce Association (IFPA), Fresh-cut produce is defined as any fresh fruit or vegetable or any combination thereof that has been physically altered from its original form, but remains in a fresh state. These fruits and vegetables have been trimmed, peeled, washed, and cut into 100% usable product that is largely bagged or prepackaged to offer consumers high nutrition, convenience, and value while still maintaining freshness (IFPA, 2002).

The idea of minimal processing of fruit has been around in different parts of the world for some time, but at a different scale. Consumers could find convenient produce items already washed and peeled or either, cut on site at a local market or street stall. No special packaging or refrigeration was used; these products were directed toward immediate consumption. In the U.S., fresh-cut produce first appeared in retail markets in the 1940s, but second-quality, misshapen produce was used; quality was unpredictable and shelf life was limited. In the mid-1970s, fast-food chains were using shredded fresh-cut lettuce and chopped onions; in the mid 1980s, salad bars opened, and fresh-cut produce start replacing canned products (Garrett, 2002). Today, vegetables (mainly salads) constitute the main segment (~70%) of fresh-cut products. In addition to the convenience, there are other reasons for the success of fresh-cut produce, such as the absence of waste material. When utilizing fresh-cut produce, 100% is consumable, and there is also a substantial decrease in labor required for home produce preparation (e.g., no need for washing, trimming, etc.) and waste disposal. The amount of waste in peeling and coring fruit can be quite elevated; in pineapple it frequently exceeds 50% of the fresh fruit weight (including the crown). These advantages make fresh-cut produce well accepted items in the foodservice sector, commercial salad bars, and fastfood outlets (Price and Floros, 1993). Examples of fresh-cut fruit available in the U.S. for retail and foodservice distribution are sliced apples, peaches, strawberries, oranges, grapefruit, mangoes, melons, watermelons, pineapple, citrus segments, and fruit salads. Fresh-cut fruit tend to be more perishable than the commodity from which they were prepared. The expected shelf life of fresh-cut fruit is around 7 to 8 d, in contrast to 10 to 14 d for fresh-cut vegetables (Kader, 2002). The limited commercial success of fresh-cut fruit is related to their perishability and consequently short shelf life.

3.2 EFFECTS OF FRESH-CUT PROCESSING ON PRODUCE PHYSIOLOGY Whereas the processing of fruits generally extends their shelf life, as seen in the case of canning, freezing, and drying, fresh-cut processing increases the perishability of fruits (Cantwell and Suslow, 1999). The physical wounding of tissue caused by the preparation (peeling, cutting, slicing, dicing, etc.) of the fresh-cut products leads to many physical and physiological responses (Brecht, 1995; Saltveit, 1997). There is an increase in respiration rate and ethylene production, which impacts fruit quality and shelf life. Increased respiration rate is related to elevated cellular metabolism, which contributes to faster quality deterioration. Table 3.1 shows examples of respiration rates of fresh-cut fruit as compared to intact fruit. Respiration rates increase with the degree of injury or processing and with storage temperature. In a study with papayas (Paull and Chen, 1997), fruit were longitudinally halved, one half fruit was kept with seeds, and the other half was deseeded.

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TABLE 3.1 Respiration Rates of Fresh-Cut Fruit and Whole Fruits Stored at Different Temperatures Respiration (mg CO2/kg/h) Fruit Kiwi Peach Muskmelon

Honeydew

whole slices whole slices large, whole large, slices small, whole small, slices whole slices

0∞ C

5∞C

10∞C

3.2 7.2 4.0 6.0 4.8 3.7 3.1 2.7 1.4 2.3

4.6 11.6 8.1 10.0 8.6 7.0 6.3 4.2 4.6 3.0

8.6 23.3 15.0 18.6 14.7 12.2 13.6 9.8 5.2 8.3

Source: From Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of fresh-cut horticultural products. Postharvest Biol. Technol. 9: 115–125.

Opposite halves of the same fruit were used. The wound caused by slicing and deseeding led to increases in ethylene production five- and ninefold higher in the halves with seeds and in the deseeded halves, respectively, as compared to the intact fruit. The injury caused by wounding through slicing and deseeding of papaya fruit also led to yellowing of the fruit skin and softening of the mesocarp. Wound-related production of ethylene is higher in fruit at the preclimacteric and climacteric stages than in postclimacteric period (Abeles et al., 1992). The response to injury may also be affected by the maturity stage of fruit. In a study by Cantwell and Suslow (1999), cantaloupe melon pieces processed from fruit at different maturity stages had similar respiration rates, but ethylene production was much higher in pieces from riper fruits. The removal of protective epidermis or skin of the fruit leads to changes in gas diffusion, accelerates water loss, and favors microbial contamination. It is important to prevent cut-surface desiccation, which negatively impacts product appearance. Another consequence of fresh-cut fruit processing is cell disruption, which allows previously compartmentalized enzymes to come in contact with their substrates, leading to undesirable reactions such as development of enzymatic browning and accelerated softening.

3.3 UNIT OPERATIONS IN FRESH-CUT FRUIT PREPARATION In order to maintain the highest possible quality of a fresh-cut fruit, it is important to observe adequate handling of the produce before preparation. It is necessary to keep produce under adequate storage conditions (temperature and relative humidity) and use gentle handling to minimize bruising and other physical injuries. Strict temperature control is the single most important factor in maintaining quality throughout the fresh-cut preparation and distribution. All unit operations should be carried out in cold processing rooms (£5∞C), and cleanliness is very important to ensure a highquality and safe product. It is of utmost importance to use sanitary equipment and adopt Good Manufacturing Practices (GMP) in combination with Hazard Analysis Critical Control Point (HACCP) (IFPA, 1996a, 1997; Beuchat, 2000). The overall steps involved in the preparation of fresh-cut fruit follow.

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3.3.1 RECEIVING, INSPECTION,

AND

STORAGE

OF

RAW MATERIAL

Fruit has to be inspected and evaluated according to high standards of safety and quality. It is well known that no processing can increase the quality of a diseased or misshapen ingredient. Therefore, it is very important that while awaiting processing, fruit should be kept under refrigeration (in general, 1 to 5∞C) according to produce specifications. If a commodity is sensitive to chilling injury, it requires appropriate storage conditions, separate from nonsensitive fruit. For example, symptoms of chilling injury are seen in pineapple stored for prolonged periods at temperatures below 12∞C (Paull and Rohrbach, 1985). Whereas some fruits, such as pears and apples, can be stored for extended periods of time while waiting to undergo processing, the effects of such storage on freshcut products may be undesirable. Gorny and coworkers (2000) observed significant shelf-life reduction in fresh-cut pears prepared from fruit that had been kept for increased storage periods at 1∞C in air or controlled atmosphere (2% O2 + 98% N2).

3.3.2 CLEANING

AND

DISINFECTION

With many fruit that have a smooth surface, such as apples and pears, the use of cold clean water may be enough for a first wash. However, some fruit may harbor a high microbial population on their external surface. Cantaloupes were one of three imported produce with the greatest incidence of pathogen contamination in a survey carried out by the Food and Drug Administration (FDA, 2002). Such fruit should be washed, scrubbed, and dipped in solutions of disinfectants. The most commonly used sanitizer is chlorinated water, but other possible chemicals for disinfection are hydrogen peroxide, surfactants, peroxyacetic acid, trisodium phosphate, and ozone, among others (Heard, 2002; Beuchat, 2000; Cherry, 1999; Sapers and Simmons, 1998). The efficacy of chlorine and hydrogen peroxide treatment on the native microflora of cantaloupes, as well as on Escherichia coli population, was compared by Ukuku and coworkers (2001). Hydrogen peroxide proved to be more efficient against the surface microflora of cantaloupe, while chlorine was more effective against E. coli ATCC 25922.

3.3.3 PEELING, DESEEDING, TRIMMING, CORING,

AND

CUTTING

In contrast to vegetables, which in many instances can be peeled and cut mechanically (such as carrots), for most fruit whose texture is soft, the removal of skins and rinds is carried out by hand. It is very important to use clean and sharp knives during this operation. The use of blunt blades causes excessive physical injury (tissue crushing) to the fruit tissue adjacent to the cut, accelerating quality deterioration. Fresh-cut cantaloupe melon prepared with sharp and blunt knives were compared (Portela and Cantwell, 2001). Melon pieces prepared using blunt knives had increased ethanol levels, off-odors, and electrolyte leakage compared to sharp-cut pieces. The marketable visual quality of sharp-cut melon lasted much longer; blunt-cut pieces developed a translucency, which is a common visual defect that indicates cell disruption in commercially prepared freshcut cantaloupe. With citrus, enzymatic peeling has been reported as an alternative to hand peeling (Bruemmer et al., 1978; Pretel et al., 1996, 1998). Whole citrus fruit can be vacuum infused with solutions of pectic enzymes that will eliminate flavedo and albedo, with limited damage to the juice vesicles. After this treatment, peeled fruit have to be rinsed by immersion in water for removal of the enzyme solution.

3.3.4 WASHING

AND

COOLING

During the cutting steps there is release of tissue fluids that should be removed to avoid undesirable microbiological or chemical reactions. It is imperative to rinse the cut surfaces of the fruit. At this stage the use of cold water (~0∞C) accomplishes the washing of the cut surfaces as well as cooling of the fruit pieces. For safety reasons, chlorinated water (usually 50 to 100 ppm) is frequently used.

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All the equipment used in the processing (cutting tools and mats) are a potential source of contamination and require adequate sanitation. In some cases it is necessary to use dipping solutions containing processing aids that will help prevent nonmicrobial quality deterioration such as surface browning and softening (Garcia and Barrett, 2002).

3.3.5 DEWATERING Excess moisture picked up during the washing operation should be removed prior to packaging. This step helps to prevent growth of microorganisms that remained after produce disinfection. Due to their delicate texture, fresh-cut fruit require passage through semi-fluidized beds with forced air to remove moisture.

3.3.6 PACKAGING

AND

DISTRIBUTION

In order to ensure the longest possible shelf life for fresh-cut fruit, it is important to choose appropriate packaging materials and storage conditions. Temperature is always a critical factor in the shelf life of fresh-cut fruit. The importance of temperature control is related to food safety and extension of fresh-cut life by slowing respiration rate and preventing quality deterioration. As an example, the shelf life of sliced pineapple is reported to range from a few hours at 20∞C and to greater than 5 weeks at 1∞C (O’Hare, 1994). Throughout the unit operations involved in preparation and up to consumption, a cold chain should be maintained; ideally temperatures should not exceed 5∞C, although preferably they should be closer to 1∞C (IFPA, 1997). During the distribution of fresh-cut products, it is also important to avoid rough handling. Very often products are subjected to shock and vibration stress, which cause injury, leading to more rapid quality loss. In a study of fresh-cut watermelon cubes, results showed increased juice leakage and darkening of product vibrated in noncompartmentalized packages. Authors suggested that the placement of compartments that decrease movement of fruit pieces during transit can improve overall quality by reducing vibration (Fonseca et al., 1999).

3.4 QUALITY ASPECTS OF FRESH-CUT FRUITS Fresh-cut fruit combine the convenience of a 100% usable product with the fresh-like quality characteristics of fresh fruit, e.g., appearance, flavor, and texture. The initial appeal of a convenient fresh-cut product will only be ensured of continued acceptability if the product quality meets the consumer’s quality criteria including appearance, product shelf-life, and relationship between perceived value and cost. Moreover, the product value as food also depends on its nutritional quality and its safety (Kader, 2002). Several preharvest factors affect the quality of a commodity, such as genetic background, agronomic practices, environmental conditions during cultivation, and stage of maturity at harvest. Final product quality is influenced to a great degree by postharvest handling and fresh-cut preparation, from the initial unit operations up to distribution. As a result of the injury (wound) brought by cutting, cells are broken and undesirable reactions may result. Fresh-cut fruit may undergo surface browning, tissue softening, loss of flavor, and other deterioration reactions. Moreover, the leakage of nutrient-rich tissue fluids can potentially lead to microbial spoilage; peeling and removal of protective tissues lead to water loss and, consequently, quality defects such as surface drying, loss of gloss, and shrinkage are more noticeable.

3.4.1 IMPORTANCE

OF

CULTIVAR SELECTION

A high-quality fresh-cut product starts with fruit of superior raw material quality and the appropriate selection of fruit cultivars (cv.). In fact, cultivar selection is one of the most important considerations.

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TABLE 3.2 Enzymatic Browning of Peara and Apricotb Purees Prepared from Different Fruit Cultivars Harvested at Commercial Maturity Pear Cultivar Abbe Fetel Comice Conference Guyot P 2198 Williams 6.30.100

DLa –16.2 –15.1 –14.1 –9.9 –15.3 –13.3 –4.8

Apricot Cultivar

DLa

Bebeco Cafona Canino Henderson Moniqui Polonais P. Tyrinthe R. Fournes R. Roussillon

5.3 11.8 16.7 26.3 21.4 16.8 3.7 17.8 17.8

a

Reflectance measurements expressed as DL, difference in lightness between oxidized and nonoxidized purees. Source: From aAmiot, M.J., Tacchini, M., Aubert, S., Oleszek, W. 1995. Influence of cultivar, maturity stage, and storage conditions on phenolic composition and enzymatic browning of pear fruits. J. Agr. Food Chem. 43(5): 1132–1137; bRadi, M., Mahrouz, M., Jaouad, A., Tacchini, M., Aubert, S., Hughes, M., Amiot, M.J. 1997. Phenolic composition, browning susceptibility, and carotenoid content of several apricot cultivars at maturity. HortScience 32(6): 1087–1091.

The most suitable cultivars should be selected, or developed, for compatibility with fresh-cut processing, packaging, and distribution. At this stage, many characteristics may be considered, such as fruit shape or size, flesh thickness, susceptibility to pre- and postharvest pathogens, susceptibility to browning, desiccation, texture loss, etc. (Romig, 1995). Different fruit cultivars may vary in their respiration rates when cut for processing. A comparison of four pear cultivars commonly available in the U.S. (cv. Bosc, Bartlett, Anjou, and Red Anjou) showed that pears of the cv. Bartlett showed no difference in respiration rate when stored at 10∞C in air and 90 to 95% relative humidity. However, slices prepared from the cv. Bosc had a 35% increase, cv. Anjou a 64% increase, and cv. Red Anjou a 232% increase in respiration in relation to the intact fruit of the same cultivar. The evolution of carbon dioxide in all cultivars was between two- and fourfold greater when temperature was increased from 0∞C to 10∞C (Gorny et al., 2000). Varying composition of different cultivars will affect final product quality. The browning tendency of different fruit cultivars is exemplified for pears and apricots in Table 3.2. The pear cultivars evaluated in this study showed almost a fourfold range in their difference in lightness (DL value) between fresh and oxidized purees. Apricot cultivars showed a similar range of difference in browning tendency. Bartlett pear slices had a large decrease (82%) in flesh firmness, in contrast with cultivar Anjou (12% decrease in firmness) after 6 d at 10∞C (Gorny et al., 2000). Variations in the shelf life (ranging from 2 to 12 d at 0∞C) of different cultivars of sliced peaches and nectarines are shown in Figure 3.1. Evaluation of shelf life was based on appearance; products developed surface browning, pit cavity breakdown, and limited dehydration, resulting in loss of sheen and gloss at the cut surface (Gorny et al., 1999).

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Peaches Nectarines Arctic Queen August Red September Red Sparkling Red Summer Diamond Summer Fire Summer Grand Zee Grand 0

2

4

6

8

10

12

14

Marketable shelf-life (days)

August Lady Cal Red Carnival Elegant Lady Flavorcrest O’Henry Red Cal Red Sun Ryan Sun Snow Giant Snow King Summer Lady Tra Zee 0

2

4

6

8

10 12 14

Marketable shelf-life (days)

FIGURE 3.1 Genotypic differences in shelf life of fresh-cut peaches and nectarines kept in air at 0∞C and 90 to 95% relative humidity. (From Gorny, J.R., Hess-Pierce, B., Kader, A.A. 1999. Quality changes in freshcut peach and nectarine slices as affected by cultivar, storage atmosphere and chemical treatments. J. Food Sci. 64(3): 429–432.)

Many agronomic (soil type, fertilizer application, water supply, etc.) and environmental (climate and rainfall) factors also affect fruit quality. Sucrose is the main soluble sugar in muskmelon flesh, and adverse weather conditions in the weeks prior to harvest can have a negative effect on fruit sweetness. Short-term shading (5 d at ~ 50% reduced daylight) of muskmelon plants prior to harvesting promoted accumulation of acetaldehyde and ethanol and reduction of sucrose content in melon flesh (Nishizawa et al., 1998). Cultivars may also respond differently to conditions adopted in modified atmosphere packaging. Different cultivars of strawberries showed wide variation in response when exposed to 20% carbon dioxide for up to 9 d. Some cultivars (such as Honeoye and Kent) accumulated large levels of fermentation products associated with off-flavor development, such as acetaldehyde and ethanol. Depending on the commodity, different traits may be relevant in selecting cultivars for freshcut preparation. While tendency to brown is important in fresh-cut apples and pears, there may be more interest in other quality attributes such as texture retention in sliced kiwifruit, absence of seeds in watermelon for fresh-cut use, reduced off-flavor production in strawberries, and a balance between sweetness and acidity in pineapple.

3.4.2 OPTIMUM DEGREE

OF

RIPENESS

FOR

PROCESSING

Quality and shelf life of fresh-cut fruit are affected by maturity stage. While a fresh-cut fruit is expected (by the consumer) to be at its optimum ripeness, with the best sensory attributes of flavor, taste, and firmness, this is usually not the best maturity stage for handling and processing. Often a fully ripe fruit may be at an advanced stage of softening for fresh-cut preparation, whereas an immature fruit may not present adequate eating quality (Kader and Mitcham, 1998). Synthesis of both flavor and color compounds occur relatively late in fruit maturation; therefore an early harvest may result in poor flavor and color. Shorter shelf life results from harvest of either immature or overripe fruit that are more susceptible to physiological disorders than fruit harvested at the proper maturity (Kader, 2002). In a study of sliced pears (cv. Bartlett), ripe fruit had a shelf life of ~2 d at 0∞C, in contrast with partially ripe and mature-green slices with a shelf life of 8 d. However, the eating quality of mature-green fruit was compromised due to poor aroma and lack of juiciness (Gorny et al., 2000). Likewise, using quality scores based on appearance, firmness, and taste, honeydew melon cubes prepared from fruit at the immature to mature threshold stage (8.8% soluble solids) were rated as poor after storage of 3 d at 10∞C, and as fair to poor after 7 d at 5∞C. Melon

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cubes from very mature fruit (13% soluble solids) were rated as good to fair after 7 d at 5∞C and fair after 3 d at 10∞C. Fresh-cut products from immature fruit did not have honeydew taste or aroma and deteriorated faster than product from mature fruit (Watada and Qi, 1999).

3.4.3 MICROBIAL SPOILAGE Although it is considered that whole fresh produce, for the most part, are among one of the safest foods (Brackett, 1987), the safety of fresh-cut products requires a great deal of attention. The processing that fruit undergo turn them more vulnerable to microbiological risks, and the sensory quality is meaningless if the product is unsafe (Brackett, 1992). Along the chain of steps involved in the production of fresh-cut products, from the growth of the raw material to the processing and distribution, there is potential for microbiological risk. Microbial contamination may (1) be of public health significance due to the presence of human pathogens, or (2) decrease the shelf life of the fresh-cut fruit due to spoilage microorganisms, and in addition render a major economic impact. Appropriate care should start in the farm environment. Contamination may result from soil, poor agronomic practices such as use of raw or improperly treated animal manure as fertilizer, irrigation with contaminated water, animal contact, etc. During fruit picking and packing, contamination may occur through contact with food handlers who may be carriers of microbial pathogens. In general, all fruit arrive at the receiving section of the food processing plant with a microbial load. In the initial steps of processing, with peeling and cutting there is removal of protective barriers (skin, peel, rind) and release of nutrient-rich cellular fluids, conditions that provide increased risk for microbial contamination of the product. After some outbreaks occurred due to the consumption of cantaloupe from salad bars, scientists concluded that microorganisms present in the melon rind were most likely introduced into the fruit during cutting of the melons (Madden, 1992). The normal microflora found on the surface of produce is diverse, but it generally includes a variety of fungi, mostly harmless bacteria and different species of yeasts. It is expected that different microorganisms will be present in produce that grows on top of the soil vs. a tree-borne fruit. Fungi constitute the majority of spoilage microorganisms found in fruits (Brackett, 1987). However, human pathogens may be present in the fruit microflora. Clostridium botulinum, Clostridium perfringens, and Bacillus cereus can be isolated from soils free of fecal contamination. Listeria monocytogenes can be found in decaying vegetation and soil, as can coliforms of nonfecal origin such as Enterobacter spp. and Klebsiella spp. Enteric microorganisms, among them E. coli, are linked to soils contaminated with feces, improperly composted manure or sewage, and contaminated irrigation water. In agricultural environments, animals (domestic and wild) constitute another source of pathogenic bacteria. Salmonellae are found in the intestine of many animals and are abundant in fecal material and sewage (Francis et al., 1999). Birds that feed at sewage works can be a vector for L. monocytogenes and enteric bacteria into farm environment. Enteric bacteria can be transmitted to flowers during pollination by insects (NACMCF, 1999). Viruses such as the Hepatitis A and Norwalk can also be found in foods. Fresh-cut product contamination may also occur during harvesting, processing and handling as a result of inadequate personal hygiene among employees, poor sanitation of the processing facilities, dirty cutting tools and work surfaces, and reuse of wash water or ice. In addition, due to certain characteristics of the fresh-cut fruit processing and packaging, there are additional risks. While one of the initial steps in the processing of fresh-cut products is sanitation, not all microorganisms are eliminated. After breaking the fruit surface integrity with peeling and cutting, rapid bacterial growth can take place. However, the low pH of most fruits does not support the growth of most pathogens (Brackett, 1987). For example, the pH of pineapple ranges between 3.5 and 4 (Siriphanich, 1994). Nevertheless, high pH values are common in different types of melons (Lund, 1992), such as honeydew (pH 5.2 to 5.6), cantaloupe (pH 6.2 to 6.5), and watermelon (pH 6.3 to 6.7), and these fruit are particularly susceptible to microbial contamination and growth.

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Storage temperature is a very important factor in controlling microbial growth. Although storage under refrigeration temperatures deters the growth of many microorganisms, some can grow at these conditions, as shown for L. monocytogenes (Nguyen-The and Carlin, 1994). With the use of certain types of packaging (such as in modified atmosphere packaging) and long periods of storage, some pathogenic microorganisms may attain numbers high enough to pose a health risk. This could result from the growth of a pathogen population under conditions where there is a suppression of the natural microflora that under normal circumstances would play a positive role in competing with pathogens. The long storage periods allow time for available pathogens to grow, and significant microbial populations may develop. Additionally, if temperature abuse occurs, anaerobic atmospheres can develop inside packages during extended storage (Francis et al., 1999). While these conditions do not favor the growth of some microorganisms, C. botulinum can grow under high CO2. Product appearance or sensory characteristics may not be affected and the potential health risk may not be perceived by the consumer (IFPA, 1996b). Following the strict guidelines of GMP and a well-designed HACCP (Hazard Analysis Critical Control Points) plan are fundamental recommendations of the International Fresh-Cut Produce Association (1996a, 1997) in order to produce a microbiologically safe product.

3.4.4 BROWNING CONTROL Surface discoloration is probably the most common quality defect of fresh-cut fruit and the factor most limiting shelf life. During the peeling and cutting operations, cells are broken, and their contents include previously compartmentalized enzymes that are suddenly freed to come in contact with their substrates. A group of enzymes called polyphenol oxidases (PPO), which occur in particularly high amounts in fruits such as banana, apple, pear, avocado, and peach, are responsible for the discoloration referred to as enzymatic browning. Enzymatic browning may be controlled through the use of physical and chemical methods; often both are employed. Physical methods commonly used include reduction of temperature and oxygen, and use of modified atmosphere packaging or edible coatings. Chemical methods depend on either treatment with compounds that inhibit polyphenol oxidase, remove its substrates (oxygen and phenolics), or function as preferred substrates. Various antibrowning agents can be used in postcutting dip solutions. An overview on preservative treatments for fresh-cut products was recently published (Garcia and Barrett, 2002). Ascorbic acid is probably the most common antibrowning agent selected for use in fresh-cut fruit. The drawback of ascorbic acid is that it confers only temporary protection because it is oxidized in the process of preventing browning. While the inhibitory effect of ascorbic acid on browning is due to its reductant action on quinones, other common types of antibrowning agents available for use in fresh-cut fruits include acidulants (e.g., citric acid, malic acid) and chelators (e.g., EDTA), which are often used in combination in browning prevention. Antibrowning treatments commonly include ascorbic acid or its isomer erythorbic acid (d-isoascorbic acid), citric acid, EDTA, cysteine, and its derivatives. Research papers have presented results on 4-hexylresorcinol, which has shown effective control of enzymatic browning in fruits (Sapers, 1993); however, 4hexylresorcinol is currently only approved for use in the prevention of shrimp discoloration (McEvily et al., 1991). No single antibrowning treatment can control enzymatic browning of all fresh-cut fruit because there is not just one possible solution for a particular fruit product. This can be illustrated by the examples shown in Table 3.3, where various antibrowning treatments selected by different authors for use in fresh-cut pears are presented. Although there has been commercial interest in the development of fresh-cut pear products, pear fruit presents some particular challenges, such as its high susceptibility to enzymatic browning and tissue softening. Fruit ripeness stage proved to be an important factor in the control of browning in pears. Overripe pears are generally prone to more severe discoloration (Sapers and Miller, 1998; Dong et al., 2000); optimal flesh firmness for

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TABLE 3.3 Examples of Treatments Suggested for Controlling Enzymatic Browning in Fresh-Cut Pears Pear

Firmness

Antibrowning Treatment

Bartlett Anjou

44.5 N 4.0 kg

1% CaCl2 + 0.5% O2 atmosphere 4% Sodium erythorbate, 0.2% CaCl2 100 ppm 4-Hexylresorcinol 1-min dip; storage at 4∞C Modified Atmosphere Packaging (MAP)

Bartlett

5.7 kg

4% Sodium erythorbate, and/or 0.2% CaCl2 and/or 4-Hexylresorcinol MAP

Bartlett

27–45 N

2% Ascorbic acid 1% Calcium lactate 1-min dip

Anjou Bartlett Bosc

36–45 N 45–67 N 27–45 N

0.01% 4-Hexylresorcinol 0.5% Ascorbic acid 1% Calcium lactate 2-min dip Storage at 2–5∞C in pouches partially vacuumed

Anjou Bartlett Bosc

Slices: 22 N 36 N 22 N

0.01 M 4-Hexylresorcinol 0.5 M Isoascorbic acid 0.05 M Potassium sorbate pH 5.5 30-sec dip Storage at 5∞C, 14 d

Bartlett

45–58 N

2% Ascorbic acid 1% Calcium lactate 0.5% Cysteine pH 7.0 5-min dip at 20∞C

Bosc

Comments

Slightly underripe fruit. Use of 0.25% NaCl holding solution after cutting With Anjou pears, neutral dip (pH 7.7) more effective than acidic (pH 3.3) dip Less firm fruit browned more, and soft Bartlett pears developed tissue breakdown Shelf life at least 14 d (Anjou and Bartlett) Trials with 0.2% cysteine did not inhibit browning, and induced red or pink discoloration 4-Hexylresorcinol prevented core tissue browning Treatment not effective on Bosc pears Product observed for 2 d at 20∞C; surface color was not significantly different from that at the start of experiment Partially ripe fruit was peeled Shelf life of 30 d for Bartlett and Bosc, and 15–20 d for Anjou pears Sensory panelists detected difference in taste as compared with water-treated control Browning inhibitor of both cut surface and edges of the slices Less inhibition in Bartlett pears In Anjou initial firmness (21–52 N) did not affect browning control When acidic (pH 3.7) dip was used, a pinkish-red discoloration was observed Dip alone was unable to control browning at the edges of flesh beneath the skin Sensory panelists did not perceive difference in taste

Reference Rosen and Kader (1989) Sapers and Miller (1998)

Gorny et al. (1998)

Dong et al. (2000)

Buta and Abbott (2000)

Gorny et al. (2002)

Note: Unless indicated, firmness was measured in whole fruit, and fresh-cut products were prepared from unpeeled fruit. N = Newtons.

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6 banana

Firmness (N)

5 4

kiwi 0 24 hr 48 hr

3 2 1 0 0

2

20 0 Ethylene in air (ppm)

2

20

FIGURE 3.2 Changes in firmness of sliced kiwifruit and bananas kept at 20∞C in atmosphere of 0, 2, or 20 ppm of ethylene in air. (From Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of freshcut horticultural products. Postharvest Biol. Technol. 9: 115–125.)

fresh-cut processing needs to be determined in order to obtain a product of high quality and long shelf life. In addition, fruit cultivar is a very important factor to consider; as discussed previously, the browning potential can vary greatly among cultivars (Amiot et al., 1995). Pear fruit size was also shown to affect surface discoloration in pears. Gorny and coworkers (2000) found that pear slices prepared from small-size fruit discolor more than slices cut from larger fruit. This may indicate a difference in maturity and activity of the polyphenol oxidase enzyme.

3.4.5 PREVENTION

OF

TEXTURAL LOSSES

Fruit texture is perceived by the consumer prior to taste. When biting into a piece of apple, crunchiness is perceived before juiciness. Softening or loss of tissue firmness is a quality defect that compromises the shelf life of many fresh-cut fruits. Examples of changes in the firmness of sliced kiwifruit and banana during storage in various ethylene concentrations are shown in Figure 3.2. Selection of appropriate fruit cultivars is important in avoiding apple fruit prone to mealiness, for instance, or choosing cv. with attractive characteristics of juiciness and crispness. However, textural defects may develop after preparation of the fresh-cut product. Figure 3.3 illustrates pear

FIGURE 3.3 Ripe fresh-cut pears (cv. Bartlett) a few minutes after cutting. Note the translucency at the edges of the pear pieces.

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pieces prepared from ripe fruit, where translucent edges were noticeable just a few minutes after cutting. Fruit had been selected based on its superior eating qualities (flavor, sweetness, juiciness, and texture), but it was evidently inadequate for fresh-cut processing. In fresh-cut kiwifruit, flesh softening is the most noticeable change after cutting (Varoquaux et al., 1990); the cut surface darkens not due to enzymatic browning, but rather to the appearance of a translucent water-soaked tissue (Agar et al., 1999). The loss of firmness increases with storage temperature and time, while removal of ethylene from the storage atmosphere improves retention of slice texture. The most common treatment used to improve texture retention is to dip fruit pieces in calcium solutions, as described for strawberries (Main et al., 1986), and pears and strawberries (Rosen and Kader, 1989). The firming effect of calcium is attributed to the formation of complexes with polygalacturonic acid residues in the middle lamella and cell wall (Van-Buren, 1979). Both calcium chloride and calcium lactate are frequently used. However, trained sensory panels have judged that calcium chloride imparts a bitter flavor to fresh-cut cantaloupe (Luna-Guzmán and Barrett, 2000). Moreover, typical melon flavor was better detected in cantaloupe treated with 1% calcium lactate as compared with 1% calcium chloride. In this study, we found that the initial firming effect of both calcium chloride and calcium lactate on melon cylinders was the same; however calcium lactate–treated samples tended to maintain higher firmness during storage. A combined treatment using low-temperature blanching prior to dipping in calcium solution has been associated with a decrease in pectin esterification by pectinmethylesterase, creating potential sites for cross-linking with calcium (Stanley et al., 1995). A study of the effect of combined calcium chloride dips and heat treatment on firmness of fresh-cut cantaloupe carried out in our group showed improved firmness as compared with a calcium dip alone. However, it may be that rather than pectinmethylesterase activation, the results observed could be related to a membrane or turgor pressure effect. No significant difference was observed in the amount of bound calcium between samples treated at 60, 40, or 20∞C (Luna-Guzmán et al., 1999). The texture of fresh-cut apples has been reported to improve with the application of heat treatments to apples prior to slicing (Kim et al., 1994). Three apple cv. (Delicious, Golden Delicious, and McIntosh) that had been kept in cold storage (2∞C, 90% RH) for less than 2 months were treated for 1.75 h in a water bath at 45∞C, then held overnight at 2∞C. Fruit were sliced and stored at 2∞C for 21 d in unsealed polyethylene bags. Whereas untreated control samples exhibited a steady loss of flesh firmness during storage, heat-treated samples showed an initial increase in firmness (up to 7 d for cv. Golden Delicious and up to 14 d for cv. Delicious), followed by a decrease in firmness. At 7 d of storage, the difference in firmness between heat-treated and control samples were 12% for cv. McIntosh, 34% for Golden Delicious, and 28% for Delicious. The greatest difference observed was with cv. Delicious at 14 d of storage, where the heat-treated sample was ~40% firmer than the untreated control.

3.4.6 APPEARANCE

AND

SENSORY QUALITY

The most appealing attributes of fresh-cut products include their perception of freshness, taste and flavor, in addition to convenience. When assessing quality, consumers take product appearance into consideration as a primary criterion. Appearance can be characterized by size, shape, color, gloss, condition, and absence of defects. However, product color contributes more than any other single factor (Kays, 1999). While appearance may have a major impact on consumers, sensory quality (taste, aroma, texture) is what will ensure consumers’ repeated acceptance of a fresh-cut product. Most of the quality indexes used by both academia and industry for these products are visual, based on appearance; little research has been done on sensory quality of fresh-cut produce (Beaulieu and Baldwin, 2002). General methods for determining quality characteristics of fresh produce are described elsewhere (Mitcham and Kader, 1998).

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The relative importance of each quality characteristic depends on the commodity; for instance, gloss is relevant for peach slices but may not be for persimmon wedges. When the optimum ripeness stage for fresh-cut preparation occurs before full ripening and fruit flavor has yet to completely develop, even a fresh-cut product with very good appearance will lack the characteristic flavor of a perfectly ripe fruit. In addition, some flavor components may disappear after processing, or in other cases off-flavors may develop due to either physiological changes in the fruit tissue or microbial spoilage after packaging. Nevertheless, it has been reported that when off-flavors of microbial origin are detected, product appearance is likely to be compromised (Huxsoll et al., 1989).

3.4.7 NUTRITIONAL ASPECTS In addition to their sensory appeal, for nutritional reasons fruit should be an important part of a healthy diet. Although it is generally low in calories due to its high moisture content, fruit supplies vitamins, minerals, phytonutrients, and fiber to the diet. One of the most significant contributions to our diet comes from the high content of vitamin C and precursors of vitamin A (carotenoids) found in fruits. While citrus fruits are well known as sources of vitamin C, kiwifruit, melons, tomatoes, and berries are also significant sources of this vitamin. Good sources of carotenoids include yellow-fleshed peaches and nectarines, mangoes, papaya, and persimmons. Apples and pears are good sources of fiber, as are many berries (Margen et al., 1992). While it has been assumed that fresh-cut products would have lowered nutrient content than the commodities they are derived from (Klein, 1987; McCarthy and Matthews, 1994), some experimental data do not indicate this. The shelf life of fresh-cut persimmons and peaches was limited before major losses of carotenoids were detected (Wright and Kader, 1997b). The same was observed with the vitamin C content of strawberries and persimmons (Wright and Kader, 1997a). However, significant oxidation of ascorbic acid was determined when washing fruit slices with chlorinated water (100 ppm sodium hypochlorite) in comparison to washing with water. An intermediate level of oxidized ascorbic acid, or dehydroascorbic acid, was obtained when washing the whole fruit in chlorinated water before slicing it. Fresh-cut kiwifruit stored for 6 d at different temperatures showed a gradual decrease in total vitamin C content with increased temperature. While a loss of 8% in relation to initial vitamin C level in kiwifruit occurred at 0∞C, there was a decrease of 13 and 21% at 5 and 10∞C, respectively. In ethylene-free storage, ascorbic acid levels were threefold higher than in control slices stored in air (Agar et al., 1999). Losses of ascorbic acid in whole pears stored under controlled atmosphere conditions (low O2 and high CO2) revealed that in the cv. Rocha, losses occurred mainly during long-term storage, while in the cv. Conference, most of the ascorbic acid decreased when pears were transferred to controlled atmosphere storage. It is a common practice to store apples and pears under controlled conditions for extended periods of time prior to processing. Lowered levels of ascorbic acid in the fruit flesh will increase susceptibility to browning (Veltman et al., 2000). In recent years there has been a great interest in phytochemicals due to a variety of potential health benefits they may have. This is an emerging field of research, and very little information is available on concentrations of such compounds in different plant tissues, and even less on possible effects of cultivar, agronomic practices, maturity stage, and postharvest handling on the retention of phytochemicals. In a study with whole apples stored at 1∞C for 2 months, the cv. Granny Smith and Delicious had increased antioxidant levels of up to 10-fold, but in longer storage substantial decreases in antioxidants were reported (Curry, 1997).

3.5 SHELF LIFE EXTENSION OF FRESH-CUT FRUIT Once the negative consequences brought about by injuries caused by fresh-cut processing are recognized, it is important to search for means of extending the shelf life of fresh-cut fruits. The

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objective is to maintain a fresh-like appearance, flavor, and nutritional quality of the product while ensuring its safety. Among the most common strategies used to extend the shelf life of fresh-cut fruits are temperature management and modified atmosphere packaging. Other benefits can be found with the application of edible coatings. It is important to consider that the consumer perceives a fresh-cut product as fresh-like produce, just minimally prepared to be ready for eating. It is not expected that many chemicals are added to such products or that they are treated in ways that are not considered wholesome by the public. The perception of fresh-cut products as “natural” is part of its appeal. In a survey on the perception of convenience products, consumers revealed the desire that such products maintain fresh characteristics longer without the use of food preservatives (Bruhn, 1994).

3.5.1 TEMPERATURE MANAGEMENT Temperature control is very critical in prolonging the shelf life of fresh-cut products, serving to minimize losses due to wound-related responses and microbial spoilage. At low temperatures, respiration rates and enzymatic activities are reduced, and general metabolic rates are also lowered, extending product shelf life. The growth of some spoilage microorganisms and foodborne pathogens is decreased under the low-temperature conditions recommended for fresh-cut fruit processing and storage. The results presented in Table 3.1 show the effect of temperature on respiration rates in some fruits. Fresh-cut fruit ideally should be kept close to 0∞C, even for chilling-sensitive fruit, because the quality deterioration that results from storage at nonchilling temperatures is more damaging than that resulting from chilling injury (Watada et al., 1996). Rapid cooling of fresh-cut produce reduces respiration and deterioration rates, as well as microbial spoilage. However, it is of paramount importance to maintain product under refrigeration throughout the processing and distribution, and up to consumption.

3.5.2 MODIFIED ATMOSPHERE PACKAGING Modified atmosphere packaging (MAP) can extend the shelf life of produce by minimizing water loss, reducing respiration and ethylene production rates, decreasing metabolic activity, and reducing microbial growth and decay. The widely used MAP aims at the creation of an ideal gas composition inside the package. The modification of the atmosphere surrounding the fresh-cut produce can be achieved either through the establishment of an active modified atmosphere (gas flushing) in the package or be generated over time by the fruit pieces respiring in the sealed package. Despite additional costs, active MAP has been preferred because of the limitations in regulating a passively established atmosphere (Kader and Watkins, 2000). In modified atmospheres, O2 levels are generally reduced and CO2 is increased; high levels of CO2 lead to a decreased sensitivity of plant tissues to ethylene (Kader et al., 1989). When O2 levels are lowered, respiration of produce begin to decrease, and generally continues to decrease with lowering O2 levels down to a level where anaerobic respiration takes place. Among the responses of plant tissue to low O2 levels include reduction in respiration, reduced ethylene synthesis and perception, reduced chlorophyll degradation, reduced cell wall degradation, and reduced phenolics oxidation. Negative responses to low O2 include induction of fermentation, accumulation of acetaldehyde, ethanol and lactate, and reduced biosynthesis of aroma compounds (Beaudry, 2000). In the establishment of safe atmospheres it is important to determine product tolerance to low O2 levels and high CO2 levels. Watkins (2000) reviewed the effects of high CO2 on produce. In general, fruit tolerances for high concentrations of CO2 and low O2 vary for the same commodity between fresh-cut products and the intact fruit. It is important to experimentally determine the appropriate gas composition for each particular product, which extends the product shelf life and also prevents the damaging

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effects of low levels of O2 or high levels of CO2. These may lead to anaerobic respiration and increase susceptibility to decay. An adequate MAP should help achieve a decrease in respiration while preventing anaerobic respiration. In the design of the optimal MAP, it is of utmost importance to know the ideal atmosphere for each particular fresh-cut fruit product and its respiration rate under a certain temperature. In addition, it is necessary to avoid temperature abuse during storage; otherwise the atmospheric composition inside the package changes, which negatively impacts the product. Recommended atmospheres for storage of selected fresh-cut fruits were reviewed by Gorny (1997). Another important aspect in the application of modified atmosphere packaging is the appropriate selection of packaging material. Common packages include flexible film pouches, rigid trays overwrapped with flexible films, and rigid plastic containers with gas diffusion windows. The most common polymeric film used in MAP is oriented polypropylene; other examples are low-density polyethylene (for bagged products), monolayer polyvinyl chloride (for overwrapped trays), and blends of low- and medium-density polyethylene with ethylene vinyl acetate; microperforated films have been studied as a way of overcoming anaerobic conditions in the package (Al-Ati and Hotchkiss, 2002). A comparison of different systems for storage of fresh-cut cantaloupe was carried out using the cv. Athena, considered adequate for fresh-cut processing (Bai et al., 2001). Cantaloupe cubes were stored either in (1) pouches where they were allowed to develop a “natural” modified atmosphere (nMAP) through produce respiration, or (2) packages that were flushed with a gas mixture containing 4 kPa of O2 plus 10 kPa of CO2 (fMAP), or (3) packages where the package film was perforated with a needle (PFP). In cantaloupe melons, quality deterioration is primarily related to the development of tissue translucency; salable quality may be limited when the level of translucency is greater than 20%. Although shelf life of cantaloupe cubes (Figure 3.4) was prolonged under both nMAP (9 d) and fMAP (12 d) conditions, a faint off-odor was detected in all samples by day 12, with the exception of one of the three replicate trials for the fMAP. Fresh-cut fruits do not respond to MAP as well as young vegetative fresh-cut tissues; the effects of MAP on control of senescence rate have been only marginally effective. Many factors influence the atmospheric composition at equilibrium inside the package. Film type, thickness, area, weight 100 PFP nMAP

Translucency (%)

80

fMAP 60

40

20

0 0

2

4

6

8

10

12

Storage (days)

FIGURE 3.4 Changes in tissue translucency of cantaloupe cubes stored in packages with overlap film perforated with 10 holes of 1.5 mm (PFP), packages containing a naturally formed modified atmosphere (nMAP), and packages in which the internal atmosphere was flushed with a gas mixture of 4 kPa O2 plus 10 kPa CO2 prior to storage. (From Bai, J.-H., Saftner, A.E., Watada, A.E., Lee, Y.-S. 2001. Modified atmosphere maintains quality of fresh-cut cantaloupe (cucumis melo L.). J. Food Sci. 66(8): 1207–1211.)

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of fresh-cut fruit in the package, temperature, relative humidity, and product respiration rate are all important. In spite of the advances occurring in film technology, currently available films are not considered as satisfying the requirements of fresh-cut fruit. Current films meet the needs of either CO2 or O2 because they allow permeability of CO2 at higher rates than O2, which constitutes a shortcoming of MAP for fresh-cut fruit application (Al-Ati and Hotchkiss, 2002). More research is needed in establishing the most appropriate atmospheres for fresh-cut fruits, which are expected to vary with cultivar, growing location, and duration of storage prior to processing (Gorny, 1997).

3.5.3 HUMIDITY Humidity is another important factor that should be controlled in fresh-cut fruit product handling and storage. Although water is the most abundant (80 to 90% of the fresh weight) component of fruit tissues, even small changes (~5%) in the water content of fruit are detrimental to quality. In fresh-cut products the removal of skin or rind and cutting lead to exposure of a large surface as seen in fruit slices, cubes, and wedges, which are prone to great water loss. Surface dehydration occurs quickly and has a negative impact on product appearance, resulting in a product exhibiting less gloss, greater wrinkling, wilting, or flaccidness. However, poor appearance is not the only possible defect resulting from water loss; cellular metabolism may be affected, fruit ripening may be accelerated, and ripening related softening significantly impacted (Paul, 1999). Desiccation may also favor the development of microorganisms that tolerate low moisture, such as fungi (Brackett, 1987). In the dewatering operation, it is important to avoid excessive water loss. Moreover, reduction of moisture loss can be achieved by decreasing the capacity of the surrounding air to hold water. This is done by lowering the temperature, increasing the relative humidity, or creating a barrier to water loss. The last mentioned is commonly done through the use of polymeric films in packaging; moisture retention can also be attained with edible coatings, as exemplified with fresh-cut papaya, where desiccation is a major problem (Siriphanich, 1994). However, in film-packaged products, water is formed as a result of respiration, and it condenses inside the package and becomes available for microbial growth (Al-Ati and Hotchkiss, 2002). This is another point demonstrating the need to avoid product temperature abuse and maintain lower respiration rates.

3.5.4 EDIBLE COATINGS The use of edible coatings is another method of extending the shelf life of fresh-cut fruit. Edible coatings consist of thin layers of protective materials applied to the surface of the fruit as a replacement for the natural protective tissue (epidermis, peel). Edible coatings are used as a semipermeable barrier that help reduce respiration, retard water loss and color changes, improve texture and mechanical integrity, improve handling characteristics, help retain volatile flavor compounds, and reduce microbial growth. It is possible to create a modified atmosphere enrobing fresh-cut produce in edible coating. (Baldwin et al., 1995; Baldwin et al., 1996; Nisperos and Baldwin, 1996). Different food additives can be incorporated into coating formulation, such as coatings with antioxidants (Baldwin et al., 1995). Control of surface browning in apples by ascorbic acid was improved when it was incorporated into an edible coating formulation compared to dipping. A carboxymethylcellulose-based coating did not control enzymatic browning of cut apples, but when such a coating was combined with additives (antioxidant, acidulant, and preservative), browning control was superior to dipping the fresh-cut produce in solutions with the same additives (Baldwin et al., 1996). Examples of browning inhibition in apple slices have been described for different edible coatings (Avena-Bustillos and Krochta, 1993; Kinzel, 1992). Edible coatings made from apple puree to which lipids and beeswax were added helped control moisture loss and browning of fresh-cut apples (McHugh and Senesi, 2000). Coated and wrapped (“apple wraps”) fresh-cut apple products were compared; apple pieces were either coated with (dipped in) a solution of 70% apple puree, 27%

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vegetable oil, 1.5% ascorbic acid, and 1.5% citric acid or wrapped in a film of the same composition. Moisture loss was significantly reduced in apple wraps during storage at 5∞C for 12 d. While coatings inhibited browning by 80% over a period of 3 d, wraps showed a 100% inhibition of browning for up to 10 d at 5∞C, in addition to maintaining texture, fruity flavor, and odor. It is important to understand the need for an integrated approach in the processing of fresh-cut fruits. In order to ensure prolonged shelf life, all steps should be carried out taking into consideration the requirement for superior fruit quality of cultivars selected for fresh-cut, adoption of good sanitation practices, gentle handling throughout processing, adequate temperature management, and appropriate choice of packaging and storage conditions. All these factors should be seen as a way of ensuring not only extended shelf life, but also a convenient high quality product that is safe, nutritionally sound, and appealing to the senses.

REFERENCES Abeles, F.B., Morgan, P.W., Saltveit, M.E. 1992. Ethylene in Plant Biology. 2nd ed., Academic Press, San Diego, CA. Agar, I.T., Massantini, R., Hess-Pierce, B., Kader, A.A. 1999. Postharvest Co2 and ethylene production and quality maintenance of fresh-cut kiwifruit slices. J. Food Sci. 64(3): 433–440. Al-Ati, T., Hotchkiss, J.H. 2002. Application of packaging and modified atmosphere to fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 305–338. Amiot, M.J., Tacchini, M., Aubert, S., Oleszek, W. 1995. Influence of cultivar, maturity stage, and storage conditions on phenolic composition and enzymatic browning of pear fruits. J. Agr. Food Chem. 43(5): 1132–1137. Avena-Bustillos, R.J., Krochta, J.M. 1993. Water vapor permeability of caseinate-based edible films as affected by pH, calcium crosslinking and lipid content. J. Food Sci. 58: 904–907. Bai, J.-H., Saftner, A.E., Watada, A.E., Lee, Y.-S. 2001. Modified atmosphere maintains quality of fresh-cut cantaloupe (cucumis melo L.). J. Food Sci. 66(8): 1207–1211. Baldwin, E.A., Nisperos-Carriedo, M.O., Baker, R.A. 1995. Use of edible coatings to preserve quality of lightly (and slightly) processed products. CRC Crit. Rev. Food Sci. Nutr. 35(6): 509–524. Baldwin, E.A., Nisperos, M.O., Chen, X., Hagenmaier, R.D. 1996. Improving storage life of cut apple and potato with edible coating. Postharvest Biol. Technol. 9(2): 151–163. Beaudry, R.M. 2000. Responses of horticultural commodities to low oxygen: limits to the expanded use of modified atmosphere packaging. HortTechnology 10(3): 491–500. Beaulieu, J.C., Baldwin, E.A. 2002. Flavor and aroma of fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 391–425. Beuchat, L.R. 2000. Use of sanitizers in raw fruit and vegetable processing. In: Minimally Processed Fruits and Vegetables: Fundamental Aspects and Applications. S.M. Alzamora, M.S. Tapia, A. López-Malo (Eds.), Aspen, Gaithersburg, MD, pp. 63–78. Brackett. R.E. 1987. Microbiological consequences of minimally processed fruits and vegetables. J. Food Qual. 10: 195–206. Brackett, R.E. 1992. Shelf stability and safety of fresh produce as influenced by sanitation and disinfection. J. Food Protect. 55(10): 808–814. Brecht, J.K. 1995. Physiology of lightly processed fruits and vegetables. HortScience 30(1): 18–22. Bruemmer, J.H., Griffin, A.W., Onayemi, O. 1978. Sectionizing grapefruit by enzyme digestion. Proc. Florida Sta. Hortic. Soc. 91: 112–114. Bruh, C. 1994. Consumer perception of quality. In: Minimal Processing of Foods and Process Optimization: An Interface. R.P. Singh, F.A.R. Oliveira (Eds.), CRC Press, Boca Raton, FL, pp. 493–504. Buta, J.G., Abbott, J.A. 2000. Browning inhibition of fresh-cut ‘Anjou’, ‘Bartlett’, and ‘Bosc’ pears. HortScience 35(6): 1111–1113. Cantwell, M., Suslow, T. 1999. Fresh-cut fruits and vegetables: Aspects of physiology, preparation, and handling that affect quality. Fresh-Cut Workshop, Sept. 14–16, University of California–Davis.

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Cherry, J.P. 1999. Improving the safety of fresh produce with antimicrobials. Food Technol. 53(11): 54–57. Curry, E.A. 1997. Effect of postharvest handling and storage on apple nutritional status using antioxidants as a model. HortTechnology 7: 240–243. Dong, X., Wrolstad, R.E., Sugar, D. 2000. Extending shelf life of fresh-cut pears. J. Food Sci. 65(1): 181–186. FDA 2002. FDA’s Produce Safety Activities. U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, http://www.cfsan.fda.gov/~dms/prodact.html. Fonseca, J.M., Rushing, J.W., Testin, R.F. 1999. Shock and vibration forces influence the quality of fresh-cut watermelon. Proc. Fla. State Hortic. Soc. 112: 147–152. Francis, G.A., Thomas, C., O’Beirne, D. 1999. The microbiological safety of minimally processed vegetables. Int. J. Food Sci. Technol. 34(1): 1–22. Garcia, E., Barrett, D.M. 2002. Preservative treatments for fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 267–303. Garrett, E.H. 2002. Fresh-cut produce: tracks and trends. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 1–10. Gorny, J.R. 1997. A summary of CA and MA requirements and recommendations for fresh-cut (minimally processed) fruits and vegetables. CA ’97: Seventh International Controlled Atmosphere Research Conference. University of California, Davis, pp. 30–66. Gorny, J.R., Cifuentes, R.A., Hess-Pierce, B., Kader, A.A. 2000. Quality changes in fresh-cut pear slices as affected by cultivar, ripeness stage, fruit size, and storage regime. J. Food Sci. 65(3): 541–544. Gorny, J.R., Gil, M.I., Kader, A.A. 1998. Postharvest physiology and quality maintenance of fresh-cut pears. Acta Hortic. 464: 231–236. Gorny, J.R., Hess-Pierce, B., Cifuentes, R.A., Kader, A.A. 2002. Quality changes in fresh-cut pear slices as affected by controlled atmospheres and chemical preservatives. Postharvest Biol. Technol. 24: 271–278. Gorny, J.R., Hess-Pierce, B., Kader, A.A. 1999. Quality changes in fresh-cut peach and nectarine slices as affected by cultivar, storage atmosphere and chemical treatments. J. Food Sci. 64(3): 429–432. Heard, G.M. 2002. Microbiology of fresh-cut produce. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 187–248. Huxsoll, C.C., Bolin, H.R., King, A.D. Jr. 1989. Physicochemical changes and treatments for lightly processed fruits and vegetables. In Quality Factors of Fruits and Vegetables. J.J. Jen (Ed.). ACS Symp. Ser. No 405, Washington, DC, chap. 16 pp. 203–215. IFPA. 1996a. Model HACCP plan: a hazard analysis critical control point system food safety program for fresh-cut produce. International Fresh-Cut Produce Association. IFPA. 1996b. Food Safety Guidelines for the Fresh-Cut Produce Industry. 3rd ed. International Fresh-Cut Produce Association, Alexandria, VA. IFPA. 1997. Fresh-Cut Produce Handling Guidelines. 2nd ed. International Fresh-Cut Produce Association, Alexandira, VA. IFPA, 2002. International Fresh-Cut Produce Association. http://www.fresh-cuts.org. Kader, A.A. 2002. Quality parameters of fresh-cut fruit and vegetable products. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 11–20. Kader, A.A., Mitcham, B. 1998. Methods for determining quality of fresh horticultural commodities. In: Freshcut Products: Maintaining Quality and Safety, Postharvest Outreach Program, University of California, Davis. Kader, A.A., Zagory, D, Kerbel, E.L. 1989. Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 28(1): 1–30. Kader, A.A., Watkins, C.B. 2000. Modified atmosphere packaging — Toward 2000 and beyond. HortTechnology 10(3): 483–486. Kays, S.J. 1999. Preharvest factors affecting appearance. Postharvest Biol. Technol. 15: 233–247. Kim, D.M., Smith, N.L., Lee, C.Y. 1993. Apple cultivar variations in response to heat treatments and minimal processing. J. Food Sci. 58: 1111–1114, 1124. Kim, D.M., Smith, N.L., Lee, C.Y. 1994. Effect of heat treatment on firmness of apples and apple slices. J. Food Process. Pres. 18: 1–8. Kinzel, B. 1992. Protein-rich edible coatings for food. Agr. Res. (May): 20–21.

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Klein, B.P. 1987. Nutritional consequences of minimal processing of fruits and vegetables. J. Food Qual. 10: 179–193. Luna-Guzmán, I., Barrett, D.M. 2000. Comparison of calcium chloride and calcium lactate effectiveness in maintaining shelf stability and quality of fresh-cut cantaloupes. Postharvest Biol. Technol. 19: 61–72. Luna-Guzmán, I., Cantwell, M., Barrett, D.M. 1999. Fresh-cut cantaloupe: effects of CaCl2 dips and heat treatments on firmness and metabolic activity. Postharvest Biol. Technol. 17: 201–213. Lund, B.M., Baird-Parker, T.C., Gould, G.W. 2000. The Microbiological Safety and Quality of Food. Aspen, Gaithersburg, MD. Madden, J.M. 1992. Microbial pathogens in fresh produce — the regulatory perspective. J. Food Protect. 55(10): 821–823. Main, G.L., Morris, J.R., Wehunt, E.J. 1986. Effect of preprocessing treatments on the firmness and quality characteristics of whole and sliced strawberries after freezing and thermal processing. J. Food Sci. 51: 391–394. Margen, S., and editors of UC Berkeley Wellness Letter. 1992. The Wellness Encyclopedia of Food and Nutrition. New York, Rebus, pp.187–189. McCarthy, A., Matthews, R.H. 1994. Nutritional quality of fruits and vegetables subject to minimal processes. In: Minimally Processed Refrigerated Fruits and Vegetables, R.C. Wiley (Ed.), Chapman & Hall, New York, pp. 313–326. McEvily, A.J., Iyengar, R., Otwell, W.S. 1991.Sulfite alternative prevents shrimp melanosis. Food Technol. 45(9): 80–86. McHugh, T.H. and Senesi, E. 2000. Apple wraps: a novel method to improve the quality and extend the shelf life of fresh-cut apples. J. Food Sci. 65(3): 480–485. Mitcham, B., Kader, A.A. 2000. Methods for determining quality of fresh horticultural commodities. In: FreshCut Products: Maintaining Quality and Safety, Postharvest Horticultural Series No. 10, Section 3b, Unversity of California, Davis. NACMCF, 1999. United States National Advisory Committee on Microbiological Criteria for Foods. Microbiological safety evaluations and recommendations on fresh produce. Food Control 10: 117–143. Nguyen-the, C., Carlin, F. 1994. The microbiology of minimally processed fresh fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 34(4): 371–301. Nishizawa, T., Taira, S., Nakanishi, M., Ito, M., Togashi, M., Motomura, Y. 1998. Acetaldehyde, ethanol, and carbohydrate concentrations in developing muskmelon fruit (Cucumis melo L. cv. Andesu) are affected by short-term shading. HortScience 33(6): 992–994. Nisperos, M.O., Baldwin, E.A. 1996. Edible coatings for whole and minimally processed fruits and vegetables. Food Aust. 48(1): 27–31. O’Hare, T.J. 1994. Respiratory Characteristics of Cut Pineapple Tissue. 1994 Report. Postharvest Group, DPI, Queensland, Australia. Paull, R.E. 1999. Effect of temperature and relative humidity on fresh commodity quality. Postharvest Biol. Technol. 15: 263–277. Paull, R.E., Chen, W. 1997. Minimal processing of papaya (Carica papaya L.O and the physiology of halved fruit. Postharvest Biol. Technol. 12: 93–99. Paull, R.E., Rohrbach, K.G. 1985. Symptom development of chilling injury in pineapple fruit. J. Amer. Soc. Hortic. Sci. 110: 100–105. Pollack, S.L. 2001. Consumer demand for fruit and vegetables: the U.S. example. In: Changing Structure of Global Food Consumption and Trade. A. Regmi (Ed.), ERS WRS No.01-1, pp. 49–54. Portela, S.I., Cantwell, M.I. 2001. Cutting blade sharpness affects appearance and other quality attributes of fresh-cut cantaloupe melon. J. Food Sci. 66(9): 1265–1270. Pretel, M.T., Lozano, P., Riquelme, F., Romojaro, F. 1996. Pectic enzymes in fresh fruit processing: optimization of enzymic peeling of oranges. Process. Biochem. 32: 43–49. Pretel, M.T., Fernández, Romojaro, F., Martinez, A. 1998. The effect of modified atmosphere packaging on ‘Ready-to-Eat’ oranges. Lebensm. Wiss. Technol. 31: 322–328. Price, J.L., Floros, J.D. 1993. Quality decline in minimally processed fruits and vegetables. In: Food Flavors, Ingredients and Composition. G. Charalambous (Ed.), Elsevier, Amsterdam, pp. 405–427. Radi, M., Mahrouz, M., Jaouad, A., Tacchini, M., Aubert, S., Hughes, M., Amiot, M.J. 1997. Phenolic composition, browning susceptibility, and carotenoid content of several apricot cultivars at maturity. HortScience 32(6): 1087–1091.

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Romig, W.R. 1995. Selection of cultivars for lightly processed fruits and vegetables. HortScience 30: 38–40. Rosen, J.C., Kader, A.A. 1989. Postharvest physiology and quality maintenance of sliced pear and strawberry fruits. J.Food Sci. 54(3): 656–659. Saltveit, M.E. 1997. Physical and physiological changes in minimally processed fruits and vegetables. In: Phytochemistry of Fruits and Vegetables. F.A. Tomás-Barberán, R.J. Robins (Eds.), Clarendon Press, Oxford, U.K., pp. 205–220. Sapers, G.M. 1993. Browning of foods: control by sulfites, antioxidants, and other means. Food Technol. 47(10): 75–84. Sapers, G.M., Simmons, G.F. 1998. Hydrogen peroxide disinfection of minimally processed fruits and vegetables. Food Technol. 52(2): 48–53. Sapers, G.M., Miller, R.L. 1998. Browning inhibition in fresh-cut pears. J.Food Sci. 63(2): 342–346. Sapers, G.M., Miller, R.L., Pilizota, V., Mattrazzo, A.M. 2001. Antimicrobial treatments for minimally processed cantaloupe melon. J. Food Sci. 66(2): 345–349. Siriphanich, J. 1994. Minimal processing of tropical fruits. In: Postharvest Handling of Tropical Fruits. B.R. Champ, E. Highley, G.I. Johnson (Eds.), ACIAR, Canberra, Australia, pp. 127–137. Stanley, D.W., Bourne, M.C., Stone, A.P., Wismer, W.V. 1995. Low temperature blanching effects on chemistry, firmness, and structure of canned green beans and carrots. J. Food Sci. 60: 327–333. Ukuku, D.O., Pilizota, V., Sapers, G.M. 2001. Influence of washing treatment on native microflora and Escherichia coli population of inoculated cantaloupes. J. Food Safety 2191): 31–47. USDA, 1998. CSFII/DHKS 1994–96 Data Set, Documentation and Technical Support Files: The 1994–96 Continuing Survey of Food Intakes by Individuals and the 1994–96 Diet and Health Knowledge Survey. Agricultural Research Service, Riverdale, MD (CD-ROM: Accession No. PB98-500457, National Technical Information Service, Springfield, VA). Van-Buren, J.P. 1979. The chemistry of texture in fruits and vegetables. J. Texture Stud. 10: 1–23. Varoquaux, P., Lecendre, I., Varoquaux, F., Sounty, M. 1990. Change in firmness of kiwifruit after slicing. Sci. Aliment. 10: 127–139. Veltman, R.H., Kho, R.M., van Schaik, A.C.R., Sanders, M.G., Ooterhaven, J. 2000. Ascorbic acid and tissue browning in pears (Pyrus communis L. cvs. Rocha and Conference) under controlled atmosphere conditions. Postharvest Biol. Technol. 19(2): 129–137. Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of fresh-cut horticultural products. Postharvest Biol. Technol. 9: 115–125. Watada, A.E., Qi, L. 1999. Quality of fresh-cut produce. Postharvest Biol. Technol. 15: 201–205. Watkins, C.B. 2000. Responses of horticultural commodities to high carbon dioxide as related to modified atmosphere packaging. HortTechnology 10(3): 501–506. Watkins, C.B., Manzano-Méndez J.E., Nock, J.F., Zhang, J., Maloney, K.E. 1999. Cultivar variation in response of strawberry fruit to high carbon dioxide treatments. J. Sci. Food Agric. 79: 886–890. Wright, K.P., Kader, A.A. 1997a. Effect of slicing and controlled-atmosphere storage on ascorbate content and quality of strawberries and persimmons. Postharvest Biol. Technol. 10: 39–48. Wright, K.P., Kader, A.A. 1997b. Effect of controlled-atmosphere storage on the quality and carotenoids content of sliced persimmons and peaches. Postharvest Biol. Technol. 10: 89–97.

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4 Juice Processing Mark R. McLellan and Olga I. Padilla-Zakour CONTENTS 4.1 4.2 4.3 4.4

4.5

4.6 4.7

4.8

4.9 4.10 4.11 4.12

Introduction ..........................................................................................................................74 Fruit Quality and Quality Products .....................................................................................74 Juice Production...................................................................................................................74 Extraction Processes ............................................................................................................75 4.4.1 Pome Fruit and Small Fruit Processing ................................................................76 4.4.1.1 Disintegration .......................................................................................76 4.4.1.2 Hot Break Process ...............................................................................77 4.4.1.3 Mash Enzyme Treatment .....................................................................77 4.4.1.4 Enzymatic Liquefaction .......................................................................78 4.4.1.5 Extraction Equipment ..........................................................................79 4.4.2 Citrus Processing ...................................................................................................80 Clarification ..........................................................................................................................80 4.5.1 Enzymatic Preparation...........................................................................................80 4.5.2 Mechanical Separation ..........................................................................................81 4.5.2.1 Using Decanters and Finishers ............................................................81 4.5.2.2 Centrifugation ......................................................................................81 4.5.2.3 Diatomaceous Earth Filtration.............................................................81 4.5.2.4 Cross-Flow Membrane Filtration ........................................................83 Concentration .......................................................................................................................86 Pasteurization and Nonthermal Processes ...........................................................................87 4.7.1 Hot Filling and Product Pasteurization for Shelf-Stable Juices ...........................87 4.7.2 Aseptic Juice Processing .......................................................................................87 4.7.3 Refrigerated Juices ................................................................................................88 4.7.4 Nonthermal Processes............................................................................................88 Packaging .............................................................................................................................88 4.8.1 Cans........................................................................................................................89 4.8.2 Glass.......................................................................................................................89 4.8.3 Semirigid and Flexible Packages ..........................................................................89 Fruit Nectars.........................................................................................................................90 Waste Treatment...................................................................................................................90 Juice Authentication To Prevent Adulteration .....................................................................91 Standard Tests for Identification of Hazes in Clear Juices .................................................91 4.12.1 Microbial Growth ..................................................................................................91 4.12.2 Starch .....................................................................................................................92 4.12.3 Protein–Tannin Complex .......................................................................................92 4.12.4 Tannin Haze ...........................................................................................................92 4.12.5 Gum Haze ..............................................................................................................92 4.12.6 Metal Ions ..............................................................................................................93

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4.13 Fruit Juice Safety and HACCP............................................................................................93 References ........................................................................................................................................94

4.1 INTRODUCTION Juice and juice products represent a very important segment of the total processed fruit industry. U.S. retail sales of fruit juice products in the year 2000 accounted for $15 billion and were projected to reach $18 billion by 2005. The per capita consumption in the U.S. of fruit juices (as single strength equivalent) was 36 l in 2000, whereas the average European consumption was 23.4 l/person. The most popular juices were orange and apple. Total sales in Europe for the same year were reported as 9100 million liters, representing a 10% increase over the 1995 levels (The Food Institute, 2002; Anon., 2001). Juice products are being marketed as refrigerated, shelf-stable, and frozen, in a variety of packages with increased emphasis on functionality, health attributes, new flavors or blends, and in some cases fortified with vitamins and minerals.

4.2 FRUIT QUALITY AND QUALITY PRODUCTS •

•

•

•

•

High-quality juice operations are dependent upon a source of high-quality raw material. No matter how good the process is, starting with poor-quality fruit for juice production will lead to a poor-quality product. Often, the quality of the fruit is dependent upon the stage of maturity or the level of ripening. Typical assessments of fruit ripening include sugar concentration, acidity, starch content, color, flavor, and firmness (Hulme, 1971). As is often the case in unit operations, efficiency drives the harvesting process, and in terms of harvester operations, this usually means some form of mechanical harvesting. Quality can be preserved with mechanical harvesting; however, extra care needs to be exercised in the design and use of harvesters. In general, all handling of the fruit should be done with sensitivity to the potential of bruising and contamination of the fruit. Special care must be made in transport of the fruit through the plant so that large drops or other impacts are avoided. Storage facilities must be optimized for the type and maturity of the fruit. Although general cooling of fruit at controlled relative humidity is standard for extending storage life of the fruit, controlled atmosphere can be used to maximize shelf life of fruits such as apples. Starting with good quality, sound fruit is important but so, too, is the cleanliness of the process operations. In all phases of the juice production, design and assurance of clean and safe operations are important. Daily cleaning and sanitation of the plant with routine shutdowns will assure maintenance of a clean operation and prevent buildup of trouble spots.

4.3 JUICE PRODUCTION The process starts with sound fruit, freshly harvested from the field or taken from refrigerated or frozen storage (Figure 4.1). Thorough washing is usually necessary to remove dirt and foreign objects and may be followed by a sanitation step to decrease the load of contaminants. Sanitizing is especially important for minimally processed juices that rely on hygienic conditions to ensure the safety of perishable products. Sorting to remove decayed and moldy fruit is also necessary to make sure that the final juice will not have a high microbial load, undesirable flavors, or mycotoxin contamination. For most fruits, preparation steps such as pitting and grinding will be required prior to juice extraction. Heating and addition of enzymes might also be included before the mash is transferred to the extraction stage. Juice extraction can be performed by pressing or by enzymatic treatment followed by decanting. The extracted juice will then be treated according to the

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Fruit (fresh or thawed) Cleaning/Washing

Sorting/culling

Sanitizing

Preparation for Extraction (pitting, crushing, heating, enzymatic treatment)

Juice Extraction (Pressing, decanting)

Cloudy Juice (coarse filtering, centrifuging)

Clarification (Depectinizing, centrifuging, filtering)

Concentration (evaporation, reverse osmosis, freeze-concentration) Pasteurization (or equivalent non-thermal treatment) Pasteurization

Filling and Storage (shelf-stable, refrigerated, frozen) Single Strength Juice

Filling and Storage (shelf-stable, refrigerated, frozen) Juice Concentrate

FIGURE 4.1 General diagram for juice production.

characteristics of the final product. For cloudy juices, further clarification might not be necessary or may involve a coarse filtration or a controlled centrifugation to remove only larger insoluble particles. For clear juices, complete depectinization by addition of enzymes, fine filtration, or highspeed centrifugation will be required to achieve visual clarity. The next step is usually a heat treatment or equivalent nonthermal process to achieve a safe and stable juice and final packaging if single-strength juice is being produced. For a concentrate, the juice is fed to an evaporator to remove water until the desired concentration level is obtained. Other processes used for water removal include reverse osmosis and freeze concentration, which are best suited for heat-sensitive juices. The concentrate is then ready for final processing, packaging, and storage (Downing, 1996).

4.4 EXTRACTION PROCESSES Once the fruit is delivered for processing, the critical operation of juice extraction begins. In general, juice extraction should be done as rapidly as possible so as to minimize oxidation of the juice by naturally present enzymes.

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AND

SMALL FRUIT PROCESSING

Generally, most pome fruit and small stone fruit can be used for juice extraction. No peeling is needed. Small stone fruit such as apricots and plums might have to be destoned (pitted) depending on the grinding–extraction equipment selection. Cherries, although containing a pit, may be pressed with the pit intact. Breakage of the pit will release benzaldehyde, the familiar aroma of maraschinotype cherries. 4.4.1.1 Disintegration The juicing process starts with the crushing step to break down the cell tissue. Grain sizes of 5 to 8 mm diameter are recommended for presses while grain sizes of 3 to 5 mm are desirable for decanters (Hamatschek et al., 1995). Hammer mills are devices used to crush the whole fruit in preparation for pressing. Hammer mills consist of heavy stainless steel bars spinning from a common axis under high-speed rotation. The fruit is disintegrated until it passes out through a screen of a specific size mounted in the bottom of the mill. With firm fruit, a small screen size should be used, and the mash will be of a finer particle size. Mash from firm fruit will press more easily, and the smaller particle size will allow greater yields. Softer fruit presses with more difficulty, and a larger particle size in the mash will enhance ease of pressing. Thus, for example, softer apples require a larger screen size in the hammer mill. Grinding mills offer an alternative method to disintegrating fruit. In the earlier models, the fruit was drawn past fixed knives mounted on a rotating cylinder. Control of the grind was accomplished by adjusting the depth of the knives and, thus, the size of the cut from the fruit. Grinding disk mills offer more flexibility and improved performance. In the Bucher-Guyer unit (Model CM 50; Switzerland) the fruit is transported by a feed screw to the grinding area. The screw pressurizes the fruit against a rotating disk equipped with grinding knives in a star pattern, and the milled fruit exits via an adjustable discharge slot. The process can be controlled by adjusting the feeder speed, the rotating speed of the grinding disk, the width of the product discharge slot (up to 10 mm), or by changing the knife size (Pinnow, 2000). Better yield is obtained due to the easy adjustment corresponding to the fruit ripeness at the time of operation. Grating mills are used in small juice operations to produce uniformly sized fruit pieces. Fruit is fed to a rotating–grating disk with fixed aperture, and the shredded fruit is discharged at the bottom. Fruit must be relatively firm with small seeds or pitted. Stemmer/crushers are used in grape juice processing to remove residual stems, leaves, and petioles from grapes and to perform the initial crush of the fruit after its arrival at the plant. These units are designed around a perforated rotating drum, with holes approximately 2.5 cm in diameter. In the process of traversing the rotating drum, grapes are caught by the perforated drum and knocked from the stems. Individual grapes are broken open or crushed in the process and dropped through the drum. Stems, leaves, and so forth continue on to the center of the drum and are discharged at the end for waste. Grapes are generally put through the crusher in order to gently express the juice and free up the flesh, yet still not break the seeds. Breakage of the seeds releases increased amounts of phenolics, adding to the astringency of the juice. Stoned fruit mills are used for plums, damsons, and apricots to crush the fruit without breaking the stones to avoid juice flavor changes and storage instability. Hard rubber-lobed wheels rotate simultaneously, forcing the fruit down and separating most of the flesh from the intact stone (Downes, 1995). Turbo extractors can be used for extraction of juice and puree from fruits and vegetables. The cold extractor unit from Bertocchi (Italy) has a feeding section with a variable speed screw and a cutting head; a softening section consisting of a stator and rotor (rotopulse); and an extraction area equipped with a rotor with paddles and a perforated cylindrical screen that continuously turns the

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product by centrifugal force. The extractor can be adjusted by changing the feeding speed, the rotor speed, the gap between the rotor and the screen, and the screen size. The fruit can be protected from oxidation by the injection of nitrogen gas or antioxidant solution to the cutting area through built-in openings. 4.4.1.2 Hot Break Process In order to maximize juice yield and color–flavor extraction, a hot break process is often used. The most common use is in grape juice processing, but other fruits such as cherries, plums, and berries may also benefit. Increased interest in highly colored juices, rich in phenolic compounds with associated health benefits, is driving the development of better techniques to preserve the functional components while maximizing the extraction. Typically crushed fruit or mash passes through a large bore, tubular heat exchanger where it is heated to 50 to 60∞C. This stage, known as the hot break process, is designed to extract a large amount of color and assist in maximizing the yield. To the hot fruit a pectolytic enzyme can be added, and in some cases such as red grape juice processing, kraft (wood pulp) paper is also added prior to pressing to serve as a press aid. The type of enzyme used is critical, as some pectolytic enzyme solutions may have some side effects that could destroy the juice color (Helbig, 2001). The addition of press aid to the mash provides coarseness and channels for the juice to exit (Hurler and Wey, 1984). Alternative press aids include rice hulls, bleached kraft-fiber sheets or rolled stock, and ground wood pulp. Ideally, a press aid should have relatively long fibers that can be separated with a minimum breakage of those fibers for maximum effectiveness. In addition, press aids should neither impart off-flavors to the juice nor remove the fruit’s flavor. If the juice is going to be extracted by decanting or centrifugation, then there is no need for press aids. 4.4.1.3 Mash Enzyme Treatment This step might not be used for the production of high quality, single-strength, cloudy and clear juices, where the preservation of the fresh flavor is imperative. Soluble pectin found in fresh juice is a result of physical breakup of the cells and the activity of pectolytic enzymes that are primarily located in the cell wall of the fruit. This soluble pectin is the cause of much difficulty in extraction due to increased juice viscosity and the lubrication it affords the press cake, resulting in reduced extraction effectiveness. Typically, the fruit mash is heated to 45 to 50∞C followed by the addition of pectolytic enzyme preparations at the dosage recommended by the enzyme supplier. Reaction time can take up to 1 to 2 h. Depectinization is designed to reduce the viscosity and slipperiness of the pulp and thus permit the effective use of decanters and presses with proper press aids as needed. It is especially useful in processing mature and stored fruit that results in low juice yield. Several depectinizing tanks are employed so that a continuous flow may be maintained to the presses or decanters. The pectic substances that need to be broken down are the structural polysaccharides in the middle lamella and the primary cell walls of fruit tissue. These pectic substances are polymeric chains with a backbone consisting of straight sections of a-D-1, 4-galacturonan regions with a dispersion of 1, 2 linked a-L-rhamnosyl residues with an alternating rhamnogalacturonan chain (Selvendran, 1985). The alternating side chains composed of neutral sugars give the pectin structure a “hairy” character (De Vires et al., 1986). The predominant sugars of pectin are D-galactose and L-arabinose. Found to a lesser degree is xylose in monomeric and oligomeric side chains (McNeil et al., 1984). Treatment of the mash with enzymes is expected to increase the yield, reduce the processing time, and improve the extraction of important or valued components of the fruit. It has been reported that the enzyme costs amount to about one third of the profits gained from the increased juice yield (Possmann, 2000).

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4.4.1.4 Enzymatic Liquefaction Fruit liquefaction is not new. Commercial enzyme preparations from Aspergillus niger with highpectinase activity for liquefaction of fruit have been studied. Effective dosages (125 to 150 g/t) for total liquefaction of apple mash during production of apple juice have been derived (Grassin and Fauquembergue, 1993). Liquefaction is accomplished using both pectolytic and cellulolytic enzymes in combination, taking advantage of observed synergistic effects (Voragen et al., 1980).

O

COOCH3

COOCH3

OH O

OH O

O OH

O

COOH OH O

O

COOH OH O

O OH

O OH

OH Polygalacturonase

+ H 2O

Pectin Esterase

O

COOH

COOH

COOH

OH O

OH O

OH O

O OH

4.4.1.4.1

O

O

OH

COOH OH O

O

OH OH

OH

OH

Commercial pectolytic enzyme preparations containing predominantly polygalacturonase (PG) and pectin and pectate lyase (PLs) are utilized in this processing step. The PG activity is targeted at the glycosidic bonds by -elimination. The enzyme treatment usually included a pectin esterase (PE) in order to improve the activity of the polygalacturonase (Pilnik et al., 1973).

O

COOH

COOH

OH O

OH O

O OH

COOCH3 O

O

4.4.1.4.2

O OH

Pectin lyase

COOH

COOH

OH O

OH O

O

OH OH

OH O

O OH

OH

Pectate lyase

O

COOCH3

OH O

H

OH

O

COOCH3

COOCH3

OH O

OH O

O

OH OH

H

OH

Commercial cellulase products capable of hydrolyzing nonpectin polysaccharides such as cellulose, glucans, and xylans are used to fractionate the cell walls and liquefy the remaining solids. Progress of the liquefaction of the pomace can be monitored by high performance liquid chromatography (HPLC) analysis of the hydrolysate for free monomeric and oligomeric sugars. The sugar content in the juice and the yield increase significantly due to the complete hydrolysis of macromolecules. The juice produced from liquefaction has a different flavor and composition compared to pressed juice. The process of fruit liquefaction for juice production is regulated in many countries with strict limitations on final usage and labeling declaration. It is mainly used for production of juice concentrate.

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4.4.1.5 Extraction Equipment 4.4.1.5.1 Rack and Frame Hydraulic Press The hydraulic rack and frame press is a very common batch press system found in small juice operations. It was the primary method of fruit juice-pressing operations for many years. Heavy cotton or nylon cloths are filled with a set amount of mash and then folded to produce what is called a cheese. The individual cheese is stacked and separated by a wooden, stainless steel, or plastic spacer platen. The combined stack is then compressed using a hydraulic ram, during which the juice is expressed. The process delivers good yield but is labor intensive. 4.4.1.5.2 Horizontal Piston Press Probably one of the most successful press systems in the fruit juice market is the Bucher horizontal piston press (Bucher-Guyer AG, Zurich, Switzerland). This press is capable of pressing berries, stone fruit, and vegetables. It operates in batch mode with loads of up to 14 t/filling. Flexible drainage elements covered with a nylon filter cloth carry the expressed juice out to a manifold. Usually, the juice has a very low level of suspended solids, more typical of a rack and frame press. The Bucher-Guyer Press is a highly automated pressing system used in a batch pressing operation. Generally, this system consists of a rotatable basket or cylinder with a hydraulic ram used for juice expression. Within the cylinder are fabric-covered flexible rubber rods with longitudinal grooves in them that allow the juice to transport easily to the discharge port. 4.4.1.5.2 Bladder Press The Willmes Press is a commonly used system for grape juice pressing. It is a pneumatic-based system that consists of a perforated, rotatable, horizontal cylinder with an inflatable rubber tube (air bag) in the center. The cylinder is filled with grape mash through a door on the cylinder wall, which is rotated to the top position. After filling, the press is rotated to ensure even filling. During this rotation, the air bag is filled, creating the mash compression action. The bag is then collapsed, and the cylinder is rotated. The rotation and pneumatic compression of the mash is repeated many times with increasing air pressure. 4.4.1.5.3 Belt Press The continuous belt press, such as the Frontier Technology Inc. system, is effective for grape and apple juice processing. In belt presses, a layer of mash is pumped onto the belt entering the machine. Press aid may be added for improved yield and reduced suspended solids. The belt is either folded over or another belt is layered on top of the one carrying the mash. A series of pressurized rollers compress the enveloped mash. Expressed juice is caught in drip pans. The cake is discharged from the last pressure roller (Figure 4.2). Fruit Mash Juice Collection Zone Press Cake (Pomace) Fruit Mash

FIGURE 4.2 A continuous belt press.

Spent Press Cake

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4.4.1.5.4 Screw Press A typical screw press consists of a reinforced, stainless steel cylindrical screen enclosing a large bore screw with narrow clearance between the screw and the screen. Breaker bars are located between the screw intervals in order to disrupt the compressing mash. Back pressure is provided at the end of the chamber and is usually adjustable. Capacities for screw presses with diameters of 30.5 and 41 cm are 5,080 kg and 15,240 kg/h, respectively (Bump, 1988). 4.4.1.5.5 Decanter Centrifuge In addition to sieving technology, the separation of juice from the mash can be performed by sedimentation through increased gravity in a decanter. Centrifugal force is used to accelerate the settling of higher density insoluble particles present in the juice. Enzyme-treated mash is best suited for juicing by decanters as the reduced viscosity and higher temperatures result in faster and more effective separation. Decanters are versatile units that can process a wide variety of fruit and vegetable mashes including hard-to-press mash from ripe or heated pears (Schobinger, 1999).

4.4.2 CITRUS PROCESSING The extraction of orange juice is based on removal of the available juice (50% by weight) in the orange. Two major types of orange juice extractors are used on the market. An FMC Citrus Juice Extractor (FMC Corporation) is designed with a squeezing process in which a hole is cut in the fruit and the juice and flesh are squeezed out of the orange. Another extractor is the Brown Extractor (Automatic Machinery Corp.) where a reaming process is used. The fruit is cut in half, and the juice and flesh are reamed out of each half. Following the extraction, the raw juice is strained to remove heavy solids, using a finisher. Although finishers can vary in design and operation, in general, all are expected to remove seeds, bits of peel, and heavy flesh from the juice. Typical models use a spinning screw that compresses the juice solids against a screen prior to ejecting the solids from the finisher. If low-pulp juice is desired, the juice may be processed with a centrifuge (Pecoroni and Gunnewig, 2001). Following the finishing step, juice from various lots is blended for optimal flavor and quality. The citrus juice is then deaerated and deoiled for standardization of juice quality. A pasteurization process not only ensures destruction of spoilage organisms, but it also inactivates the pectic enzymes responsible for juice separation. From this point on, the juice is ready for processing into chilled, aseptic, and hot-filled frozen concentrate or other products.

4.5 CLARIFICATION Solids separation in juice processing is a unit operation, often requiring multiple steps and possible pretreatment. Clarification is required for the production of clear juices.

4.5.1 ENZYMATIC PREPARATION Clarification is a process by which the semistable emulsion of colloidal plant carbohydrates that support the insoluble cloud material of a freshly pressed juice is “broken” such that the viscosity is dropped and the opacity of the cloudy juice is changed to an open splotchy look. This can be accomplished in one of two general ways: enzymatically and nonenzymatically. It has been suggested that by using pectinase enzymes, the pectin coat surrounding the protein particulates in this emulsion is broken down (Figure 4.3). This process allows the particles to aggregate and then drop to the bottom of the tank (Yamasaki et al., 1963). Detailed reviews of enzyme use with apples have been written by Kilara and others (Kilara and Van Buren, 1989; Kilara, 1981; Kulp, 1975).

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−− − − − −− ++++++ − +− − − ++ −− + +++ −− + + − + − −− − − − − − − −− ++++++ − +− − − ++ −− + +++ −− −− +++ − − − −− − − − −− +++++ − +− − ++ −+ −− + +++ −− Pectin −− +++ − − −

81

Protein

Stable pectin suspension due to electrostatic forces of repulsion.

− − +++ − ++ ++ −− +− + + ++ − −−++++−−−− − −

−− − − −−+++++ − + + − − ++ + −− + ++ −−−+++ −− −− − −− − − −−+++++ − + + − − ++ + −− + ++ −−−+++ −− −− −

Juice treated with pectic enzymes. Exposure of opposite charged surfaces.

−− + − +++ ++ − + + − + − + + + − + −− +++ ++−−− ++++−−− −+ − − + − −− + ++ −− + −−++++++ −−− ++ ++ −− + ++ −− −−−− ++ −−− + + −− +++ − −− −−−

Formation of floc due to agglomeration via electrostatic attraction.

FIGURE 4.3 A suggested theory of floc formation during enzyme treatment of the juice.

Nonenzyme clarification involves breaking the emulsion by other means, the most common of which is heat (Smock and Neubert, 1950). Other techniques include addition of gelatin, casein, and tannic acid–protein combinations (Kilara and Van Buren, 1989). Additionally, the use of honey and combined honey–pectinase treatments have been found to be effective clarification agents (Kime, 1982). It is believed that the proteinaceous component of honey is responsible for a synergistic effect when honey and pectinase are used in combination (McLellan et al., 1985).

4.5.2 MECHANICAL SEPARATION 4.5.2.1 Using Decanters and Finishers A high-solids stream can be partially clarified using decanters and finishers. Both pieces of equipment operate on the same principle with a spinning central cone, drum, and set of paddles pushing the juice through a screen of some type. The unit is typically mounted horizontally, and throughput is relatively high. Total suspended solids may be reduced to 1% or less during operation, depending upon the characteristics of the feed stream and operating conditions of the separator. 4.5.2.2 Centrifugation A very common unit used for removal of juice-insoluble solids is the centrifuge. A centrifuge places the juice under high gravimetric force induced by centrifugal action. This is effective in producing a juice that is opaque but free of visible solids. Modern centrifuges are highly automated and run continuously with timed solids ejection. Centrifuges with a high force of gravity are capable of producing clear juice under optimized conditions. Operation of the centrifuge must be done in a way that minimizes the introduction of excessive oxygen in the product. Possible remedies include the use of inert gas. 4.5.2.3 Diatomaceous Earth Filtration The use of filter aids in filtration operations is one of the most traditional techniques to achieve a clarified juice. It is not as popular as in the past due to safety restrictions in handling the material and cost of waste disposal. It involves a three-step operation in which a precoat of filter aid is built up on a filtration element (paper, cloth, or screen), and then filtration is conducted using the continuous addition of filter aid to the juice (Figure 4.4). This continuous addition is called body feed. Finally, the built-up cake is removed, and the entire cycle is started again.

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FIGURE 4.4 Component parts of a diatomaceous earth filter.

The basic system utilized in one of these operations included the following elements: a filter feed pump, a body feed injection pump, the precoat mixing tank, the body feed mixing tank, the filter element, connecting pipes, and openings or method for filter cake removal. 4.5.2.3.1 Pressure Filtration The final filtration process is typically accomplished through the use of a diatomaceous earth (DE) filtration system. This system utilizes suspension of diatomaceous earth in the juice as a means of renewing the filter surface in a plate and frame filter. When done properly, this filtration process is highly efficient and very effective. Its success can be highly operator dependent. The rate and effectiveness of filtration is dependent upon the use of the appropriate grade and amount of DE. In the filtration process, suspended filter aid deposits on a precoated layer as the filtration progresses. This deposition is permeable and prevents the extended buildup and subsequent clogging of the filter by the suspended solids. The following are examples of filtration equipment: •

•

•

•

•

The simplest of the pressure filtration type is the filter press. The cost is typically lower than other types of pressure filters. Although some of the systems are partially automated, it does require a significant amount of manual labor. The system can be dismantled easily for inspection and cleaning. Filter cakes can be easily washed from the system once disassembling has progressed. In the filter press, the amount of unfiltered liquid is relatively low once the shutdown process is terminated. Another type of pressure filter is the cylindrical element filter. In this system, tubular elements are suspended vertically in a closed tank system. Juice enters from the base of the system and filters through the elements, and the filtrate exits from the top of the system. Wash down and automation of this system are relatively straightforward. A third type of pressure filter is a vertical leaf filter. It is a low-cost system because of the inherent simplicity of its design. It offers an easy cake removal system and can be automated. A modified version of the leaf filter is the horizontal tank vertical leaf filter that accommodates a very large area of filtration surface, up to 2000 ft2 (180 m2). Filter leaves can easily be removed, inspected, and repaired. Due to the size of this equipment, costs can be very significant. A rotating leaf filter, in which the filtration elements are circular leaves suspended on a central axis, is another type of pressure filter. The leaves are rotated only during cleaning and discharging, which allows for an automated and rapid cake removal and cleanup system. Another type of pressure filter is a horizontal rotating leaf filter, essentially identical to the vertical rotating leaf system, except that it is available in much smaller models.

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Removal of the rotation capability leaves a horizontal leaf filter. This filter, although lowcost, must be manually disassembled for cleaning. It is usually used only for polishing operations, with liquids containing very low solids, giving relatively long filtration cycles. 4.5.2.3.2 Rotary Vacuum Filtration This filtration technology is dealt with separately here because of the unique character of the filtration system. This system has a large rotating element creating a vacuum that draws onto the drum element a buildup of precoat and body feed on the cake. Juice is fed into the base of the tub where vacuum action draws the juice through the filter cake. Once the cake is coated, a knife blade removes solids and filter aid material from the element itself. This system can deal with large amounts of suspended solids. It has a relatively high capital cost, including maintenance of the system. These drum filters can be up to 1000 ft2 (90 m2). Use of the rotary vacuum precoat (RVP) filter is called for when test filtration rates drop to near zero within seconds after only a very small amount of fluid has passed through the filter cake. It is particularly effective for small, slimy material that may blind a filter cake. Other conditions, in which particles are so small that they pass in significant numbers before a thick-enough cake is formed are appropriate for considering an RVP filter. The rotary drum is precoated with a thickness of up to 6 in. (15 cm). This precoat must be put on uniformly, with a variability of no more than ±1/4 in. (6 mm). The precoat has to be firm and should be applied at maximum vacuum, at maximum drum speed (2 rpm), and with a drum submergence of 10 to 15% of the drum diameter. For a 6 in. (15 cm) precoat thickness, precoating should take about 1.5 h. Precoat slurry should be approximately 7 to 10%. Possible cake cracking can be minimized with a row of flat sprays at 10 to 15 psig (69 to 103 kPa). Operation of the RVP filter is affected by many factors, including filter aid grade, vacuum level, drum speed, knife advance rate, and drum submergence level, as well as all of the other typical factors involved with precoat and bodyfeed filtration. Tests should be conducted to arrive at optimum filtration conditions. The first test should be to determine the coarsest grade of precoat filter aid that shows no penetration of solids into the precoat. The second test should be to determine filtration rates vs. submergence time of the drum. The third and final set of tests measures penetration of solids into the precoat at various knife advance rates. 4.5.2.4 Cross-Flow Membrane Filtration Membrane filtration is a pressure-driven technology with pore sizes ranging from 100 mol wt cutoff (MWCO) to 5 µm, offering a wide spectrum of applications for the juice industry. Membrane filtration processes include reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. For juice clarification, ultrafiltration and microfiltration are now commonly used, representing membranes with pore sizes from 10,000 MWCO to 0.6 µm. Different configurations and membrane materials are available in the market to fit specific separation applications. Although tubular polymeric membranes were the first to be used in juice clarification, hollow fiber membranes are currently used, also. Advantages of membrane filtration over traditional clarification methods include reduced processing time, increased juice yield, elimination of filter aid and filter presses, better product quality, and reduced enzyme usage (Cheryan, 1998). 4.5.2.4.1 Polymeric Membranes Ultrafiltration (UF) is a membrane filtration process that separates particles based on molecular weight (Milnes, 1984; Cheryan, 1986). The process uses a cross-flow method of operation, as opposed to depth filtration used in DE filtration (Figure 4.5). It can be utilized to clarify apple juice, as well as other fruit juices (Heatherbell et al., 1977). It has been used commercially for this purpose in several plants in Europe, the U.S., and South Africa (Cheryan, 1998; O’Sullivan et al., 1988; Möslang, 1984).

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Feed membrane surface

Polarized layer

Low permeate passage

Depth Filtration (Perpendicular Flow) Feed

Retentate

membrane surface

High permeate passage Cross-flow Filtration

FIGURE 4.5 A comparison of cross-flow filtration with depth filtration.

Polymeric ultrafiltration membranes, with a wide range of pore size ranging from 500 to 750 kDa MWCO, are available in the market (Swientek, 1986). The juice manufacturer prefers to buy the largest possible effective pore size to minimize the filtration time. The decision is then based on two factors: quality of the ultrafiltered juice and filtration rates. There are very few studies that deal with both factors at the same time. Baumann et al. (1986) reported that apple juice ultrafiltered with membranes of pore sizes between 10 kDa and 0.22 mm presented large variations only in color. Flow rates were not reported, and the storage stability at room temperature of the prepared juices was not studied. In the juice operation, the UF system can be set up as a single pass batch configuration or as a feed-and-bleed system (Figure 4.6). In the standard batch system, the membrane is run in a closed system where the permeate is continually drained out of the system. The juice retained by the membrane is identified as the retentate. The ratio of the initial volume of cloudy juice to the final volume of permeate is called the concentration factor. Generally, the final concentration factor is achieved when the centrifugal or spin solids level of the retentate is in the range of 40 to 80%, depending on the design of the system. As with any system, the initial solids level of the juice depends upon the type of press utilized, the application of any mash enzyme treatment, the use of filter aid, and the degree of depectinization, as well as the variety and condition of the fruit brought into the operation. Typically, the initial solids level of juice and streams entering the UF systems used nowadays range from 2 to 5% solids. The rate of permeate flow from the UF system is dependent on two primary variables: the temperature of the feed stream and the degree of depectinization. Typical flux rates for UF can range at room temperature from 15 gal/ft2/d (630 l/m2/d) for a raw juice to 25 gal/ft2/d (1050 l/m2/d) for a depectinized juice. By elevating the temperature to 125∞F (52∞C), feed streams will range from 30 gal/ft2/d (1260 l/m2/d) for raw juice up to a level of 50 gal/ft2/d (2100 l/m2/d) for depectinized juice. Several studies have been published concerning the quality of the ultrafiltered fruit juice. Möslang (1984) compared apple juice clarified by conventional procedures, i.e., gelatin and pectinase treatment followed by DE filtration with juice ultrafiltered with 18 kDa MWCO tubular

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Single pass processing Feed tank Retentate

Permeate

Feed pump

Retentate Feed tank

Batch operation

Permeate

Feed pump

Feed tank

Recycle loop

Feed

Feed and bleed

Permeate

Retentate

FIGURE 4.6 Various modes of operation for juice ultrafiltration.

membranes. The only difference noted was for higher calcium and magnesium in the DE-filtered juice. Drake and Nelson (1987) compared the quality and storage stability of ultrafiltered apple juice with plate-and-frame filtered juice using a 50 kDa MWCO membrane. Results showed that the ultrafiltered juice was as acceptable as the filtered one, even though sensory flavor was rated lower. The juice maintained a very low turbidity through 12 months of storage at 1, 21, and 32∞C. 4.5.2.4.2 Inorganic Membranes Inorganic membranes for cross-flow filtration of foods offer advantages to processors, such as resistance to abrasion and chemical tolerance, thus opening the range of applications and facilitating the cleaning cycles. They are more common in microfiltration processes with pore sizes of 0.1 to 0.6 µm. They can be operated at high temperatures and pressures, and they are autoclavable and have virtually unlimited life. Their major drawback is the high cost per membrane area compared to polymeric membranes. Ceramic membranes are used widely in the dairy industry and moderately in the beverage industry, the latter due in part to the lack of information and to low reported flow rates (Merin and Daufin, 1989). Baumann et al. (1986) compared the quality of apple juice clarified with microfiltration membranes (0.2 µm) made of polysulfone and alumina. They concluded that the filtered apple juice was sterile and presented no significant differences due to the membrane material. The steady-state permeate fluxes (flow rate per membrane area) reported for both modules were in the range of 100 to 120 l/h/m2 at 16 to 23∞C. Another study by Wu et al. (1990) compared the quality of apple juice clarified using two polymeric ultrafiltration membranes (5 and 50 kDa) and one ceramic microfiltration element (0.1 µm). The results showed that the microfilter produced a darker and more turbid juice, as was expected, due to the much larger pore size of the ceramic element. They reported that the average permeate flux of the ceramic membrane was 112 l/h/m2, but no operating parameters were specified to compare against other studies.

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Ben Amar et al. (1990) examined the clarification of apple juice using two mineral membranes of 0.2-µm pore size: a ceramic membrane and a carbon membrane. The effect of the temperature, inlet velocity, time, transmembrane pressure, and pectinase treatment on the permeate flux, as well as methods to reduce or control fouling, were studied on test benches. Optimum operating parameters were temperatures of 50 to 55∞C, transmembrane pressure of 3 to 3.5 bar, and inlet velocity of 4 to 4.5 m/s, giving juice flux from 100 to 150 l/h/m2. Permeate backflushing for 2 to 5 sec at 6.5 to 7 bar every 2 to 5 min was beneficial for the ceramic membrane (average flux increased to 200 l/h/m2) but had no effect on the carbon membrane. The use of pulsating entry flow (1.33 Hz frequency, 84 cm3 displaced volume) increased the permeate flux from 200 to 250 l/h/m2 for the ceramic membrane and from 175 to 225 l/h/m2 for the carbon membrane. The increase in flux obtained by the use of backflushing and pulsating flow seemed to indicate that substantial improvement in juice fluxes could be achieved by further optimizing the operating conditions to minimize membrane fouling and polarization through higher inlet velocities. In addition, these experiments were carried out by changing pressures while recycling the juice, so the results did not account for the potential decrease in flux due to fouling that could occur at the previous set of experimental conditions. The trials also tested each operating parameter separately, which made it impossible to evaluate any interactions that could affect the permeate flux. The performance of a 0.2-µm ceramic membrane for clarification of depectinized apple juice was studied. The results showed that the flux is higher at high feed velocities (14.6 m/sec) and high temperatures (50∞C), with the transmembrane pressure being a positive factor only at high temperatures. The juice flux at optimal conditions was between 400 and 500 kg/h/m2. Filtration of juice with pectin resulted in flux decreases of 40 to 50% compared to depectinized juice. Periodic backflushing during processing at optimal conditions, i.e., high temperatures, high feed velocity at the membrane surface, and low pressure, did not increase the juice flux significantly (Padilla and McLellan, 1989, 1993).

4.6 CONCENTRATION Fruit juice concentration can offer significant advantages to the processor; by concentrating the juice, the bulk is reduced, thereby reducing storage volume requirement and transportation costs. Concentration allows a more complete deposition of insoluble solids and tartrates in grape juice. Storage of the cold concentrate is less likely to exhibit yeast growth because of the high sugar concentration and also if frozen concentrate is the primary end product. Storage as a concentrate prior to canning and freezing is an appropriate process. Hartel (1992) and Rao (1989) have published reviews that cover the technical aspects of concentration of fruit juices. An indepth discussion concerning fluid-food evaporators, energy use in the concentration of fluid foods (Schwartzberg, 1977), principles of freeze concentration and its economics (Thijssen, 1974), and engineering principles of aroma recovery (Bomben et al., 1973) have been presented in the literature. Fruit juice is concentrated by controlled evaporation of water, the major constituent of the juice. Because of possible loss of aroma constituents, the first major step is stripping of volatiles from which aromas can be recovered. Stripping is primarily handled by partial evaporation. Alternatively, a steam-stripping process could be employed. Many different evaporator designs can be employed for juice concentration including: evaporators with single-pass recirculating concentrators, single or multiple stages, single or multiple effect, and with or without a surface mixing type of condenser (Hartel, 1992). The word stage is used to identify the flow of juice through the system. Single-strength juice enters the first stage, and final concentrate exits the last stage. Effect describes the path of steam through the system. The first effect of an evaporator system receives steam from a boiler, and the second effect receives steam and vapors boiled off from the first effect.

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A Schmidt Sigmastar multieffect evaporator is commonly used for grape juice concentration. The removed volatiles from this system are concentrated 100- to 150-fold and added back to the concentrate prior to storage. Concentration of “clean” juice streams is also possible using a combination of reverse osmosis and evaporation. Cross-flow membrane systems are ideally suited for this application due to the self-cleaning turbulence effect. The reverse osmosis technology is effective in concentrating a lowsolids juice (7 to 8∞ Brix) two- or threefold. Afterwards, evaporation technology would be appropriate. Modified reverse osmosis systems can achieve a 50 to 60∞ Brix concentrate (Cross, 1989).

4.7 PASTEURIZATION AND NONTHERMAL PROCESSES 4.7.1 HOT FILLING

AND

PRODUCT PASTEURIZATION

FOR

SHELF-STABLE JUICES

Pasteurization of the finished juice is accomplished by hot-filling or bottled juice pasteurization. Hot filling is conducted by passing the final juice through a heat exchanger and raising the temperature such that the temperature of the juice filling the container reaches a recommended fill temperature of 88 to 95∞C. A holding time of at least 3 min is normally applied to the juice prior to cooling. This hot-fill process is adequate for highly acidic beverages such as apple, cranberry, cherry, and grape juice. The maximum pH allowed for the hot-fill process is normally 4. A shelf life of high-quality retention can range from 9 to 12 months in glass bottles or in high-barrier containers, though, typically, juice movement through U.S. markets to consumers will take only 3 to 6 months. Postbottling pasteurization is also applicable to acidic juices and is currently used for carbonated juices. A continuous tunnel pasteurizer or a batch system can be selected to perform the heating and cooling operations.

4.7.2 ASEPTIC JUICE PROCESSING This process requires that the product be commercially sterile at the time of packaging. For each product, a commercial sterilization process should be determined and verified by a process authority. Additionally, the package itself must be free of any microorganisms at the time of filling, and, finally, the filling and sealing process must be done such that no recontamination is possible (Figure 4.7). Use of high-temperature, continuous processes can produce a sterile product with a minimal effect on product quality. This is because spoilage organisms are more sensitive to increasing temperatures than many quality characteristics. Packaging materials for aseptic processing are generally cheaper than those used in traditional hot-fill operations. Package sterilization procedures include heat, chemicals (such as hydrogen peroxide), high-energy radiation, or a combination of these (Gavin and Weddig, 1995). Aseptic processing is not a new concept. It was commercially practiced in Europe with the filling of milk during the 1950s; however, the widespread use of this technology has awaited the development of more suitable packaging designed for the process. Today, a wide variety of rigid and flexible materials are available for retail, institutional (food service), and bulk packaging. The aseptic process itself requires an effective, continuous, and closed loop system for heating the product. This might include tubular, plate and scraped-surface heat exchangers, or direct steam injection and steam infusion. For fruit juice products, the most heat-resistant microbial component for process determination should be heat-resistant molds identified with the specific product being processed (Cousin and Rodriguez, 1987). An alternative method appropriate for high-acid juice sterilization might be sterile filtration. This technology would be used for fully clarified juice with no remaining particulate matter. Membranes with a pore size small enough to exclude microorganisms (< 0.45 mm) would be required.

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The Aseptic Process

Commercial Sterilization of Foods-cooled to ambient temperature (Closed System)

Sterilization of the Container

Product Packaged in Sterile Environment at Ambient Temperature

Room Temperature Product Storage Extended Shelf life

FIGURE 4.7 A layout of the component steps for an aseptic operation.

4.7.3 REFRIGERATED JUICES The consumer demand for fresher juices has created the need to develop milder pasteurization procedures that enable the retention of nutrients and fresh flavor profiles. This category includes minimally processed and unheated juices with limited refrigerated shelf life. The main concern is to apply a process that will render the juice safe by eliminating microorganisms and contaminants that could pose a health risk to the consumer. Pasteurization temperatures in the 70 to 90∞C range for a few seconds are typically used. If the juices are not pasteurized, careful control of the microbiological quality of the raw fruit and final juice, and proper sanitizing procedures to eliminate pathogens from the fruit prior to juice extraction are necessary (see section “Fruit Juice Safety and HACCP”). Extended shelf life of refrigerated juices can be achieved by combining pasteurization with clean-fill operations. Addition of chemical preservatives, if permitted by law, will also extend the life of the product. Typical compounds used in the juice industry are potassium sorbate and sodium benzoate at levels not to exceed 0.1% as regulated by the FDA in the U.S.

4.7.4 NONTHERMAL PROCESSES Alternatives to thermal pasteurization have become a reality in the food industry. The production of cold-treated juices offers the advantage of fresh-like characteristics with extended shelf life. Technologies that are being utilized or developed include ultraviolet light (UV), high-pressure processing, pulsed electric fields, electron beam irradiation, high carbon dioxide processing, and chemical sterilants (Knorr et al., 2002; Odebo, 2001, Worobo et al., 1998). Iu et al. (2001) demonstrated that a combination of pulsed electric fields and heat treatments was very effective against E. coli O157:H7 in apple cider. The use of UV at 14 mJ/cm2, a method approved by the FDA to deliver safe refrigerated apple juice, has been described by Tandon et al. (2003). The UVtreated juice retained the fresh flavor and was favorably compared to juice pasteurized at 71∞C for 6 sec.

4.8 PACKAGING The primary functions of food packaging are to retard or prevent the loss of quality, to contain the food adequately, and to give protection against environmental contamination (Crosby, 1981; Paine

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and Paine, 1983). Packaging also provides the consumer with information, helps sell the product, and adds convenience. A package should not interact adversely with the food contained within it. The packaging systems used for juices have two primary goals: to retain a hermetic environment such that recontamination is unlikely and to minimize further quality degradation due to oxygen permeation into the product. New consumer expectations are driving the juice industry to offer a variety of choices based on flavors, freshness, health, environmental stewardship, convenience, closure features, shapes, and sizes. For shelf-stable juices, glass, cans, and cartons are still the dominant packaging types, although plastic (PET and HDPE) bottles layered with an oxygen barrier are becoming popular. The expected trend is for the PET bottles to eventually dominate the aseptic juice market. Plastic containers are increasing in the fresh and extended shelf-life refrigerated category, where the carton containers have been strong. Active packaging, where oxygen absorbers are used to extend the life of a product, can improve the utilization of polymeric materials. Oxygen absorbing packaging materials offer barrier properties superior to other products currently available in the marketplace (Castberg et al., 2000).

4.8.1 CANS Cans are made of steel or aluminum. Steel cans are coated to prevent corrosion from acidic juices. Using cans for juice has fallen into disuse due to concerns over flavor loss and interaction with the can coatings. A properly made and closed metal can is an absolute barrier between the canned product and the external environment. Cans provide high-strength, retortable containers for processed foods. Processing standards for canned foods are well defined; however, highly acidic foods such as fruits may corrode can interiors, causing extensive detinning, particularly at the product headspace interface. Aluminum cans are heavily used in beverages. For frozen juice concentrate, composite cans are typically used. They are made of spiral wound laminated board with metal or plastic ends, although all-plastic containers are also available.

4.8.2 GLASS Glass enjoys a continued and strong use in this industry. The primary advantages of glass are its chemical inertness, clarity, and heat resistance. In food applications, its transparency is considered to be a significant marketing advantage, conveying the image of a quality product (Osborne, 1980). Its heat resistance ensures that containers will not deform during hot filling; however, glass containers are subject to thermal shock and may shatter. Failure may also result from frictional stresses caused by repeated bottle contact along a packaging line. In addition, glass is heavier than most other packaging materials.

4.8.3 SEMIRIGID

AND

FLEXIBLE PACKAGES

A wide variety of polymeric materials are available for the manufacture of single or multicompound packages. Because plastic and carton packages are lighter in weight than glass or metal containers, they provide economic advantages in terms of lower shipping costs (Kashtock, 1988). Production is also less energy intensive than for glass or metal cans. Flexible films are more efficiently stored as roll stock prior to packaging of products. Rigid tubs can be nested to minimize space utilization; however, the higher permeability of some plastics may lead to reduced product shelf life. Difficulty in recycling multilayer packaged can also be a limitation. Product–package interactions are a major concern in the use of plastic polymers for food packages (Hotchkiss, 1988; Gray et al., 1987). Possible interactions include: • • •

Chemical and physical interactions between the food and polymer Absorption or scalping of flavor notes or other components from the food Migration of components from the plastic to food

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Foods may actively leach components from the surrounding polymer, or components may migrate independently (Briston and Katan, 1974). The permeability of the packaging material to gases, water vapor, and volatiles also impacts the packaged foods. Various mathematical models have been developed to describe migration phenomena (Gray et al., 1987). Low-molecular-weight components of plastics that may migrate into foods include polymerization residues and processing aids such as antioxidants, antiblock agents, lubricants, and plasticizers (Crosby, 1981; Harte and Gray, 1987). Considerations in selecting a composite polymer packaging material include the compatibility of the material with the food to be packaged; the tolerance levels of the food to uptake or loss of oxygen, water, and volatile food components; and requirements of the package based on the established food tolerance levels (Brown, 1987).

4.9 FRUIT NECTARS Fruit nectar is a blend of sugar syrup, fruit acid, and very high-solids juice or puree, generally containing most of the solids from the fruit. In the U.S., a standard of identity for fruit nectars has been defined in the Federal Register (Anon., 1968). Usually, these nectars are produced with fruit that are either low in flavor as a single-strength, clarified juice or too strong in flavor and need dilution. The process of making nectars generally starts with a softening and cooking of the fruit flesh followed by a pulping and finishing of the puree. Acidified sugar syrup is added, and the entire mixture is hot-filled into appropriate containers. Typical nectar products include apricot, peach, pear, prune, mango, cranberry, and individual and blended citrus.

4.10 WASTE TREATMENT Large quantities of liquid and solid waste are produced from the manufacture of fruit juices. The disposal of these materials is regulated by law and can represent a significant cost in the juice plant operation. When possible, reduction in waste generation and processing of the waste effluents into value-added products represent the best alternatives. In rural agricultural areas, where land is available, the simplest solution is to spray the fields with liquid waste and to use the solid waste as soil conditioner and animal feed. For disposal on land, the wastewater must be screened through 10 to 20 mesh screens to remove silt and other insoluble solids. The pH of the wastewater is usually adjusted to 6.4 to 8.4 because the pH outside this range will render some nutrients in the wastewater inaccessible to plants. The most commonly used methods for waste disposal on land are sprinkler irrigation and surface irrigation. Possible value-added products from solid waste are fuels, proteins, and biochemicals. In other cases, the effluents will need to be processed substantially to be safely discharged into the public systems (Hang, 2000). The effluents from the processing lines consist of three broad categories: wash water (low polluted), process water (high polluted), and residual solids (extremely high polluted). Lowpolluted liquid effluents, with biological oxygen demand (BOD) of less than 200 mg/l, should be treated in aerobic systems such as aerated lagoons. Anaerobic biological treatment systems are better suited for high-polluted process water with BOD up to 20,000 mg/l. Efficiencies of 70 to 95% reduction of BOD are expected with these systems. For higher reductions, the treated effluent might need to be processed in an aerobic system (Koevoets, 2002). The activated sludge process is another widely used system for wastewater treatment. The treatment unit consists of a bioreactor that provides an environment for converting soluble waste solids into insoluble microbial cells under aerobic conditions and a clarifier where microbial cells are allowed to settle. The settled cells or sludge may be returned to the bioreactor or wasted. The removal of BOD by this method

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ranges from 80 to 99% depending on the waste characteristics, loading rate, and other operating conditions (Hang, 2000). Solid waste poses a more difficult dilemma due to reduced opportunities to dispose of the solids. Fruit solids can be used as animal feed or fertilizer, and apple and citrus solids have been studied extensively for this use. Not only have cattle been fed these, but to a lesser extent, sheep, pigs, horses, and deer have also used fruit solids as a feed supplement. If necessary, solid fruit waste can be disposed of in local landfills (Koevoets, 2002). Proper composting offers an excellent opportunity to dispose of a waste product and produce a valuable end product. The challenge in this is getting a substrate that offers adequate aeration such as wood chips, ground newspaper, hay, or already composed material to mix with the waste. The application of liquefaction to fruit pomace as a waste reduction process and to increase juice yield is an option being utilized by some manufacturers.

4.11 JUICE AUTHENTICATION TO PREVENT ADULTERATION Extensive texts have been written on this subject and should be referenced for complete details (Hammond, 2001; Brause, 1998; Nagy et al., 1988). Adulteration has been and will continue to be a serious problem due to the incentives to adulterate fruit juice, the primary one being economics, even though adulteration can lead to legal issues that include the potential for imprisonment. The extent and type of adulteration have changed over the years from a simple sweetening and diluting of the juice to a high-tech chemical masking of the adulterated product. This sophistication leads to the need for a more detailed understanding of the unique chemistries of these fruit systems.

4.12 STANDARD TESTS FOR IDENTIFICATION OF HAZES IN CLEAR JUICES When working with juice systems, a number of tests are available that can help identify problem situations, especially those related to juice stability. Van Buren (1988) has discussed these tests in detail.

4.12.1 MICROBIAL GROWTH A most obvious question to ask when faced with a clear juice that has turned cloudy is whether the cloud is microbial in nature. Microscopic examination should answer this question quickly. Probably the most common type of microbial growth in fruit juice is due to yeast contamination. Lactic acid and acetic acid bacteria are also able to grow in acid foods such as fruit juices (Beech and Carr, 1977). Finally, molds of the genus Byssochlamys, Talaromyces, and Neosartorya produce ascospores that can survive the thermal process given to fruit juices. Fruit commonly comes into contact with the soil, which is a common source of ascospore contamination. Mold growth is usually in the form of a localized mycelial mass; however, under certain conditions, the growth may be broken up to produce a generalized haze (Splittstoesser and King, 1984). A microscope capable of 500 to 1000 times magnification should allow for identification of specific microorganisms. In each of these cases, the finding of microbial growth should cause immediate concern as to the adequacy of the process, the cleanliness of the equipment, and possible tampering with the bottle. The yeast and bacteria that can grow in fruit juice possess little heat resistance. Their presence, therefore, would indicate a gross underprocessing or leakage following the process. Heavily contaminated cooling water can be the source of non-heat-resistant microorganisms (Gavin and Weddig, 1995). Spoilage of fruit juice by heat-resistant molds is rare. The level of ascospore contamination is usually low, and the washing of fruit prior to milling and pressing will eliminate many of the spores.

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The polishing–filtration treatments will also remove ascospores. As a result of the latter, “natural style” juice that has a cloud is more susceptible to contamination by ascospores than clear juice.

4.12.2 STARCH Starch buildup is an easy, quick problem to identify and address. The first question that might come to mind is the stage of maturity of the fruit at harvest. Was it a relatively early harvested fruit used in making the fruit juice in question? Is the fruit prone to production of starch? If so, then the possibility exists that a bottling haze is due to starch. In fact, heavy starch accumulation in early harvest can lead to a white powder precipitate in the tank bottoms of early pressed juice. A relatively simple test for soluble starch consists of combining juice with an iodine test solution to determine the presence of starch. The iodine test solution can be made by dissolving 0.1 g of iodine crystals and 2 g of potassium iodide in 100 ml of distilled water. Warm a 1-ml sample of juice in a test tube by immersing in a boiling water bath for 5 min. Cool the sample and add 1 ml of iodine test solution. A blue color is a positive reaction for soluble starch. If this problem is confirmed, all starch precipitate should be filtered from the juice, and then the soluble starch should be treated with amylase.

4.12.3 PROTEIN–TANNIN COMPLEX Protein–tannin complexes are a common cause of turbidity in bottled apple juice and other juices. In this situation, the tannins (polyphenolic compounds) act as bridges and glue to aggregate the haze-activating protein chains together. A complete explanation of the haze formation is given by Siebert (1999). These hazes can usually be decreased by heating the juice, but then the haze reforms rapidly again. The most common cause of this haze is proline-rich proteins such as gelatin or enzymes that are added in excess quantities by the processors. In clarified juice without any protein addition, there are seldom any significant levels of proteins because natural tannins will precipitate them prior to filtration (Yokotsuka et al., 1978). A test for protein in the juice can be made, and if appreciable amounts of protein exist, then additional high-molecular-weight tannin can be added to help precipitate the excess protein. Removal of protein can also be handled by ultrafiltration or by the addition of bentonite and silica sol. To test for protein, make a tannic acid test solution by dissolving 1 g of tannic acid in 20 ml of 95 % ethanol and enough distilled water to makeup a volume of 100 ml. Add 1 ml of the tannic acid solution to 10 ml of juice. Observe for haze formation. An increase of ten or more nephelous turbidity units (NTU) indicates a relatively high level of protein.

4.12.4 TANNIN HAZE Tannin hazes can be formed in juices with small amounts of protein or starch. Tannin hazes can also form slowly as the polyphenolic molecules polymerize to form large moieties. Their growth and formation are maximized in apple juice by low pH and high storage temperature. Tannin complexes will tend to form after long-term storage. For normal bottling, distribution, and sales histories, this type of haze is seldom a significant issue. Various methods for determining tannin content are described by Amerine and Ough (1980).

4.12.5 GUM HAZE Gums can also be possible contributors to haze formation. These hazes form under cool conditions and settle very slowly. Determination of this type of haze usually requires an analysis of the hyrdolyzed sugars present in the precipitate. If the analysis found an appreciable amount of arabinose, galactose, or xylose, then the sediment probability indicates plant gums as the cause. Treatment of the juice with hemicellulose enzymes may be able to solve this problem by helping to digest the gum material.

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4.12.6 METAL IONS Hazes can also be caused by metal ions in the juice. Typical levels of iron in apple juice are 3.7 mg/kg and of copper, 0.2 mg/kg. If quantities of these metals rise to levels five times or more than these natural levels, they could be a causative agent for haze (Gebhardt et al., 1982; Kilara and Van Buren, 1989). Sources of these metals would usually be from corroding pipes or fittings, or the presence of tools or scrap metals in tanks or the flow stream of the juice. Treatment is mainly preventative, in the sense that all sources of copper, brass, bronze, and iron should be eliminated from the process stream. To test for iron and copper, test solutions need to be made up. A ferrocyanide solution should be made by dissolving 0.5 g potassium ferrocyanide in 100 ml of distilled water. A 3 N HCI solution should be made by diluting 258 ml of 36% HCI to 1 l distilled water. To test for the presence of copper, add 2 ml of the ferrocyanide solution to 20 ml of juice. A brown-red flocculant indicates the presence of copper. To test for the presence of iron, add 2 ml of 3 N HCI to 20 ml of juice : a blue color resulting indicates the presence of iron.

4.13 FRUIT JUICE SAFETY AND HACCP Fruit juices were considered safe due to their high acid content until a number of outbreaks in the 1990s became associated with the consumption of fresh apple and orange juices. The pathogens responsible for the illnesses were Salmonella spp., Escherichia coli O157:H7 and Cryptosporidium parvum. As a result of these outbreaks, the FDA issued a final rule in 2001 titled “Hazard Analysis and Critical Control Point (HACCP); Procedures for the Safe and Sanitary Processing and Importing of Juice” (21 CFR Part 120). The rule requires processors and importers of juices to establish a HACCP plan to minimize the risk of juice contamination with biological, chemical, or physical hazards. The HACCP plan must be developed individually for each processing establishment by a team of knowledgeable individuals that includes persons trained in juice HACCP. Any company producing 100% juice or juice puree used in manufacture of juices and beverages is required to comply with this regulation. The law requires the juice to be treated with a process that achieves at least a 100,000-fold decrease in the number of pertinent pathogens likely to occur in the juice. This requirement is known as the 5-log reduction performance standard. The pertinent pathogen that may occur in the juice is the most heat-resistant microorganism of public health concern. For apple juice, the processors must address critical control points to eliminate the risk of E. coli O157:H7 and C. parvum. For orange juice, the most likely risk comes from Salmonella spp. Approved processing methods to control microbiological hazards are thermal pasteurization (minimum of 71 to 73∞C for 6 to 11 sec) and UV light. Citrus processors have the option of treating the surface of the fruit prior to juice extraction because it is unlikely that the pathogens enter intact, sound fruit under current industry processing practices. Proper monitoring, verification, and validation procedures are necessary to ensure that the HACCP plan is effective. In addition to microbial hazards, the rule also established the maximum level of patulin allowed in a juice at 50 ppb. Patulin is a metabolite of certain fungi such as Aspergillus clavatus, Aspergillus claviforme, Byssochlamys fulva, Penicillium patulum, and Penicillium expansum and, as a mycotoxin, could pose a health risk if found in significant quantities in fruit juices. Patulin is typically associated with unsound, rotting apples (Stott and Bullerman, 1975) and must be prevented from entering the juice by culling infected fruit before juice extraction. There is one exemption in the HACCP rule for retail establishments. A retail establishment is an operation that provides juice directly to consumers. Even though these operations are not required to implement a HACCP plan and therefore might not pasteurize or UV-treat the juice, they must label fresh juices with a warning statement that describes the risk of consuming untreated juices (Juice HACCP Alliance, 2002). Strict sanitary conditions, good agricultural practices, adherence

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to current good manufacturing practices, and utilization of clean healthy fruit are necessary to minimize the presence of microbiological and chemical contaminants in freshly squeezed juices. Other bacterial spore formers have been associated with acidic fruit juices. For a more complete reference to all of these food safety issues in juice processing, the reader is referred to Chapter 10 in this book by D.F. Splittstoesser and R.W. Worobo.

REFERENCES Amerine, M.A. and Ough, C.S. 1980. Wine and Must Analysis. John Wiley & Sons, New York. Anon. 2001. Steady growth in European fruit juice market. Fruit Process., 11(12): 494–495. Anon. 1968. Canned fruit nectars: order establishing definitions and standards of identity. Fed. Reg., 33: 6862–6864. Baumann, G., Strobel, B., and Gierschner, K. 1986. Microfiltration and ultrafiltration of apple juice — comparison of inorganic/organic membranes and conventional deep-bed filters. Flüssiges Obst., 53:251. Beech, F.W. and Carr, J.G. 1977. Cider and perry. In Economic Microbiology, Vol. 6, Alcoholic Beverages, A.H. Rose (Ed.), Academic Press, London, pp. 139–313. Ben Amar, R., Gupta, B.B., and Jaffrin, M.Y. 1990. Apple juice clarification using mineral membranes: fouling control by backwashing and pulsating flow. J. Food Sci., 55: 1620. Bomben, J.L., Bruin, S., Thijssen, H.A.C., and Merson, R.L. 1973. Aroma recovery and retention in concentration and drying of foods. Adv. Food Res., 20: 1–111. Brause, A. 1998. Detection of apple juice adulteration. Fruit Process., 8(7): 290–297. Briston, J.H. and Katan, L.L. 1974. Plastics in Contact with Food. Food Trade Press, London. Brown, W.E. 1987. Selecting packages and composite barrier systems for food packages. In Food ProductPackage Compatibility Proceedings, J.I. Gray, B.R. Harte, and J. Miltz (Eds.), Technomic Publishing, Lancaster, PA. Bump, V.L. 1988. Apple processing and juice extraction. In Processed Apple Products, D.L. Downing, (Ed.), AVI Publishing, New York, p. 3. Castberg, H.B., Fredsted, L.B., and Rysstad, G. 2000. Marketing and packaging of fruit juices and noncarbonated juice based beverages. Fruit Process., 10(4): 130–135. Cheryan, M. 1986. Ultrafiltration Handbook. Technomic Publishing, Lancaster, PA. Cheryan, M. 1998. Ultrafiltration and Microfiltration Handbook. Technomic Publishing, Lancaster, PA. Cousin, M.A. and Rodriguez, J.H. 1987. Microbiology of aseptic processing and packaging. In Principles of Aseptic Processing and Packaging, P.E. Nelson (Ed.), Food Processors Institute, Washington, D.C. Crosby, N.T. 1981. Food Packaging Materials: Aspects of Analysis and Migration of Contaminants. Applied Science Publishers, London. Cross, S. 1989. Membrane concentration of orange juice. Proceedings of the Florida Horticultural Society, 102: 146–152. De Vires, J.A., Voragen, A.G.J., Rombouts, F.M., and Pilnik, W. 1986. Structural studies of apple pectin with pectolytic enzymes. In Chemistry and Function of Pectins, M.L. Fishman and J.J. Jen (Eds.), American Chemical Society, Washington, D.C., pp. 38–48. Downes, J.W. 1995. Equipment for extraction of soft and pome fruit juices. In Production and Packaging of Non-Carbonated Fruit Juices and Fruit Beverages, 2nd ed., P.R. Ashurst (Ed.), Blackie Academic & Professional, London, pp. 197–220. Downing, D.L. 1996. Canning of juices, fruit drinks and water. In A Complete Course in Canning and Related Processes 13th ed., Book III Processing Procedures for Canned Food Products. CTI Publications, Baltimore, MD. Drake, S.R. and Nelson, J.W. 1987. Apple juice quality as influenced by ultrafiltration. J. Food Qual., 9: 399. Gavin, A. and Weddig, L.M. 1995. Canned Foods — Principles of Thermal Process Control, Acidification and Container Closure Evaluation. The Food Processors Institute, Washington, D.C. Gebhardt, S.E., Cutrufelli, R., and Matthews, R.H. 1982. Composition of Foods: Fruits and Fruit Juices. Agricultural Handbook 8–9. U.S. Department of Agriculture. Grassin, C. and Fauquembergue, P. 1993. Enzymatic liquefaction of apples. Flüssiges Obst., 60(7); supplement Fruit Proc. 3(7): 242–245.

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Gray, J.I., Harte, B.R., and Miltz, J. (Eds.). 1987. Food Product-Package Compatibility Proceedings. Technomic Publishing, Lancaster, PA. Hamatschek, J., Buhler, K.H., Schottler, P., and Gunnewig, W. 1995. Separators and Decanters for the Production of Fruit and Vegetable Juices. Technical scientific documentation No. 18. Westfalia Separator AG, Oelde, Germany. Hammond, D.A. 2001. Synergy between liquid chromatographic-pulsed amperometric detection and capillarygas chromatographic methods for the detection of juice adulteration. J. AOAC Int., 84(3): 964–975. Hang, Y.D. 2000. Waste management: fruits and vegetables. In Encyclopedia of Food Science and Technology. John Wiley & Sons, New York. Harte, B.R. and Gray, J.I. 1987. An overview of food component interaction during processing and storage. In Food Product-Package Compatibility Proceedings, J.I. Gray, B.R. Harte, and J. Miltz (Eds.), Technomic Publishing, Lancaster, PA. Hartel, R.W. 1992. Evaporation and freeze concentration. Handbook of Food Engineering, Marcel Dekker, New York, chapt. 8: 343–393. Heatherbell, D.A., Short, J.L., and Strübi, P. 1977. Apple juice clarification by ultrafiltration. Confructa, 22(5–6): 157. Helbig, J. 2001. Production of colour-intensive and colour-stable coloured juices. Fruit Process., 11(9): 342–347. Hotchkiss, J.H. 1988. Food and Packaging Interactions. ACS symposium series, Series No. 365 ACS, Washington, D.C. Hulme, A.C. 1971. The Biochemistry of Fruits and Their Products, Vols. 1 and 2, Academic Press, New York. Hurler, A. and Wey, R. 1984. Technique of grinding and making the mash. Confructa Stud., 28: 125–130. Iu., J., Mittal, G.S., and Griffiths, M.W. 2001. Reduction in levels of Escherichia coli O157:H7 in apple cider by pulsed electric fields. J. Food Prot., 64(7): 964–969. Juice HACCP Alliance. 2002. Juice HACCP Training Curriculum, 1st Edition. Developed by the Juice HACCP Alliance as recognized by the Food and Drug Administration. Kashtock, M.E. 1988. New packaging for processed foods: Opportunities and challenges. In Food and Packaging Interactions, J.H. Hotchkiss (Ed.), American Chemical Society, Washington, D.C. Kilara, A. 1981. Enzymes and their uses in the processed apple industry. In Proceedings of the Processed Apple Institute. Research Seminar at The Pennsylvania State University, University Park, PA. Kilara, A. and Van Buren, J. 1989. Clarification of apple juice. In Processed Apple Products, D.L. Downing (Ed.), Van Nostrand Reinhold, New York. Kime, R.W. 1982. Clarification of fruit juice with honey. United States Patent 4,327,115. Knorr, D., Ade-Omowaye, B.I.O, and Heinz, V. 2002. Nutritional improvement of plant foods by nonthermal processing. Proceedings of the Nutrition Society 61(2): 311–318. Koevoets, W.A.A. 2002. Reliable and sustainable treatment solution for fruit processing effluents. Fruit Process., 12(3): 102–106. Kulp, K. 1975. Carbohydrases. In Enzymes in Food Processing, G. Reed (Ed.), Academic Press, New York. McLellan, M.R., Kime, R.W., and Lind, L.R. 1985. Apple juice clarification with the use of honey and pectinase. J. Food Sci., 50: 206. McNeil, M., Darvill, A.G., Frey, S.C., and Albersheim, P. 1984. Structure and function of the primary cell walls of plants. Ann. Rev. Biochem., 53: 625–663. Merin, U. and Daufin, G. 1989. Separation processes using inorganic membranes in the food industry. Paper presented at the 1st International Conference on Inorganic Membranes, Montpellier, France, July 3–6. Milnes, B.A. 1984. The application of ultrafiltration to apple juice processing. New York State Agricultural Experimental Station Special Report, No. 54, D.L. Downing (Ed.), Geneva, NY. Möslang, H. 1984. Ultrafiltration in the fruit juice industry. Confructa, 28(3): 219. Nagy, S., Attaway, J.A., and Rhodes, M.E. 1988. Adulteration of Fruit Juice Beverages. Marcel Dekker, New York. Odebo, U. 2001. Fresher under pressure — a fully commercial cold pasteurization method for fruit products. Fruit Process., 11(6): 220–221. Osborne, D.G. 1980. Glass. In Developments in Food Packaging — 1, S.J. Paling (Ed.), Applied Science Publishers, London. O’Sullivan, T., Simons, and Epstein, A. 1988. For winemakers — a simplified introduction to ultrafiltration. Vineyard Winery Management, 3: 65.

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Padilla, O.I. and McLellan, M.R. 1989. Molecular weight cut-off of ultrafiltration membranes and the quality and stability of apple juice. J. Food Sci., 54(5): 1250–1254. Padilla, O.I. and McLellan, M.R. 1993. Optimization and modeling of apple juice cross-flow microfiltration with a ceramic membrane. J. Food Sci., 58(2): 369–374, 388. Paine, F.A. and Paine, H.Y. 1983. A Handbook of Food Packaging. Leonard Hill, Glasgow. Pecoroni, S. and Gunnewig, W. 2001. Citrus technology — improved quality and economics through the use of high performance centrifuges. Fruit Process., 11(2): 46–48. Pilnik, W., Rombouts, F.M., and Voragen, A.G.J. 1973. On the classification of pectin depolymerases: Activity of pectin depolymerases on glycol esters of pectate as new classification criterion. Chem. Microbiol. Technol., 2: 122–128. Pinnow, D. 2000. Higher yields by means of an optimised mash preparation. Fruit Process., 10(9): 346–350. Possmann, P. 2000. European way of apple processing. Fruit Process., 10(3): 88–93. Rao, M.A. 1989. Concentration of apple juice. In Processed Apple Products, D.L. Downing (Ed.), Van Nostrand Reinhold, New York. Schobinger, U. 1999. Progress in fruit juice technology during the last 50 years — survey. Fruit Process., 9(7): 275–281. Schwartzberg, H.G. 1977. Energy analysis in food process operations. Food Technol., 31: 49–88. Selvendran, R.R. 1985. Developments in the chemistry of pectic and hemicellulosic polymers, J. Cell. Sci. Suppl., 2: 51–88. Siebert, K.J. 1999. Effects of protein-polyphenol interactions on beverage haze, stabilization, and analysis. J. Agric. Food Chem., 47(2): 353–362. Smock, R.M. and Neubert, A.M. 1950. Apples and Apple Products. Interscience Publishers, New York. Splittstoesser, D.F. and King, A.D., Jr. 1984. Enumeration of Byssochlamys and other heat resistant molds. In Compendium of Methods for the Microbiological Examination of Foods, M.L. Speck (Ed.), American Public Health Association, Washington, D.C., pp. 203–210. Stott, W.T. and Bullerman, L.B. 1975. Patulin: A mycotoxin of potential concern in foods. J. Milk Food Technol., 38: 695–705. Swientek, R.J. 1986. Ultrafiltration’s expanding role in food & beverage processing. Food Process., 4: 71. Tandon, K., Worobo, R.W., Churey, J.J., and Padilla-Zakour, O.I. 2003. Storage quality of pasteurized and UV treated apple juice. J. Food Process. Preserv., 27(1): 21–34. The Food Institute. 2002. The Almanac of the Canning, Freezing, Preserving Industries 2001-2002. 84th ed. Elmwood Park, NJ. Thijssen, H.A.C. 1974. Freeze concentration. Advances in Preconcentration and Dehydration of Foods, A. Spicer (Ed.), John Wiley & Sons, New York, pp. 115–149. Van Buren, J.P. 1988. The sources and prevention of turbidity in apple juice. In Processed Apple Products, D.L. Downing (Ed.), VNR/AVI Publishing, Westport, CT. Voragen, A.G.J., Heutink, R., and Pilnik, W. 1980. Solubilization of apple cell walls with polysaccharidedegrading enzymes. J. Appl. Biochem., 2: 452–468. Worobo, R.W., Churey, J.J., and Padilla-Zakour, O. 1998. Apple cider: treatment options to comply with new regulations. J. Assoc. Food Drug Off., 62(4): 19–26. Wu, M.L., Zall, R.R., and Tzeng, W.C. 1990. Microfiltration and ultrafiltration comparison for apple juice clarification. J. Food Sci., 55: 1162. Yamasaki, M., Yasui, T., and Arima, K. 1963. Pectic enzymes in the clarification of apple juice. 1. Study on the clarification in a simplified model. Agric. Biol. Chem., 28: 779. Yokotsuka, K., Kato, A., and Kushida, T. 1978. Microdetermination of proteins in juices and wines by trichloroacetic acid (perchlori acid)-dye method. J. Ferment. Technol., 56: 606.

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in the Fruit Juice and 5 Enzymes Wine Industry Alberecht Höhn, Daqing Sun, and Francois Nolle CONTENTS 5.1 5.2

Introduction ..........................................................................................................................97 Enzymes for Apple Juice Production ..................................................................................99 5.2.1 Juice Clarification ................................................................................................100 5.2.2 Mash Treatment ...................................................................................................101 5.2.3 Advanced Maceration ..........................................................................................102 5.3 Enzymes for Berry Processing ..........................................................................................103 5.4 Enzymes for Citrus Processing..........................................................................................105 5.4.1 General Citrus Processing ...................................................................................105 5.4.2 Citrus Processing Enzymes .................................................................................106 5.4.2.1 Oil Extraction Enzymes.....................................................................106 5.4.2.2 Pulpwash Enzymes ............................................................................106 5.4.2.3 Cloudifier Production Enzymes.........................................................107 5.4.2.4 Clarified Citrus Juice Production ......................................................108 5.5 Enzymes in Wine Making..................................................................................................108 5.5.1 General Remarks .................................................................................................108 5.5.2 Use of Pectolytic Enzymes..................................................................................108 5.5.2.1 Mash Treatment .................................................................................108 5.5.2.2 Must Treatment ..................................................................................109 5.5.3 Use of Proteases ..................................................................................................109 5.5.4 Other Enzyme Applications in Wine Making.....................................................111 References ......................................................................................................................................111

5.1 INTRODUCTION The use of technical enzymes has been an essential part of the entire technology of fruit juice production from the beginning. In the early 1930s, the first steps were made to process fruits into juices that could be stored for a longer time without the danger of alcoholic fermentation or other forms of undesired spoilage. The purposes of using enzymes in fruit juice or wine are (1) to increase extraction of juice from raw material, (2) to increase processing efficiency such as pressing and solids settling or removal, and (3) to render the final product sparkling clear and attractive. Fruit juices were and still are considered to be an excellent alternative to alcoholic beverages like beer or wine. Starch-degrading enzymes for fruit juice clarification and, later, pectolytic enzymes represented the first enzymatic products that were successfully used in fruit juice production. New processing technologies have been made feasible by new enzyme preparations. For example, yields of juice from apples have been pushed higher and higher, making modern fruit processing more efficient and economical. Typical of the enzyme activity of commercial fruit juice processing 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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activity at different temperatures

activity at different pH values % relative activity

% relative activity

100

100

80

80

60

60

40

40 20

20 pH value 0

0 5 7 4 6 50 1 3 20 30 40 10 2 reaction time: 90 min - temperature: 55°C reaction time: 90 min - pH value: 3.5 properties of pectinases

°C 60

FIGURE 5.1 Pectinase activity — dependence on pH and temperature.

enzymes is their dependence on reaction conditions like pH and temperature (Figure 5.1). Most of them show their highest activity at a pH range of 3.5 to 4.5. As can be seen in the general production line depicted in Figure 5.2, there are two possibilities for using enzymes in clear juice processing: mash treatment and juice clarification. In the mash treatment stages of juice production, the objective is to obtain as much juice as possible from the fruit mash. Pectolytic enzymes help considerably at this stage. At the clarification stage, pectolytic enzymes help to clarify juice and make juice filtration much easier. In the citrus industry, on the other hand, special pectolytic enzymes are used in the pulp wash process and to reduce viscosity in order to avoid gel formation of pectin during concentration and to recover citrus oil. Traditionally, the term pectinases includes the three enzymes pectin methyl esterase, pectin lyase, and polygalacturonase. Sufficient hydrolysis of pectin is the primary key to increase yield and clarity of clear juice. The early model of the structure of pectin was that of a smooth chain of partially esterified polygalacturonic acid. Therefore, it was believed that as long as there is sufficient pectinases, i.e., pectin lyase, pectin methyl esterase, and polygalacturonase, the pectin molecule could be totally hydrolyzed. More recent studies of the structure of pectin molecule have concluded that the structure of pectin is composed of smooth regions and hairy regions (Figure 5.3). The smooth region is called homogalacturonan, which is partially esterified polygalacturonic acid. The hairy region is complex and may be composed of rhamnogalacturonan I, rhamnogalacturonan II, and other galacturonic acidcontaining polysaccharides. There are a large number of neutral sugar branches attached to the hairy regions of a pectin molecule. To hydrolyze both the smooth region and the hairy regions, and therefore to achieve total hydrolysis of the pectin molecule, requires the activity of more enzymes than the three basic pectinase enzymes mentioned. Based on our current knowledge of the pectin molecule, the following enzymes may be necessary to achieve total hydrolysis of pectin: pectin lyase, pectin methyl esterase, polygalacturonase, arabinanase, rhamnogalacturonase, rhamnananase, galactanase, and other glycanses. Different fruits have different types of pectin, in which the weight, composition, and structure of the hairy region are different. In principle, commercial enzyme preparations for juice processing should be formulated in such a way that they cover necessary enzyme activities to achieve a high grade of hydrolysis of the targeted fruit or vegetable pectin. The same principle applies to the white wine making process because, before the fermentation step, grapes need to be crushed and solids need to be removed just like in any other juice making process. In addition, as recently as in 2002, a protease was found to be of important use in hydrolysis of a heat labile protein in white wine. In the case of red wine making, the situation is different

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FRUITS WASHING, SELECTING

STEMMING, DESTONING, PEELING

BLANCHING IF NECESSARY

CRUSHING, GRINDING, MILLING MASH (HEATING) ENZYME TREATMENT

PRESSING

DECANTING FINISHING

TURBID JUICE

PUREE

ENZYME TREATMENT ULTRAFILTRATION

FLASH HEATING

FLOCCULATION CLARIFICATION STABILIZATION

CENTRIFUGING

FILTRATION

CLEAR JUICE

CLEAR JUICE

CLOUD STABLE JUICE

FIGURE 5.2 General fruit juice line.

Hairy Region

Smooth Region

Hairy Region

FIGURE 5.3 Pectin structure.

from white wine making because red wine is fermented on the skin. Still, certain commercial pectinases are specifically useful in red color extraction and in a process called thermovinification.

5.2 ENZYMES FOR APPLE JUICE PRODUCTION Apples are by far the most important fruit for the production of clear juices and clear juice concentrates worldwide. The most important producers of apple juice concentrates are the U.S., China, Chile, and Argentina and, in Europe, Germany, Italy, and Poland.

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aroma recovery

juice from the press

pectinase amylase

aroma

enzyme treatment

fine filter

clarification

bentonite gelatin silica sol

pre-filter

FIGURE 5.4 Schematic of apple juice clarification.

5.2.1 JUICE CLARIFICATION The principle of apple juice clarification can be taken as representative for juices from other fruits as well. Apple juices that are running off the press are always cloudy due to the presence of a wide range of natural colloidal polysaccharides, cellulosic fragments from skin and pulp, and other small particles like protein fragments or polyphenols and tannins with protein. The polysaccharides are part of the intercellular cement or pectin, which holds together the plant tissue, whereas proteins, polyphenols, and other compounds are constituents of the plant material. During crushing of the fruit and the dejuicing process, these constituents enter the juice in solubilized and unsolubilized colloidal form. The most important of these high molecular substances is pectin, which is responsible for a series of technical problems encountered during processing. As a cloud stabilizer, it impedes the clarification of juices, which leads to increased viscosity and gelling and thus can be easily scorched to burn the juice in the evaporator. Pectin can also complex with calcium to form hazes and precipitates in later processing steps. Pectolytic enzymes are used in order to facilitate the clarification process. Through enzymatic depectinization, the juice can be clarified and filtered easily. Starch can often be found in apple juices. It is normally present in ripe fruit in the form of microscopically small, insoluble granules. When the juice is heated, for example, for aroma recovery or during pasteurization, the starch becomes hydrated and gelatinized. However, after filtration of the juice, the starch once again becomes insoluble as it retrogrades and precipitates. This causes an unattractive postbottling clouding of the juice or concentrate. Complete enzymatic degradation of the starch by means of special amylases is only possible if the juice was previously heated to at least 85∞C. The heat treatment hydrates and gelatinizes the starch and makes it ready for enzymatic degradation. This production process in Figure 5.4 describes the hot-clarification process, which is by far the most frequently used for the production of clear apple juice concentrate worldwide. In this process, enzyme treatment and subsequent fining are carried out in the juice coming from the aroma recovery where the juice has been heated up briefly in order to remove the aroma and to solubilize the starch. The aroma has to be removed before concentration and stored separately due to quality reasons. The de-aromatized juice has a temperature of 50 to 55∞C after passing through a heat exchanger where it is cooled down by preheating the juice coming from the press. This temperature range is almost ideal for enzyme activity. Compared to cold clarification of apple juice, which is done at ambient temperature and takes 6 to 8 h, the hot clarification usually requires

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a time period of only 2 to 3 h. This process and the higher temperatures during processing practically exclude the danger of alcoholic fermentation or other forms of undesired microbial activity. Only after the enzyme treatment has been completed is the process ready to be continued. Fining agents like bentonite, gelatin, and silica sol may be added for further flocculation and precipitation of cloudy material. Compared to the fining procedure, ultrafiltration is a relatively new technology in the fruit juice industry. The benefits of ultrafiltration are as follows: 1. 2. 3. 4.

No clarifying and fining agents are necessary. Continuous processing is possible. Less tank space is required. There is a saving of labor costs because of automatic controls.

In respect to the enzyme application, both the fining process and ultrafiltration process require complete enzymatic breakdown of pectin and starch or other polysaccharides. The practical application of enzymes for apple juice clarification is fairly simple. Liquid enzymes, which are by far the most widespread in fruit juice technology, are simply poured into the juice tank while filling up the tank with juice. The correct dosage is usually determined with the help of pretests in the laboratory. A mechanical stirrer in the tank leads to a good distribution of the enzyme. Granulated enzyme preparations have to be diluted in cold tap water before they are added to the juice. After the enzyme reaction, which has to be checked with specific test methods, the juice is ready for clarification, filtration, and concentration. The most important test method for pectin degradation is the so-called alcohol test. The test is simple and rather reliable; therefore, it is well-suited to be used during production. The test is carried out in the following way. Two parts of acidified alcohol are added to one part of juice. Gel formation in the presence of pectin can be observed 10 to 15 min after addition of the alcohol. Using this method, even very small amounts of pectin can be detected; therefore, this test has become very important as a quality control tool in international trade with clear juice concentrates. The degradation of starch is tested by using the iodine test. The addition of a 1% iodine solution to the juice sample provokes a typical bluish-purple coloration of the juice if starch is present. The disappearance or absence of this color is proof that starch was completely degraded.

5.2.2 MASH TREATMENT Whereas enzymes for clarification were part of apple juice production right from the beginning, the use of enzymes for apple mash treatment is fairly recent. There are two main reasons for using pectinases for mash treatment: higher juice yields and an increase of the press capacity and efficiency. Many stored apples are processed, particularly in the late part of the processing season. The quality of this raw material for juice production is not as good as that of fresh fruit. The longer the storage period, the greater is the change in composition of the apple tissue: pectin is converted from its insoluble form, the so-called protopectin, into soluble pectin. The latter leads to increased mash viscosity and impedes the flow of the juice. The whole pressing process takes much longer. Measures to increase the juice yield are often indispensable to producing juice with profit. Enzymatic hydrolysis of the soluble pectin by means of a special mash enzyme removes these drawbacks and provides a better yield. The quantity of juice can be increased significantly. In years with a good harvest, the main emphasis is on the maximum use of the press capacity. By using a special apple mash enzyme, large quantities of raw material can be processed with ease. The whole pressing capacity, expressed by hourly throughput, is stepped up considerably. The use of a mashing enzyme to increase the throughput could help avoid the need to purchase additional expensive presses. Due to the high investment costs for juice presses, the increase of the press capacity is a very strong argument for using mash enzymes.

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Special pectinase for apple mash treatment

apple mash enzyme continuous dosage into the mill

Time: 15–60 min Temperature: 10–30°C

belt press

horizontal press

One pressing step

two pressing steps

FIGURE 5.5 Processing line — apple mash treatment.

The enzyme is usually added as a 1 to 2% dilution into the mill. Optimal distribution is essential for a good effect of the mash enzyme. The principle is shown in Figure 5.5. Extraction of the pomace with water after the first pressing can further increase the total yield. Addition of mash enzyme in this extraction process, together with warm water, is necessary to wash out residual sugar that can still be found in the pomace after one press run. This water extract can be processed only into clear apple juice concentrate. By combining pressing and extraction processes in the production of apple juice concentrate with the help of enzymes, yields of more than 90% as measured by total brix are realistic. Furthermore, enzymatic mash treatment facilitates cleaning of the presses. Especially when stored apples have been pressed, a lot of residue remains in the pomace, making it sticky and difficult to remove. Much time has to be used cleaning the presses, which results in a decrease in the total processing capacity.

5.2.3 ADVANCED MACERATION Decanters are horizontal centrifuges that are able to separate juice from mash with high contents of solids. Progress in the construction of decanters and the availability of specially designed advanced maceration enzymes have supported this new development. As is known from various studies and tests, it is preferable to achieve a rapid drop in mash viscosity to 50% or 60% of the initial value. This is only possible with enzymatic treatment. Enzymes used in this application are a family of products able to perform advanced pulp maceration before juice separation in decanters or can be used to perform advanced pomace maceration, i.e., first pressing on belt press, then liquefaction of residual pomace before separation in a decanter (Figure 5.6) or a Bucher press. Those processes result in much better yield, but require a different technology from the classical mash treatment. This technology is not popular in some parts of the world because people do not like the whole apple to be totally macerated into juice. Some in the industry have a concern that the liquefaction process may introduce undesirable components into the juice. For example, the presence of trace levels of cellobiose in juice has been attributed to the use of enzymes in the liquefaction process. However, it has been found that cellobiose, the breakdown product of cellulose, is present in commercial applesauce where no enzymes have been employed. This misperception is increased when the process is incorrectly described as “total liquefaction.” It should be pointed out that none of the enzymes used in either the mash or liquefaction processes are capable of total

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apples mill pips and cores

finisher reaction tank

pomace

decanter

liquefaction enzyme 200–300 ppm 1–3 hrs. reaction time

juice fining filtration concentration

FIGURE 5.6 Processing line — mash liquefaction.

liquefaction of the entire fruit. In fact, in the liquefaction process about 25% of the mash is discarded as waste after juice extraction is completed. In the liquefaction process the enzymes function to assist in the physical separation of the juice from the pomace and to reduce the viscosity of the juice so that it can be efficiently separated from the pomace by the decanter. The end result is that additional juice is recovered in the liquefaction process that may not be extracted during traditional pressing operations. The differences between the “classical” mash treatment and advance maceration are as follows:

Dejuicing system Enzyme dosage Reaction time Reaction temperature Enzyme activities Process

Mash Treatment

Advanced Mash Maceration

Horizontal press 50–100 ppm 30–60 min 10–30∞C Pectinase Batchwise

Decanter 100–300 ppm 1–3 h 30–50∞C Pectinase, Hemicellulase Continuous

A further advantage of advance maceration is the saving of an extra step for juice treatment with enzymes. The enzyme dosages for the advanced maceration are usually high enough to achieve complete pectin and starch degradation of compounds already in the mash. Due to the fact that the alcohol test cannot be carried out with the mash itself, it is necessary to separate the serum (juice) from the solids by centrifuging a small sample of mash in a lab centrifuge. The supernatant is taken to check whether pectin and starch have been degraded completely and the mash is ready for further processing. When alcohol and iodine tests are negative, indicating that no residual pectin or starch is present, the highly macerated mash is ready to be extracted with the decanter.

5.3 ENZYMES FOR BERRY PROCESSING Compared to apples, the pectin content in some berry fruits is much higher (Figure 5.7). When berry fruits are crushed without using pectolytic enzymes, the soluble pectin causes gelling of the mash before the juice can run off the press (Figure 5.8). Adding pectolytic enzymes to the mash prevents the formation of the pectin gel and makes it possible to produce good quality berry juices for a reasonable price. The enzymes that are used for processing berry fruits have to meet special requirements, which are caused by the production technology and the character of the raw material.

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FIGURE 5.7 Pectin content of apples compared with black currants. Dosage to the mash: 80–300 g/t (Blackberries) 40–150 g/t (Others) mash tanks pectinase for berry processing

roller mill

heater

control: alcohol test

pomace extraction

belt press

enzyme treatment and fining tanks filtration enzyme application for berries and stone fruits

FIGURE 5.8 Processing line — berry juice production.

The berry mash is usually heated after crushing. Special tubular heat exchangers have to be used for this production step. Heating the mash can improve extraction of the cell wall material and color. Hot pressing of black currant or cranberry pulp after a 1 h enzyme treatment at 55∞C is widely practiced. The process is preferred to cold clarification because of better yield and retention of good color, aroma, and vitamin C. The pH value of most berry juice is very low. As an example, a typical blackcurrant or raspberry juice has a pH of 2.8. Anthocyanins are the largest group of water soluble pigments responsible for most of the red, pink, and blue color of fruits. Berry color is primarily given by anthocyanins. Most commercial pectolytic enzymes contain certain anthocyaninase side activity, which degrade anthocyanins into a colorless form called anthocyanidin. To ensure berry juice color stability, the level of anthocyaninase side activity must be minimal. A good pectolytic enzyme preparation for berry processing therefore needs to possess the following characteristics: heat tolerance and low pH tolerance, with little or no anthocyaninase activity. In addition, it was found that juices from blanched currants contain a substantially larger amount of alcohol-precipitable material (pectin, polysaccharide, and protein) and that an acid fungal protease improved the action of pectinase in the secondary clarification of the juice. Cellulase and hemicellulase may be useful in color extraction from berry fruits.

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5.4 ENZYMES FOR CITRUS PROCESSING Among citrus juices, orange juice is by far the most important. Lemon, lime, grapefruit, and tangerines are also processed as juice. Lemon juice is still a by-product of cold pressed lemon oil. Bergamotes and other citrus species are processed just for their essential oil. Brazil is by far the biggest producer of orange juice, followed by the U.S., Greece, Italy, Spain, and South Africa. The three biggest producers of lemon juice are Argentina, Spain, and Italy. One of the characteristics of citrus juice is that it is to be consumed mainly as a cloudy drink. Although the biggest percentage of that juice is made of diluted first extraction juice that does not require enzyme treatment most of the time, there is an appreciable production of by-products that are made from the pulp and the peels, and these are marketed either as second-quality juice or as natural cloudifiers. The processes involved in the production of such by-products require specific enzymatic treatments that are designed to extract sugars and reduce juice viscosity in order to allow concentration without destabilizing the juice cloud. Some citrus juices, especially lemon juice, are to be marketed as clarified juice. In that specific case, the enzymatic treatment is designed to depectinize the juice before filtration and concentration.

5.4.1 GENERAL CITRUS PROCESSING After the fruits are washed and sorted out, the juice extraction is performed on specific juicing equipment. Two technologies are widely used: The FMC system extracts the juice at the same time an oil emulsion is obtained by spraying water on the fruit, while Brown technology is designed to extract the oil emulsion first before the fruit is squeezed. The first extraction therefore results in the production of a pulpy juice on one hand and an oil-in-water emulsion on the other hand. Juice yields in this step are generally within the 40 to 50% (w/w) range. The pulpy juice then goes to a series of finishers where the pulp content is reduced, and the de-pulped juice can be concentrated as cloudy citrus juice. While orange juice is marketed mostly as 60 or 65∞ brix concentrate, cloudy lemon juice is sold based on its citric acid content. Generally, it is concentrated to 400 or to 530 gpl (grams citric acid per liter). It is important to note that citrus fruits usually have substantial endogenous pectin methyl esterase content. This enzyme is able to destabilize the juice cloud through the production of calcium pectates that rapidly precipitate. It is therefore of premium importance to pasteurize the juice as soon as possible after squeezing to denature that pectin methyl esterase. The oil-in-water emulsion generally goes through two steps of centrifugation in order to isolate the essential oil. It is then stored at cold temperatures to remove the waxes before it can be sold. Enzymes are used to increase the oil recovery. The by-products resulting from the general oil extraction process are mainly peel, pulp, and core. The peel can be dried to be used as cattle feed or for pectin extraction. However, a new trend is to produce natural cloudifiers from those peels (sometimes added to pulp) that are used in fruit based drinks such as lemonades. Enzymatic treatment is of primary importance in this process. The peel and pulp are finely crushed and water is added. Sometimes, pasteurization can be performed at this step in order to destroy fruit pectin methyl esterase and to solubilize more pectin. Then an enzymatic maceration is performed in order to extract sugar and to reduce the molecular weight of the pectin. The macerated mash then goes to a centrifugal separator (decanter) to separate the juice from the remaining pulp. This step is often performed at 80∞C in order to make the separation easier. Then the juice is centrifuged and concentrated. Later the key enzymatic treatment linked to that process will be described. The pulp is generally used to produce a cloudy concentrated pulpwash juice. The process consists of counter-current sugar extraction with leaching water. The resulting juice has a high soluble pectin content that impedes its concentration. An enzymatic treatment has to be performed

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PEELS (RAGS/PULP) Water (1:1)

Grinding Heating 45°C

pectinase 40–60 ppm

Enzyme Reaction approx. 60 min

Finisher

Extract Liquid

pectinase 10–30 ppm

Enzyme Reaction 20–30 minutes continuous Viscosity Control

Only if enzymatic Pulp treatment has not been applied

Pasteurization Centrifuge Concentration CLOUDY PEEL PRODUCTS

FIGURE 5.9 Citrus oil recovery process.

to allow its concentration without gelling. In some cases, an initial enzymatic maceration of the pulp plus water mix is performed in order to increase the sugar recovery from the pulp.

5.4.2 CITRUS PROCESSING ENZYMES 5.4.2.1 Oil Extraction Enzymes Due to the low price of orange essential oil, this application is seldom used in orange processing. However, it is economically justified to use enzymes in lemon, tangerine, grapefruit, and bergamote oil production. Enzymatic cocktails based on pectolytic enzymes and cellulytic enzymes are widely used to increase the oil recovery. These enzymes are added to the oil-in-water emulsion before centrifugation or between the two centrifugation steps (Figure 5.9), and help in breaking down the emulsion, resulting in easier separation and increased yield. These enzymes are mainly used when the extraction water is recirculated to the extraction system after it is de-oiled. However, even in open systems, an important yield increase can be obtained. Typical yield increases vary from 5 to 10%, and the oil recovery can go up to around 80%, based on fruit oil content, if enzymes are properly used. 5.4.2.2 Pulpwash Enzymes As mentioned above, pulpwash juice has a high soluble pectin level. The soluble pectin tends to gel when the sugar concentration increases during the concentration step. Pectolytic enzymes can

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be used to reduce the molecular weight of pectin, thus decreasing dramatically their ability to gel. However, since the juice should remain cloudy, this enzymatic treatment must be designed to reduce pectin molecular weight without causing juice clarification. Although most pectolytic enzymes can achieve the pectin molecular-weight reduction in the process conditions, using common pectolytic enzymes can be risky. If the enzyme dosage is properly calculated and the contact time is completely under control, an acceptable viscosity reduction can be achieved with acceptable cloud stability. However, in industrial conditions, reaction temperature and contact time can seldom be completely under control, and common enzymatic products can easily induce clarification or cloud loss if contact time is longer than expected or if the reaction temperature is not exactly as expected. This is why very specific enzymatic products have to be used in that application, so that the pulpwash juice cloud remains stable when enzymatic treatment conditions vary within an acceptable industrial range. The most efficient viscosity reduction with stable cloud effect can be achieved with pectolytic enzymes that have been previously purified from some other undesirable activities that render cloud instability. Specific hemicellulases also have a beneficial effect on viscosity reduction, without inducing any risk of cloud instability. An adequate combination of those specific pectolytic enzymes and hemicellulases allow achievement of fast and efficient viscosity reduction, without initiating any cloud instability when contact time and temperature are reasonably under control. Such enzymes are designed to allow processors a sufficient margin to cope with process variability. It has to be noted that the same enzymatic products can also be used, although at much lower dosages, to decrease first extraction juice viscosity. In some cases, the first extraction juice tends to gel at the high Brix levels that are required for the market place. A light enzymatic treatment with adequate pulpwash enzymes can allow a dramatically reduced juice viscosity, thus eliminating the gelling risk and making the processing easier. Lower concentrate viscosity means easier pumping, even higher concentration capacity, etc. 5.4.2.3 Cloudifier Production Enzymes These enzymes are designed to solubilize solids from the peels and to decrease juice viscosity in order to make concentration possible (Figure 5.10). Specific enzymatic blends are required in order to have an acceptable soluble substance extraction, reduce viscosity while maintaining the cloud stability, and prevent concentrate gelling. Specific pectinases, with high cellulase side activity content, as well as specific hemicellulases are necessary to achieve this goal. Careful control of processing conditions such as particle size, maceration temperature, and contact time are of premium importance to achieve a good result. citrus fruits

Water recycling

oil extraction emulsion (1–3% oil) desludger pectinase proteinase

Water

cream (60–80% oil)

filter

make up tank pectinase

polisher dewaxing citrus oil essence

FIGURE 5.10 Processing of natural cloudy peel concentrate — extraction process.

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5.4.2.4 Clarified Citrus Juice Production While orange juice clarification can be achieved with most common pectinases, acid juices such as lemon and lime juice will dramatically reduce the activity of the above mentioned common pectolytic enzymes. It is therefore of premium importance to use acid-stable pectolytic enzymes to economically depectinize lemon and lime juice, whose pH can be as low as 2. Most pectolytic enzymes have an optimal pH varying between 4.5 and 3.8. At pHs lower than 3.5, the remaining activity is reduced dramatically and the activity at pH values lower than 2.5 can be quite low. Only a very small number of pectolytic enzymes can retain enough activity at those low pHs to perform economical pectin degradation, and therefore allow acceptable filtration and trouble free concentration of the filtered juices. Side activities present in such pectinases are also of premium importance to allow production of haze free concentrates. In the case of using ultrafiltration, the hemicellulase content of the pectolytic enzymes used is also of premium importance to achieve good ultrafiltration rates. Last but not least, in some cases, the use of acid-stable proteases can be useful in achieving perfect concentrate stability. Other applications of enzymes in the citrus industry have been developed on a large scale, such as juice debittering. Naringinase, for instance, is able to convert bitter naringin into sweet prunin, thus reducing the bitter taste of grapefruit juice. In this section, we emphasized the importance of hemicellulase activity working together with pectolytic enzymes. In fact, the “hemicellulase” works against the pectin hairy region; therefore, strictly speaking it should be considered as part of the group of pectolytic enzymes. Generally speaking, enzymes are very useful tools in citrus juice processing. Specific processes can be implemented using specific enzymes in many applications, and new alternatives are being investigated on a daily basis. It would be difficult in one chapter to mention all the possible applications, and all the investigations going on. One should refer to specialists in enzyme companies for specific needs.

5.5 ENZYMES IN WINE MAKING 5.5.1 GENERAL REMARKS Many years after the fruit juice industry’s recognition of the value of applying enzymes, wine makers have also come to acknowledge their usefulness. In many wine growing countries, enzyme use had long been difficult or impossible for reasons of tradition or because of legal regulations. In Germany, the Wine Act of 1971 opened the door for the industrial use of enzymes by expressly permitting the addition of pectinases. In Switzerland, the U.S., Australia, and South Africa, proteases and b-glucanases are also approved for use in wine treatment, and their approval is currently being discussed in Europe as well. Meanwhile, enzymes have become an indispensable element of processing, both from the aspect of quality and of cost reduction.

5.5.2 USE

OF

PECTOLYTIC ENZYMES

Depending on the quality of the grapes and the processing method used, pectolytic enzymes can be used for various purposes. Their use in the treatment of grape mash, must, and hazy young wines is generally acknowledged. 5.5.2.1 Mash Treatment Treatment of the de-stemmed grape mash is particularly recommended for some white grape varieties that are difficult to press or for better flavor enhancement of bouquet grapes. With dessert grapes and similar types, enzymatic mash treatment is, in fact, almost indispensable to press the juice at an acceptable rate. Enzymatic liquefaction of grapes containing pectin improves so-called

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preliminary juice extraction, i.e., free-run juice from the mash before pressing. This juice, which runs without any mechanical pressure, is considered to be of superior quality and is usually used for the production of high-quality wines. Similar to fruit juice production, the total juice yield is also increased. In red and rose wine making, enzymatic mash treatment speeds up the color extraction. Pectolytic enzymes open up the fruit tissue, and thereby enhance pigment diffusion. In the mash fermentation process, which is considered to be the “classical” red wine technology, the fermentation period is reduced. Drawing off can be performed earlier and without loss of color and wine quality, which is a great advantage for automated processes. In thermovinification, the mash is heated up in order to extract the color before alcoholic fermentation. The juice that runs off the press contains most of the anthocyanins from the skin, which is responsible for the typical red wine color. The juice is then fermented as in the production of white wine. In this technology, the enzyme unfolds its full activity in the optimum temperature range of 45 to 65∞C, thereby improving the color extraction, preliminary dejuicing, and pressing. 5.5.2.2 Must Treatment Musts that are coming from the press contain considerable amounts of soluble pectin, which stabilizes the suspended matter and prevents it from settling rapidly. Preclarification is a very important measure in quality wine production because it reduces the content of undesired microorganisms and makes the alcoholic fermentation easier to control. Therefore, it must be the aim to remove the soluble pectin until the musts subjected to fermentation contain a certain minimum content of suspended matter. Pectolytic enzymes hydrolyze pectin quickly, so that it takes only a short reaction time for the preclarification process, either as natural sedimentation or with the help of mechanical separation. In the case of normal settling, enzymatic must treatment leads to more effective clarification and a smaller lees volume. After a reaction time of 4 to 6 h, a compact lees has formed, from which the supernatant can be drawn off with ease. After fermentation, young wines can be clarified and filtered easily, so that no further enzymatic treatment is required. Red wines from thermovinification process usually cause problems during clarification and filtering because their natural enzyme system has been destroyed by heating. To avoid problems during clarification and filtering, it is therefore imperative to add enzymes either to the mash or to the juice obtained from the press.

5.5.3 USE

OF

PROTEASES

Brilliance and clarity are important features for a wine to be perceived as good quality. The formation of haze or precipitate in wine after it is bottled causes consumers to be suspicious as to the quality of the wine. For consumers, the formation of haze or precipitates in wine indicates that the wine may be of poor quality or microbiologically spoiled. The aggregation of heat-labile proteins to form precipitate in white wine has been a concern in wine making for many years. Wine makers have attempted to use a number of approaches for the removal of wine proteins. The most commonly used preventative measure involves addition of bentonite, a montmorillonite clay that principally consists of aluminum and silicone oxides. The adsorption of compounds, including wine proteins, on bentonite is principally through a cation-exchange action. A protease was recently developed to replace bentonite to prevent a heat instability problem in white wine. The protease can be added at various stages of white wine fermentation (Figure 5.11). The benefits of using this protease instead of bentonite include but are not limited to: 1. Recovering more wine. Bentonite retains large volumes of wine, resulting in a significant loss in wine volume that is very difficult to recover. Using protease results in no loss of wine.

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Obtain Fruit

Extract Juice Add Protease Add Yeast

Fermentation

Remove Solids

Bottle

FIGURE 5.11 Using protease in white wine making.

2. Improving wine quality. Large amounts of added bentonite have the potential for changing the sensory characteristics of the wine by removal of desirable aroma or flavor and other components contributing to wine body. Using this enzyme will not affect wine sensory characteristics. 3. Reducing operation cost associated with bentonite application. 4. Positive environmental impact. Unlike the use of bentonite, the use of protease will not raise the issue of disposal because there is nothing to dispose of in such a case. Possibly due to low substrate concentration and inhibiting factors in wine environment, the action of protease in white wine stabilization should be considered a slow and constant process. This is different from the effect of bentonite, which eliminates protein rather quickly by absorption. The role of protease in white wine protein stabilization may not be to eliminate unstable grape protein preemptively but rather to act upon unstable grape proteins to hydrolyze them into smaller peptides and keep them from giving haze or precipitate during prolonged storage. Even though the enzyme can be applied at virtually any stage of white wine making for the purpose of heat stabilization, researchers found some other benefits when applying the protease at the beginning of fermentation. It was found that juice treated with the enzyme did not produce a significant amount of foam during fermentation compared with the control (Figure 5.12). This can help to save tank capacity requirement and to prevent the contamination problem. The benefit can be seen more obviously when fermentation is at relatively high temperatures or with grape varieties that tend to foam severely. For low temperature fermentation, faster and more complete fermentation was achieved by using enzymes. The enzyme-treated fermentation produced 0.1 to 1.5% more alcohol than the control. Moreover, the enzyme can consistently shorten fermentation time by at least a day and reduce the risk of “stuck fermentation,” that is, fermentation stopping before all sugars are converted to alcohol by yeast. Most white wines are fermented cold (13 to 15∞C) for development of better aroma, fruitiness, and richer mouth feel. A “stuck” wine will soon develop volatile acid (vinegar) and hydrogen sulfide. Understanding and eliminating stuck fermentation is the most active research project in wine industry and universities worldwide. There are many theories, but no solutions. This enzyme is a good candidate to be used universally to prevent stuck fermentation so that wine makers can perform cold fermentation with confidence.

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111

Control

Crystalzyme Vino-ProTM 180 ppm

Enzymes in the Fruit Juice and Wine Industry

FIGURE 5.12 Antifoaming effect of protease in wine fermentation.

5.5.4 OTHER ENZYME APPLICATIONS

IN

WINE MAKING

Enzyme treatment in wine production can also be done after fermentation. Most wine makers prefer to do an enzyme treatment in the mash or in the must. The reason is that, at this early stage, the reaction conditions for the enzyme are more favorable than after fermentation when alcohol, sulphur dioxide, and cold storage temperatures have an enzyme-inhibiting effect and therefore require long reaction times for the enzyme treatment. Nevertheless, due to some technical problems, the enzyme treatment can only be carried out after fermentation. At this production step, problem-solving enzymes are needed, so it is often more effective to use enzymes that are provided with defined secondary activities instead of standard pectinases. In the treatment of wines from grapes that were infected with noble rot during ripening. This fungus, Botrytis cinerea, converts a certain part of the grape sugar into a polysaccharide that is known as b-glucan. Although these wines are of superb quality due to their high concentration of sugar and flavor compounds, they cause serious problems when they are filtered. When sheet filters are used, particularly, many sheets have to be used, resulting in high filtration costs. Furthermore, used filter sheets are solid waste and must be disposed of. These filtering problems can only be solved by hydrolyzing the high molecular b-glucan with the aid of specific b-glucanases. Another example is pectinases with b-glucosidase side activity for natural essence release. In some white wine varieties, the content of bound terpenes is rather high. These substances are a potential bouquet reservoir and can be released when enzymes with b-glucosidase activity are added to the wine immediately after fermentation. Especially in bouquet grapes like Morio Muskat, Gewuerztraminer, and Scheurebe, this treatment leads to a noticeable enhancement of the characteristic fragrance, which can be proved, both sensorially and with analytical methods.

REFERENCES Braddock, R.J., By-products of citrus fruit, Food Technol., 49, 74, 1995. Browska, J., Application of enzymes in the production of pulpy carrot juice, Fruit Process., May, 162, 2000. Clahorst, S., Enzymes solutions for fruit processors, Food Product Design, Jan., 73, 2003.

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Ducroo, P., Arabans in apple juice, Flüssiges Obst., 54, 265, 1987. Endo, A., Studies on the enzymic clarification of apple juice, Agric. Biol. Chem., 29, 129, 1965. Fugelsang, K.C., Gump, B.H., and Zoecklein, B.W., Review of protein stability in wine, Vineyard Winery Manage., Sept./Oct., 28, 1988. Fundira, M., Blom, M., Pretorius I.S., and Van Rensburg P., Comparison of commercial enzymes for the processing of marula pulp, wine, and spirits, J. Food Sci., 67, 2346, 2002. Garzon, G.A. and Wrolstad, R.E., Comparison of the stability of pelargonidin-based anthocyanins in strawberry juice and concentrate, Food Chem. Toxicol., 67, 1288, 2002. Labib, A.S., Enzymes in guava fruit processing, Fruit Process., Sept., 13, 1999. Lagace, L.S., Survey of yeast acid proteases for effectiveness of wine haze reduction, Am. J. Enol. Vitic., 41, 147, 1990. Van der Maarel, M.J.E.C., van der Veen, B., Uitdehaag, J.C.M., Leemhuis, H., and Dijkhuizen, L., Properties and applications of starch-converting enzymes of the a-amylase family, J. Biotechnol., 94, 137, 2002. Miller, G.C., Amon, J.M., Gibson, R.L., and Simpson, R.F., Loss of wine aroma attributable to protein stabilization with bentonite or ultrafiltration, Aust. Grapegrower Winemaker, April, 46, 1985. Modra, E.J. and Williams, P.J., Are protease active in wines and juices? Aust. Grapegrower Winemakers, April, 42, 1988. Nakamura, T., Hours, R.A., and Sakai, T., Enzymeatic maceration of vegetables with protopectinases, J. Food Sci., 60, 468, 1995. Paigh, J.G., Enzyme formulations for optimizing juice yields, Food Technol., 49, 79, 1995. Pinik, W., Enzymes in the beverage industry, in Use of Enzymes in Food Technology, Dupuy, P., Ed., Lavoisier, Paris, 1982, 425. Sun, D. and Harris, J., Effective use of protease in winemaking, U.S. patent pending, 2002. Underkofler, L.A., Enzymes, in Handbook of Food Additives, 2nd ed., Furia, T.E., Ed., CRC Press, Boca Raton, FL, 1980, p. 57. Uriaub, R., Enzymes in Citrus Processing. Rohm GmbH, Darmstadt, Germany, 1992, 32. Waters, E.J., Wallace, W., and Williams, P., Identification of heat-unstable wine proteins and their resistance to peptidases, J. Agric. Food Chem., 40, 1514, 1992. Wightman, J.D., Price, S.F., Watson, B.T., and Wrolstad, R.E., Some effect of processing enzymes on anthocyanins and phenolics in pinot noir and cabernet Sauvignon wines, Am. J. Enol. Vitic., 48, 39, 1997. Yamasaki, M., Kato, A., Chu, S.Y., and Arima, K., Studies on the clarification of apple juice, Agric. Biol. Chem., 31, 552, 1967.

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6 Fruit Preserves and Jams Robert A. Baker, Norman Berry, Y.H. Hui, and Diane M. Barrett CONTENTS 6.1 6.2

Introduction ........................................................................................................................113 Pectin ..................................................................................................................................114 6.2.1 Pectin Structure....................................................................................................114 6.2.2 Pectin and Gelation .............................................................................................115 6.2.3 Effect of Methylation on Function......................................................................115 6.2.4 Low-Methoxyl Pectins.........................................................................................116 6.2.5 Interaction with Sugar Cosolute..........................................................................117 6.2.6 Effect of pH .........................................................................................................117 6.3 Processing Technology.......................................................................................................118 6.3.1 General Considerations........................................................................................118 6.3.2 Plate Evaporation Process ...................................................................................118 6.3.3 Vacuum Batch Process ........................................................................................120 6.3.4 High-Pressure-Treated Fruit Preserves and Jam .................................................121 6.4 Federal Standards...............................................................................................................121 6.4.1 21 CFR 150.160 Fruit Preserves and Jams.........................................................121 6.5 Phytonutrient Properties of Fruit Preserves and Jams.......................................................123 References ......................................................................................................................................124

6.1 INTRODUCTION Historically, jams and jellies may have originated as an early effort to preserve fruit for consumption in the off-season. As sugar for their manufacture became more affordable, the popularity and availability of these fruit products increased (Anon., 1983). Jellies, jams, preserves, and marmalades are primarily distinguished by the form in which their fruit component is incorporated. In jellies, only strained fruit juice is used, while jams are made with crushed or ground fruit material. Preserves are made with whole fruit (if sufficiently small) or large pieces of fruit (Ahmed, 1981). Marmalades are basically clear jellies in which slices or shreds of (usually) citrus peel are suspended. Regardless of their form, all are sugar–acid–pectin gels or low-methoxyl pectin–calcium gels. Their structure, appearance, and mouthfeel result from a complex interaction between pectin level and functionality, PH, sugar type and content, setting temperature, and, in the case of low-methoxyl pectin gels, calcium content. Originally, jam or jelly production relied on the native pectins of incorporated fruit for gel formation. Fruit was cooked with sugar, extracted acids, and pectins, and if the proper balance of sugar level, pH, and pectin content were achieved, a satisfactory jelly was obtained; however, modern manufacturing requirements for uniform gel strength and appearance preclude reliance on fruit component pectins, which may vary in content and quality, depending on fruit maturity and variety. In spite of the current availability of other gelling agents, pectin remains the universal 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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choice for jams and jellies, in part because of its presence as a natural fruit ingredient and also because of the characteristic consistency that pectin imparts to a gel. Pectins of known quality and gelling capacity (usually derived from citrus or apple by-products) are added to jelly and jam formulations to achieve a desired gel strength. It is estimated that 80 to 90% of commercial pectin production, which totals 6 to 7 million kg, is used in the production of jellies and jams (Crandall and Wicker, 1986). Although the basic steps in the production of jams remain the same, there is a continuous demand for improvement in processing performance and product quality. Important aspects to consider include close cooperation between preserve manufacturers and engineers to meet changing consumer preferences (e.g., preserves with higher fruit content or low-sugar jams), optimization of the preserve and jam manufacturing process, new developments in processing equipment and automatic controls, and strict sanitation controls. The next section gives an updated account on pectin and gelation.

6.2 PECTIN 6.2.1 PECTIN STRUCTURE An understanding of pectin structure is a prerequisite to defining its role in gel formation, and much research has focused on elucidating the finer points of this molecule’s primary, secondary, and tertiary structures. Pectin is a plant cell-wall polysaccharide consisting of a linear chain composed of D-galacturonic acid residues linked via a(1-4)-glycosidic bonds. At intervals in this homogalacturonan chain are regions in which the polymer backbone consists primarily of alternating units of galacturonic acid and rhamnose (Toman et al., 1976). Such regions are described as “hairy,” because, within these areas, short side chains of xylose occur, interspersed with larger, more elaborately branched side chains of arabinose and galactose. Approximately 95% of pectin’s galacturonic acid residues are located in the “smooth,” or homogalacturonan, regions (de Vries et al., 1986). Xylose residues in the hairy regions are primarily attached to galacturonic acid residues, while most galactose side chains are attached to rhamnose residues of the backbone (Schols et al., 1990). These arabinogalactan-rich regions are located at regular intervals along the pectin chain, but this regularity may vary depending on the pectin source. Recent work suggests that the hairy regions of apple pectin comprise, at least in part, smaller repeating units (Colquhoun et al., 1990). In commercial pectins, rhamnose and pentose linkages of these hairy regions may be partially hydrolyzed by acids used in extraction (de Vries, 1988; Mort, 1993a). As a long-chain polymer, pectin’s molecular weight (MW) may vary widely, depending on source and method of extraction. Its MW is also susceptible to reduction by chemical, enzymic, and even physical treatments. Average MWs of commercial pectins may range from 5 ¥ 104 to 1.8 ¥ 105 (Ahmed, 1981; Fishman et al., 1991; Smit and Bryant, 1969), but milder extraction procedures can yield pectins of considerably higher MWs (Kontominas and Kokini, 1990; Miyamoto and Chang, 1992). In addition, pectin is an association colloid and can aggregate to form even larger macromolecular-sized units (Fishman et al., 1992). Galacturonic acid residues of the pectin backbone are esterified to varying degrees, usually with methanol; the extent to which this occurs is referred to as the degree of esterification (DE) or methoxyl content. There is some evidence of a repeating structure in the distribution of these sites as well, with a suggested repeating unit of five esterified galacturonate residues and one free acid residue (de Vries, 1988). This would imply a DE of 83% for unaltered intercellular pectin; however, extraction and processing of pectins alter these in situ ratios, such that commercial pectins may range from 20 to 70% DE. Certain pectins may be naturally low in ester content, such as those from cotton cell cultures (Mort et al., 1993b) or sunflower head residues (Miyamoto and Chang, 1992).

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Recent studies have claimed that esterified residues in pectin are randomly distributed (Garnier et al., 1993; de Vries et al., 1986) or nonrandomly distributed (Mort, 1993a, 1993b). Using liquid hydrofluoric acid (HF) solvolysis to cleave cultured cotton cell pectin at the rhamnose inserts of the pectin backbone, Mort (1993a, 1993b) isolated two homogalacturonan fractions, one approximately 40% esterified and the other 10 to 15%. Isolation of residues differing in esterification suggested a nonrandom distribution in the original pectin. However, ion-exchange chromatography showed a random distribution of charge in a citrus pectin that had undergone acid-catalyzed deesterification (Garnier et al., 1993). Such disparate findings may, in part, be due to the length of galacturonate residues being examined or to differences in pectin source. From the above discussion, it should be apparent that pectin is an extremely heterogeneous material and any discussion of pectin’s molecular weight or degree of esterification refers only to the average for the molecular species present in solution. Such average values alone will not define the functionality of a particular pectin. Different methods of preparation may result in pectins identical in average MW or DE but with differing MW and DE distributions, which will, in turn, confer entirely different properties (Baker, 1979).

6.2.2 PECTIN

AND

GELATION

Pectin gels have been aptly described as intermediate between a solid and a liquid state, consisting of a three-dimensional network of pectin immobilizing the aqueous component (Oakenfull, 1987). The solvent water, pH, and accompanying cosolutes (usually sugar) influence the intermolecular forces contributing to gel structure; conversely, the gel structure prevents the aqueous phase from separating. In high methoxyl pectin solutions, gelation occurs via noncovalent bonding between adjacent polymer chains, with both hydrogen bonding and hydrophobic interactions between juxtaposed chains contributing to formation of junction zones (Oakenfull, 1987). Bonding between adjacent pectin chains occurs primarily at these junction zones, which can range from 18 to 250 galacturonic-acid units (Oakenfull and Scott, 1984). Although hydrogen bonding in a higher methoxyl (70% DE) pectin is approximately double that of the hydrophobic contribution, hydrogen bonding alone is insufficient to initiate gelation. Thus, hydrophobic interaction between adjacent ester methyl groups, which is enhanced by cosolutes such as sucrose, is essential to gel formation (Oakenfull and Scott, 1984). More recent work with high-methoxyl pectin gels utilizing ethanol, 1-butanol, and dioxan as cosolutes confirmed the dependence of junction-zone formation on hydrophobic interactions (Brosio et al., 1993). Although formed by noncovalent bonds, these junction zones can be of sufficient strength to prevent gel melting without polymer breakdown (May and Stainsby, 1986).

6.2.3 EFFECT

OF

METHYLATION

ON

FUNCTION

Of the features influencing pectin gelation, the degree of esterification may arguably be the most significant. Pectins can be modified chemically to produce a continuum of esterification levels, from 0% (pectic acid) to almost 100%. In practical terms, for food use, the upper limit of esterification in commercial pectins is set by the naturally occurring ester content of pectin in the raw material source. Although raw material origin may provide an upper bound to esterification levels, both DE and gel-forming capacity of pectins can be altered by extraction time, temperature, and pH (Woodmansee and Baker, 1954; Aravantinos-Zafiris and Oreopoulou, 1992). Pectin functionality is altered by changes in DE, and a broad distinction is made between high-mehoxyl pectins (DE values from 55 to 80%) and low-methoxyl pectins (DE values below 50%); however, it should be understood that this is an arbitrary distinction, and pectins with DE values near 50% will display features intermediate between those of high and low DE values. Commercial low-methoxyl pectins usually have DE values ranging from 20 to 50% because pectins with excessively low DE will precipitate rather than gel.

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High-methoxyl pectins gel only under acidic conditions and when the sugar (sucrose) content is at least 55% (Oakenfull and Scott, 1984). Low pH suppresses dissociation of free carboxylic acid groups, reducing their electrostatic repulsion (Watase and Nishinari, 1993), whereas sugars stabilize hydrophobic interactions between the methyl ester groups (Oakenfull and Scott, 1984; Brosio et al., 1993; Rao et al., 1993). Junction-zone size and the standard free energy of gelation increase as the degree of esterification increases, being proportional to the square of the DE (Oakenfull and Scott, 1984). As pectin ester content is lowered to 50%, jelly strength increases, but only at progressively lower pH values (Smit and Bryant, 1968). Commercial high-methoxyl pectins are further divided into rapid set (20 to 70 sec), medium set (100 to 150 sec), and slow set (180 to 250 sec) categories, depending on the time required for a gel to form under standard conditions. Gel-setting time is a function of DE, with rapid-set pectins possessing a DE of 72 to 75, medium-set pectins a DE of 68 to 7l, and slow-set pectins a DE of 62 to 66 (Crandall and Wicker, 1986). Such distinctions are of value in processing where speed of gelation can influence product quality. Rapid-set pectins are useful in the manufacture of jams and ensure uniform dispersal of fruit pieces and prevent floating. When flotation of fruit is not a problem, such as in clear jellies, then slow-set pectins are preferable because they allow entrained air bubbles to rise before gelation. Esterification levels of pectins can also have an effect on the flavor perception of jellies. For a substance to be tasted, it must contact the taste buds; therefore, if a gel delays diffusion of the substance (flavor) to the taste bud surface, taste perception may be controlled not by the taste reaction but by diffusion. Guichard et al. (1991) found that, at similar gel consistencies, highmethoxyl pectin reduced taste intensity more than low-methoxyl pectin.

6.2.4 LOW-METHOXYL PECTINS Low-methoxyl pectins, those with DE of 50% or less, are also able to form gels but by an entirely different mechanism than that described previously. These pectins do not require high levels of sugar or low pH to initiate gelations, but gel in the presence of divalent cations such as calcium. Such divalent cations form associations between sequences of charged species on adjacent chains. As stated earlier, pectins with DE in the upper range of this class (45 to 50% DE) exhibit properties intermediate between high- and low-methoxyl pectins; that is, they can also form acid–sugar gels (Smit and Bryant, 1968). However, the low pH requirements for such gels preclude their use in most foods. Low-methoxyl pectins with DEs near the upper limit of the range require some sugar for gelation, a further indication of their intermediate properties (Oakenfull, 1987). Low-methoxyl pectin’s ability to form gels with less sugar content allows the production of dietetic jams or jellies, whereas their ability to gel with calcium at higher pH enables them to produce gels in acid sensitive foods such as milk. The free carboxyl groups available on the galacturonic acid residues of low-methoxyl pectin may form calcium bridges (Figure 6.1) with adjacent pectin polymers. This results in a stronger gel network and firmer structure, and less occurrence of syneresis. Low-methoxyl pectins can be generated from high-methoxyl pectins by various treatments, including acid-, base- or enzyme-catalyzed deesterification. Acid-catalyzed deesterification can be done simultaneously with initial extraction of pectin from the raw material source (Woodmansee, 1954). Such treatment tends to lower the MW of pectin, but methods have been developed to produce high-MW, low-DE pectins (Wiles and Smit, 1971). In addition, some sources of pectins (e.g., sunflower) are naturally low in DE, and careful extraction of these pectins provides highMW, low-DE pectins (Chang and Miyamoto, 1992; Miyamoto and Chang, 1992). While acid- and base-catalyzed deesterification tends to randomly remove methyl esters, enzyme-catalyzed deesterification with pectin methylesterase cleaves esterified sites nonrandomly, producing blocks of completely deesterified galacturonic acid units interspersed with unmodified original material. Activation energies for low-methoxyl pectin gels are much lower than for high-methoxyl pectin

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Methyl esterified,

Free carboxylic acid,

No Ca binding

Ca binding

FIGURE 6.1 Pectin polymers indicating methylation (left) which prevents calcium binding or free carboxylic acid (right) that may bind calcium.

gels, suggesting that shorter sections of the polymer backbone are involved in the gelation process in low-methoxyl pectin gels (Garnier et al., 1993).

6.2.5 INTERACTION

WITH

SUGAR COSOLUTE

As stated earlier, high-methoxyl pectins will not form a gel with less than 55% sugar as cosolute. As pectin DE levels drop below 50%, the amount of sugar becomes less significant because lowmethoxyl-calcium gelation becomes the predominant gelling mechanism; however, even in lowmethoxyl pectin gels, the addition of sugar can increase gel strength and reduce syneresis (Axelos and Thibault, 1991). For high-methoxyl pectin gels, any of a number of other sugars, alcohols, and polyols will permit gelation. From a practical standpoint, it may be advantageous to substitute other sugars for sucrose, either because of cost, to reduce the likelihood of crystallization, or for flavor modification (Ahmed, 1981). Partial replacement of sucrose with other sugars such as maltose, glucose syrups, or high-fructose syrups altered the setting times and certain rheological properties of model gels (May and Stainsby, 1986). For example, addition of maltose reduced the setting time and extended the pH range of gelation, whereas fructose delayed setting time. Partial or complete replacement of sucrose with other sugars alters the water activity of the system and can modify the hydrophobic interactions contributing to gelation. The use of dietary fiber as a thickener was evaluated in peach jam (Grigelmo-Miguel and Martin-Belloso, 2000). Commercial amidated pectin was either partially or totally substituted by peach dietary fiber, and jams with soluble solids contents of 40, 45, 50, and 55∞ Brix were produced. Jam color was unaffected because both the dietary fiber and puree originated from peaches. The rheological behavior of the jams with substituted dietary fiber was pseudoplastic in nature, similar to that of conventional jams. The sensory characteristics of the high peach dietary fiber jams were also deemed as acceptable as conventional jams. Therefore, the authors concluded that such a jam might serve as a dietary supplement with no adverse effects.

6.2.6 EFFECT

OF PH

Control of pH is critical to successful gel formation with pectins, particularly high-methoxyl pectins. Low pH increases the percentage of unionized carboxyl groups, thus reducing electrostatic repulsion between adjacent pectin chains. High-DE, rapid-set pectins will gel at higher pHs than lower-DE, slow-set pectins; however, this difference is slight, with the optimum pH for slow-set pectins being

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about 3.1 and for rapid set pectins 3.4 (Crandall and Wicker, 1986). Substitution of other sugars for sucrose, by modifying hydrophobic interactions between chains, allows gels to be formed at somewhat higher pHs (May and Stainsby, 1986). Because they rely on calcium bonding to effect gelation, low-methoxyl pectins can form gels at higher pHs than high-methoxyl pectins. Gels can be made at pH values near neutrality (Chang and Miyamoto, 1992; Garnier et al., 1993), an advantage in producing dairy-based products.

6.3 PROCESSING TECHNOLOGY 6.3.1 GENERAL CONSIDERATIONS This section concerns the manufacture of jams. The four necessary ingredients in manufacturing jams include fruit, pectin, sugar, and acid. Optional ingredients include spice, buffering agents, preservatives, and antifoaming agents. The exact process selected will depend upon the type of product to be manufactured, raw materials available, and scale of production. Essentially, the main stages in jam manufacture are as follows: • • •

Blending together of ingredients Evaporation to the desired total solids level Heat treatment to pasteurize the product

Traditionally, all of the ingredients were blended together at the first stage of the process; however, with modern demands for high consistency of quality, it has been common to add some critical ingredients at later stages in the process. Thus, for example, citric acid may be added at a late stage to give precise pH control. Volatile flavoring may also be added after the evaporation stage to avoid evaporative loss. While it is possible with a simple atmospheric batch evaporation to carry out the whole process in one vessel, the requirements for consistent, economic, high-speed production and improved product quality mean that this technique has been generally superseded, except for very small-scale manufacture. Most modern plants are based on low temperature or vacuum evaporation, which may necessitate the addition of an extra pasteurizing stage to give a product of suitable microbiological quality to allow prolonged storage. In general, the main factors to be considered when selecting process are as follows: • • • • • •

What What What What What What

is the required total solids (TS) level in the finished production? is the required fruit percentage? requirement is there for fruit particle integrity? will the fruit content be? are the required production rates? will the range of ingredients be?

Although it is not possible to discuss in detail all of the possible process variables, the following two processes cover the main needs of the larger scale jam manufacturer.

6.3.2 PLATE EVAPORATION PROCESS This process is particularly suitable for jams that do not have any large particles in suspension but where there is a need for a high-quality product and good process economy. It may be used for

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PLATE EVAPORATION PROCESS CONDENSATE

FRUIT PULP SUGAR PECTIN

STEAM

DEAERATOR

STEAM

PRE-MIX

VESSELS

PRE-HEATER

CONDENSATE

VAPOR SEPARATOR

IN-LINE MIXER

TANK

PLATE EVAPORATOR

S.S.H.E.

BUFFER

COOLANT

ACID/FLAVOR DOSING

FILLER

FIGURE 6.2 A typical processing plant.

the production of standard consumer products and is particularly suitable for special bakery and confectionery jams with very high solids levels and for jams for portion packing. Figure 6.2 shows a typical plant. The first stage is the mixing together of ingredients that would normally include fruit pulp, sugar, pectin, and possibly corn syrup (liquid glucose). The various ingredients may be weighed or metered into the batch premix vessels. Two vessels are provided so that one may be used for the recipe preparation while the other is feeding the process. Thus, with flip/flop operation, a fully continuous feed is possible. Although in most cases this type of batch premix system is used, where long runs on a single recipe are likely, a continuous inline metering and blending system may be more appropriate. The premix then goes through a Paraflow plate heat exchanger where it is heated by condensate and steam. If sulphited fruit pulp is included in the recipe, the hot mix enters a desulphiting column. At the top of the column is a variable orifice spray device. This allows the production of a good spray pattern with large surface area exposure to steam, which rises up the vessel to give effective stripping of sulphur dioxide. The opening of the nozzle may be adjusted to take into account both the velocity and viscosity of the feed. A second trimming stage of desulphiting takes place in the premix held in the bottom of the vessel by steam being sparged up through the mix. A level-control device allows the residence time in the vessel to be controlled according to the particular recipe’s sulphite content. Where nonsulphited pulps are used, this stage may be omitted. The hot mix is now fed to the APV plate evaporator, which is held under vacuum. This is a rising and falling film-type evaporator. Flashing occurs, giving good distribution in the wide gap plates, and further heating by steam passing through alternate plates ensures that very rapid evaporation takes place, minimizing loss of fruit volatiles and giving a product with a good natural fruit color. Typically, the temperature of evaporation is 60 to 65∞C (140 to 149∞F).

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A vapor recompressor (thermocompressor) may be used to recompress the product vapor with incoming high-pressure steam. This reduces the evaporation steam requirement by almost 50%. Further steam economy is achieved by utilizing the steam condensate in the Paraflow for the partial preheating of the premix. The concentrated jam and resulting vapor are discharged through a large rectangular port to a snail shell separator where jam is separated from the vapor. The vapor passes to the condenser, which may be of the spray or surface type. The jam is extracted by a rotary pump and passes through an inline refractometer. This is used to monitor the solids level, and may be used to control the steam rate to the evaporator and thus accurately control the finished product concentration. Inline pH metering allows control of citric acid solution addition to give the desired acidity level. Postprocess addition of any flavorings avoids volatile loss during the evaporation. The product now passes through a scraped surface cooler before passing to buffer storage and on to filling. This type of process may also be used for certain types of particulate-bearing products where the particulate material is presyruped in a strong sugar syrup. With this technique, it is possible to manufacture products with very good particle identity and a surrounding liquid phase of sparkling clarity.

6.3.3 VACUUM BATCH PROCESS Figure 6.3 shows the general details of this process, which is suitable for the production of highquality jams containing fruit pieces or whole fruit. Evaporation is carried out at low temperature, reducing thermal degradation. Postprocess pasteurization allows the process to be used for jams of a wide range of TSs with or without preservatives. It is ideal for linking to aseptic or ultraclean filling equipment. The various ingredients are metered or weighed into the premix vessel, which is jacketed for the preheating of the mix and dissolution of sugar. A scraped surface agitator and baffle combination in this vessel gives both good heat transfer and rapid but gentle blending together of the various ingredients. After mixing, the premix is drawn under vacuum into the vacuum cooking vessel. This has zoned dimple panel heating elements so that there is efficient heat transfer using the low-speed scraper agitator without problems of burn-on even in preparing part batches. Rapid vacuum evaporation takes place at 60 to 65∞C (140 to 149∞F), and the vapor is separated in a cyclone separator with any product carryover being returned to the vessel. Normally, the vapor is condensed in a surface and may be recovered for use in recipe makeup, in-place cleaning, and so on; however, where required, a partial condensation volatile recovery system may be added for recovery of volatile flavorings. After completion of this rapid low-temperature cooking process, the product is transferred using top-filtered air pressure to the buffer tank. From here, it is pumped to an APV scraped-surface heat VACUUM

VACUUM BATCH PROCESS

VAPOR EVAPORATOR

METERS CORN SYRUP SUGAR SYRUP PECTIN

COOLING WATER

FRUIT

METERING PUMP

PREMIX VESSEL

FIGURE 6.3 Processing details.

VACUUM BOILING PAN

BUFFER TANK

PUMP

FILLER

SCRAPED SURFACE HEAT EXCHANGER

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exchanger flash-pasteurizing plant prior to filling. This stage is necessary because the low-temperature vacuum evaporation may not be effective in killing any spoilage microorganisms present in the ingredients. The rapid pasteurization usually takes place at 85 to 95∞C (185 to 203∞F), depending on the TS and acidity of the recipe. Pasteurizing is under pressure to avoid any volatile loss and, after a short hold time, is followed by rapid cooling to the required filling temperature. This process may also be adapted for use during the fruit harvesting period for the pasteurization of fruit pulp or fruit in syrup, which can then be filled into aseptic tanks or containers for later use in recipes. While the two systems described above may not be suitable for every circumstance, they have proved effective for a wide range of applications throughout the world. As mentioned at the beginning, it is important to carefully analyze the requirements and then select the most appropriate process accordingly.

6.3.4 HIGH-PRESSURE-TREATED FRUIT PRESERVES

AND JAM

Jam preparation using a new method (e.g., the application of high pressures) has been studied extensively in Japan in recent years. In a study by Horie et al. (1991), the use of high pressure (4000 to 6000 kg/cm2) resulted in a fresh fruit jam of superior color and flavor. In addition, vitamin C retention of fresh strawberries was 95% in pressure-treated jam. The authors reported that these pressure levels resulted in reduced levels of Zygosaccharomyces rouxii, Saccharomyces cerevisiae, Staphylococcus and Salmonella. In addition, a taste panel preferred the high-pressure-treated jam over the heat-processed product. Due to the presence of residual enzymes, refrigeration of the jam is required. Pressure-treated jam and preserves are commercially available in Japan and offer the consumer a more colorful, flavorful, and perhaps nutritionally sound product.

6.4 FEDERAL STANDARDS U.S. federal standards and definitions do not differentiate between preserves and jams. A preserve is minimally 45 parts prepared fruit with 55 parts of sugar and is concentrated to 65% or higher solids, resulting in a semisolid product. Jellies are similar to preserves, with 45 parts of clarified fruit juice and 55 parts of sugar, resulting in a minimum of 65% solids. Both categories can utilize a maximum of 25% corn syrup for sweetness, as well as pectin and acid to achieve the gelling texture required. Fruit butters are prepared from mixtures containing not less than 5 parts by weight of fruit to 2 parts of sugar. The following is the 1994 U.S. Food and Drug Administration standard of identity for fruit preserves and jams (21 CFR 150.160).

6.4.1 21 CFR 150.160 FRUIT PRESERVES

AND JAMS

(a) The preserves or jams for which definitions and standards of identity are prescribed by this section are the viscous or semisolid foods, each of which is made from a mixture composed of one or a permitted combination of the fruit ingredients specified in paragraph (b) of this section and one or any combination of the optional ingredients specified in paragraph (c) of this section which meets the specifications in paragraph (d) of this section, and which is labeled in accordance with paragraph (e) of this section. Such mixture, with or without added water, is concentrated with or without heat. The volatile flavoring material from such mixture may be captured during concentration, separately concentrated, and added back to any such mixture, together with any concentrated essence accompanying any optional fruit ingredient. (b)(1) The fruit ingredients referred to in paragraph (a) of this section are the following mature, properly prepared fruits that are fresh, concentrated, frozen, and/or canned:

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Group I: Blackberry (other than dewberry), black raspberry, blueberry, boysenberry, cherry, crabapple, dewberry (other than boysenberry, loganberry, and youngberry), elderberry, grape, grapefruit, huckleberry, loganberry, orange, pineapple, raspberry, red raspberry, rhubarb, strawberry, tangerine, tomato, yellow tomato, youngberry Group II: Apricot, cranberry, damson, damson plum, fig, gooseberry, greengage, green-gage plum, guava, nectarine, peach, pear, plum (other than greengage plum and damson plum), quince, red currant, currant (other than black currant) (2) The following combinations of fruit ingredients may be used: (i) Any combination of two, three, four, or five of such fruits in which the weight of each is not less than one fifth of the weight of the combination; except that the weight of pineapple may be not less than one tenth of the weight of the combination. (ii) Any combination of apple and one, two, three, or four of such fruits in which the weight of each is not less than one fifth and the weight of apple is not more than one half of the weight of the combination; except that the weight of pineapple may be not less than one tenth of the weight of the combination. In any combination of two, three, four, or five fruits, each such fruit is an optional ingredient. For the purposes of this section the word “fruit” includes the vegetables specified in this paragraph. (c) The following safe and suitable optional ingredients may be used: (1) Nutritive carbohydrate sweeteners (2) Spice (3) Acidifying agents (4) Pectin, in a quantity which reasonably compensates for deficiency, if any, of the natural pectin content of the fruit ingredient (5) Buffering agents (6) Preservatives (7) Antifoaming agents, except those derived from animal fat (d) For the purposes of this section: (1) The mixture referred to in paragraph (a) of this section shall be composed of not less than: (i) In the case of a fruit ingredient consisting of a Group I fruit or a permitted combination exclusively of Group I fruits, 47 parts by weight of the fruit ingredient to each 55 parts by weight of the saccharine ingredient, and (ii) in all other cases, 45 parts by weight of the fruit ingredient to each 55 parts by weight of the saccharine ingredient. The weight of the fruit ingredient shall be determined in accordance with paragraph (d)(2) of this section, and the weight of the saccharine ingredient shall be determined in accordance with paragraph (d)(5) of this section. (2) Any requirement with respect to the weight of any fruit, combination of fruits, or fruit ingredient means: (i) The weight of fruit exclusive of the weight of any sugar, water, or other substance added for any processing or packing or canning, or otherwise added to such fruit. (ii) In the case of fruit prepared by the removal, in whole or in part, of pits, seeds, skins, cores, or other parts; the weight of such fruit, exclusive of the weight of all such substances removed therefrom. (iii) In the cases of apricots, cherries, grapes, nectarines, peaches, and all varieties of plums, whether or not pits and seeds are removed therefrom, the weight of such fruit, exclusive of the weight of such pits and seeds. (iv) In the case of concentrated fruit, the weight of the properly prepared fresh fruit used to produce such concentrated fruit. (3) The term “concentrated fruit” means a concentrate made from the properly prepared edible portion of mature fresh or frozen fruits by removal of moisture with or without the use of heat or vacuum, but not to the point of drying. Such concentrate is canned or frozen without the addition of sugar or other sweetening agents and is identified to show or permit the calculation of the weight of the properly prepared fresh fruit used to produce any given quantity of such concentrate. The volatile flavoring material or essence from such fruits may be captured during concentration and

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separately concentrated for subsequent addition to the concentrated fruit either directly or during manufacture of the preserve or jam, in the original proportions present in the fruit. (4) The weight of any optional saccharine ingredient means the weight of the solids of such ingredient. (5) The soluble-solids content of the finished jam or preserve is not less than 65%, as determined by the method prescribed in Official Methods of Analysis of the Association of Official Analytical Chemists, 13th ed. (1980), section 22.024, under “Soluble Solids by Refractometer in Fresh and Canned Fruits, Jellies, Marmalades, and Preserves — Official Final Action,” which is incorporated by reference, except that no correction is made for water-insoluble solids. Copies may be obtained from the Association of Official Analytical Chemists, 2200 Wilson Blvd., Suite 400, Arlington, VA 22201-3301, or may be examined at the Office of the Federal Register, 1100 L St. NW, Washington, D.C. 20408. (e)(1) The name of each preserve or jam for which a definition and standard of identity is prescribed by this section is as follows: (i) If the fruit ingredient is a single fruit, the name is “Preserve” or “Jam,” preceded or followed by the name or synonym whereby such fruit is designated in paragraph (b) of this section. (ii) If the fruit ingredient is a combination of two, three, four, or five fruits, the name is “Preserve” or “Jam” preceded or followed by the words “Mixed fruit” or by the names or synonyms whereby such fruits are designated in paragraph (b) of this section, in the order of predominance, if any, of the weights of such fruits in the combination. (2) Each of the optional ingredients specified in paragraphs (b) and (c) of this section shall be declared on the label as required by the applicable sections of Part 101 of this chapter, except that: (i) The name(s) of the fruit or fruits used may be declared without specifying the particular form of the fruit or fruits used. (ii) When the optional ingredients listed in paragraphs (c) (3), (4), and (5) of this section are declared on the label, the declaration may be followed by the statement “used as needed” on all preserves or jams to which they are customarily, but not always, added to compensate for natural variations in the fruit ingredients used.

6.5 PHYTONUTRIENT PROPERTIES OF FRUIT PRESERVES AND JAMS Fruits are widely revered for their macronutrient properties; indeed, they serve as a primary source of vitamins and minerals in the diets of many people. Recent attention has focused on the phytonutrients naturally present in many fruits. These compounds include flavonoids (including polyphenolics and anthocyanins), carotenoids, and other antioxidant compounds that epidemiological studies show play a role in prevention of cancer and heart disease. Many of these nutritional components are also responsible for the color of fruit crops; for example, the flavonoids may be yellow, red, blue, and purple colors whereas carotenoids are typically yellow, orange, and red. Jams have been determined to contain flavonoid profiles similar to those of the natural fruit. Tomas-Lorente et al. (1992) analyzed the phenolic compounds in a number of commercial jams as a means of determining the authenticity of the fruit jam. They compared between three and six commercial preparations of apricot, peach, plum, strawberry, sour orange, apple, and pear jam for both total phenolics and specific phenolic compounds. It was interesting to note that, even when there were significant differences in total phenolics content, a single pattern of individual phenolics was determined. The total phenolic content was influenced by cultivar of fruit, maturity stage of fruit, and the industrial process utilized.

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REFERENCES Ahmed, G.E. 1981. High methoxyl pectins and their uses in jam manufacture — A literature survey. The British Manufacturing Industries Research Association, Scientific and Technical Surveys (127):July. Anon. 1983. Jams and jellies. Institutional Distribution, 19(12): 218, 222–224. Aravantinos-Zafiris, G. and Oreopoulou, V. 1992. The effect of nitric acid extraction variables on orange pectin. J. Sci. Food Agric., 60: 127–129. Axelos, M.A.V. and Thibault, J.F. 1991. The chemistry of low-methoxyl pectin gelation. In The Chemistry and Technology of Pectin. R.H. Walter (Ed.), Academic Press, New York, pp. 109–118. Baker, R.A. 1979. Clarifying properties of pectin fractions separated by ester content, J. Agric. Food Chem., 27: 1387–1389. Brosio, E., Delfini, M., Di Nola, A., D’Ubaldo, A., and Lintas, C. 1993. 1H and 23Na NMR relaxation times study of pectin solutions and gels. Cell. Mol. Biol., 39: 583-588. Chang, K.C. and Miyamoto, A. 1992. Gelling characteristics of pectin from sunflower head residues. J. Food Sci., 57: 1435–1438, 1443. Colquhoun, I.J., de Ruiter, G.A., Schols, H.A., and Voragen, A.G.J. 1990. Identification by n.m.r. spectroscopy of oligosaccharides obtained by treatment of the hairy regions of apple pectin with rhamnogalacturonase. Carbohydr. Res., 206: 131–144. Crandall, P.G. and Wicker, L. 1986. Pectin internal gel strength: Theory, measurement and methodology. ACS Symposium Series, 310: 88-102, American Chemical Society, Washington, D.C. de Vries, J.A., Voragen, A.G.J., Rombouts, F.M., and Pilnik, W. 1986. Structural studies of apple pectin with pectolytic enzymes. ACS Symposium Series, 310:38-48, American Chemical Society, Washington, D.C. de Vries, J. 1988. Repeating units in the structure of pectin. In Gums and Stabilizers for the Food Industry. G.O. Phillips, P.A. Williams, and D.J. Wedlock (Eds.), IRL Press, Washington, D.C., pp. 25–29. Fishman, M.L., Gillespie, D.T., Sondey, S.M., and El-Atawy, Y.S. 1991. Intrinsic viscosity and molecular weight of pectin components. Carbohydr. Res., 215: 91–104. Fishman, M.L., Cooke, P., Levaj, B., Gillespie, D.T., Sondey, S.M., and Scorza, R. 1992. Pectin microgels and their subunit structure. Arch. Biochem. Biophys., 294: 253–260. Garnier, C., Axelos, M.A.V., and Thibault, J.-F. 1993. Dynamic viscoelasticity and thermal behaviour of pectincalcium gels. Food Hydrocolloid., 5(112): 105–108. Grigelmo-Miguel, N. and Martin-Belloso, O. 2000. The quality of peach jams stabilized with peach dietary fiber. Eur. Food Res. Technol., 211: 336–341. Guichard, E., lssanchou, S., Descourvieres, A., and Etievant, P. 1991. Pectic concentration, molecular weight, and degree of esterification: Influence on volatile composition and sensory characteristics of strawberry jam. J. Food Sci., 56: 1621–1627. Horie, Y., Kimura, K., Ida, M., Yosida, Y., and Ohki, K. 1991. J. Agric. Chem. Soc. Jpn., 65(6): 975–980. Kontominas, M.G. and Kokini, J.L. 1990. Measurement of molecular parameters of water soluble apple pectin using low angle laser light scattering. In Flavors and Off-Flavors: Proceedings of the 6th International Flavor Conference, Dev. Food Sci., Elsevier Scientific Publication, 24: 375–384. May, C.D. and Stainsby, G. 1986. Factors affecting pectin gelation. In Gums and Stabilisers for the Food Industry. G.O. Phillips, D.J. Wedlock, and P.A. Williams (Eds.), Elsevier Applied Science, London, pp. 515–523. Miyamoto, A. and Chang, K.C. 1992. Extraction and physicochemical characterization of pectin from sunflower head residues. J. Food Sci., 57: 1439–1443. Mort, A.J., Qiu, F., and Maness, N.O. 1993a. Determination of the pattern of methyl esterification in pectin. Distribution of contiguous nonesterified residues. Carbohydr. Res., 247: 21–35. Mort, A.J., Maness, N.O., Qiu, F., Otiko, G., An, J., West, P., and Komalavilas, P. 1993b. Extraction of defined fragments of pectin by selective cleavage of its back-bone allows new structural features to be discovered. J. Cell. Biochem., issue S17A, 9 pp. Oakenfull, D. and Scott, A. 1984. Hydrophobic interaction in the gelation of high methoxyl pectins. J. Food Sci., 49: 1093–1098. Oakenfull, D. 1987. Gelling agents. CRC Crit. Rev. Food Sci. Nutr., 26: 1–25. Rao, M.A., Van Buren, J.P., and Cooley, H.J. 1993. Rheological changes during gelation of high-methoxyl pectin/fructose dispersions: Effect of temperature and aging. J. Food Sci., 58: 173–176, 185.

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Schols, H.A., Posthumus, M.A. and Voragen, A.G.J. 1990. Structural features of hairy regions of pectins isolated from apple juice produced by the liquefaction process. Carbohydr. Res., 206:117–129. Smit, C.J.B. and Bryant, E.F. 1968. Ester content and jelly pH influences on the grade of pectin. J. Food Sci., 33: 262–264. Smit, C.J.B. and Bryant, E.F. 1969. Changes in molecular weight of pectin during methylation with diazomethane. J. Food Sci., 34: 191–193. Toman, R., Karacsonyi, S., and Kubackova, L. 1976. Studies on pectin present in the bark of white willow (Salix alba L.). Structure of the acidic and neutral oligosaccharides obtained by partial acid hydrolysis. Cellulose. Chem. Technol., 10: 561–565. Tomas-Lorente, F., Garcia-Viguera, C., Ferreres, F., and Tomas-Barberan, F.A. 1992. Phenolic compounds analysis in the determination of fruit jam genuineness. J. Agric. Food Chem., 40: 1800–1804. Watase, M. and Nishinari, K. 1993. Effects of pH and DMSO content on the thermal and rheological properties of high methoxyl pectin-water gels. Carbohydr. Polymers, 20(3): 175–181. Wiles, R.R. and Smit, C.J.B. 1971. Method for producing pectins having high resistance to breakage and high capability for gelling in the presence of calcium. U.S. Patent No. 3,622,559. Woodmansee, C.W. and Baker, G.L. 1954. The preparation of calcium pectinates and the effect of the degree of esterification on their gel properties, University of Delaware Agricultural Experiment Station Bulletin No. 305, Newark, DE.

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7 Drying of Fruits Cristina Ratti and Arun S. Mujumdar CONTENTS 7.1 7.2

Introduction ........................................................................................................................127 Drying Characteristics of Fruits ........................................................................................129 7.2.1 Properties of Fruits ..............................................................................................129 7.2.1.1 Sorptional Equilibrium ......................................................................129 7.2.1.2 Density and Thermal Properties ........................................................130 7.2.1.3 Shrinkage............................................................................................132 7.2.1.4 Mass Transfer.....................................................................................135 7.2.1.5 Dielectric Properties...........................................................................136 7.2.1.6 Radiation ............................................................................................136 7.2.2 Drying Kinetics....................................................................................................139 7.2.2.1 Air-Drying..........................................................................................139 7.2.2.2 Freeze-Drying ....................................................................................141 7.2.3 Quality Considerations ........................................................................................141 7.3 Types of Dryers..................................................................................................................142 7.3.1 Classification and Selection.................................................................................142 7.3.2 Specific Drying Systems .....................................................................................144 7.3.2.1 Conventional Hot-Air Drying ............................................................144 7.3.2.2 Solar Drying.......................................................................................146 7.3.2.3 Microwave Drying .............................................................................147 7.3.2.4 Osmotic Dehydration .........................................................................147 7.3.2.5 Explosion Puffing...............................................................................148 7.3.2.6 Freeze-Drying ....................................................................................149 7.3.3 Novel Dryers........................................................................................................150 7.4 Design Considerations .......................................................................................................150 7.4.1 Heat and Mass Balances......................................................................................150 7.4.2 Energy Aspects ....................................................................................................152 7.4.2.1 Heat Pump Dryers .............................................................................152 7.5 Concluding Remarks..........................................................................................................152 7.6 Nomenclature .....................................................................................................................153 7.6.1 Subscripts.............................................................................................................153 7.6.2 Greek Symbols ....................................................................................................154 References ......................................................................................................................................154

7.1 INTRODUCTION Two definitions of fruit (botanical and commercial) can be found in the literature. Botanically, a fruit is “a ripened ovary” usually developing as part of the plant that contains the ovules that become the seeds of the plant. This definition is frequently too strict for the idea of fruit that people normally 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

127

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FRUITS

Whole

Sliced/Chopped / Particulate

Pastes / Suspensions

1. Sun drying

1. Through circulation

1. Spray dryer

2. Solar drying in cabinets

2. Conveyor

2. Drum dryer

3. Freeze dryer

3. Solar dryer

3. Through circulation dryers

4. Tray dryer 5. Vacuum dryer

FIGURE 7.1 Various types of dryers for drying of fruit.

have; for example, it does not include apples or strawberries in its classification (Wills et al., 1981). The other definition of fruit can be stated as “the parts of a tree or bush that contain seeds and are often eaten for their usually sweet flesh” (Longman Dictionary of Contemporary English, 1990). Under this more common definition, the tomato is eaten as a vegetable because it is not sweet although it is technically a fruit because it contains seeds. Fruits are essentially dessert foods (Wills et al., 1981). Artificial drying of fruit is an important method of preservation and production of a wide variety of products. Storage technologies are now available to keep fruit fresh over extended periods of time without appreciable change in the original physical form of the fruit. Drying, on the other hand, changes the physical and biochemical form of the fruit leading to shrinkage and change of colour, texture, taste, and so on. If the water activity is reduced to appropriate levels (depending on the fruit variety and sugar content), the dried product can have a shelf life exceeding 1 year if properly packaged. Fruit may be dried as a whole (e.g., grapes, various berries, apricot, plum, etc.), in sliced form (e.g., banana, mango, papaya, kiwi, etc.), in puree form (e.g., mango, apricot, etc.), as leather, or as a powder by spray or drum drying. Depending on the physical form of the fruit (e.g., whole, paste, slices), different types of dryers must be used for drying. Figure 7.1 illustrates the wide assortment of dryers that may be found in practice for drying of fruits. Fruits are characterized by the following features, which must be taken into account when selecting the type of dryer as well as the operating conditions: 1. Very high initial moisture content 2. High temperature sensitivity (color, flavor, texture, nutritional value subject to thermal deterioration) 3. High susceptibility to microbial attack 4. High sugar content (problems of stickiness, fermentation, etc.) 5. Presence of a “skin” on some fruits that has poor permeability for water or moisture (e.g., grapes, blueberries) It is clear from the above list that the drying of fruit is necessarily a very slow process carried out under gentle drying conditions, leading to large dryers for a given throughput. Where feasible, solar drying often provides the most cost-effective drying technique although open sun drying is practiced widely in California and the tropical countries. Pretreatment of the feed to speed the drying process and use of additives to avoid biochemical damage during the extended drying period

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are often necessary. The type of treatment and additives used depends on the species of fruit being dried. No generalization can be made. The objective of this chapter is to provide a brief overview of the principles of drying and to discuss some applications in the drying of selected fruits. Some thermophysical property data useful in the design of such dryers are presented. Finally, examples of various dryers used commercially for drying of fruit are presented. For details, the reader is referred to the literature cited.

7.2 DRYING CHARACTERISTICS OF FRUITS 7.2.1 PROPERTIES

OF

FRUITS

7.2.1.1 Sorptional Equilibrium Fruits are highly hygroscopic materials. The water contained in a fruit is bound to the solid matrix such that the vapor pressure it exerts is lower than that for pure water at the same temperature. Sorptional equilibrium data for water vapor–foodstuff systems are used to describe the hygroscopic properties of a product. This information is required in numerous processes involving foods. As pointed out by Wolf et al. (1985), useful information on the sorption and desorption enthalpies, as well as microbiological stability of the product, can be obtained from the sorptional data. In drying, desorption equilibrium moisture content determines the final moisture content to which a material can be dried using the specified drying conditions. Also, such data are needed for estimation of the drying times and energy requirements (Shatadal and Jayas, 1992). The ratio of the water vapor pressure in a food material to that for pure water at the same temperature is termed water activity, aw. Sorptional equilibrium is the relationship between the water activity of a solid with its water content and temperature: aw = aw ( X , T )

(7.1)

Values of aw as functions of X (water content) and T (temperature) for various fruits and fruit powders are given in Table 7.1 and Table 7.2. A typical plot of Equation (7.1) for foodstuffs exhibits a sigmoidal shape. This plot is shown in Figure 7.2 for sorptional equilibrium of prunes at different temperatures (Maroulis et al., 1988). Usually, an increase of temperature results in a decrease of the amount of water adsorbed. The effect of temperature on isotherms can be well represented by the Clausius–Clapeyron relationship (Crapiste and Rotstein, 1986; Okos et al., 1992): DHs È ∂ ln aw ˘ Í ∂(1 / T ) ˙ = - R Î ˚

(7.2)

where Hs is the heat of sorption. A plot of the heat of sorption for selected fruits is shown in Figure 7.3 (Román et al., 1982; Kapsalis, 1987). Other factors that affect the sorption isotherms are chemical composition, physical structure, and pretreatment of the sample among others related to the way of handling and drying the product (Van der Berg, 1986). For design or simulation of processes, an empirical correlation of the sorptional data is convenient because a continuous rather than discrete representation is required to perform the calculations. A large number of empirical correlations have been proposed in the literature to represent the isotherms (Brunauer et al., 1938; Halsey, 1948; Henderson, 1952; Chung and Pfost, 1967). They require extensive experimental data at each temperature in order to evaluate the parameters in the model. The number of correlations that allow for a variation with temperature as well is much smaller. Table 7.3 summarizes some of them. There are some requirements that a good

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TABLE 7.1 Sorptional Data of Selected Fruits aw Fruit

X (%)

Apricot

7.75 14.79 24.2 60.2 10 15 20 10 15 20 6.25 9.01 24.7 6.10 12.8 15.88 26.13 52.8 92.8

Grapefruit

Peach

Persimmon

Plum

5∞C

20∞C

25∞C

30∞C

45∞C

60∞C

90∞C

0.10 0.35 0.60 0.90 0.46 0.52 0.60

Source Ginzburg and Savina (1982)

0.46 0.52 0.60 0.40 0.50 0.60

0.10 0.30 0.40 0.60 0.80 0.90

0.50 0.59 0.65 0.40 0.50 0.60 0.25 0.40 0.70

0.66 0.71 0.75

Okos et al. (1992)

0.50 0.65 0.78

Okos et al. (1992)

Ginzburg and Savina (1982) Ginzburg and Savina (1982)

correlation must achieve (Ratti et al., 1989). For example, the correlation must predict accurately the effect of water content and temperature, and give a good estimation of the partial derivatives (∂X/∂aw)T and (∂X/∂T)aw and also of the heat of sorption. Following these criteria, the correlations from Table 7.3 that represent the sorption data on fruits rather well are those proposed by Iglesias and Chirife (1976), Crapiste and Rotstein (1986) and Ratti et al. (1989). However, the Guggenheim–Anderson–de Boer (GAB) equation is the correlation the most used to represent sorption data of foodstuffs. A list of GAB coefficients for several fruits can be found in Rahman (1995). 7.2.1.2 Density and Thermal Properties The design and optimization of any process in which the transfer of heat is involved requires knowledge of the density, as well as of the thermal properties, of the materials being processed. The density of fresh fruits is typically in the range 865 to 1067 kg/m3, whereas for frozen fruits it is between 625 and 801 kg/m3 (Lewis, 1987). Some other thermophysical properties of fresh fruits are presented in Table 7.4. It is important to note that the product composition, as well as temperature, changes during drying. In addition, fruits being materials of biological origin, their chemical compositions are not fixed but vary with parameters like variety, maturity, or the location where they are grown. Figure 7.4 shows the specific heat and thermal conductivity of selected fruits as a function of water content (Ratti, 1991; Lozano et al., 1979). Density and specific heat of apple juice (Constenla et al., 1989) as a function of concentration and temperature, are presented in Figure 7.5. Several literature sources present empirical equations to model these properties (Heldman, 1975; Choi and Okos, 1986; Singh and Mannapperuma, 1990; Singh, 1992). The most common equations are those that describe the properties of the material in terms of their component fractions and the properties of individual components. The effect of change on the property when there is a

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TABLE 7.2 Sorptional Data of Selected Fruit Powders aw Fruit

X (%)

Banana

2.47 5.50 15.62 26.89 34.93 2.06 6.78 19.54 29.34 56.17 2.13 6.82 17.00 29.42 49.20 2.18 6.81 15.34 29.23 45.73 0.09 0.34 2.22 6.82 23.0 0.42 1.08 4.13 17.94 44.35 3.53 7.64 23.57 33.69 70.91 3.68 7.75 18.87 34.03 64.13 2.45 7.41 17.82 34.45 55.52

Blueberry

Mango

Strawberry

5∞C

15∞C

25∞C

37∞C

0.10 0.30 0.60 0.80 0.90

Source Siddappa and Nanjundaswamy (1960)

0.11 0.34 0.65 0.76 0.88

Khalloufi et al. (2000a)

0.11 0.33 0.61 0.76 0.86 0.11 0.33 0.58 0.75 0.84 0.10 0.30 0.70 0.80 0.90

Siddappa and Nanjundaswamy (1960)

0.32 0.54 0.75 0.79 0.91 0.11 0.34 0.65 0.76 0.88

Khalloufi et al. (2000a)

0.11 0.33 0.61 0.76 0.86 0.11 0.33 0.58 0.75 0.84

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80 15 C 70 30 C

X (dry basis, percent)

60

45 C

50

60 C

40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

aw

FIGURE 7.2 Sorptional equilibrium of prunes at different temperatures. (From Maroulis, Z.B., E. Tsami, D. Marinos-Kouris, and G.D. Saravacos. 1988. Application of the GAB model to the moisture sorption isotherms for dried fruits. J. Food Eng., 7, 63–78.) 80 75 Pinneaple Banana Apple Grapefruit Water

∆Hs (kJ/mol)

70 65 60 55 50 45 40 0

5

10

15

20

25

30

35

X (dry basis, percent)

FIGURE 7.3 Heat of sorption of selected fruits. (From Román, G.N., Rotstein, E., and Urbicain, M.J. 1982. Kinetics of water vapor desorption from apples. J. Food Sci. 44(1), 193–197; Kapsalis, J.G. 1987. Influences of hysteresis and temperature on moisture sorption isotherms. In L.B. Rockland and L.R. Beuchat (Eds.), Water Activity: Theory and Applications to Food, IFT Basic Symposium Series, Marcel Dekker New York, pp. 173–214.)

phase change is taken into account in these models. The reader may refer to the references cited for more details. 7.2.1.3 Shrinkage Considerable changes in the physical structure of the product, such as reduction in volume and decrease in internal porosity can be found during drying (Krokida and Maroulis, 1997; Jankovié,

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TABLE 7.3 Correlations for Sorptional Equilibrium Source

Equation

Ratti et al. (1989) Crapiste and Rotstein (1986) GAB, Guggenheim (1966) Iglesias and Chirife (1976) Pfost et al. (1976) Thompson (1972)

ln aw = -k1X-k2 + k3 exp(-k4X)Xk5 ln pwo ln aw = -k1(1/T - 1/k2)w-k3 exp(-k4w) X/Xm = ckaw/(1 - kaw)(1 - kaw + ckaw) ln aw = -exp(k1 + k2T)Xk3 ln aw = -(k1/R(T + k2))exp(-k3X) ln (1 - aw) = -k1(T + k2)Xk3

TABLE 7.4 Thermal Properties of Selected Fresh Fruits Product

Ta (∞C)

ktb

Cpafc

Cpbfc

Apple Apricot Grapefruit Orange Pineapple Plum Strawberry

2 to 36 — 30 30 — — -14 to 25

0.393 — 0.450 0.431 0.549 0.551 0.675

3.651 3.684 3.818 3.751 3.684 3.718 3.852

1.892 1.905 1.955 1.930 1.905 1.918 1.968

a

Range of temperature for kt. kt is given in [W/m K]. Source: From Singh, R.P. 1992. Heating and cooling processes for foods. In D.R. Heldman and D.B. Lund (Eds.), Handbook of Food Engineering, Marcel Dekker, New York, pp. 247–255; Mohsenin, N.N. 1980. Thermal Properties of Foods and Agricultural Materials, Gordon and Breach, New York, pp. 161. c Cp is given in [J/kg ∞C]. The subscripts af and bf mean above and below freezing. Source: From Rao, M.A. 1992. Transport and storage of food products. In D.R. Heldman and D.B. Lund (Eds.) Handbook of Food Engineering, Marcel Dekker, New York, pp. 225–245. b

1993; Ratti, 1994). Shape and size changes during drying modify both dimensions and transport properties of individual particles and also thickness and porosity of the packed bed in the dryer. Volume changes of individual particles are usually expressed as a bulk shrinkage ratio of sample volume at any time to initial volume, (V/Vo). Experimental data on shrinkage of fruits during airdrying reported in previous studies (Kilpatrick et al., 1955; Lazar and Farkas, 1971; Lozano et al., 1980, 1983) showed shrinkage of different foods only as a function of total water content. In practice, however, this phenomenon is also dependent on drying conditions (Van Arsdel, 1963; Ratti, 1994). There are few reported models for shrinkage (Kilpatrick et al., 1955; Suzuki et al., 1976; Lozano et al., 1980, 1983; Ratti, 1994) and most of them fit the data only under specific operating conditions. Constant porosity and no shrinkage are often stated as key assumptions in the model for dryer design. However, to properly predict the drying time and average drying rates, it is necessary to know the relationship between porosity and average water content of the bed of particles. Although scarce, some experimental data on changes in bed volume and porosity of a packed bed during airdrying of pieced fruits, and as a function of average water content, can be found in the literature (Ratti, 1994). Another parameter also important in the design of dryers is the particle surface

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4.5 4

Cp (J/g°C) or Kt (mW/cm K)

3.5 3 2.5 2 1.5

Cp, apple Cp, peach k (apple, 22°C) k (apple, 80°C)

1 0.5 0 0

2

4

6

10

8

12

14

X (dry basis)

FIGURE 7.4 Thermal properties of apple and peach. (From Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina; Lozano, J.E., Urbicain, M.J., and Rotstein, E. 1979. Thermal conductivity of apples as a function of moisture content. J. Food Sci., 44(1), 198–199.) Cp 䡲 30°C ● 50°C ⽧70°C 多 90°C ρ ▫ 20°C 䡩 40°C 〫60°C 夞 80°C 1.4

1.1 ▫

1.2

多 ⽧ ● 䡲

多 ⽧ ● 䡲

1.1

1

0.9 0

多 ⽧ ● 䡲

▫

▫ 䡩 〫 夞

䡩〫 夞

20

▫ 䡩 〫 夞

多 ⽧ ● 䡲

▫ 多 〫 䡩 ⽧ 夞 多 ● 䡲 ⽧ 多 ▫ ⽧ 䡩 ● 䡲 〫 夞 ● 䡲

40

▫ 䡩 〫 夞

1

0.9

多 ⽧ ● 多 䡲 ⽧ 多 ⽧ ● 䡲 ● 䡲

60

0.8

多 ⽧ ● 䡲

Cp (cal /g°C)

Density (g/cm3)

1.3

䡩 〫 夞

0.7

0.6 80

Concentration (°Brix)

FIGURE 7.5 Density and specific heat of apple juice. (From Constenla, D.T., Lozano, J.E., and Crapiste, G.H. 1989. Thermophysical properties of clarified apple juice as a function of concentration and temperature. J. Food Sci., 54(3), 663–668.)

area-to-volume ratio (av ). This parameter has been shown to be practically independent of the drying conditions but to be dependant on the sample geometry and type of foodstuff (Ratti, 1994). Figure 7.6 presents experimental data for av related to the initial value for slices and cylinders of apples. The solid state of water during freeze-drying protects the primary structure and the shape of the products with minimal reduction of volume. Less than 10% of shrinkage is normally expected in final products due to freeze-drying as compared to 80 to 90% when air-dried. Volumetric changes due to freeze-drying have been quantified as 8% for whole strawberries and less than 2% for slices

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3 2.8 Slices Cylinders

2.6 2.4 av /avo

2.2 2 1.8 1.6 1.4 1.2 1 0

0.2

0.4

0.6

0.8

1

X /Xo

FIGURE 7.6 Area/Volume ratio for slices and cylinders of apple. (From Ratti, C. 1994. Shrinkage during drying of foodstuffs. J. Food Eng., 23(1), 91–105.)

(Shishehgarha et al., 2002). These results are similar to those obtained by Jancovié (1993) for raspberries and blackberries. Although volume reduction is not usually pronounced, in freeze and spray drying of high-sugar content foodstuffs such as fruits or fruit juices, collapse is a frequent problem if operational variables are not well set (Roos, 1995; Slade and Levine, 1991). This phenomenon occurs when the solid matrix of the foodstuff can no longer support its own weight, leading to drastic structural changes shown as a marked decrease in volume, increase in stickiness of dry powders, loss of porosity, and change in color, etc. (Chuy and Labuza, 1994; Levi and Karel, 1995; Roos, 1995). In the case of air-drying, volume reduction is usually accompanied by wrinkles, deformation, and even change in color, indicating a certain degree of collapse in air-dried products. 7.2.1.4 Mass Transfer Diffusion coefficients are often used to characterize mass transfer during drying. A rigorous and realistic approach to the phenomena taking place in biological multiphase materials (e.g., fruits) during air-drying was developed by Crapiste et al. (1988). This theory was defined in terms of a reduced dimensionless coordinate that follows the movement of the nonaqueous material due to shrinkage. On this basis, the water transport equation is given by ∂W * 1 ∂ È ∂W * ˘ = 2 D ∂t Lo ∂Z ÍÎ r ∂Z ˙˚

(7.3)

where Dr is effective reduced diffusivity, a function of temperature, water content, shrinkage, and also of the water transport mechanism within the material. The evaluation of Dr becomes extremely complex if the effect of all variables is to be taken into account. In addition, the experimental measurement of Dr is almost impossible due to the lack of reliable instrumentation required to measure the water content profiles inside the solid. A mathematical model must be used to determine Dr . A simple but very useful way to estimate an approximate value of Dr is to suppose this property to be constant over a certain range of water content, so within that range, Equation (7.3) can be written as

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∂W * Dr ∂ 2 W * = 2 ∂t Lo ∂Z 2

(7.4)

The solution of Equation (7.4) for internal mass transfer, planar geometry, and uniform initial concentration can be found in Crank (1967):

W - We 8 = 2 1 - We p

•

 i=0

È D (2i + 1) 2 p 2 t ˘ exp Í- r ˙ 4 L2o Î ˚ (2i + 1) 2

(7.5)

For long drying times, the first term of the series is adequate to represent the series, so Equation (7.5) is reduced to È - D p 2t ˘ W - We 8 = 2 exp Í r 2 ˙ 1 - We p Î 4 Lo ˚

(7.6)

The effective diffusivity can be obtained from experimental drying data through a linear regression of the linearized form of Equation (7.6). Table 7.5 shows a compilation of effective diffusivity values for selected fruits. The same table also indicates the water content ranges over which the reported values were obtained. The functionality of Dr with temperature can be represented through the well-known Arrhenius relationship DE ˘ Dr = Dro exp ÈÍÎ RT ˚˙

(7.7)

where DE is called activation energy for diffusion. Typical values of DE for different fruits are given in Table 7.6. 7.2.1.5 Dielectric Properties Moist solids such as foodstuffs are classified as “lossy dielectric materials” (Schiffmann, 1987). Such materials absorb electromagnetic energy and convert it into heat. There are two properties that characterize dielectric heating: the relative dielectric constant e¢ and the loss factor e≤. The loss tangent or dissipation factor is then defined as tan d =

e ¢¢ e¢

(7.8)

Table 7.7 shows a compilation of these dielectric properties for selected fruits (Nelson, 1983, 1980; Mohsenin, 1984). Details can be found in the literature cited. Values of the loss factors are important if dielectric (microwave or radio frequency) heating is used to enhance drying rates. This is not always desirable in view of its potentially negative effects on product quality. 7.2.1.6 Radiation Specifically for infrared (IR) drying, knowledge of the material properties may be the clue to accomplish a safe and efficient process because radiation properties of both the radiator and the

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TABLE 7.5 Effective Diffusivity of Selected Fruits Product

Temp (∞C)

Apple: McIntosh Granny S.

Red Delicious

Avocado

Banana chip Coconut albumen

Pear Raisin

66 30 30 71 76 40 40 51 51 61 61 31 43 56 58 58 58 25 45 60 80 100 66 25

Water Content

Dr(m2/s)

Reference

— X > 0.15 X < 0.15 — — Dry zone Wet zone Dry zone Wet zone Dry zone Wet zone 14.7% 14.7% 14.7% 4.7% 7.9% 9.2% 0.0096–0.0102 0.6 (initial) 0.6 (initial) 0.6 (initial) 0.6 (initial) 6.5 (initial) 0.27 (initial)

1.1 e-9 2.6 e-10 4.9 e-11 1.6 e-9 3.6 e-9 1.9 e-10 9.4 e-10 2.2 e-10 1.3 e-9 5.4 e-10 1.7 e-9 1.1 e-10 2.1 e-10 3.3 e-10 1.8 e-9 1.3 e-9 1.2 e-9 2.911 e-11 4.6 e-11 7.7 e-11 10.4 e-11 11.8 e-11 9.63 e-10 4.167 e-11

Labuza and Simon (1970) Rotstein and Cornish (1978) Román et al. (1979) Alzamora (1979) Ratti (1991)

Alzamora et al. (1979)

Alzamora (1979)

Hong et al. (1986) Bimbenet et al. (1985)

Saravacos and Charm (1962) Lomauro et al. (1985)

TABLE 7.6 Activation Energy for Diffusion in Selected Fruits Product Apple: Red Delicious Apricot Avocado Coconut albumen Grapes

Composition

Activation Energy (kJ/mol)

Dry zone Wet zone —

42.26 24.4 22–26

— X > 0.4 X < 0.4 —

39.71 13.0 33.9 43.0

Reference

Ratti (1991) Berna et al. (1990); Abdelhaq and Labuza (1987) Alzamora et al. (1979) Bimbenet et al. (1985) Berna et al. (1991)

material to be dried must be matched in order to obtain the most efficient results (Ratti and Mujumdar, 1995). Again, quality parameters determine the suitability of radiant heating. Emissivity (e), absorptivity (a), reflectivity (r), and transmissivity (t) are the four key radiative properties of a material. The relative magnitudes of a, r, and t depend not only on the material, its thickness, and surface finish, but also on the wavelength of the radiation applied (Kreith, 1965). The emission of electromagnetic waves is a material property.

Pear

—

—

79.7

Pineapple pieces

23 23 23 23 50 75 100 125 50 75 100 125 -3.9 -6.7 -9.4 -15.0 -3.9 -9.4 -15.0

T (∞C)

—

—

—

—

0.840 0.944 0.847 0.912

7.48 3.22 4.10 1.77 13.6

3.0 1.97 0.50 0.19 4.56

9.8 3.84 5.79 2.58 11.2 5.9

3.56 0.82 0.61 0.29 5.85 2.94

e’’

1.5 GHz e’

e’’

1.0 GHz e’

4.56 3.14 10.4 5.70 4.17

8.45 0.85 0.38 6.0 2.30 1.12

4.22

e’’

2.0 GHz e’

58.3 58.9 55.2

e’

12.7 15.2 13.6

e’’

2.45 GHz

43

e’ 9

e’’

2.5 GHz

Dielectric properties

54.1 53.5 52.0 49.9 62.0 56.5 51.4 46.0

15.7 15.2 15.7 16.9 11.0 8.9 8.5 8.4

e’’

2.8 GHz e’

49.4 50.4 50.4

e’

5.2 2.6 3.7

e’’

11.7 GHz

49.9 49.9 49.1

e’

1.8 1.3 1.2

e’’

22.0 GHz

138

Peach

57.6

86 88 91 92

w%

Apple Peach Watermelon Cantaloupe Concentrated orange juice

Product

Tissue Density (g/cm3)

TABLE 7.7 Dielectric Properties of Several Fruits

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The material to be dried by IR should have low reflectivity in order to minimize the power required to heat it. In drying thick moist materials such as foodstuffs, it is necessary to have a reasonable level of transmissivity in order to avoid excessive heating and thermal damage to the product surface. It is important to point out that, if the absorptivity of a material is low, its transmissivity is high and vice versa. Properties like absorptivity and transmissivity of moist materials are not readily found in the literature. In addition to the dependency with wavelength and thickness, these properties also depend on the water content. One of the most extensive reports on experimental values of these properties for foodstuffs can be found in Ginzburg (1969). The variation of absorptivity of moist materials with wavelength is difficult to estimate without experimental data. Foodstuffs, for example, are complex mixtures of different large biochemical molecules and polymers, inorganic salts, and water (Sandu, 1986), and the absorption bands of each of these constituents are not the same. Generally, many fully wet materials have their minimum absorptivity at those wavelengths where water has its maximum transmissivity; this points out the important role that water plays in radiation absorption. For many materials, transmissivity is generally higher at lower wavelengths (Ginzburg, 1969). As drying proceeds, the material being dried undergoes a change in its radiation properties, increasing its reflectivity and, consequently, lowering its absorptivity at low water contents. It is then possible to adequately change the temperature of the emitter in order to improve the absorption of radiation during drying. Further, transmissivity decreases with increase of layer thickness, while absorptivity is increased. An approximate way of representing experimental transmissivity data as a function of thickness is presented by Ginzburg (1969). A compilation of experimental data about the variation of transmissivity of foodstuffs with thickness, maturity, water content, and wavelength can be found in Mohsenin (1984).

7.2.2 DRYING KINETICS 7.2.2.1 Air-Drying Air-drying curves generally have two well-defined periods: constant rate period and falling rate one. The constant rate period is characterized by an almost free water evaporation from the surface of the solid and may be predicted by the following well-known heat and mass transfer relationships: nw = kg( pws - pw• ) ms (1 + X )C p

dT = hg As (Tg - T ) - nw As DHs dt

(7.9)

(7.10)

Note that the classical drying curve is obtained under constant drying conditions. On the other hand, the falling rate period is an extremely complex phenomena and it is not well understood yet where the control for mass transfer occurs within the particle. The constant rate period is governed by external heat and mass transfer rates and is thus not dependent on the material being dried. Drying of food particles is a complex problem involving simultaneous mass and energy transport in a hygroscopic, shrinking system. In general, the constant rate period does not appear in the drying of these materials, and in addition, the equations that represent the mass transfer during the falling rate period demand additional calculations if the effect of shrinkage and the dependence of diffusivity with water content and temperature are to be taken into account. The heat transfer rate is not appreciably affected by the change in mass transfer, and since the Biot number for heat transfer generally remains small during the whole drying process (Bruin and Luyben, 1980), Equation (7.10) is still valid for the falling rate period.

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Drying rate, -dX/dt (kg/(kg sec))

1.7E-04

0.5 m/sec

1.5E-04

1 m/sec

1.3E-04

2 m/sec

1.1E-04 9.0E-05 7.0E-05 5.0E-05 3.0E-05 1.0E-05 0

5

10

15

20

25

30

X (kg water/kg dry matter)

FIGURE 7.7 Drying of prunes at different air velocities. (From Bimbenet, J.J., Daudin, J.D., and Wolf, E. 1985. Air Drying Kinetics of Biological Materials. In A.S. Mujumdar (Ed.), Drying ’85, Hemisphere, New York, pp. 178–185.)

Figure 7.7 shows the effect of air velocity on the drying rate of prunes (Bimbenet et al., 1985). The effect of temperature on the drying kinetic of slices of apple is presented in Figure 7.8 (Ratti, 1991). In summary, the influence of external factors is as follows: An increase in temperature or velocity or a decrease in air humidity results in an increase in the drying rate. As the dimensions of the particle increase, the drying rate decreases. Another characteristic of the drying curves is the initial warming-up period that occurs when the temperature of the solid at the start of the process is lower than the dew point temperature of the air-water vapor mixture; therefore, condensation of water vapor occurs upon initial exposures. Fick’s diffusion law is commonly used to represent the drying kinetics in the falling rate period; however, this simple model is not always adequate to represent the complex process of drying. Any simple theory idealizes the process and generally cannot be used safely in the design of dryers (Keey, 1980). On the other hand, there are some complex theories that represent the drying process from the microscopic standpoint of mass and heat transfer between each phase inside the food particles (Gekas, 1992; Crapiste et al., 1988). This careful approach, which uses the volume 1

X /Xo

0.9 0.8

61°C

0.7

51°C

0.6

40°C

0.5 0.4 0.3 0.2 0.1 0 0

100

200

300

400

500

t (min)

FIGURE 7.8 Drying kinetics of slices of apple. (From Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina.)

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averaging method to solve the governing transport equations, is too complex for practical use. For practical purposes, it is often useful to use a lumped-parameter model supported by carefully designed experimentation at laboratory scale. The so-called characteristic drying curve (Van Meel, 1958) is a semiempirical approach, which minimizes the experimental data required to evaluate a kinetic drying model. This effectively allows scale-up of kinetic data to other operating conditions than those at which the original data were taken. This theory defines a characteristic function f that is independent of drying conditions but is dependant on the product and water content: nw = fnwc

(7.11)

Numerous simplifying assumptions are implicit in the definition of Equation (7.11). Application of Equation (11) to food systems presents several problems because it is derived for nonhygroscopic, nonshrinking systems. A new model is needed for vegetables and fruits (Ratti and Crapiste, 1992). This model is represented by the following equation: nw =

kg[aw pws - pw• ] [1 + (F / Xo ) Bimrs ]

(7.12)

in which the parameter F must be obtained empirically. This parameter was shown theoretically and experimentally to be independent of drying conditions and particle geometry, and in the case of apple, potato, and carrot, the results of the regression yield (Ratti and Crapiste, 1992): F = 5.320e - 3( X / Xo ) -1.079

(7.13)

7.2.2.2 Freeze-Drying Freeze-drying is a process by which the product is first frozen and then the ice from the frozen material is removed by sublimation, usually under conditions of low pressure and temperature. The sublimation leaves place to a dry porous layer that continuously recedes during the process. Several theoretical models that can be used to represent food freeze-drying kinetics can be found in the literature (Lombraña et al., 1997; Lombraña and Izkara, 1996; Liapis and Bruttini, 1995a; Mellor, 1978; Karel, 1975). However, in most cases adjustable parameters are needed to match the model predictions with experimental data (Sheehan and Liapis, 1998; Sadikoglu and Liapis, 1997; Sharma and Arora, 1995; Millman et al., 1985). In other cases, no comparison with experimental data is presented (Liapis and Bruttini, 1995b; Nastaj, 1991). Khalloufi et al. (1999) have recently developed a model that has proved valuable in predicting experimental freeze-drying kinetics data for fruits and vegetables.

7.2.3 QUALITY CONSIDERATIONS Special attention on quality is required in processes dealing with foodstuffs. As mentioned by Karel (1991), the quality of a dehydrated product depends strongly on the quality of the raw materials, but their processing and storage must be carried out, minimizing any undesirable change in quality. Figure 7.9 shows that the required quality characteristics of a foodstuff may be different, depending on the end use of the product. The quality characteristics may be separated into two major groups (Salunkhe et al., 1991): sensory and hidden. Color, gloss, size, shape, defects, texture, flavor, and aroma belong to the former group, and the nutritive value and the presence of adulterants and toxic components belong to the latter group. Attention has recently been focused on degradation of nutraceutical compounds

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ease of cultivation

GROWER

size

disease resistance

yield color texture

PROCESSOR

flavor

CONSUMER

nutritive value presence of toxins

FIGURE 7.9 Quality characteristics of foodstuffs.

during drying. Such is the case for tomato drying, in which the deterioration of lycopene has been extensively studied (Zanoni et al., 1999; Shi et al., 1999; Inakuma et al., 1998; Yoon et al., 1997). The whole process of drying involves also the treatment of foodstuffs before and after dehydration — for example, transport, peeling, cutting, and storage — so quality restrictions have to be taken into account in all these sub-processes. The important changes occurring in a foodstuff due to dehydration are (Bruin and Luyben, 1980) the influence of drying on microorganisms, enzymatic and chemical conversions, and physical changes. The estimation of these changes as a function of the wide, diverse range of conditions that can be expected for individual foods is extremely complicated (Karel and Flink, 1983). The application of the knowledge learned in the area of glass transition of polymers to food systems succeded in improving understanding and predicting the behavior of foodstuffs during processing (Schenz, 1995). Glass transition temperature, Tg, can be defined as the temperature at which an amorphous system changes from the glassy to the rubbery state (Karmas et al., 1992; Roos and Karel, 1991b). Recently, Tg of food products has been pointed out to be responsible for the deterioration mechanisms during processing, and an indicator of food stability (Chirife and Buera, 1995; Roos, 1995; Slade and Levine, 1991; Kerr et al., 1993; Roos and Karel, 1991a; Schenz, 1995; Peleg, 1995). It has also been reported that when temperature of some processes exceeds Tg, the quality of foodstuffs is seriously altered (Peleg, 1996). Although numerous, most of the published articles on glass transition and water content of foodstuffs are limited to the study of liquid model foods such as solutions of pure sugars (Roos, 1995; Slade and Levine, 1991). There is little literature concerning the glass transition of solid foodstuffs such as fruits, vegetables, and meats as a function of water content. Figure 7.10 shows the glass transition temperature of several fruit powders (Khalloufi et al., 2000b) as a function of water content, showing a strong relationship between variables. Process-quality relationships could be a solid basis to optimize both existing and novel dehydration methods (Nijhuis et al., 1996). However, although this parameter is important in analyzing the impact of processing on the final quality of a product, quantifiable expressions between quality parameters and glass transition have not been found yet. For more details on this topic, the reader can refer to the references cited.

7.3 TYPES OF DRYERS 7.3.1 CLASSIFICATION

AND

SELECTION

Over 200 types of dryers are available commercially to process the diverse physical forms of foods and feeds and impart the required product characteristics. Menon and Mujumdar (1987) have given detailed schemes for classification of dryers based on

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340 Strawberry

320

Blueberry 300 Tg (K)

Raspberry 280

Blackberry

260 240 220 200 0

0.1

0.2

0.3

w (water content, wet basis)

FIGURE 7.10 Glass transition temperature of several fruit powders as a function of water content. (From Khalloufi S., El Maslouhi, Y., and Ratti, C. 2000b. Mathematical model for prediction of glass transition temperature of fruit powders. J. Food Sci., 65(5), 842–848.)

1. Mode of processing (batch, continuous, and semi-batch) 2. Operating pressure (vacuum, atmospheric, and high pressure) 3. Mode of heat transfer (conduction, convection, radiation, dielectric, and a combination of different modes) 4. Adiabatic or nonadiabatic (e.g., with heat exchangers immersed within the dryer) 5. Physical form of feed (e.g., granular, pasty, continuous web, etc.) 6. Physical state of product being dried (e.g., stationary, moving, conveyed, fluidized, dispersed, spouted, vibrated, atomized, stirred, coated on inert surfaces, etc.) This classification scheme readily gives an idea of the large number of potential configurations for dryers. The selection of the dryer for a specific task, however, is governed by a number of criteria that limit the possible candidate dryers significantly. Among the important criteria to be considered are: 1. Physical form of feed and product. For example, if it is a paste or slurry, the choice of dryer is limited to spray, drum, or fluidized beds of inert particles. The dryer must be able to handle the physical form of the feed as well as the product. 2. Desired quality of product. 3. Heat-sensitivity of product (e.g., mode of heat transfer and maximum permissible product temperature must be selected to be consistent with the physical form of the feed). For extremely heat-sensitive products, it may be necessary to use a vacuum dryer or even a freeze dryer. 4. Energy efficiency. Some dryer types are inherently more energy-efficient than others. For example, contact (conduction or direct) dryers are more efficient than convection dryers; however, they are not suitable for all types of feeds. 5. Production rate (e.g., small, medium, large). For drying of fruits and vegetable materials, conventional types of dryers are commonly used because of their simplicity of construction and operation, as well as low cost. Most such products

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TABLE 7.8 Selection of Dryers Based on the Form of Feed Form of Feed Particulate or granular

Large cubes or slices

Paste-like (or suspension)

Possible Dryer Types

Form of the Product

Packed bed dryer Fluid bed dryer Conveyor dryer Spouted bed dryer Tray (batch or continuous) Rotary Vibrated bed Flash (only if surface moisture is present) Freeze dryer Packed bed dryer Conveyor dryer Tunnel dryer Impinging jet (jet-zone) Vibrated bed Tray (batch or continuous) Freeze dryer Spray dryer Drum dryer Fluid or spouted bed of inert solid particles

Particulate or granular

Cubes/slices (deformed)

Powder

are seasonal in nature, which means that dryers are operated only over a fraction of the year. Thus, the units must have low capital costs for economic reasons. Also, fruits and vegetables may be dried in their original form (e.g., grapes, slices or cubes of papaya, mango, apple, pineapple, etc.) or in processed form (e.g., puree of mango, papaya, etc.). Depending on the physical form of the feed, different dryers must be used. The presence of sugar in most fruits leads to the danger of undesirable caramelization during drying if the operating temperature in the dryer is chosen incorrectly. Also, the hygroscopicity and stickiness of a product may lead to problems of deposits in dryers, as well as caking during storage. These and other factors must be considered in selecting both the dryer type and the operating conditions. Table 7.8 is a brief, simple summary of types of dryers one may use, depending on the form of the feed. Numerous other variants are possible (see Menon and Mujumdar [1987] for details). It is important to carry out laboratory or pilot tests to ensure feasibility of handling the product in a given dryer. For example, because of their particle size, shape, or stickiness characteristics, some particulate materials may not fluidize at all. In such cases, it is useful to test if back-mixing the feed with the dried product makes the mixture fluidizable (or dispersible for a flash dryer).

7.3.2 SPECIFIC DRYING SYSTEMS 7.3.2.1 Conventional Hot-Air Drying In this type of drying, the heat needed for drying is provided by convection with hot air in contact with the product. The most common of these dryers used for fruits are: kiln, cabinet, tunnel, and continuous belt. The kiln dryer is basically an oven heated by gas burners in which the product is placed in slotted trays. This equipment is operated in a batch mode and is still widely used to dry slices of apples and several other fruits. It takes from 6 to 8 h to dry a batch of slices or rings of apples to a final moisture content of 14 to 40% (Somogyi and Luh, 1986).

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The cabinet dryer is also a type of batch equipment, but there is a forced flow of air along or through the trays where the material is placed. This air is heated indirectly, generally by steam coils, after entering the cabinet. The operation can be controlled, and the drying is more uniform in this case than in the kiln dryer. This unit is more suitable for small-scale operations of pieced fruits (Salunkhe et al., 1991), and although the cost of the equipment is low, its operating (labor) cost is high. The thermal efficiency is low as well. The tunnel dryer is the most commonly used equipment to dry fruits. It is similar to the cabinet dryer but the trays are made to move along a tunnel in which hot air flows in parallel or counter flow to the product. This type of dryer is efficient yet simple. It is more suitable to industrial application because they can handle larger production rates. This method is often used to dry apricots, peaches, pears, apples, figs, dates, and so on. The continuous belt dryer is formed by an endless (permeable and impermeable) belt on which the product to be dried is placed and carried through a counter- or co-current flow of hot air. The main advantage of this dryer is the ease of automatic continuous operation that minimizes labor requirements (Salunkhe et al., 1991). Conventional hot-air drying is the main process used in obtaining fruit leather. A flow diagram for the fruit leather process is shown in Figure 7.11. Apples, apricots, berries, cherries, nectarines, peaches, pears, pineapples, and strawberries have been determined to be the best fruit sources to obtain this type of product (Reynolds, 1993). Fruits should first be selected for their ripeness (ripe or slightly overripe fruits are required) and afterwards washed, peeled, and cut into chunks. The fruit pieces are pressed until a smooth puree is obtained. In order to obtain the desired texture and to avoid fruit darkening, water and ascorbic acid are added to the puree. The fruit puree is poured SELECTION

WASHING

PEELING

CUTTING

Ascorbic Acid

FRUIT MASHING

POURING

DRYING

STORING

FIGURE 7.11 Flow diagram for the fruit leather process.

Liquid

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into trays that are placed in a kiln or cabinet dryer at 60∞C (maximum temperature to avoid case hardening) until the fruit leather is dry (Reynolds, 1993). 7.3.2.2 Solar Drying The drying of fruits and vegetables using solar energy has been used since antiquity in order to preserve foods. There are two main types of drying using solar energy: direct and indirect. In direct solar drying, commonly called open-sun drying or just sun-drying, the product is placed directly on trays in open air where only the incident solar radiation (or incident and reflected radiation) is used as the energy source for drying. Such dryers have the following main disadvantages (Imre, 1987): large area requirement, inability to control degradation of product by biochemical or microbiological reactions, insect infestation, and so on. In addition, the drying is uneven in the tray, and the drying time is extremely long. For instance, drying apricots to 24% moisture requires almost 50 h using direct sun-drying; this decreases to 31 h when the dryer is improved with the use of reflected solar radiation (Bolin and Salunkhe, 1982). In the case of peaches, the drying time to a final moisture content of 24% is 150 h due to the large fruit size, and it decreases by 45% when reflected radiation is also used (Bolin and Salunkhe, 1982). The indirect solar dryers use solar energy to heat the drying air in special heat exchangers (solar collectors), and then this air flows through or over the product by natural or forced convection. Figure 7.12 shows one of the most common and simple solar dryers for vegetables and fruits — the cabinet dryer (Kalra and Bhardwaj, 1981). It is composed of a solar collector followed by a packed-bed-type dryer. The air conditions at the inlet of the collector (air humidity and temperature) vary with time, depending on the hour of the day and other factors like weather conditions; therefore, the air conditions at the entrance of the dryer also vary in time required. A description of more complicated solar dryers can be found in Imre (1987). The main advantage of these dryers is that the energy source is renewable, free, and nonpolluting (Imre, 1987). On the other hand, this type of drying is only useful in climates with a hot sun and dry atmosphere (Somogyi and Luh, 1986) over extended periods. The problem of periodicity in solar energy can be solved with the storage Air Exit

Batch Dryer

Solar Collector

Air Inlet

FIGURE 7.12 Cabinet solar dryer. (From Kalra, S.K. and Bhardwaj, K.C. 1981. Use of simple solar dehydrator for drying fruit and vegetable products. J. Food Sci. Technol. (India), 18, 23.)

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of part of this energy (e.g., rock storage), but any improvement in this type of equipment must be done carefully because the technoeconomics are not always favorable. Fruits commonly dried with solar energy include figs, prunes, peaches, apricots, and to a large extent grapes from which raisins are obtained. An updated review of these processes can be found in Jayaraman and Das Gupta (1992). Note that a backup fossil-fuel-fired heater is needed when the product is subjected to biochemical or microbiological degradation during periods of no insolation (e.g., night) or poor insolation (e.g., cloudy, rainy days). Thermal storage adds to the cost of solar drying very significantly. 7.3.2.3 Microwave Drying Conventional drying of foods is a slow process. Attempts to enhance this process have been made, but only few of them are applied presently in the industry. Microwave heating increases the temperature of the interior, wetter parts of the solid. In addition, moisture transport to the evaporation surface is also enhanced by an internal pressure gradient. Microwave heating has three main advantages (Van Arsdel et al., 1973): (1) a penetrating quality that leads to uniform drying (conventional drying may cause damage to the surface of the product; it is dry and uses high temperatures compared to the interior); (2) selective adsorption by liquid water, which leads to a uniform moisture profile within the particle; and (3) ease of control due to the rapid response of such heating. Advantages (1) and (2) increase the quality of the final product and make it easier to be rehydrated. But on the other hand, such equipment is complicated and expensive, and, unfortunately, no industry uses it for fruit dehydration (Somogyi and Luh, 1986). Work done on the finish drying of fruits demonstrated a synergistic effect between hot-air and microwave drying (Salunkhe et al., 1991). Apple, mango, and pineapple were investigated recently for microwave drying under vacuum (Xiufan et al., 1993). The rate of drying and the core temperature were measured in this work. The experimental results showed that the geometry of the samples and the power used have a strong influence on the microwave drying rate of fruits, as well as on product quality. 7.3.2.4 Osmotic Dehydration Hot-air drying notably reduces the quality of the processed foodstuffs (changes in color, shape, losses of aroma and nutrients, etc.). Osmotic dehydration is an alternative technology to reduce the water content, as well as to improve the quality of the final product. This process is being used in industry to dehydrate fruits, vegetables, meat, and fish, but the industrial application is still limited. Osmotic dehydration involves the immersion of cut foods in concentrated solutions of sugars and salts. A flux of water out of the food and of other solutes into the foodstuff develops due to the difference in osmotic pressure. The product thus loses some water to the external solution. Ponting et al. (1966) first applied osmotic dehydration for reduction of weight by up to 50% of apples prior to vacuum drying. The osmotic dewatering rate can be enhanced by increasing the concentration of the osmotic solution or the temperature. Rogachev and Kislenko (1972) coupled alternate heating and cooling with acoustic fields to increase osmosis. More recently, Shi and Maupoey (1993) applied vacuum (100 mb) treatment to increase water transport during osmosis in 65∞Brix sugar solutions for dehydration of pineapple and apricot cubes. They claimed higher dehydration rates at lower temperatures, thus improving the quality of dehydrated fruit products. Sugar gain is related to the biological characteristics of fruits; it is unaffected by application of vacuum. Grabowski and Mujumdar (1992) have proposed a novel scheme for solar-assisted osmotic dehydration of fruits. Solar energy is used to dry the osmotically dehydrated slices of fruits and also to concentrate the osmoticum. Figure 7.13 shows the flow diagram for osmotic dehydration of fruits. Mango, banana, pear, apple, cranberries, and strawberries (as whole or cut in pieces) are the usual fruits transformed by

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FRUIT (whole or pieces)

PRE-TREATMENT (blanching, SO2, etc.)

OSMOTIC DEHYDRATION

DRAINING

Fruits

Syrup

AIR-DRYING

STORING

FIGURE 7.13 Flow diagram for the osmotic dehydrated fruit process.

this process. Fruits can be pretreated if necessary to avoid darkening. The osmotic medium is commonly a sugar solution with a fruit/syrup ratio of 1/3–5 at a temperature between 20 to 50∞C and slightly agitated (Garrote et al., 1992). The duration of the process and syrup concentration are specific to the fruit being processed (Dauthy, 1995). After osmotic dehydration has been completed (reducing up to 50% of the initial water content), the fruits are drained and placed into a hot air dryer to achieve the final desired moisture content. Syrup solution can be recuperated and after the sugar concentration has been adjusted, it could be used again for osmotic dehydration. 7.3.2.5 Explosion Puffing The incorporation of explosion puffing into a hot-air dehydration process facilitates faster dehydration and leads to a final product with a highly porous structure that is capable of rapid rehydration. This process has long been applied to rice and wheat breakfast cereals and now is being applied to fruit and vegetables (Salunkhe et al., 1991). Explosion puffing has been successful with apples, blueberries (Somogyi and Luh, 1986), and banana slices (Saca and Lozano, 1992). The process starts with conventional hot-air drying of the fruit pieces until a certain water content is attained. Then the pieces are placed in a closed chamber, the gun, where the pressure is increased by means of superheated steam (Figure 7.14, Saca and Lozano, 1992). After some time, the pressure is suddenly released by opening the lid of the gun. Due to the high temperature and the sudden decompression, the water in the particles flashes explosively. Then the solids are placed again in the air drying equipment to be dried to the desired final water content. Due to the highly porous structure after puffing, the drying time is reduced notably. The key parameters that affect this process are the initial water content of the particles entering the explosion puffing process, the temperature and pressure of the steam in the gun, and the dwell

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OPENING MECHANISM

O-RING

GUN CYLINDER THERMOCOUPLES SAMPLES STEAM

LID

TEFLON TRAY

GAS BURNER

FIGURE 7.14 Schematic of an explosion-puffing dryer. (From Saca, A.S. and Lozano, J.E. 1992. Explosion puffing of bananas. Int. J. Food Sci. Technol. 27, 419–426.)

time inside the gun. The optimal initial water content is between 20 to 30% for apples, 19 to 30% for blueberries (Somogyi and Luh, 1986), and 27 to 38% for banana slices (Saca and Lozano, 1992). In the latter case, it was found that the explosion puffing process decreased by 25% the drying time of the whole process, with an increase of porosity of the product from 10.3% to 46.8%. This fact makes the product easily rehydratable. 7.3.2.6 Freeze-Drying Freeze-drying (also called lyophilization) is characterized by heating (i.e., conduction, radiation, radio-frequency, microwaves, etc.) a moist frozen product in a vacuum chamber maintained at an absolute pressure below the vapor pressure of the ice within the product (Hanson, 1976). When the material is heated, the ice sublimes. The main advantages of this drying method are related to the high quality of the final product compared with other drying methods. These advantages can be summarized as follows (Okos et al., 1992; Jayaraman and Das Gupta, 1992; Salunkhe et al., 1991; Somogyi and Luh; 1986): (1) high flavor and aroma retention; (2) high retention of nutritional value; (3) minimal shrinkage; (4) minimal change in shape, color, and appearance; (5) practically no damage in structure and texture; (6) porous final structure; and (7) easily rehydratable. As was pointed out by Salunkhe et al. (1991), the low temperature of this process leads to a decrease in the kinetics of all the degradative processes (nonenzymatic browning, protein deterioration, and enzymatic reaction, etc.). The industrial application of freeze-drying to a wide range of fruits has been limited by its main disadvantage: the high capital and operating costs. In addition, the final product has to be properly packaged in special materials to avoid oxidation and moisture pickup (Jayaraman and Das Gupta, 1992), which also increases the cost of the final product. Pilot and industrial scale freeze dryers are carefully described in Liapis (1987) and Liapis and Bruttini (1995a). Among the industrial freeze dryers, the most common are the tray and tunnel type. In general, they are only of the batch type. This drying technique has been applied successfully to raspberries, strawberries, peaches, cherries, and figs (Shishehgarha et al., 2002; Salunkhe et al., 1991; Somogyi and Luh, 1986), but the commercial application has been limited to exotic fruit and orange juices (Jayaraman and Das Gupta, 1992). It has been shown that this process is viable because of the higher quality of production of fruit powder for retail consumption (Holdsworth, 1986).

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7.3.3 NOVEL DRYERS Kudra and Mujumdar (1994) have extensively surveyed scores of novel drying techniques, some of which are applicable to dehydration of fruits in sliced, diced, or pureed forms. In principle, dielectric drying, which provides preferential heat to the polar free water molecules volumetrically, should be a desirable mode of heating for heat-sensitive products. In actual fact, this process tends to be highly capital-intensive and requires expensive electrical energy, as noted earlier. For products that are nonfluidizable, e.g., apple cubes, strawberries, and blueberries, it is possible to employ the so-called technique of two-component fluidization. For example, Mujumdar and Macdonald (1993) have shown that it is possible to fluidize whole grapes and blueberries in beds of granulated sugar. Using hot air as the fluidizing and heating medium, the hygroscopicity of sugar helps improve the mass transfer rate from the fruit pieces. Heat transfer rates from fine particles to immersed bodies is known to be of an order-of-magnitude greater than that to air flow. It is found that the sugar-to-grape weight ratio should be at least six to ensure smooth fluidization. In principle, this idea could be extended to other fruits. Care is needed to ensure that the sugar does not granulate and eventually defluidize the bed. For “weak” fruits (e.g., blueberries), fluidization leads to structural damage and, hence, is not recommended. For products in sliced form, the so-called jet-zone dryer is used, wherein downwardly diverted jets of hot air impinge on a bed of the product conveyed on a vibratory or oscillatory conveyor. The product achieves a state of pseudo-fluidization and is yet handled gently (see Kudra and Mujumdar, 1995). In a novel process tested recently by Mujumdar and Macdonald (1993), blueberries were dried in a two-step process. First, the berries were treated in a warm dilute solution of ethyl oleate in caustic soda to make the skin permeable to water and solute flow and then immersed for several hours in a saturated sugar syrup. The osmotically dehydrated berries were dried in a vacuum dryer (100 mm Hg pressure) at low temperatures (-20 to 20∞C). The berries retained their color and physical form and had enhanced taste due to some sugar uptake. Better optimization of the process conditions for osmotic dehydration followed by freeze-drying has been shown through experimentation to yield high quality dried blueberries for special applications, such as additives to cereals, desserts, etc. Sometimes fruit purees may be added as flavoring components to products such as mashed potato. The mixtures may be dried in spray, drum, flash, or the novel impinging stream dryers (Kudra et al., 1995). In the latter, the feed is dispersed finely in hot air jets turned in opposite directions for very rapid and efficient drying to produce fine powders directly. Such dryers are not currently available commercially.

7.4 DESIGN CONSIDERATIONS 7.4.1 HEAT

AND

MASS BALANCES

The main problem in the design of dryers for biological materials arises when the diverse physicochemical changes that occur in the product during the process have to be taken into account. As an example, the effect of shrinkage is often neglected due to the complexity in the derivation and solution of the corresponding model equations. Ratti (1991) developed a model for packed bed batch dryers, which includes the effect of shrinkage and the variation of the product properties with water content and temperature for application to fruit and vegetable products. The transient heat and mass balances equations are expressed in appropriately defined moving coordinates to account for particle shrinkage during drying. The key assumptions employed in this model are: 1. One-dimensional transport of heat and mass 2. Uniform air velocity distribution in the dryer (plug flow of drying air)

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TABLE 7.9 Heat and Mass Balances for a Batch Dryer Partial Differential Equations System Mass balance in the gas phase: nw av (1 - e ) 1 Gs r s (1 - e ) ∂Y È ∂Y ˘ ÍÎ ∂t ˙˚ = ra e SLo r a e r s,o (1 - e o ) ∂L L

(1)

Mass balance in the solid: nw av È ∂X ˘ ÍÎ ∂t ˙˚ = - r L s

(2)

av È ∂Ts ˘ Í ∂t ˙ = r (1 + X )Cp [hg (Tg - Ts ) - nw DH s ] Î ˚L s sh

(3)

Energy balance in the solid:

Energy balance in the gas phase: hg av (1 - e ) È ∂Tg ˘ 1 Gs r s (1 - e ) ∂Tg (Tg - Ts ) Í ˙ =r a eCpah SLo r a e r s,o (1 - e o ) ∂L Î ∂t ˚ L

(4)

Boundary and Initial Conditions Initial Profile (t = 0): Ï X = Xo Ô ÔTs = Tso L=0Ì ÔY = Ygo ÔTg = Tgo Ó

Ï X = Xo ÔT = T Ô s so Lπ0Ì ( ) ÔY = Ysat Tso ÔTg = Tso Ó

(5)

Boundary Conditions: L = 0 Y = Ygo, Tg = Tgo

3. Adiabatic system (well-insulated dryer) 4. Purely convective heat transfer 5. Negligible conduction heat transfer between particles in the bed and negligible contact diffusion 6. Shrinking particles Table 7.9 summarizes the governing differential heat and mass balance equations and the initial and boundary conditions for constant inlet air drying conditions. Most of the parameters and properties needed to complete the model have been referred to in previous sections. An extension of this model for varying inlet air conditions can be found in the literature (Ratti and Mujumdar, 1993). Models for different types of dryers can be found in the literature (Keey, 1978; Mujumdar, 1987). Models for drying of foodstuffs in particular also exist, e.g., concurrent-flow packed bed and cascading rotary dryers (Bakker-Arkema, 1986), spray dryers (Sokhansaj and Jayas, 1987; Bakker-Arkema, 1986), or freeze dryers (Okos et al., 1992; Sokhansaj and Jayas, 1987). In most cases, the reader must carefully revise the model assumptions in order to apply these models to drying of highly deformable shrinking material such as fruits. Recently, a model for a tunnel

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dehydrator for tomatoes was developed (Unadi et al., 2002). This model was used to determine the performance of such a dryer in different modes, as well as the influence on the quality of dried tomatoes. The authors claimed that counterflow mode gives the highest throughput, the lowest specific energy consumption, and satisfactory quality of dried tomatoes. 7.4.2

Energy Aspects

A dryer is an energy-consuming piece of equipment (Strumillo and Lopez-Caicedo, 1987), and when applied to foodstuffs, it becomes more energy consuming because mild operating conditions are needed in order to obtain a final product with acceptable quality. Convective drying requires electric energy and heat. For instance, drying of apples up to 20% moisture content requires 5 MJ/kg of evaporated water (Lewicki and Lenart, 1992), whereas only 3% of the energy needed is electric. An interesting description of the energy consumption during osmo-convection drying of fruits and vegetables, together with operating data, can be found in the previous reference. This author claimed that depending on the design of an osmo-convective process, the savings in energy consumption can be as high as 75% compared with regular convection drying because osmotic dehydration reduces the amount of water that must be thermally removed. The most highly energy-consuming process is obviously freeze-drying. This operation has three main energy-consuming steps: freezing, sublimation, and water vapor removal. Fruits are generally frozen by cold air prior to dehydration (Salunkhe et al., 1991). It is known that the freezing temperature and the freezing rate are important factors that affect the quality of the final product in terms of its structure (Jayaraman and Das Gupta, 1992). Fast freezing leads to small crystals so new techniques applied to fruit freezing are based on their immersion in liquid CO2, freon, or liquid N2 (Salunkhe et al., 1991). The second step is carried out under vacuum, and the heat for sublimation is provided by conduction or by microwave heating in order to accelerate the process. The process of water removal depends on the size, shape, and tortuosity of the pores (Somogyi and Luh, 1986), so the faster the freezing rate, the slower the water removal. 7.4.2.1 Heat Pump Dryers Drying of heat-sensitive products (e.g., fruits and vegetables) requires large amounts of lowtemperature heat. Use of high-grade energy sources (e.g., gas, oil, electricity, etc.) for such tasks leads to low energetic efficiencies. Heat pumps can take advantage of the energy of high-grade energy sources. Indeed, benefits of heat pumps increase as the target air temperature for heating approaches the ambient temperature. Several different configurations are possible to couple heat pumps with conventional convective dryers. They may be single-pass or of a recirculating type. Aveces-Saborio (1993) has presented some simulation results on energy consumption in heat pump dryers and recirculating conventional dryers. Stromman (1986) has discussed energy-optimal temperature differences in heat-pump dryers. Pendyala et al. (1986) have evaluated the economics of heat pump-assisted drying systems. Zylla and Strumillo (1987) have reviewed heat pumps in drying. Recently, Jia et al. (1993) developed, modeled, and tested a heat pump-assisted microwave dryer for vegetables (ginger, carrots, etc.) and showed that energy consumption in such systems is comparable to conventional drying while the product quality may be enhanced. They also showed that the specific moisture evaporation rate (SMER) is strongly dependent on the product being dried. While no tests are reported on fruits, it is likely the product quality may be better at a lower cost.

7.5 CONCLUDING REMARKS Industrial drying of fruits is a technology based both on engineering and empiricism. Experimental evaluation of the effects of dryer operating variables on the drying kinetics (which govern the dryer

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size), as well as the quality of the product, are keys to successful design and economic drying of fruits. Energy consumption is often a secondary consideration to quality, but it is possible to enhance efficiency without compromising on quality through a fundamental understanding of the phenomena involved. Drying of biological materials such as fruits is extremely complex since it is accompanied by biochemical and physical changes that govern product quality, such as color, flavor, shape, size, taste, etc. Extensive laboratory and pilot testing is needed before a full-scale dryer can be designed without prior experience with the same feedstock.

7.6 NOMENCLATURE As = surface area [m2] av = particle surface area/volume [m -1] aw = water activity [–] Bimrs = mass Biot number (see Ratti and Crapiste, 1992) [–] Cp = specific heat [J/kg K] c = constant of GAB equation, Table 7.3 Dr = effective diffusivity [m2/sec] f = Van Meel’s characteristic drying function [–] hg = heat transfer coefficient [J/m2 sec K] k = constant of GAB equation, Table 7.3 kg = mass transfer coefficient [kg water/m2 sec kPa] ki = regression constants, Table 7.3 kt = thermal conductivity [W/m K] L = characteristic length [m] ms = dry mass [kg] nw = water mass flux [kg water/m2 sec] nwc = water mass flux in the constant rate period [kg water/m2 sec] pw = water vapor pressure [kPa] pwo = pure water vapor pressure [kPa] R = universal gas constant [J/mol K] T = temperature [K] t = time [sec] V = volume [m3] X = water content (dry basis) [kg water/ kg dry matter] Xm = constant of GAB equation, Table 7.3 W = dimensionless water content (dry basis) relative to the initial = X/Xo [–] W* = local dimensionless water content w = water content (wet basis) [kg water/ kg total matter] Z = reduced moving coordinate [–]

7.6.1 SUBSCRIPTS af bf e g o s •

= = = = = = =

above freezing below freezing equilibrium gas initial at the surface at the bulk

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7.6.2 GREEK SYMBOLS DE DHs e¢ e≤ r F

= = = = = =

activation energy for diffusion [kJ/mol] heat of sorption [kJ/mol] relative dielectric constant [–] loss factor [–] density [kg/m3] generalized drying parameter (see Ratti and Crapiste, 1992) [–]

REFERENCES Abdelhaq, E.H. and T.P. Labuza. 1987. Air drying characteristics of apricots. J. Food Sci., 52(2), 342–345. Acevez-Saborio, S. 1993. Analysis of energy consumption in heat pump and conventional dryers. Heat Recov. Syst. CHP., 13(5), 419–428. Alzamora, S.M. 1979. Thesis. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Argentina. Alzamora, S.M., J. Chirife, P. Viollaz, and L.M. Vacarezza. 1979. Heat and mass transfer during air drying of avocado. In A.S. Mujumdar (Ed.), Drying ’80, Vol. I: Developments in Drying, Science Press, NJ. Bakker-Arkema, F.W. 1986. Heat and mass transfer aspects and modelling of dryers — A critical evaluation. In D. MacCarthy (Ed.), Concentration and Drying of Foods, Elsevier Applied Science, London, pp. 165–202. Berna, A., C. Roselló, J. Cañellas, and A. Mulet. 1990. Drying kinetics of apricots. In W.E.L. Spiess and H. Schubert (Eds.), Engineering and Food. 1. Physical Properties and Process Control, Elsevier, Amsterdam, pp. 628–636. Berna, A., C. Roselló, J. Cañellas, and A. Mulet. 1991. Drying kinetics of a Majorcan seedless grape variety. In A.S. Mujumdar and I. Filková (Eds.), Drying ’91, Elsevier, Amsterdam., pp. 455–462. Bimbenet, J.J., J.D. Daudin, and E. Wolf. 1985. Air Drying Kinetics of Biological Materials. In A.S. Mujumdar (Ed.), Drying ’85, Hemisphere, New York, pp. 178–185. Bolin, H.R. and D.K. Salunkhe. 1982. Food dehydration by solar energy. CRC Crit. Rev. Food Sci. Nutr., April, 327. Bruin, S. and K.Ch.A.M. Luyben. 1980. Drying of food materials: A review of recent developments. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. 1, Elsevier Science, Amsterdam, pp. 155–213. Brunauer, S., P.H. Emmet, and E. Teller. 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc., 60, 309. Chirife, J. and M. Buera. 1995. A critical review of some non-equilibrium situations and glass transition on water activity values of foods in microbiological growth range. J. Food Eng., 25, 531–552. Choi, Y. and M.R. Okos. 1986. Effects of temperature and composition on the thermal properties of foods. In M. Le Maguer and P. Jelen (Eds.), Food Engineering and Process Applications, Vol. 1: Transport Phenomena, Elsevier Applied Science, London, pp. 93–101. Chung, D.S. and H.B. Pfost. 1967. Adsorption and desorption of water vapour by cereal grains and their products. Trans. ASAE, 10, 549. Chuy, L.E. and T.P. Labuza. 1994. Caking and stickiness of dairy-based food powders as related to glass transition. J. Food Sci., 59: 43–46. Constenla, D.T., J.E. Lozano, and G.H. Crapiste. 1989. Thermophysical properties of clarified apple juice as a function of concentration and temperature. J. Food Sci., 54(3), 663–668. Crank, J. 1967. The Mathematics of Diffusion. Oxford University Press, London. Crapiste, G.H. and E. Rotstein. 1986. Sorptional equilibrium at changing moisture contents. In A.S. Mujumdar (Ed.), Drying of Solids, Wiley Eastern. Crapiste, G.H., S. Whitaker, and E. Rotstein. 1988. Drying of cellular material. Parts 1 and 2. Chem. Eng. Sci., 43(11), 2919–2936. Dauthy, E.M. 1995. Fruit and Vegetable Processing. FAO Agricultural Services Bulletin No.119, Food and Agriculture Organization of the United Nations, Rome.

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Garrotte, R.L., E. Silva, and R.A. Bertone. 1992. Osmotic concentration at 5 and 25∞C of pear and apple cubes and strawberry halves. Lebensm.-Wiss.u.-Technol., 25, 133–138. Gekas, V. 1992. Transport Phenomena of Foods and Biological Materials, CRC Press, Boca Raton, FL, pp. 150–160. Ginzburg, A.S. 1969. Application of Infra-red Radiation in Food Processing, Chemical and process engineering series, Leonard Hill, London. Ginzburg, A.S. and I.M. Savina. 1982. Mass and Moisture Transfer Characteristic of Food Products (in Russian), Lyegkaya Pischevaya Promyshlennost, Moscow. Grabowski S. and A.S. Mujumdar. 1992. Solar-assisted osmotic dehydration. In A.S. Mujumdar (Ed.), Drying ’92, Elsevier Science, Amsterdam, pp. 1689–1700. Guggenheim, E.A. 1966. Applications of Statistical Mechanics, Clarendon Press, Oxford. Halsey, G. 1948. Physical adsorption on non-uniform surfaces. J. Chem. Phys., 16, 931. Hanson, L.P. 1976. Commercial processing of fruits. Food Technology, Review No. 30, Noyes Data Corporation, New Jersey, pp. 41–100. Heldman, D.R. 1975. Food Process Engineering, AVI Publishing, Westport, CT pp. 96–103. Henderson, S.M. 1952. A basic concept of equilibrium moisture. Agr. Eng., 33, 29. Holdsworth, S.D. 1986. Advances in the dehydration of fruits and vegetables. In D. MacCarthy (Ed.), Concentration and Drying of Foods, Elsevier Applied Science, London, pp. 293–303. Hong, Y.C., A.S. Bakshi, and T.P. Labuza. 1986. Finite element modeling of moisture transfer during storage of mixed multicomponents dried foods. J. Food Sci., 51(3), 554–558. Iglesias, H.A. and J. Chirife. 1976. Prediction of the effect of temperature on water sorption isotherms of food materials. J. Food Technol., 11, 109. Imre, L.L. 1987. Solar Drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 1st ed., Marcel Dekker, New York, pp. 357–417. Inakuma, T., M. Yasumoto, M. Koguchi, and T. Kobayashi, 1998. Effect of drying methods on extraction of lycopene in tomato skin with supercritical carbon dioxide. J. Jpn. Soc. Food Sci. Technol., 45(12), 740–743. Jankovié, M. 1993. Physical Properties of Convectively Dried and Freeze-dried Berrylike Fruits. A publication of the Faculty of Agriculture, Belgrade, 38(2), 129–135. Jayaraman, K.S. and D.K. Das Gupta. 1992. Dehydration of fruits and vegetables — recent developments in principles and techniques. Drying Technol., 10(1), 1. Jia, X., C. Shave, and P. Jolly. 1993. Study of heat pump assisted microwave drying. Drying Technol., 11(7), 1583–1616. Kalra, S.K. and K.C. Bhardwaj. 1981. Use of simple solar dehydrator for drying fruit and vegetable products. J. Food Sci. Technol. (India), 18, 23. Kapsalis, J.G. 1987. Influences of hysteresis and temperature on moisture sorption isotherms. In L.B. Rockland and L.R. Beuchat (Eds.), Water Activity: Theory and Applications to Food, IFT Basic Symposium Series, Marcel Dekker New York, pp. 173–214. Karel, M. 1975. Heat and mass transfer in freeze-drying. In S.A. Golblithm, L. Rey, and W.W. Rothamayr (Eds.), Freeze-Drying and Advanced Food Technology, Academic Press, New York. Karel, M. 1991. Physical structure and quality of dehydrated foods. In A.S. Mujumdar and I. Filková (Ed.), Drying ’91, Elsevier Science, Amsterdam, pp. 26–35. Karel, M. and J.M. Flink. 1983. Some recent developments in food dehydration research. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. 2, Elsevier Science, Amsterdam, pp. 103–153. Karmas, R., M.P. Buera, and M. Karel. 1992. Effect of glass transition on rates of nonenzymatic browning in food systems. J. Agric. Food Chem., 40, 873–879. Keey, R.B. 1978. Introduction of Industrial Drying Operations. Pergamon Press, Oxford. Keey, R.B. 1980. Theoretical foundations of drying technology. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. I, Hemisphere, Amsterdam, pp. 1–20. Kerr, W.L., M.H. Lim, D.S. Reid, and H. Chen 1993. Chemical reaction kinetics in relation to glass transition temperatures in frozen food polymer solutions. J. Sci. Food Agric., 61, 51–56. Khalloufi S., Y. El Maslouhi, and C. Ratti. 2000b. Mathematical model for prediction of glass transition temperature of fruit powders. J. Food Sci., 65(5), 842–848. Khalloufi S., J. Giasson, and C. Ratti. 2000a. Water activity of freeze-dried berries and mushrooms. Can. Agric. Eng., 42(1), 51–56.

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Nelson, S.O. 1980. Microwave dielectric properties of fresh fruits and vegetables. Trans. ASAE., 23, 1314. Nelson, S.O. 1983. Dielectric properties of some fresh fruits and vegetables at frequencies of 2.45 to 22 GHz. Trans. ASAE., 26, 613. Nijhuis, H.H., E. Torringa, H. Luyten, F. René, P. Jones, T. Funebo, and T. Ohlsson. 1996. Research needs and opportunities in the dry conservation of fruits and vegetables. Drying Technol., 14(6), 1429–1457. Okos, M.R., G. Narsimhan, R.K. Singh, and A.C. Weitnauer. 1992. Food dehydration. In D.R. Heldman and D.B. Lund (Eds.), Handbook of Food Engineering, Marcel Dekker, New York, pp. 437–562. Peleg, M. 1995. Description of mechanical changes in foods at their glass region. In G.V. Barbosa-Canovas and Welti-Chanes (Eds.) Food Preservation by Moisture Control: Fundamentals and Applications. Technomic Publishing, Lancaster, PA. Peleg, M. 1996. On modeling changes in food and biosolids at and around their glass transition temperature range. Crit. Rev. Food Sci. Nutr., 36,49–67. Pendyala, V.S. Devotta, and V.S. Patwardhan, 1986. The economics of heat pump assisted drying systems. J. Heat Recov. Sys., 6, 433–442. Pfost, H.B., S.G. Maurer, D.S. Chung and G.A. Milliken. 1976. Summarizing and Reporting Equilibrium Moisture Data for Grains. A.S.A.E. Paper 76-3520. 1976 ASAE Meeting, St Joseph, MI. Ponting, J.D., G.G. Watters, G.G. Forrey, R.R. Jackson, and R. Stanley. 1966. Osmotic dehydration of fruits. Food Technol., 20, 125. Rahman S. 1995. Food Properties Handbook. CRC Press, Boca Raton, FL. Rao, M.A. 1992. Transport and storage of food products. In D.R. Heldman and D.B. Lund (Eds.) Handbook of Food Engineering, Marcel Dekker, New York, pp. 225–245. Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina. Ratti, C. 1994. Shrinkage during drying of foodstuffs. J. Food Eng., 23(1), 91–105. Ratti, C. and G.H. Crapiste. 1992. A generalized drying curve for shrinking food materials. In A.S. Mujumdar (Ed.), Drying ’92, Elsevier Science, Amsterdam, pp. 864–873. Ratti, C., G.H. Crapiste, and E. Rotstein. 1989. A new water sorption equilibrium expression for solids foods based on thermodynamics considerations. J. Food Sci., 54(3), 738–742. Ratti C. and A.S. Mujumdar. 1993. Fixed-bed batch drying of shrinking particles with time varying drying air conditions. Drying Technol., 11(6), 1311. Ratti, C. and A.S. Mujumdar. 1995. Infrared drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying. 2nd ed., Marcel Dekker, New York, pp. 567–588. Reynolds, S. 1993. So Easy to Preserve, Bulletin 989, 3rd ed., Cooperative Extension Service. The University of Georgia, College of Agricultural and Environmental Sciences. Rogachev, V.I. and I.I. Kislenko. 1972. Combined Methods of Decreasing Water Activity in Fruits under Preservation. Acta Aliment., Hung. 2, 245. Román, G.N., E. Rotstein, and M.J. Urbicain. 1979. Kinetics of water vapor desorption from apples. J. Food Sci. 44(1), 193–197. Román, G.N., M.J. Urbicain, and E. Rotstein. 1982. Moisture equilibrium in apples at several temperatures: experimental data and theoretical considerations. J. Food Sci. 47(5), 1484–1488. Roos, Y. 1995. Phase Transitions in Foods. Academic Press, London. Roos, Y. and M. Karel. 1991a. Applying state diagrams to food processing and development. Food Technol., 45, 66–70, 107. Roos, Y. and M. Karel, 1991b. Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. J. Food Sci., 56, 38–43. Rotstein, E. and A.R.H. Cornish. 1978. A transport phenomena analysis of air drying of foodstuffs. In A.S. Mujumdar (Ed.), Proceedings of the 1st International Symposium on Drying, Montreal, Canada, Science Press, Princeton, NJ. Saca, A.S. and J.E. Lozano. 1992. Explosion puffing of bananas. Int. J. Food Sci. Technol. 27, 419–426. Sadikoglu, H. and A.I. Liapis. 1997. Mathematical modelling of the primary and secondary drying stages of bulk-solution freeze-drying in trays: parameter estimation and model discrimination by comparison theoretical results with experimental data. Drying Technol., 15(3, 4), 791–810. Salunkhe, D.K., H.R. Bolin, and N.R. Reddy. 1991. Storage, Processing, and Nutritional Quality of Fruits and Vegetables, 2nd ed., Vol. 1 and 2. CRC Press, Boca Raton, FL.

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8 Fruit Freezing David S. Reid and Diane M. Barrett CONTENTS 8.1 8.2

Introduction ........................................................................................................................162 The Freezing Process .........................................................................................................162 8.2.1 The Influence of Temperature Change................................................................162 8.2.2 The Freezing Profile ............................................................................................162 8.2.3 The Freezing Process Described by Phase Diagram ..........................................163 8.2.4 Freezing in Tissue Systems .................................................................................164 8.2.4.1 Freezing in the Presence of Cells......................................................165 8.2.5 The Freezing Process and Freezing Damage......................................................165 8.2.5.1 Osmotic Damage................................................................................165 8.2.5.2 Solute-Induced Damage.....................................................................165 8.2.5.3 Structural Damage .............................................................................166 8.3 Industrial Freezing Methods ..............................................................................................166 8.3.1 Freezers ................................................................................................................166 8.3.2 Prefreezing Handling and Preparation ................................................................167 8.4 Freezing Methods for Specific Fruits ................................................................................167 8.4.1 Apple....................................................................................................................167 8.4.2 Apricot .................................................................................................................167 8.4.3 Avocado ...............................................................................................................168 8.4.4 Berries ..................................................................................................................168 8.4.5 Cherry ..................................................................................................................168 8.4.6 Coconut ................................................................................................................168 8.4.7 Cranberry .............................................................................................................169 8.4.8 Dates ....................................................................................................................169 8.4.9 Figs.......................................................................................................................169 8.4.10 Mango ..................................................................................................................169 8.4.11 Melon ...................................................................................................................169 8.4.12 Papaya ..................................................................................................................169 8.4.13 Peach ....................................................................................................................169 8.4.14 Pineapple..............................................................................................................169 8.4.15 Plum .....................................................................................................................170 8.4.16 Rhubarb................................................................................................................170 8.4.17 Strawberry............................................................................................................170 8.4.18 Tomato .................................................................................................................170 8.5 Effects of Freezing on Nutritional Components ...............................................................170 References ......................................................................................................................................171

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8.1 INTRODUCTION Most fruits have limited harvest periods. In order to have extended availability, some form of storage and preservation is needed. A variety of preservation systems exist, each of which results in an extended shelf life. Freezing provides a significantly extended shelf life and has been successfully employed for the long-term preservation of many fruits. In this chapter, we will discuss the application of freezing preservation to fruits. In order to do so, it is necessary to first discuss the freezing preservation process briefly and to consider the special problems of preservation that are presented by fruits.

8.2 THE FREEZING PROCESS Freezing involves the use of low temperatures. In general, reactions take place at slower rates as temperature is reduced. One of the more common temperature dependences of rate is expressed by the Arrhenius equation: log K = const – EA/RT where K is the reaction rate, EA is the activation energy, R is the gas constant, and T is the absolute temperature. Based on this equation, the reduction in rate can be quantified through the activation energy. All other things being equal, therefore, storage at a low temperature would give an extended storage life, and the lower the temperature the better. All other things are not equal, however. The Arrhenius expression describes the temperature dependence of reaction rates when the mechanism of the reaction does not significantly change. It also describes single reaction rates and does not necessarily describe the temperature dependence of a series of reactions that have different individual temperature dependences. Furthermore, the Arrhenius expression does not describe reaction rates where the direction of the equilibrium changes, for example, phase change, where the favored form is temperature-dependent. For all of these reasons, it is necessary to examine the effect of temperature change in rather more detail.

8.2.1 THE INFLUENCE

OF

TEMPERATURE CHANGE

As temperature is lowered, many processes will slow. In tissue systems, there may be changes in membrane and organelle properties that produce altered metabolic pathways. Should this happen, the mere act of chilling can produce product quality loss, known as chilling damage (Lyons, 1973; Wilson, 1987), If chilling damage is not a problem, refrigeration close to the freezing point can lead to a significant extension of shelf life. Freezing and the freezing point are the next sources of complications to the simplified “lower temperatures give longer storage” picture. Why should this be? Freezing implies phase change. The aqueous component of the tissue separates into at least two phases, one of which is ice. Because some of the water has separated out as ice, the remaining liquid phase has to have increased solute concentrations. The presence of ice and the increase in solute concentration can have significant effects upon the state of the tissue that is being frozen (Brown, 1979; Reid, 1983). Let us therefore follow a freezing process in some detail, from this particular viewpoint.

8.2.2 THE FREEZING PROFILE In order to freeze, heat must be removed from the product. Figure 8.1 shows a typical freezing profile for a point close to the surface of a product. In region A, the temperature is falling but is still above the freezing point. At the surface, as cooling progresses, the temperature will reach the freezing point. Due to difficulty in seeding ice, freezing does not immediately initiate. The

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temperature

A

C

B D

time

FIGURE 8.1 Schematic cooling curve. See text for an explanation of segments.

temperature continues to fall. At some point, seeding (or nucleation) initiates freezing, and the temperature rises to close to the freezing point. This is region B in the plot. Closer to the center of the product, this undercooled region is not seen, and freezing initiates at the freezing point. As heat continues to be removed, the temperature now falls more slowly. The reason for the slower fall, given that the rate of heat removal is unchanged, is that heat is released by the phase change from water to ice. This heat, termed latent heat, is additional to the sensible heat loss that accompanies temperature change. This region of slower temperature change due to the heat release of ice formation is region C in the plot. As the process continues, the rate of ice formation decreases, the contribution of latent heat decreases, and the temperature begins to fall more rapidly. This is region D. Many workers, e.g., Persson and Lohndahl (1993), have labelled these regions as follows: region A — prefreezing, regions B and C — freezing, region D — reduction to storage temperature.

8.2.3 THE FREEZING PROCESS DESCRIBED

BY

PHASE DIAGRAM

Another view of this freezing process can be obtained through the use of a simplified phase diagram. If we assume that in the initial freezing process only ice will separate out, we can utilize the schematic binary phase diagram of Figure 8.2 to help describe the process. The product composition

FIGURE 8.2 Schematic phase diagram for a binary system. See text for an explanation of labels.

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is assumed to be represented by X. Initially, as the product is cooled, the composition stays unchanged. Segment PQ represents this initial cooling and corresponds to region A of Figure 8.1. Region B, the period of undercooling prior to the initiation of crystallization, is represented by the short section QR that lies below the liquidus curve (the liquidus curve shows the concentration dependence of the melting point, indicating the one temperature at which a solution of a given composition and ice can coexist in equilibrium). When freezing initiates, the system separates into two phases: (1) ice, represented by the left axis (i.e., 100% water) and (2) a more concentrated solution, where the concentration is defined by the liquidus coordinate for that temperature. This is represented by the short curve from R to Q¢, where Q¢ is a port on the liquidus curve QT. The curved segment of the liquidus from Q¢ to S illustrates the change in concentration of the nonice matrix as we move through region C. The segment of the liquidus from S to T is region D, where the rate of ice formation has reduced. A useful property of a phase diagram of this type is that it allows for the estimation of the amount of ice at any temperature. For example, the line S¢ S at temperature T1 crosses the lute PX at point L. For our system of overall composition X, at temperature T1, the ratio of the amount of ice to the amount of solution of composition represented by point S is simply S/LS¢. The overall composition is still X. This diagram shows that, as the temperature decreases, the amount of ice increases, and the composition of the unfrozen phase increases. At some temperature and liquid phase composition, a second phase may start to separate out, yielding what is termed a eutectic mix. This would happen at point E, where the solubility curve of the crystallized material intersects the liquidus. Solute crystallization does not always take place, due to kinetic constraints (Franks, 1982). Should this prove to be the case, the unfrozen phase continues to cool until it crosses a kinetic threshold and becomes effectively solid (in a glassy state). Since this glassy phase is produced by the freeze-concentration process just described, it is often referred to as the glassy state of the maximally freeze-concentrated matrix. The temperature of transformation to this glassy matrix can be measured and has some significance for frozen storage stability because it has been suggested that, once the unfrozen matrix enters the glassy state, the rate of change in storage will significantly reduce (Levine and Slade, 1989; Slade and Levine, 1991). The relevance of this in producing freezing is discussed briefly in Reid (1990). Current research in my laboratory seeks to establish this critical temperature for many frozen products, including fruits.

8.2.4 FREEZING

IN

TISSUE SYSTEMS

Up to this point, the discussion has been of freezing in a uniform system. Consider now the freezing process as it might occur in plant tissue. In addition to the complexities introduced by the formation of ice as temperature is lowered, an additional complexity is introduced. In plant tissues there are cells with cell walls. In other words, there are at least two distinct environmental situations. There are cell interiors that are individual, separate entities, and there are the extracellular spaces in between that exist in a connected network. This additional state, “inside or outside,” interacts with the phase state of the aqueous system. The interaction depends upon the properties of the cell wall boundaries. If the cell wall and cell membranes are intact and in functional form, they provide a barrier that is permeable only to certain small molecules, including water. If this barrier is somehow damaged, molecular movements through the barrier become much easier. Damage to the barrier can result from a variety of causes. In processing, the most common cause of damage is heat treatment, such as might be applied in blanching. If the barrier is intact, it will allow the process of osmosis (or selective water transfer) to occur between the cell contents and the external environment of the cell. This assumes particular relevance if the external environment is changing due to ice formation. A damaged barrier does not support osmotic processes. The process of osmosis is the passage of solvent through a barrier permeable to solvent but not to solute, in such an amount as to tend to equalize the concentration of the solutions on either side of the barrier.

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8.2.4.1 Freezing in the Presence of Cells Let us look at the freezing process again, taking into account the presence of cells. The first cooling, A, is the same. Once we reach regions B and C, the presence of cells changes the detailed picture. Ice forms external to cells, in general, because even if some ice growth initiates within a cell, it can only reach another cell by growing into the external matrix between cells. If ice is in the external matrix and the cell wall barrier is intact and effective, ice does not penetrate into the cell. Because water can permeate through the membrane, an osmotic process will occur. Water will leave the cell, forming additional extracellular ice and, at the same time, increasing the concentration of the internal cell contents in the direction of the concentration of the external unfrozen matrix. As the temperature falls and the external unfrozen matrix concentration increases as described by the liquidus line in the phase diagram, the concentration of the internal medium will tend to increase in the same manner. The maximum rate at which water can leave the cell is important to the effectiveness of this process, as this governs the maximum rate at which the concentration of the internal medium can increase. If water cannot be exported sufficiently rapidly, the internal contents will be more dilute than required for equilibrium (described by a coordinate point below the liquidus). The contents are therefore undercooled by an amount described by the difference between t h e liquidus temperature coordinate for the actual internal solution composition and the actual internal temperature. If the undercooling exceeds a threshold value characteristic of the particular tissue, internal seeding of ice and consequent ice growth may occur. Once ice forms within the cell, the concentrations of the internal and external unfrozen matrices match, and there is no longer an osmotic driving force for water transfer. The system-dependent variation in cross-barrier water transport rates of the osmotic process accounts, in part, for the differences between fast freezing and slow freezing. In fast freezing, there is insufficient time to remove the water from the cell through osmosis. The cell contents undercool and seed, and ice forms within the cell. In slow freezing, there is enough time to r e m ove t h e appropriate amount of water from the cell. The concentration of the cell contents increases sufficiently rapidly to prevent the cell contents from being significantly undercooled. Ice does not form within the cell. Note that, if any c h a n g e should occur on freezing that would prevent this water from returning to the cell on thawing, then the water will become a source of drip loss. It may be for this reason that drip loss is often more marked in slowly frozen fruits. Fast freezing and slow freezing are therefore operational definitions, and the threshold freezing rate separating fast freezing from slow freezing will be system dependent.

8.2.5 THE FREEZING PROCESS

AND

FREEZING DAMAGE

8.2.5.1 Osmotic Damage When heat is removed rapidly, ice forms rapidly. These ice crystals tend to be small. Because the ice grows rapidly, the concentration of the external unfrozen matrix rises rapidly. Osmotic transfer of water is limited: the cells freeze internally, and little water translocates. In slow cooling, the ice forms slowly, external to the cells, and there is sufficient time for a large amount of osmotic transfer of water from the cells. This results in cell shrinkage that can damage the membranes (Meryman, 1971; Steponkus, 1984). A considerable amount of water translocates. D u e t o c e l l wa l l damage consequent upon the freezing process, this water does not return to the cells on thawing but, rather, becomes drip loss. 8.2.5.2 Solute-Induced Damage In addition to this cell shrinkage mechanism for damage, primarily linked to the extensive cellular dehydration accompanying slow freezing, there are other mechanisms of damage. The high-solute concentrations of the unfrozen matrix, in particular the high salt concentrations, can cause damage to many polymeric cell components and may kill the cell (Mazur, 1977). To prevent this, some

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form of solution-based protection might be needed (Meryman et al., 1977). A typical method for reducing salt concentration-induced damage is to add sugars to the aqueous phase that is undergoing “freeze-concentration.” Note that these sugars must actually be incorporated into the solution that is freezing. It is not enough to add the sugar to the overall system. The concentration effect is present whether freezing is fast or slow. 8.2.5.3 Structural Damage In fast freezing, additional to the concentration effect, the formation of ice within the cell may cause damage to the delicate organelle and membrane structures of the cell. As one consequence, enzyme systems may be dislocated. This may result in uncontrolled enzyme action, leading to a variety of effects, including the production of off-flavors. Prevention of such enzyme-mediated damage can be achieved by utilizing blanching, a prefreezing heat treatment that denatures the enzymes and, hence, terminates their catalytic activity; however, it has to be remembered that blanching, because it is a heat treatment, will influence the semipermeable properties of the cell membrane and also destroy cell turgor. Cell turgor is an important component of the eating quality of many fruits. It is produced by the internal pressure of the cell contents. Lack of turgor is perceived as softness and lack of crispness and juiciness. Where turgor is an important product characteristic, blanching may not be an acceptable procedure, and other steps may be necessary to control enzymically initiated degradative processes. Blanching is not the only cause of reduced turgor. If cells become leaky or lose some of their contents, turgor is reduced or destroyed, and the texture of the fruit becomes much softer (Brown, 1977; Mohr, 1971). A loss of turgor caused by freezing is particularly evident in fruits such as strawberry (Szczesniak and Smith, 1969). Loss of turgor due to processing procedures is of most relevance to fruits that are customarily eaten raw, rather than fruits that are customarily cooked. Cooking, a more severe thermal treatment than blanching, destroys turgor so that the retention of turgor through earlier processing procedures is not necessary. Familiarity with the molecular picture of the freezing process is necessary if we are to appreciate the sources of the freezing damage that results in a reduction in consumer-perceived quality in comparison to the fresh raw product. Through an awareness of the mechanisms of damage it is possible to identify whether careful design and control of the freezing processes applied to the product might avoid or minimize some of the quality degradation.

8.3 INDUSTRIAL FREEZING METHODS 8.3.1 FREEZERS It is now appropriate to consider the industrial freezing processes that are applied to fruit products. In order to remove heat, the product must be brought into contact with a cold medium. This can be cold air, in a blast freezer, cryogenic liquids or gases in a cryogenic freezer, or cold surfaces in a plate freezer. The heat transfer mechanism in cold air is through convection. This can be assisted by blowing the air over the product. Cryogenic gases are similarly blown past the product and, due to their lower temperature, result in more rapid heat transfer. Cryogenic liquids have a higher thermal density and remove heat more rapidly due to an improved heat transfer capability. Heat transfer to a cold surface tends to be by conduction, and its effectiveness depends on the quality of the thermal contact. Reid (1991) discusses briefly the many types of commercial freezers that exist and that can be utilized in the freezing of plant tissues. This discussion includes consideration of the methods by which large quantities of the product can be exposed to freezing conditions, utilizing both batch and continuous methods. A more extensive discussion of freezers is given by Persson and Londahl (1993). The choice of appropriate freezer is, in part, governed by the product size and, in part, by the required freezing rate. Small fruits, which need to be frozen rapidly, might be frozen in a low-temperature, high air velocity blast freezer or in a cryogenic freezer to produce

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an individually quick frozen (IQF) product. Fruits packed in large cans or drums, on the other hand, will not freeze quickly due to heat transfer limitations and so are usually frozen in a cold room with a high-capacity cooling unit and reasonable air circulation. Note that it is poor practice to freeze such product in a cold storage room. A separate room should be set aside for product freezing. Retail packs of approximately rectangular brick shape are often frozen in plate (i.e., conduction) freezers. This method is reasonably successful, provided that the package is well filled. Air spaces within the package will slow down heat removal significantly.

8.3.2 PREFREEZING HANDLING

AND

PREPARATION

Prior to entering the freezer, the fruit must be prepared appropriately. Methods of preparation are specific to the individual fruit, but there are some common factors that can be identified. The preparation begins at harvest. Poor handling at this stage can irreversibly degrade final product quality. Methods of harvesting are discussed in several major textbooks, for example, Woodroof and Luh (1975) and Tressler et al. (1968). Acceptable harvesting methods are designed to minimize mechanical damage to the fruit. After harvest, cleaning will be required. The method of cleaning is fruit specific. Sorting and trimming will also be required to remove undesirable materials. Many fruits are peeled, or they may be sliced. A variety of specially designed pieces of equipment exists for these tasks. It is often necessary to remove field heat rapidly prior to these process steps, especially for highly perishable fruits, if unacceptable damage is to be avoided. The processor has the responsibility of delivering the fruit to the freezing equipment at as high a quality level as is practical.

8.4 FREEZING METHODS FOR SPECIFIC FRUITS In this section, details of the processing procedures for selected fruits are given. The information summarized in this section comes from a variety of sources, including Woodroof and Luh (1975), Tressler et al. (1968), and TRRF (1993).

8.4.1 APPLE Not all varieties of apple result in an acceptable product when frozen, whether the end use is for the bakery trade or for other purposes; however, some cultivars of apple are suitable for freezing. These are sorted, washed, peeled, cored, and sliced. The sliced fruit is treated to minimize enzymatic browning. This can be achieved by application of antioxidant solutions, including some proprietary treatments. One common treatment is salt brining, soaking slices in a 1% salt solution in order to remove intercellular air. Ascorbic acid solutions can also be used, but these tend to be expensive. A surface blanch, using steam or boiling water, can also be employed, though it results in a softened slice that may not be suited for some uses. The treated slices in a ratio of five parts fruit to one part dry sugar are frozen in large 30- to 50-lb containers in a blast freezer below –10∞F. Some apples may be frozen using a dehydrofreezing process where about 50% of the water is removed from the apple slices by standard drying equipment prior to freezing the slices.

8.4.2 APRICOT Though some apricots are frozen whole for later processing, the major proportion of apricots are usually frozen as peeled apricot halves. This enhances the tendency for browning and, therefore, requires steps to be taken to minimize browning. The apricots are peeled, halved and pitted, and dipped in ascorbic acid solution to minimize browning, or blanched for a short time to inactivate the enzymes. The halves are packed in sugar or sugar syrup prior to freezing at a 3:1 or 4:l ratio of fruit to sugar. Air blast freezers are adequate. It is best to freeze the apricots on trays or on a

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belt prior to packing into barrels or 30-lb containers. This helps minimize discoloration. Storage should be below 0∞F. For good retention of ascorbic acid, storage should be at –20∞F.

8.4.3 AVOCADO Avocados present a challenge to the commercial freezer due to their high oil content that readily becomes rancid, and also because of a very active oxidative browning system. Pureed avocado is a successful product. Preservation life is enhanced by lowering the pH of the puree to below 4.5 through the addition of lemon juice, lime juice, and salt. Packaging under nitrogen also enhances shelf life. Vacuum packaging has also been employed. Any reasonably rapid freezing method can be employed. Storage should be around 0∞F for a reasonable shelf life.

8.4.4 BERRIES Many varieties of berries are frozen. Berries can be frozen in syrup or as individual berries. As individual berries, they may be tray frozen or IQF frozen on a belt in an air blast or cryogenic freezer. Individually frozen berries will be discussed after bulk methods for freezing for retail or the processing trade. Red raspberries for retail are packed in an approximately 50% syrup, in the proportion of six parts berry to four parts syrup, and 10- and 16-oz containers are used. Any reasonably rapid freezing method may be employed. Black raspberries are used for further processing and are packed in 30-lb containers or larger. In order to successfully freeze berries in a large container, the following procedures have been shown to be necessary (TRRF Commodity Storage Handbook, 1993). The temperature of the fruit should not exceed 60∞F at the time of filling. The containers should be moved to the freezer as quickly as possible. The temperature on entering the freezer should be below 70∞F. Freezer conditions (air temperature < –15∞F and airflow velocity high) should allow for the center of the container to reach a temperature of 32∞F or less within 48 h. 5. Freezing should be continued until the center temperature is 0∞F. This should take no more than 4 to 5 d. 6. Storage should be below 0∞F.

1. 2. 3. 4.

Reasons for these recommendations were discussed previously in the text. Blackberries, boysenberries, loganberries, and others are frozen utilizing the same procedures as have been described for raspberries. Blueberries are frozen in 20-lb containers, with steps being takers to minimize or eliminate air in the package.

8.4.5 CHERRY The major portion of cherries frozen are tart cherries, though some sweet cherries are also frozen. The procedures for freezing are essentially the same. Tart cherries are harvested when bright red, sweet cherries when mature. The cherries are held and transported in ice-cold water, which reduces losses due to crushing and bruising and makes the fruit firmer for pitting. Fruits are size graded, pitted, packed with sugar in large cans, and frozen in a blast freezer.

8.4.6 COCONUT Shredded coconut can be frozen without any particular preparation. The rate of freezing is not critical so long as cooling is sufficiently rapid to minimize microbiological contamination. Storage, in large containers, is at 0∞F.

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8.4.7 CRANBERRY Cranberries are frozen at 0∞F using conventional techniques. The majority of the frozen crop is used for processing.

8.4.8 DATES Fresh dates may be frozen. The use of a good moisture-proof and vapor-proof wrapping is recommended to prevent moisture loss during freezing or storage.

8.4.9 FIGS Figs can be frozen as whole fruit in heavy syrup or as sliced fruit as four parts fruit to one part water. Standard freezing methods are employed. Storage temperatures should be below 0∞F.

8.4.10 MANGO Mango is frozen as slices in syrup. The syrup contains ascorbic acid to inhibit polyphenol oxidaseinduced browning. In addition, mango puree is a significant frozen product. Purees can be single or double strength. Storage should be at or below 0∞F. Browning can be a significant problem at higher storage temperatures due to nonenzymic browning.

8.4.11 MELON Melon is frozen when the texture is firm enough to allow for cutting of cubes or balls that retain their integrity. If too ripe, a very mushy product will result because fully thawed melon loses considerable texture. Melon is usually frozen in syrup.

8.4.12 PAPAYA Papaya puree is prepared from ripe papaya. Steamed fruit can be sliced and crushed, and the pulp can be separated from the skin. The acidified pulp is passed through a heat exchanger to inactivate enzymes before cooling and freezing to –10∞F.

8.4.13 PEACH In general, freestone peaches are used for freezing. Yellow fleshed varieties are preferred for better texture and lower susceptibility to oxidative browning. Fruit for freezing is usually harvested while still firm and then ripened under control. The peaches are pitted, peeled, and sliced prior to freezing. The usual pack is in syrup (one part syrup to five parts peach) containing around 250 ppm ascorbic acid to help protect against browning. Freezing is usually in packages; 32- to 40lb packs are common. Larger barrels are also available. Freezing methods are, in general, as described for other bulk frozen fruit products. Some lQF slices are frozen for special markets. Storage should be at temperatures below 0∞F if extended shelf life is required. The limiting change is the browning.

8.4.14 PINEAPPLE Pineapple for freezing is prepared in the same way as pineapple for canning. Rectangular chunks are filled in syrup into cans or bulk containers and frozen. The cans are frozen in a blast tunnel, the bulk containers in a blast freezer. The Smooth Cayenne variety should be frozen. The Red Spanish variety has a tendency to develop off-flavors on freezing.

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8.4.15 PLUM A small volume of purple plums and prunes are frozen for institutional markets and for further processing. The fruit is halved, pitted, and packed in syrup in barrels. Freezing is by standard methods. Storage is at or below 0∞F.

8.4.16 RHUBARB Rhubarb freezes easily and requires no special treatment, though a short blanch can extend the storage life significantly. Rhubarb can be frozen with or without sugar. Stalks are trimmed to fit the package. Storage life at 0∞F is at least 6 months.

8.4.17 STRAWBERRY Not all varieties of strawberry freeze well. The selection of varieties for freezing should be made in conjunction with agricultural advisors familiar with the production state. Strawberries are frozen in several forms, depending on the final end use. Most strawberries are frozen as a raw material for use in further processing. Depending on the final product, different freezing procedures might be appropriate. For use in jam manufacture or ice cream, strawberries are packed in syrup and frozen. This can be in 30-lb tins or 50-gal barrels. The strawberries may be sliced and sugared for this process. The procedures described under berries are appropriate. Because strawberries are even more fragile than many other berries, it is recommended that the critical times be shorter. For example, a core temperature of 15∞F should be reached in no more than 24 to 36 h. Storage should be at 0∞F or below for a reasonable shelf life. Flavor and color are lost rapidly if the storage temperature is too high. Prefreezing treatments may be applied to strawberries to stabilize the integrity of the tissues. Suutarinen et al. (2000) studied the effects of various calcium chloride or sucrose prefreezing treatments on the textural integrity and drip loss in frozen strawberries. These authors examined CaCl2 concentration of the dipping solution (1, 5.5, or 10 g/l), dipping time (0.25, 7.625, or 15 min) and solution temperature (25, 37.5, or 50∞C) and found the greatest firmness resulted from the combination of 5.5 g/l CaCl2 applied for 7.625 min at 37.5∞C. Crystallized sucrose was compared to dips in water–sucrose solutions (350 and 700 g sucrose/l) and dipping times of 1 and 15 min were utilized. Sucrose prefreezing treatments also resulted in greater cellular integrity, with those strawberries sprinkled with crystalline sucrose having the highest firmness. IQF methods are used to produce frozen whole strawberries for both institutional and retail trade. Freezing utilizes air blast, liquid nitrogen, or carbon dioxide belt freezers. Storage of IQF fruit should be at a stable, low temperature to prevent clumping of the berries (due to moisture migration) and loss of the lQF character.

8.4.18 TOMATO Whole tomatoes are not an item of frozen commerce. They lose turgor and, hence, texture on freezing and are no longer suited to the uses common for fresh tomatoes. Chopped or pureed whole tomatoes can be frozen and stored for 6 to 9 months at 0∞F for use in further processing. Other tomato products such as purees, sauces, and pastes can readily be frozen. They are commonly employed as ingredients in other frozen products. Freezing provides an advantage of color stability compared to other storage methods.

8.5 EFFECTS OF FREEZING ON NUTRITIONAL COMPONENTS Consumers are becoming increasingly aware of the importance of nutritional components in their diets, and the potential for fruit and vegetables in particular, to provide beneficial health effects.

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Fruits are a relatively significant source of various antioxidant compounds, including the polyphenolics, carotenoids, and vitamins. Preservation of fruit by freezing, and the effect of this process on various antioxidant components, have been the subject of many recent investigations. A select group of publications on this topic will be highlighted in this section. Asami et al. (2003b) evaluated the effects of storage at refrigeration and frozen temperatures on the concentration of total phenolics in clingstone peaches. Maturity stage III peaches of the Ross variety were peeled, pitted, sliced, and frozen at –12∞C for a period of 3 months. There appeared to be a statistically significant increase (P < 0.05) in total phenolic content following freezing, and this higher content was retained after 2 and 3 months of frozen storage. It was postulated that the freezing process may have resulted in cellular disruption and more facilitated extraction of phenolics. The effect of freezing and frozen storage on raspberry phytochemicals and volatiles was the subject of two manuscripts by de Ancos and colleagues (de Ancos et al., 2000a, 2000b). These authors compared two early-season and two late-season raspberry cultivars and found differential effects of freezing. In the early-season cultivars, freezing resulted in increased anthocyanin content, while in the late-season cultivars, which initially had higher concentrations of anthocyanins, freezing caused an overall reduction. The authors suggested that the preservation of anthocyanins during freezing depends on the pH of the fruit, organic acid content, sugar concentration, initial anthocyanin concentration, and initial cyaniding-3-glucoside content. They did not find a relationship between polyphenol oxidase activity and anthocyanin content. De Ancos and colleagues (2000b) also found that freezing had a slight effect on ellagic acid, vitamin C, and total phenolics, depending on the raspberry cultivar. Free radical scavenging capacity was decreased as a result of the freezing process, anywhere from 4 to 26%, again related to cultivar. Frozen storage of raspberries at –20∞C for a 1-year period did not appear to affect total phenolics or free radical scavenging capacity, but did cause a decline in ellagic acid vitamin C. In another study of the effects of freezing on raspberry phenolics, ellagitannins, flavonoids, and antioxidant capacity (Mullen et al., 2002), these authors found that the antioxidant capacity of the fruit and vitamin C levels were not affected by freezing. The raspberry cultivar used in this study differed from those evaluated by de Ancos, however, and this may have affected the results. Freezing preservation of fruit and vegetables is less destructive toward some antioxidant compounds, in particular total phenolics and ascorbic acid, than other means of preservation. One illustration of this is a recent publication (Asami et al., 2003a) in which Marionberries, strawberries, and corn were preserved using freezing, freeze-drying, and air-drying methods. The highest levels of both total phenolics and ascorbic acid (reduced form) were consistently found in the extractions of frozen samples, followed by those of freeze-dried and then air-dried samples. Freezing may cause some damage to cell structure, and application of a drying procedure following freezing, even though this is under vacuum at reduced temperatures, may result in even greater losses of beneficial nutrients. Air-drying at temperatures above 60∞C may result in oxidative condensation or decomposition of thermolabile compounds, such as (+)catechin and ascorbic acid. Therefore, the presence of total phenolics and ascorbic acid in the air-dried products was lower than that in either frozen or freeze-dried products.

REFERENCES Asami, D.K., Hong, Y.-J., Barrett, D.M., and Mitchell, A.E. 2003a. A comparison of the total phenolic and ascorbic acid contents of freeze-dried and air-dried Marionberry, strawberry, and corn grown using conventional, organic, and sustainable agricultural practices. J. Agric. Food Chem., 51, 1237–1241. Asami, D.K., Hong, Y.-J., Barrett, D.M., and Mitchell, A.E. 2003b. Processing-induced changes in total phenolics and procyanidins in clingstone peaches. J. Sci. Food Agric., 83, 56–63. Brown, M.S. 1977. Texture of frozen fruits and vegetables. J. Texture Stud., 7: 491–494. Brown, M.S. 1979. Frozen fruits and vegetables: Their chemistry, physics, and cryobiology. Adv. Food Res., 25: 181–235.

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de Ancos, B., Ibanez, E., Reglero, G., and Cano, M.P. 2000a. Frozen storage effects on anthocyanins and volatile compounds of raspberry fruit. J. Agric. Food Chem., 48, 873–879. de Ancos, B., Ibanez, E., Reglero, G., and Cano, M.P. 2000b. Ellagic acid, Vitamin C, and total phenolic contents and radical scavenging capacity affected by freezing and frozen storage in raspberry fruit. J. Agric. Food Chem., 48, 4565–4570. Franks, F. 1982. The properties of aqueous solutions at subzero temperatures. In Water: A Comprehensive Treatise, Vol. 7, F. Franks (Ed.), Plenum, New York, pp. 215–338. Levine, H. and Slade, L. 1989. A food polymer science approach to the practice of cryostabilization technology. Comments Agric. Food Chem., 1: 315–396. Lyons, J.M. 1973. Chilling injury in plants. Annu. Rev. Plant Physiol., 24: 445–466. Mazur, P. 1977. The role of intracellular freezing in the death of cells cooled at supraoptimal rates. Cryobiology, 14: 251–272. Meryman, H.T. 1971. Osmotic stress as a mechanism of freezing injury. Cryobiology, 8: 489–500. Meryman, H.T., Williams, R.J., and Douglas, M.St.J. 1977. Freezing injury from solution effects and its prevention by natural or artificial cryoprotection. Cryobiology, 14: 287–302. Mohr, W.P. 1971. Freeze thaw damage to protoplasmic structures in high moisture edible plant tissue. J. Texture Stud., 2: 316–327. Mullen, W., Stewart, A.J., Lean, M.E.J., Gardner, P., Duthie, G.G., and Crozier, A. 2002. Effect of freezing and storage on the phenolics, ellagitannins, flavonoids, and antioxidant capacity of red raspberries. J. Agric. Food Chem., 50: 5197–5201. Persson, P.O. and Londahl, G. 1993. Freezing Technology. In Frozen Food Technology, C.P. Mallett (Ed.), Blackie Academic and Professional, London, pp. 20–58. Reid, D.S. 1983. Fundamental physiochemical aspects of freezing. Food Technol., 37: 110–115. Reid, D.S. 1987. The freezing of food tissues. In The Effects of Low Temperature on Biological Systems, B.W.W. Grout and G.J. Morris (Eds.), Edward Arnold, London, pp. 478–488. Reid, D.S. 1990. Optimizing the quality of frozen foods. Food Technol., 44: 78–84. Reid, D.S. 1991. Freezing. In Vegetable Processing, D. Arthey and C. Dennis (Eds.), Blackie, Glasgow, Scotland, pp. 102–122. Reid, D.S. 1993. Basic physical phenomena in the freezing and thawing of plant and animal tissues. In Frozen Food Technology, C.P. Mallett (Ed.), Blackie Academic and Professional, Glasgow, Scotland, pp. 1–19. Slade, L. and Levine, H. 1991. Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Crit. Rev. Food Sci. Nutrition, 30: 115–360. Steponkus, P.L. 1984. Role of the plasma membrane in freezing injury and cold acclimation. Annu. Rev. Plant Physiol., 35: 543–584. Suutarinen, J., Heiska, K., and Autio, K. 2000. The effects of calcium chloride and sucrose prefreezing treatments on the structure of strawberry tissues. Lebensm.-Wiss. Technol., 33: 89–102. Szczesniak, A.S. and Smith, B.J. 1969. Observations on strawberry texture, a three pronged approach, J. Texture Stud., 1: 65–89. Tressler, D.K., van Arsdel, W.B., and Copley, M.J. (Eds.). 1968. The Freezing Preservation of Foods. 4 volumes, AVI, Westport, CT. TRRF. 1993. Commodity Storage Handbook. The Refrigeration Research Foundation, Washington, D.C. Wilson, J.M. 1987. Chilling injury in plants. In The Effects of Low Temperatures on Biological Systems, B.W.W. Grout and G.J. Morris, (Eds.), Edward Arnold, London, pp. 271–292. Woodroof, J.G. and Luh, B.S. 1975. Commercial Food Processing. AVI, Westport, CT.

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9 Thermal Processing of Fruits Hosahalli S. Ramaswamy CONTENTS 9.1 9.2

Introduction ........................................................................................................................174 Thermal Process Considerations........................................................................................174 9.2.1 Thermal Resistance of Microorganisms and Enzymes.......................................175 9.2.1.1 Oxygen, pH and Temperature Sensitivity .........................................175 9.2.1.2 Microflora in Canned Fruits ..............................................................176 9.2.1.3 Composition of Fruits ........................................................................177 9.2.1.4 Microbial Destruction Kinetics .........................................................177 9.2.1.5 Heat Penetration Curve......................................................................180 9.2.1.6 Heat Penetration Parameters..............................................................181 9.3 Thermal Process Calculations............................................................................................181 9.3.1 Original General Method.....................................................................................181 9.3.2 Improved General Method ..................................................................................183 9.3.3 The Formula Methods .........................................................................................183 9.3.3.1 Ball’s Method.....................................................................................183 9.3.3.2 Stumbo’s Method ...............................................................................184 9.4 Process Calculation Methodology for Pasteurization of Fruits ........................................184 9.5 Fruit Canning Operations...................................................................................................187 9.5.1 Raw Material Selection .......................................................................................188 9.5.2 Washing................................................................................................................188 9.5.3 Sorting/Grading....................................................................................................189 9.5.4 Blanching .............................................................................................................189 9.5.5 Prevention of Fruit Browning..............................................................................190 9.5.6 Peeling/Preparation ..............................................................................................191 9.5.7 Filling...................................................................................................................191 9.5.8 Exhausting and Vacuum ......................................................................................193 9.5.9 Can Seaming and Closing ...................................................................................193 9.5.10 Container Coding.................................................................................................194 9.5.11 Retort Operations.................................................................................................194 9.5.12 Cooling.................................................................................................................195 9.5.13 Labeling and Storage...........................................................................................195 9.6 Quality Control of Canned Fruits......................................................................................196 9.7 Grades for Canadian and U.S. Canned Fruits...................................................................196 9.8 Nomenclature .....................................................................................................................198 References ......................................................................................................................................199

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9.1 INTRODUCTION Since ancient times, several techniques have been used to preserve fruits and their products: drying, concentration, freezing, fermentation, and chemical preservation by use of vinegar, wine, sugars, and spices. Thermal processing is relatively a more recent technique used for fruit preservation, but it has proven to be one of the most effective (Woodroof and Luh, 1986). Nicholas Appert, in 1809, was awarded a prize by the French government for developing a new method for heat preservation of food. The method has since been recognized as Appertization (named after Appert), sterilization, or more commonly, canning. Appert utilized his method to preserve more than 50 kinds of foods, some of which included fruits, vegetables, and meats. In the early stages of the method, food was filled into wide-mouthed glass bottles or jars and carefully corked, after which they were cooked in boiling water. The process time varied according to the product type. Although the method was successful in producing shelf-stable foods which did not undergo spoilage, the scientific basis for the preservation was not really known until almost half a century later, when another Frenchman, Louis Pasteur, discovered that food spoilage was caused by microorganisms which are destroyed at elevated temperatures. The technique which bears his name, pasteurization, as we know today, refers to a relatively mild heat treatment intended to destroy pathogenic microorganisms in foods, providing short-term extension of shelf life under refrigerated conditions. By the turn of the century, significant progress achieved in the microbiology of food spoilage, understanding of thermal destruction kinetics of microorganisms, and heating behavior of packaged foods led to scientific approaches in thermal process calculations. The intervening period also saw significant developments in the manufacturing of thermal processing equipment in the form of improved versions of retort systems. Next in the line of process equipment were continuous systems for thermal sterilization of food cans and glass jars and systems for handling high-temperature short time (HTST) processes in batch or continuous modes in still or rotary autoclaves. Developments such as aseptic processing and packaging, thin profile processing, fully automated agitating retorts, and retort systems based on different media have revolutionized the food industry in the last few decades. New processes such as combined methods technology (Alzamora et al., 1993) are continually being introduced, especially for heat-sensitive products, such as delicate fruits and fruit juices, to preserve overall color, flavor, and other quality attributes. Fruits, in general, are commodities with special organoleptic properties which must be carefully preserved when establishing operating conditions for a thermal process. Although the quality of canned fruits is generally considered to be inferior to that of frozen fruits, the total U.S. per capita consumption of canned fruits in 1992–1993 was reported to be approximately four times that of dried and frozen fruits together (Almanac, 1993). Per-capita consumption of frozen and dried fruit was 4.66 and 3.24 lb (product weight basis), respectively, whereas the total was 14.40 lb for canned fruits. A 1989 survey conducted in 2000 American households (Market Reserve Corporation of America Information Service) indicated that the greatest per-capita consumers of canned fruit were those in the 0 to 5 and +55 age groups. The tendency for canned fruit consumption lies in lower income groups and those who demand more convenience type foods. The California canned-fruit industry, which is one of the most competitive leaders in the industry, has an estimated value of $600 million for canned peaches, pears, apricots, and fruit cocktail (Moulton, 1992).

9.2 THERMAL PROCESS CONSIDERATIONS Canning is the most commonly used technique to heat-sterilize foods in order to prevent microbiological and enzymatic spoilage. A variety of foods are canned such as meats, fish, poultry, fruits, vegetables, dairy, and vegetable products. Heat processes used for these foods are dependent on the type of food, its chemical composition and type(s) of microorganism(s) that cause spoilage or

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public health concern, in addition to properties related to container material, shape, and size as well as the properties related to the heating medium. Any thermal process for a food should be designed to achieve three basic objectives, the most important being to reduce the number of microorganisms to statistically small levels, whether they are of public health concern or of the spoilage type which cause off-flavors and odors. The second objective is to create an environment in the container which would suppress the growth or activity of spoilage type microorganisms by utilizing one or more of the following methods: (1) oxygen removal, (2) pH control, and (3) control of storage temperature. The third objective is to assure an adequate or hermetic seal of the container to prevent recontamination following processing and during storage. Thermal processing is not intended to completely sterilize the packaged food. Such an approach might produce a stable product, but it will be at the expense of severe destruction of product quality. The success of thermal processing depends on selectively destroying the microorganisms of spoilage and public health concern while creating an environment around the product to minimize the growth and activity of other microorganisms. In order to determine the extent of heat treatment, several factors must be known (Fellows, 1988): (1) The type and the heat resistance of the target microorganism, spore, or enzyme present in the food; (2) The pH of the food; (3) The storage conditions following the process; (4) The heating conditions; and (5) The thermophysical properties of the food, and container shape and size.

9.2.1 THERMAL RESISTANCE

OF

MICROORGANISMS

AND

ENZYMES

9.2.1.1 Oxygen, pH and Temperature Sensitivity Target microorganisms and enzymes for thermal destruction in a food vary according to the type of food and its composition. Thus, these target components and their respective thermal resistance determine the thermal process itself. The first step in designing the thermal process is to verify the type of test microorganism or enzyme upon which the process should be based. Several factors should be considered in this aspect. For example, hermetic conditions that are used to package foods with extremely low oxygen levels (under vacuum such as in sous vide products) provide an atmosphere which does not support the growth of microorganisms that require oxygen (obligate aerobes) to cause food spoilage or public health problems. Further, the spores of obligate aerobes are less heat resistant than the microbial spores that grow under anaerobic conditions (facultative or obligate anaerobes). The growth and activity of these anaerobic microorganisms are largely pH dependent. Consequently, the most important factor affecting microbial spoilage is acidity, and heat processing requirements for various foods depend mainly on pH. From a thermal processing standpoint, foods are categorized into three pH groups: (1) high-acid foods (pH < 3.7), (2) acid or medium-acid foods (3.7 < pH < 4.5), and (3) Low-acid foods (pH > 4.5). The pH classification of thermally processed foods is extremely important, especially for the determination of processing requirements and specific criteria. The establishment of the resulting thermal process is generally based on Clostridium botulinum, which is a highly heat-resistant, rodshaped, spore-forming, anaerobic pathogen. The mere presence of the pathogen alone in a product does not constitute a health hazard; however, concern in this respect is warranted if the spores are permitted to germinate in favorable conditions for growth and produce a deadly botulism toxin. It has been generally recognized that C. botulinum does not grow and produce toxin below a pH of 4.6, which is the pH of most fruits and fruit products. Berries and most fermented foods are included in this category because they are considered high-acid foods (pH < 3.7). Consequently, the designated pH to separate the low acid group from the acid group is set at 4.5 such that in the mediumand high-acid foods (pH < 4.5), C. botulinum would not pose any potential threat. On the other hand, in the low-acid foods (pH > 4.5), the most heat-resistant spore former, C. botulinum, may cause foodborne illness and death following production of the fatal toxin. This can easily occur under anaerobic conditions that prevail inside a sealed container, provided, of course, the pathogen

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is not destroyed by the heat treatment. There are other microorganisms, for example, Bacillus stearothermophilus, Bacillus thermoacidurans, and Clostridium thermosaccolyaticum, that are more heat resistant than C. botulinum. These are generally thermophilic in nature (optimal growth temperature ~ 50 to 55∞C) and hence are of little concern if the processed cans are stored at temperatures below 30∞C. The phrase minimal thermal process was introduced by the U.S. Food and Drug Administration in 1977 and defined as “the application of heat to food, either before or after sealing in a hermetically sealed container, for a period of time and at a temperature scientifically determined to be adequate to ensure the destruction of microorganisms of public health concern.” For low-acid foods, C. botulinum is designated the microorganism of public health concern. Due to its high heat resistance, temperatures of 115 to 125∞C are commonly employed for the processing of these foods. Thermal processes for acid foods and medium-acid foods are usually based on the destruction of heatresistant spoilage-type vegetative bacteria, yeasts, or molds, or the inactivation of enzymes at temperatures below 100∞C. Thus, thermal processes for such foods are normally carried out in boiling water. If not inactivated following thermal processing, several heat-resistant enzymes in fruits (including peroxidase, pectinesterase, lipoxygenase, catalase, and polyphenoloxidase) may cause undesirable quality changes in the final canned fruit product during storage, especially related to color, texture, and flavor attributes. For the thermal processing of acid foods (including pasteurization), the inactivation of these enzyme systems is often used as a basis because they usually possess a higher thermal resistance than the microorganisms present in the food. As peroxidase is known to have a very high heat resistance, its destruction is often used as an adequate marker for the destruction of all heat-resistant enzymes present in the food. Nevertheless, the heat resistance of enzymes varies with the fruit, its variety, pH, and TSS (total soluble solids) (Ranganna, 1986). Despite the established processing criteria for acid foods and high-acid foods, fruits that receive a heat treatment such as blanching or cooking have recently been designated as potentially hazardous by the Food and Drug Administration Retail Foods Branch (Madden, 1992). All fruits are subject to a variety of conditions during growth, harvest, and distribution, all of which provide sources of microbial contamination. Although most raw fruits possess a protective barrier that prevents penetration of potentially pathogenic microbes, it is recognized that thermal processing may destroy this barrier, allowing potential pathogens to penetrate the produce and to grow in conditions conducive to their growth. In order to prevent contamination and foodborne illnesses, it is recommended that chlorinated water be used to remove soil from produce and that proper refrigeration, storage, and shipping conditions be maintained. 9.2.1.2 Microflora in Canned Fruits Spoilage of fruits in cans is mainly caused by microorganisms that are sensitive to heat in humid conditions and are readily destroyed at temperatures between 60 and 100∞C within a few minutes. Spore-forming bacteria such as Bacillus thermoacidurans and Clostridium pasteurianum may be found in acid foods but are destroyed by sterilization temperatures. The former is an aerobic thermophile which produces flat sour. The latter is a spore-former anaerobic saccharolytic gas producer. Nonsporulating bacteria include Lactobacillus and Leuconostoc that occur in stewed fruits and sweet and sour cucumbers and that cause can-bulging due to carbon dioxide production. They are the two most resistant types of nonsporulating bacteria that require heating to 88∞C or a hold at 64.5∞C for 10 min. Yeasts that are of major concern are those that have low heat resistance and that cause spoilage in canned foods only in cases of gross underprocessing or can leakage resulting from faulty seams. A 5-min exposure to 66∞C destroys living forms and the same exposure time at 80∞C destroys spores. Molds in canned fruits are quite insignificant because some are destroyed together with spores in 30 min at 65 to 70∞C, the effect being greater in anaerobic conditions. Some molds

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possess an extremely high resistance to heat in canned acid foods, and these can be found in the species Byssochlamys, Paecilomyces, and Phialophora. Byssochlamys fulva is exceptionally heat resistant as it breaks down pectinous materials disintegrating fruit and sometimes producing gas. Its limited ability to grow in the presence of air permits growth in products such as sterilized compotes, jams, and fruit preserves. It has been found that B. fulva is more heat resistant in foods containing malic acid, citric acid, and tartaric acid as compared to foods containing higher quantities of lactic acid and acetic acid (Kyzlink, 1990). Obviously, the heat process for the preservation of fruits will not necessarily kill spore-forming microorganisms such as C. botulinum that will not normally grow in acidic conditions but may nevertheless still be present in the acidic food as viable bacterial spores even after the product has undergone an adequate thermal process for the destruction of aciduric bacteria, yeasts, and molds. The thermal resistance of microorganisms in fruits is dependent upon various factors such as the amount and type of sugar present, the pH, and type of acid (Desrosier, 1977). Organic acids have a detrimental effect on microorganisms due to the toxicity of the hydrogen ion and the undissociated molecule. Lower pH levels are more toxic to bacteria, which explains the addition of an acidulant to adjust the pH to a standard value often allowing for a shorter sterilization time. In some instances, however, decreasing the temperature is also possible. Different acids possess various levels of effectiveness in lowering the heat resistance of microorganisms. This order is lactic > citric > acetic. Based on the pH of the product, the order is acetic > citric > lactic (Ranganna, 1986). 9.2.1.3 Composition of Fruits The acids present in fruits are advantageous for preservation, especially because they have a bacteriostatic effect. In good-quality fruit ready for processing, the organic acids most frequently encountered are citric, malic, and tartaric acids, although quinic acid is sometimes present in considerable quantities in apples and pears (Kyzlink, 1990). In blueberries, 40% of the organic acids present is quinic acid. Lactic and acetic acid are also present in varying amounts. Malic acid is present in significant amounts in stone fruits such as cherries, apples, and apricots. Citric acid is found in berries, red currants, strawberries, raspberries, and citrus fruits. Organic acids reach peak levels in fruit just as they reach the stage of ripeness. Consequently, normal pH values in fruits correspond to an organic acid content of between 0.3 and 3.1% (averaging approximately 1%). Malic acid, for example, reaches a peak of 3% in apples at ripeness. The organic acid content will tend to decrease in several types of fruit at the end of the ripening period through conversion to sugars. Thus, the acid content decreases while sugar content increases. During storage, the acid is consumed through respiration. Fruits are rich in sugars; however, their levels depend on a variety of factors, such as species, soil, location, and the ripening stage. Sugar levels generally range between 0.5 and 25%. At first when the fruit is detached from the mother plant, sugar levels decrease due to the increase in respiration rate (cells consume sugar). In the maturing fruit, the total sugar content rises due to two main reasons: (1) hydrolysis of polysaccharides and (2) formation of sugars as secondary products following acid conversion. 9.2.1.4 Microbial Destruction Kinetics In order to establish the thermal processing schedule consisting of an appropriate heating time at a specified temperature, the thermal destruction rate of the test microorganism or enzyme must be determined under the conditions that normally prevail in the container. In addition, the temperature dependence of the microbial destruction rate or enzyme inactivation rate must be known in order to integrate the destruction or inactivation effect through the temperature profile, mainly due to the come-up time (CUT) required to achieve processing temperatures.

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Number of Survivors

105

104

103 D 102

101 0

2

4

6

8

10

Heating Time (min)

FIGURE 9.1 A typical survivor curve.

Survivor curves and D value: The thermal destruction of microorganisms generally follows a first-order reaction or a logarithmic order of death. The logarithm of the surviving number of microorganisms following a heat treatment at a particular temperature is plotted against heating time to give a straight line “survivor” curve. Thus, the same percentage of surviving microorganisms will be destroyed in each successive unit of time irrespective of the number present. The microbial destruction rate is defined as a decimal reduction time (D value) which is the heating time in minutes at a given temperature required to result in one decimal reduction (or 90% destruction) in the surviving microbial population. This is depicted graphically as the time between which the survival curve passes through one logarithmic cycle (Figure 9.1). The same concept may be extended to enzyme inactivation kinetics where a heat-resistant enzyme in a fruit product is subjected to a thermal process at a particular temperature. The resulting plot of residual enzyme activity vs. time would yield the D value of the enzyme, which is the heating time in minutes required to inactivate 90% of total enzyme activity. Thermal death time (TDT) and D value: The thermal death time (TDT) is the heating time required to cause microbial death or destruction and is obtained by subjecting the microbial population to a series of heat treatments at a given temperature and testing for survivors. Theoretically, complete destruction of a microbial population or inactivation of enzymatic activity is not possible because a decimal fraction of the population would ultimately survive even after numerous D values. The microbial death would indicate the failure to show positive growth in a subculture media after heating. The TDT value differs from the D-value approach in that the former depends on the initial microbial load, whereas the D value does not. In practice, calculated fractional survivors are treated by a probability approach; for example, a surviving population of 10–8/unit would indicate one survivor in 108 units. Thermal inactivation time (TIT) of enzymes: The heat resistance of enzymes is dependent on a variety of factors, some of which include the fruit and its variety, the pH, and TSS. A thermal inactivation curve is established in a manner similar to that of the TDT curve for bacteria. This is done by subjecting the food sample to a series of heat treatments at a specific temperature and testing for residual enzyme concentration. When there is no measurable residual activity, the enzyme is considered inactivated and the time taken as TIT. Since most enzyme systems in fruits (peroxidase, pectinesterase, and polyphenoloxidase) possess higher thermal resistance than microorganisms, the calculation of the process time for canned fruits is based on the TIT of the most heat-resistant enzyme present in the product. Temperature dependence and z value: The D value of a particular microorganism or enzyme is strongly dependent on the processing temperature. Increased heating temperatures

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D Value, TDT or TIT (min)

103

102

101 z 100 220

230

240

250

260

270

Temperature (°F)

FIGURE 9.2 A typical thermal resistance curve.

clearly yield lower D values. A thermal resistance curve with logarithm of D values plotted against temperature would yield a temperature sensitivity indicator, z value, which is defined as the temperature range that results in a 10-fold change in D values. Graphically represented (Figure 9.2), the z value indicates the temperature range between which the D value curve passes through one logarithmic cycle. Lethality concept: The effectiveness of the thermal process in killing microorganisms in a food product may be denoted by the F value or the lethality. The lethality or sterilizing value of a thermal process is a measure of the effectiveness of the heat treatment. In order to compare the relative sterilizing capacities of different heat processes, a unit of lethality has been established as the equivalent heating of 1 min at a reference temperature (such as 250∞F for sterilization or 180∞F for pasteurization). Heating times at other temperatures (FT) can be converted to equivalent minutes (Fo) at the reference temperature using Equation (9.1): Fo = FT 10(T – To)/z

(9.1)

In thermal processes which involve a food product passing through a time–temperature profile, the lethal effects at each time–temperature combination may be integrated using the following equation: Fo = Ú 10(T – To)/z dt

(9.2)

The resulting combined lethality is known as the process lethality and may be represented by the symbol Fo. The term always applies to a specific location in the product container or the slowest heating point (cold spot). For conduction heating foods, the cold spot is the thermal center, whereas for convection heating foods, it is located approximately 1/10th the length of the can from the bottom. Generally, it is assumed that if the cold spot receives an adequate process lethality, then all other points within the container will have received an equal or greater process lethality. From a microbiological safety point of view, the assurance of a minimal lethality at the cold spot is of utmost importance, whereas from a quality standpoint it is desirable to minimize the overall destruction throughout the container. The criteria for the adequacy of a thermal process is based on microbiological considerations, especially for low-acid foods for which the minimal criterion for processing is the destruction of C. botulinum spores. It has been established that this minimum process should be severe so as to reduce the population of C. botulinum through at least 12 decimal reductions (bot cook). A decimal

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FIGURE 9.3 Schematic of thermal processing principles.

reduction time of 0.21 min at 250∞F is normally assumed for C. botulinum (Stumbo, 1973). A 12decimal reduction would thus be equivalent to an Fo value of 12 ¥ 0.21 = 2.52 min. Therefore, the minimal process lethality (Fo) required for all low-acid foods is 2.52 min; however, several lowacid foods are processed beyond this minimum value. An Fo value of 5 min is perhaps more common for these foods simply due to the occurrence of more heat-resistant spoilage-type of microorganisms that are not of public health concern. In the case of acid foods such as fruits, the criterion for the adequacy of the process is based more specifically on the reduction in the number of spoilage-causing bacteria and the inactivation of the most heat-resistant enzymes present. The two objectives of thermal process calculations are (1) to estimate how much destruction a given process will accomplish and (2) to arrive at an appropriate process time required to accomplish the desired level of destruction or inactivation (in the case of enzymes). In order to attain these two objectives, thermal destruction kinetics and the product heating profile must be obtained. The basis for establishing a thermal process can be represented as shown in Figure 9.3. Oxygen is intentionally excluded from the cans so as to eliminate aerobic bacterial growth. The pH of food differentiates the severity of the process (sterilization vs. pasteurization). Processed cans are recommended to be stored in a cool place to reduce spoilage by thermophilic bacteria. 9.2.1.5 Heat Penetration Curve In order to establish a thermal process schedule, the temperature history of the product and the thermal resistance characteristics of the test microorganism (z and Fo) are required. The temperature history of the product undergoing the process depends on several factors:

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1. The heating process: still vs. agitated cook; in-package vs. aseptic processing 2. The heating medium: steam, water (immersion or spray) with or without air over pressure, steam/air 3. The heating conditions: retort temperature, initial temperature, loading pattern 4. Product type: solid, semisolid, liquid, particulate liquid; thermophysical properties of the product 5. Container type, shape, and size The heating process may include in-package, which is the conventional way (e.g., canning), or aseptic processing, which involves heating and cooling a liquid product (with or without particulates), and then filling into sterile packages and sealing under aseptic conditions. The latter process generally offers a higher quality product due to rapid heating and cooling. Container agitation is another method that promotes higher quality and shorter process times through mixing of the contents and more rapid, uniform heating (Eisner, 1988). The product type greatly influences the mode of heat transfer and the temperature history of the product. Due to a variety of ingredients in many heat processed foods, packaged foods on the market rarely heat by pure conduction or pure convection. Generally, conduction-heating products that are tightly packed and quite viscous do not move within the container during heating or cooling. They exhibit an initial lag followed by a straight-line heating curve. Examples include creamy soups, purees, fruit jams, and fruit compotes. Convection heating products include those which are more loosely packed and may contain particulates. The semilog curve is generally steeper than that of conduction-heating products. Examples include thin soups with vegetables, peas in brine, and fruit slices or halves in syrup solution. Some products show a broken heating behavior. They heat by convection at the beginning of the process and then change to conduction following a certain temperature. Examples include products containing starch or other thickening agents. Basically, process calculation methods can be divided into two major types: (1) general methods and (2) formula methods. General methods, the first scientific approaches to calculate the adequacy of thermal processes, are based on destruction data of a particular test organism and are involved in a graphical or numerical integration procedure whereby time–temperature data during heat processing is used to integrate lethal effects at the cold spot in the container. Common temperature measuring devices known as thermocouples are placed at the appropriate cold spot and used to gather time–temperature data necessary for the calculations. Formula methods utilize heat penetration data obtained from experimental time–temperature data. 9.2.1.6 Heat Penetration Parameters Heat penetration parameters are obtained from a plot of the temperature difference (Tr – T during heating and T – Tw during cooling, on log scale) against heating time (on linear scale). The key parameters are the heating rate index, fh, and heating and cooling lag factors, jch and jcc. Details on calculating the various parameters are given in Nomenclature (see Figures 9.4 and 9.5):

9.3 THERMAL PROCESS CALCULATIONS The purpose of the thermal process calculations is to arrive at an appropriate process time under a given set of heating conditions to result in a given process lethality or, alternately, to estimate the process lethality of a given process.

9.3.1 ORIGINAL GENERAL METHOD This method is mostly of academic interest today; however, it has laid the foundation for all subsequent process calculation methods. Originally described by Bigelow et al. (1920), it is based

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Temperature Difference, Tr−T (log scale)

182

2.25

log (Tr−Tpih) jch =

2.00

Tr−Tpih Tr−Tih

log (Tr−Tih)

1.75 1.50 1.25 1.00 fh 0.75 0

2

4

6 8 10 12 14 16 18 20 Heating Time (min)

Temperature Difference, T−Tw (log scale)

FIGURE 9.4 Heating curve and heating parameters.

2.50

log (Tpic−Tw) jcc =

2.25

Tpic−Tw Tic−Tw

log (Tic−Tw)

2.00 1.75 1.50 1.25 fc 1.00 0

2

4

6 8 10 12 14 16 18 20 Cooling Time (min)

FIGURE 9.5 Cooling curve and cooling parameters.

on a graphical procedure for integrating the lethal effects of the time–temperature data at the cold spot in a container obtained for a given processing condition. It was based on the destruction data of a selected microorganism. The destruction kinetics are integrated over the heating process. In the original method, the heat penetration data is first obtained as a time–temperature curve. The temperatures are converted to lethal rates (reciprocal of TDT at the existing product temperature) to get a lethal rate vs. time curve. The area under the lethal rate curve so obtained represents the integrated sterilization value of the process which indicates the extent of destruction at the cold spot after the entire thermal process. Thus, a sterilization value of one indicates that the effect is the same as heating a spore suspension at any given temperature to a time equal to the TDT at that temperature. The sterilization value or value of unity is the minimal requirement with respect to a particular test microorganism. If the resulting unity value is greater than one or less than one, it indicates that the product is overprocessed and underprocessed, respectively. Thus, in the former case, the heating time should be reduced, and in the latter it should be increased. In this way, the process is repeated until the desired sterilization value is obtained and the corresponding process time is noted. This method is rather tedious and is somewhat limited to the experimental setup.

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Consequently, it has undergone several modifications that resulted in the development of the improved general method.

9.3.2 IMPROVED GENERAL METHOD The improved general method suggested by Ball (1923) employs the same graphical approach as the original method and the process calculations are quite similar. However, the improved general method is based on a hypothetical thermal destruction curve or reference TDT curve of the test microorganism with an Fo value of 1 min at the reference temperature. The z value is conventionally taken as 18∞F for sterilization and 10∞F for pasteurization; however, this value may vary depending on the product. The lethal rate or L value at any temperature may be calculated using the following equation: L = 10(T To)/z

(9.3)

The L value is the number of minutes at To equivalent to 1 min at temperature T. The time–temperature data from the heat penetration test are integrated with respect to the L values as calculated above. Generally, a numerical integration approach is later employed to facilitate computer calculation of the area under the curve that represents the accumulated lethality or process lethality (Fo) as shown by the following equation: Process lethality = Fo = Ú L dt

(9.4)

Briefly, the sum of the lethal effects of the entire process (heating and cooling) is expressed as the number of minutes at a reference temperature. Limitations of the general methods include tedious adjustments that need to be made with the process calculations.

9.3.3 THE FORMULA METHODS 9.3.3.1 Ball’s Method The formula methods are more versatile than the general methods and can accommodate several variables. They are based on characterizing heat penetration data and combining kinetic data with heat penetration parameters. Ball’s formula method in particular is one of the most widely used techniques for thermal process calculations. The method is based on the following equation derived from the heat penetration curve (using the same symbols as detailed earlier): B = fh log (jch • Ih/gc)

(9.5)

Heat penetration data is arranged in such a way that the log of the difference between the retort temperature and the product center temperature during heating is plotted against time. After an initial hyperbolic heating lag, the heating process is denoted by a logarithmic straight line. The time required for the straight-line portion of this curve to pass through one log cycle is the heating rate index, fh, and the lag factor is jch. The cooling curve is also constructed in a similar fashion, where the log of the difference between the product temperature and the cooling water temperature is plotted against the time after the steam is shut off and the product starts to cool. The cooling rate index, fc, and cooling lag factor, jcc, are calculated in a manner similar to fh and jch. Ball’s formula method is based on several assumptions: 1. Based on the reference TDT curve concept, the F value (or U) at the retort temperature is equivalent to the product of the desired process lethality Fo and Fi.

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2. 3. 4. 5. 6.

The The The The The

z value is 18∞F. heating rate index is equivalent to the cooling rate index. CUT effectiveness is 42%. cooling curve is initially hyperbolic followed by logarithmic. cooling lag factor may be approximated by an average jcc value of 1.41.

Several implications of Ball’s assumptions include: The calculations are not valid for z not equal to 18∞F. If fc > fh (cooling is slower than heating), then there is a possibility of overprocessing. If fh > fc (heating is slower than cooling), an underestimation of the process is possible. If jcc > 1.41, there is a greater lag and the process is safe; however, overprocessing is quite likely. 5. If jcc < 1.41, the product is cooling much faster causing underprocessing since insufficient lethality is contributed by the cooling phase.

1. 2. 3. 4.

Despite some inaccuracies in the development of this method (Merson et al., 1978), Ball’s method is still the most widely used method for thermal process calculations in the food industry. 9.3.3.2 Stumbo’s Method Stumbo’s method is similar to Ball’s method in the sense that it is based on heating and cooling curves and assumes that fh is equal to fc. The difference lies in its being versatile in accounting for the lethality contributed by the cooling phase when the cooling lag factor jcc is not equal to 1.41, as assumed by Ball. Stumbo’s method accommodates several jcc values and z values as outlined in published tables (Stumbo and Longley, 1966). Stumbo’s method is very flexible; it can accommodate the destruction of bacterial spores and vegetative cells or nutrients, all of which possess different thermal resistances (D and z values). All types of thermal processes may be calculated; for example, the z value of a typical pasteurization process is 10∞F (vegetative bacteria), that of a sterilization process is 18∞F (C. botulinum), and that of nutrients is typically 40 to 50∞F. The steps followed are essentially similar to Ball’s procedure with the calculation of the process time and process lethality using similar equations. Table 9.1 shows the fh/U vs. g relationships when z = 10∞F necessary for typical pasteurization processes.

9.4 PROCESS CALCULATION METHODOLOGY FOR PASTEURIZATION OF FRUITS Heat processing is quite clearly the most destructive method of fruit processing. Since many fruits are high in organic acids such as lactic, acetic, and citric acid, their overall pH is quite low and they require much less rigorous heat treatments as compared to low-acid foods such as meats. In order to establish a processing schedule for canned fruits subjected to pasteurization, the most heat-resistant microorganism of public health concern in the container must be identified and evaluated for its thermal destruction rate. In most cases, however, it is the most heat-resistant enzyme system and vegetative bacteria that are used as the basis for establishing the thermal process. The thermal inactivation time for the particular enzyme, which is usually peroxidase, should be adequate to provide a microbiologically safe fruit product. In contrast to destruction of spores during sterilization processes, pasteurization processes target vegetative cells only. The D and z values of these vegetative cells are much smaller than those of spores. This decreased thermal resistance permits the use of lower processing temperatures. With reference to processing of fruits or pasteurization process, first, the reference temperature is changed from 250∞F to a lower value, typically 180∞F. In addition, the reference z value is changed

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TABLE 9.1 fh/U Relationships When z = 10∞F Values of g When j of Cooling Curve Is fh/U

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.20 0.50 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

2.68–05 2.01–02 0.282 1.14 1.83 2.33 2.71 3.01 3.25 3.47 3.67 3.84 5.22 6.27 7.14 7.87 8.51 9.07 9.56 10.0 10.4 13.0 14.3 15.2 15.8 16.3 16.8 17.1 17.4

2.78–05 2.21–02 0.294 1.17 1.88 2.41 2.81 3.15 3.43 3.68 3.90 4.11 5.67 6.81 7.72 8.49 9.15 9.72 10.2 10.7 11.1 13.7 15.2 16.1 16.8 17.4 17.8 18.2 18.5

2.88–05 2.40–02 0.305 1.19 1.92 2.48 2.92 3.29 3.61 3.89 4.14 4.38 6.12 7.34 8.31 9.10 9.78 10.4 10.9 11.4 11.8 14.5 16.0 17.0 17.8 18.4 18.9 19.3 19.7

2.98–05 2.60–02 0.317 1.21 1.97 2.55 3.03 3.43 3.78 4.10 4.38 4.64 6.57 7.88 8.89 9.72 10.4 11.0 11.6 12.0 12.5 15.2 16.8 17.9 18.7 19.4 19.9 20.4 20.8

3.07–05 2.79–02 0.329 1.24 2.01 2.63 3.14 3.57 3.96 4.30 4.62 4.91 7.01 8.41 9.48 10.3 11.1 11.7 12.2 12.7 13.1 16.0 17.7 18.8 19.7 20.4 21.0 21.5 22.0

3.17–05 2.99–02 0.340 1.26 2.05 2.70 3.24 3.72 4.13 4.51 4.85 5.17 7.46 8.95 10.1 11.0 11.7 12.3 12.9 13.4 13.8 16.8 18.5 19.7 20.6 21.4 22.1 22.6 23.1

3.27–05 3.18–02 0.352 1.29 2.10 2.77 3.35 3.86 4.31 4.72 5.09 5.44 7.91 9.48 10.7 11.6 12.3 13.0 13.6 14.1 14.5 17.5 19.3 20.6 21.6 22.4 23.1 23.7 24.3

3.36–05 3.38–02 0.364 1.31 2.14 2.85 3.46 4.00 4.49 4.93 5.33 5.70 8.35 10.0 11.2 12.2 13.0 13.6 14.2 14.7 15.2 18.3 20.1 21.5 22.6 23.4 24.2 24.8 25.4

3.46–05 3.57–02 0.376 1.33 2.19 2.92 3.57 4.14 4.66 5.14 5.57 5.97 8.80 10.6 11.8 12.8 13.6 14.3 14.9 15.4 15.9 19.0 21.0 22.4 23.5 24.5 25.3 25.9 26.6

Source: Adapted from Stumbo, C.R. 1973. Thermobacteriology in Food Processing. 2nd ed. Academic Press, Orlando, FL.

from 18∞F to 10∞F. Process calculations can then be performed either using the general or formula methods. A typical calculation using the numerical integration technique is shown in Table 9.2. The formula methods may also be employed using one of two approaches. The first approach is based on the criterion of achieving a certain minimal center point temperature in the product, for example 185∞F or 85∞C. Using the heat penetration parameters and the above criterion, the process time can easily be calculated using Ball’s formula: B = fh (log jchIh – log gc)

(9.6)

The parameters fh and jch are obtained from the heat penetration data, and Tih and Tr are known. The value for gc is also known because gc = Tr – 185 (using the above criterion). An example calculation for peach processing is shown in Table 9.3. The other approach using the formula method is based on Ball and Stumbo methods using a reference temperature of 180∞F and z value of 10∞F. Examples of process calculation for achieving an F10180 = 10 min are shown in Table 9.4 to Table 9.7.

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TABLE 9.2 Process Calculation by the Improved General Method (Numerical Integration Technique)a Time (min)

TEMP (∞F)

L = 10(T–180)/10

L¥t

SL¥t

–4 –2 0 (CUT) 1 2 3 4 5 6 7 8 9 10 11 12

80 91 130 145 160 170 180 188 195 190 185 170 160 140 130

0.000 0.000 0.000 0.000 0.010 0.100 1.000 6.310 31.62 10.00 3.160 0.100 0.010 0.000 0.000

0.000 0.000 0.000 0.000 0.010 0.100 1.000 6.310 31.62 10.00 3.160 0.100 0.010 0.000 0.000 FO, min

0.000 0.000 0.000 0.000 0.010 0.110 1.110 7.420 39.04 49.04 52.20 52.30 52.31 52.31 52.31 52.31

a

Example: pasteurization of canned apple slices Tr = 212∞F (boiling water).

TABLE 9.3 Process Calculation Using Ball’s Formula Approacha 1. fh 2. jch 3. Retort temperature (Tr) 4. Initial temperature (Ti) 5. Ih = Tr – Ti 6. jchIh 7. log (jchIh) 9. Target temperature (Tc) 10. g = Tr – Tc 11. log (g) 12. log (jchIh) – log (g) 13. B = fh [log (jchIh) – log (g)] Conveyor speed = tunnel length/ process time (B) = 50/14.4 = 3.47 ft/min a

17.6 min 1.16 212∞F 60∞F 152∞F 176 2.25 185∞F 27∞F 1.43 0.82 14.4 min

Example: Pasteurization of pouch-packed peach slices in syrup. Pouches subjected to pasteurization in a steam tunnel (Tr = 212∞F; 50-ft-long tunnel). The calculation computes the process time (conveyor speed) to achieve a target temperature of 185∞F in slices.

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TABLE 9.4 Calculation of Process Time Using Ball’s Method

TABLE 9.5 Calculation of Process Lethality Using Ball’s Method

1. 2. 3. 4. 5. 6. 7. 8. 9 10. 11. 12.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

jch fh Process Lethality (Fo) Retort Temperature (Tr) Initial Temperature (Ti) Ih = Tr – Ti jch.Ih log (jch·Ih) Fi = 10[(180–Tr)/10] fh/U = fh/(F0 ¥ Fi) From Ball Table obtain log g B = fh [log (jch·Ih/g)]

1.46 30 min 40 min 200∞F 60∞F 140∞F 204.4 2.310 0.01 75 1.328 29.5 min

jch fh Process time Retort temperature (Tr) Initial temperature (Ti) Ih = Tr – Ti jch·Ih log (jch·Ih) Fi = 10[(180 – Tr)/10] log (g) = [log (jch·Ih) – B/fh] From Ball Table obtain fh/U Fo = fh/[fh/U ¥ Fi)

1.60 25 min 30 min 200∞F 60∞F 140∞F 224.0 2.350 0.01 1.150 24.367 102 min

TABLE 9.6 Calculation of Process Time by Stumbo’s Method 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

jch 1.5 fh 10.6 min Process lethality (Fo) 40 min Retort temperature (Tr) 200∞F Initial temperature (Ti) 56∞F Ih = Tr – Ti 144∞F jch·Ih 216 log (jch·Ih) 2.334 z 10∞F Fi = 10[(180 – Tr)/10] 0.01 fh/U = fh/[F0 – Fi) 26.5 jcc 1.8 From Stumbo’s Table for z = 10∞F (jcc = 1.8), obtain g value by interpolation fh/U g value 25.0 9.26 30.0 10.02 Interpolate 26.2 9.488 14.4 min B = fh [log (jch·Ih/g) ]

9.5 FRUIT CANNING OPERATIONS The peach is the fruit which is canned most frequently and the major processors are the U.S. and Greece. In 1990, the U.S. canned peach production was 50% larger than that of Greece despite Greece being the largest exporter of canned peaches and apricots in the world. Compared to 1989 figures, the world production of peaches is expected to increase by 10% by the year 1995 (Moulton, 1992). The fruit is often inspected not only at the beginning of the process but also at various checkpoints during processing. Due to variations in the canning process for different types of fruits,

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TABLE 9.7 Calculation of Process Lethality by Stumbo’s Method 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15.

jch 1.75 fh 15.0 min Process time 20.0 min Retort temperature (Tr) 200∞F Initial temperature (Ti) 56∞F Ih = Tr – Ti 144∞F jch·Ih 252 log (jch·Ih) 2.401 z 10∞F Fi = 10[(250 – Tr)/z] 0.01 B/fh 1.333 log (g) = log (jch·Ih) – B/fh 1.0168 g 11.69 jcc 1.6 From Stumbo’s Table for z = 10∞F (jcc = 1.6), obtain fh/U by interpolation fh/U g value 50 11.57 60 12.33 Interpolate 51.7 11.69 51.8 Fo = fh/[fh/U) ¥ Fi] 29.0 min

the following operations are described in general for all fruits which are canned, with particular reference to specific fruits whenever appropriate. The flowchart for the typical operation is shown in Figure 9.6.

9.5.1 RAW MATERIAL SELECTION The quality of processed fruit depends largely on the quality of incoming fruit, and this in turn depends on how the fruit is harvested, handled, and stored (Prussia and Woodroof, 1986). Harvesting at the proper maturity is an important step in thermal processing of fruits. Plums, grapes, whole olives, gooseberries, brandied peaches, and maraschino cherries are harvested and processed in the “firm-ripe” stage. Exceptions include bananas, pears, and some apples that, when harvested at a mature stage, produce a higher quality processed product than those harvested at the “soft-ripe” stage (Wiley and Tolby, 1960). However, most types of fruits are canned in the “mellow-ripe” stage to capture maximum natural nutrients, flavor, aroma, and color. Different ingredients may be added such as spices, salt, sugar, colors, flavors, and nutrients (e.g., vitamin C) to compensate for the underdeveloped full natural flavor, aroma, and nutrient loss. Fruits that are needed to produce juices, purees, preserves, marmalades, and sauces are processed in the “soft-ripe” stage because flavor and aroma are more important than fruit texture (Prussia and Woodroof, 1986).

9.5.2 WASHING Fruits are washed with water to remove dust, dirt, and mold spores that will affect their color, aroma, and flavor. This process should be carried out thoroughly to also ensure the removal of the heat-resistant micromycete N. fischeri, which has been linked to mold formation in canned fruits

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Raw Material

Filling

Washing

Preparation (peeling, slicing, etc.)

Exhausting (vacuum)

Storage

Sorting/Grading

Blanching

Sealing

Thermal Processing

Labeling

Cooling

FIGURE 9.6 Typical fruit canning operations.

(Jesenka et al., 1991). In addition, the water is also used to cool the produce by removing the field heat following harvest. Fruits that are peeled, such as peaches, pears, apples, and apricots, are seldom washed before peeling (Prussia and Woodroof, 1986). On the other hand, washing after peeling removes vitamins and minerals and should be kept to a minimum. The volume of water required varies with the method of preparation of the fruit for canning and the kind of fruit (Lopez, 1987). Detergents are frequently used in the wash or rinse water. The water temperature should be kept low because it will keep the fruit firm and reduce leaching. The effectiveness of the washing operation depends on the amount, temperature, acidity, hardness, and mineral content of the water and the force at which it is applied. High pressure sprays are sometimes used; however, caution should be used so as to not injure the fruit. Browning may take place before peeling due to bruising, but is accelerated once the skin is broken or the tissue cells are ruptured (Prussia and Woodroof, 1986).

9.5.3 SORTING/GRADING This operation ensures the removal of inferior and/or damaged produce. An inspection belt may be used in addition to trained personnel who detect poor quality produce unsuitable for canning. Sorting labor costs may be reduced by new technologies that are noninvasive, such as magnetic resonance imaging (Moulton, 1992). In certain cases, produce may be sorted by its difference in optical properties for the determination of various chemical and physical measurements such as chlorophyll, yellow pigments, and brix (Dull et al., 1980). Sugar content may be measured using high-resolution magnetic resonance (Cho et al., 1991), and quality evaluation at this stage may also include the glycoside content of raw stone fruits such as cherries, peaches, and apricots, which may be associated with cyanogenesis in the final canned product (Voldrich and Kyzlink, 1992). Acceptable fruits are then size-sorted, where they are mechanically passed over a screen with different sizes of holes or slits. Undersized fruit may be sorted out and used as concentrate or for baby food (Moulton, 1992). Fruits with aesthetic defects may be used for juices and concentrates.

9.5.4 BLANCHING The blanching of fruits or a “partial cook” is an important step in the canning process, especially with respect to overall final quality evaluation. The product is usually immersed in hot water (88 to 99∞C) or exposed to live steam. It is an energy-intensive unit operation and can account for as

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high as one third of the total energy required for processing. Most of the water-soluble nutrients, such as ascorbic acid, are lost in this operation. During the blanching process, it is imperative that certain enzymes that have the potential to cause flavor and textural changes be inactivated. The process involves a brief heat treatment applied to most vegetables but also to some fruits in order to inactivate oxidative enzyme systems such as catalase, peroxidase, polyphenoloxidase, ascorbic acid oxidase, and lipoxygenase. When the unblanched tissue is disrupted or bruised and exposed to air, these enzymes come in contact with substrates, causing softening, discoloration, and the production of off-flavors. Since this action can potentially occur during the period prior to heat processing, it is most often standard practice to blanch fruits in order to prevent quality deterioration. Although the primary purpose of blanching is enzyme inactivation, especially with preservation techniques such as freezing and dehydration, there are several other benefits: blanching initially cleanses the product, decreases the microbial load, and preheats the product before processing. The mild heat treatment also softens the fruit, which facilitates compact packing in the can. At the same time, intercellular gases in the raw fruit are expelled, preventing excessive pressure buildup in the container and allowing for an improved heat transfer during heat processing. Consequently, a higher vacuum can be achieved in the final product as well as a reduction in internal can corrosion. For fruits that are blanched, the most common methods are conventional water and steam blanching. Water blanching is generally of the immersion type or could be a spray as the product moves on a conveyor. Steam blanching often involves belt or chain conveyors upon which the product moves through a tunnel containing live steam. Other methods less frequently used include hot gas blanching and microwave blanching. Adequacy of blanching is usually assessed based on inactivation of one of the heat-resistant enzymes (peroxidase or polyphenol oxidase).

9.5.5 PREVENTION

OF

FRUIT BROWNING

Some fruits cannot be subjected to blanching due to delicate tissues that may be disrupted during the process. They undergo alternate treatments that are used to prevent the oxidative browning which can occur due to exposure to oxygen, especially during peeling and slicing operations. Oxidative browning is caused by the action of oxidase with catechol tannins and is an important problem, especially in fruits such as peaches, apples, bananas, cherries, nectarines, apricots, grapes, and persimmons. Pineapples, tomatoes, and melons are not as prone to browning. The most common means of preventing browning in fruits are the following: SO2 or sulfite treatment: A 2000- to 4000-ppm SO2 solution (sodium or potassium metabisulfite) may be used to dip the fruits for approximately 2 to 5 min. SO2 may also be used as a gas dissolved in water (sulfurous acid) for dipping or in the gaseous form in fumigation chambers as is commonly done for grapes to control fungal growth prior to dehydration. The Food and Drug Administration has recommended a maximum residual sulfur dioxide level of 300 ppm in fruit juices and 2000 ppm in dried fruit (IFT, 1993). Acids: Acids used to raise acidity (acidifying antioxidants) include citric, fumaric, tartaric, acetic, phosphoric, ascorbic, and citraconic acid (Woodroof and Luh, 1986). For example, acidification with citric acid is especially important for sliced fruit. The cut surfaces may be protected from browning by immersion into a 1 to 2% solution of citric acid. Antioxidants: Ascorbic acid is a commonly used antioxidant in most fruit juices and canned fruits. Prestano and Manzano (1993) found ascorbic acid to be an effective inhibitor of peroxidase in selected fruits such as kiwi. It may be used on its own, in dry sugar/citric acid/ascorbic acid mixes, or in syrups. Citric acid is sometimes necessary because it acts as a stabilizer for the ascorbic acid. Usually, very small amounts are used that do not affect the taste of the final canned product. Ascorbic acid acts to reduce quinones, which are generated by polyphenol oxidase–catalyzed oxidation of polyphenols, back to phenolic compounds, preventing their conversion to brown pigments (IFT, 1993).

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Sugars: Although the main purpose of sugar is to sweeten the fruit, it is also used to prevent browning in peeled and sliced fruit, whether it is in dry or syrup form. It acts in inhibiting oxidation by partially excluding air in the tissues. This ability seems to increase with pH. In many cases, the sugar is mixed with ascorbic acid and citric acid as an effective agent against loss of texture, color, and flavor. Sapers et al. (1989) evaluated the performance of a variety of novel browning inhibitors in raw apple and the open cut surfaces of apples. They demonstrated that ascorbic acid-2 phosphate (AAP) and triphosphate (AATP) inhibited browning for apple surfaces but not in juices. Effective treatments for juice alone included cinnamate, benzoate, and combinations of beta-cyclodextrin with ascorbic acid, AAP, or ascorbyl palmitate. Recently, the addition of chitosan in filtered apple and pear juices has been found to be successful in the prevention of enzymatic browning (Sapers, 1992).

9.5.6 PEELING/PREPARATION Many different types of equipment are used in peeling and/or coring fruits. Some peeling and coring methods are advantageous for only certain kinds of fruits due to their structure or physical resistance to bruising. Lye peeling: Lye peeling is a common method of peeling for various fruits such as peaches, nectarines, apricots, and pears. Lye is an aqueous solution of approximately 10 to 15% caustic soda (sodium hydroxide) or potassium hydroxide. The operation requires an ample water supply, lye, and a heat source. The lye peeler is a heated tank containing the hot lye solution (60 to 90∞C) that allows for the passage of fruit at a specific rate. The caustic lye solution dissolves the fruit skin. Several variables should be controlled at all times, namely lye concentration and temperature, product holding time, and agitation in the caustic solution. Larger fruits require more water than smaller fruits during lye peeling. Following the treatment, the fruits are subjected to a water wash with pressure sprays to remove the peel and remove the lye from the surface. An acid dip is used (citric acid) after washing to neutralize any remaining traces of caustic soda. This method is advantageous in that there is a relatively low loss in usable fruit tissue as compared to other methods, and it is economical. Mechanical peeling: Mechanical peelers are also very popular with fruits, especially apples and pears. The fruit should be free of bruises and other blemishes in order to ensure the efficient operation of the peeling and coring (if necessary) operation (Downing, 1989). Either the fruit is made to rotate against a stationary knife or vice versa. The operation may consist of a continuous system to peel, core, and slice in a high-speed, continuous manner. Peeled fruits are washed with high-pressure water and then mechanically cored. Coring is usually done before slicing. If there is a delay after peeling and coring, fruit may be held in a 2 to 3% salt solution to prevent browning, treated in one of the ways previously mentioned. Examples of fruits that usually require peeling before canning include peaches, apples, tomatoes, and pears. Generally, fruits that are not peeled prior to canning retain more nutrients as compared to peeled fruits.

9.5.7 FILLING Filling of the cans is usually done by automatic machines, although it may be done by hand for very soft fruits that have a tendency to bruise easily. Mechanical fillers are adjusted to dispense each can a predetermined volume of fruit from a chamber and add a given amount of syrup or juice. These quantities must be uniform in order to ensure accurate and constant fill weights for the final product. Some fruit products undergo a “hot fill” whereby they enter the can at a sufficiently high temperature (approximately 80 to 85∞C). This preheating of the contents before filling, which drives out much of the dissolved gases in the product, may be done for the removal of air and the production of a vacuum. In such cases, a headspace is not necessary. Generally, these products are not processed

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at high sterilization temperatures and hence space is not needed for the expansion of the can contents, which may cause a strain on the seal. Canned applesauce is an example of a “hot-fill” product. Some fruit products require a headspace in the can, especially those that are processed in an agitating retort. The headspace bubble in the can is crucial for the movement of the contents during agitation. The amount of headspace in a can is important; insufficient space may cause can ends to bulge, whereas excessive space can cause underprocessing and even collapse of cans during processing as well as lead to can corrosion during storage due to insufficient vacuum. Type of pack: Fruits are available for canning in a variety of forms: whole fruit, slices, halves, sauce, purée, juice, or in mixed fruit packs such as fruit cocktail. Peaches are usually canned as halves, slices, or in fruit cocktail. Pears canned as halves or slices are also known as “grade pack,” which is used to distinguish them from pears added to fruit cocktail. This unit operation involves the addition of prepared fruit and the covering liquid, when necessary, into the can. Type of covering liquid: During thermal processing of canned fruits, heat is first transferred from the heating medium (steam or water) to the container surface and then to the covering liquid. The covering liquid may include syrups, water, mixtures of fruit juices, and water or fruit juices alone. Heat from the covering liquid is then transferred to the fruit product. Besides facilitating the heat transfer to fruits, the covering media also serves to sweeten the product and improve the quality characteristics (odor and color), as well as to fortify the nutrients. The most common syrups used in canned fruit products are sucrose syrup (cane or beet sugar), corn syrup, invert sugar syrup, dextrose, and high-fructose corn syrup (Lopez, 1987). The different designations for syrups range from light fruit juice syrups to extra heavy syrups. Their designations followed by their brix measurement are: extra heavy syrup (E): 22 to 35∞; heavy syrup (H) 18 to 22∞; light syrup (L) 14 to 18∞, and light fruit juice syrup or water (W) < 14∞ (Lopez, 1987). Other options include canning in natural fruit juices (Vyas and Joshi, 1981). Syrup strengths may be verified using a refractometer or a Brix hydrometer. Recent market trends, however, have found a growing popularity toward packing in concentrated juice of the same fruit with no sugar added (Moulton, 1992). According to Canadian labeling laws, when fruits are packed without sugar, the label may indicate “no sugar added” or “unsweetened.” Juices such as grape juice and apple juice, which are labeled “vitamin C added,” must contain no less than 18 mg and 35 mg ascorbic acid, respectively. This vitamin fortification is usually done in order to offset the minimal loss during unit operations and storage. Container specifications and types: In the U.S., Canada, and a majority of other countries, can dimensions are expressed using two numbers, each number consisting of three digits. The first digit of both numbers refers to whole inches, and the second and third digits together refer to the additional fraction of the dimension expressed in sixteenths of an inch. The first three-digit number refers to the can diameter and the second three-digit number refers to the can height. For example, a can with the dimensions 401 ¥ 411 would be 4 1/16 in. in diameter (from the outside edge of both double seams) and 4 11/16 in. in height (outside edge of both terminal seams). Specifications for dimensions of glass containers are similar to cans, except that specific numbers for jars are assigned to the equivalent can dimensions. Glass containers are used frequently for products such as applesauce and cherries. Three numbers are used for rectangular cans to denote the dimensions of length, width, and height, respectively. In addition to the traditional can that is recyclable but costly with respect to its requirements for tin, steel, and aluminum, other types of containers are competing for an advantage in the processed fruit industry. Plastic-ring six-pack carriers for beverages, thinner wall metal cans, and paper-based composites (e.g., paper, plastic, and metal foil laminates) will undoubtedly be used on a larger scale in the future due to current concern for convenience, cost, and environmental issues (Moulton, 1992). Popular new ideas include individual aseptically processed laminated paper juice packs and vacuum-packed fruit without a cover syrup or liquid. Container sizes: In the California canned fruit industry, the most popular consumer sizes are the buffet (8 oz), the No. 303 (16 oz), and the No. 2 1/2 (1 lb 13 oz). Canned fruits in Canada are

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usually packed in standard sizes of 5 fl oz (142 ml), 10 fl oz (284 ml), 14 fl oz (398 ml), 19 fl oz (540 ml), 28 fl oz (796 ml), 48 fl oz (1.36 l), and 100 fl oz (2.84 l). For information on “Suggested Net Quantity Statements for Fruits” refer to The Almanac (1993). Suggested container sizes are listed for a variety of canned fruits with different levels of syrup (from light to extra heavy) along with the imperial and metric measures for net capacity. A table with label measurements for various can sizes are also included in The Almanac, with the common name of the can, can size from 202 ¥ 204 to 603 ¥ 700, and label size. Can lacquer: Fruits such as apricots, grapefruit, peaches, pears, and pineapples need an acidresisting lacquer that is mostly used for high-acid foods. Highly colored fruits such as boysenberries, raspberries, strawberries, and red plums also require it, as do cans for jams made from fruits high in anthocyanin pigments. Two kinds of lacquered cans are available: an acid-resisting (AR) lacquer (for acid foods, mostly fruits) and a sulfur-resisting lacquer (SR), for nonacid foods such as vegetables, beans, and meat. The “R” enamel protects the natural pigment of highly colored fruits such as dark colored berries and cherries. Peaches and pears that are packed in cans completely enameled inside will be somewhat darker in color and slightly different in flavor than if packed in cans lined with plain tin (Lopez, 1987).

9.5.8 EXHAUSTING

AND

VACUUM

The primary reason for exhausting and vacuum is to create an anaerobic environment in the can that would inhibit microbial spoilage. Vacuum treatment also removes occluded gases from fruit tissue, which is necessary in order to increase its specific gravity. Generally, three methods of can exhausting are used in order to remove headspace gas and produce a vacuum. The conventional technique is thermal exhausting, which involves the passage of cans through a steam chamber or exhaust box. The steam replaces the air inside the can and it is sealed while still hot. Vacuum is created in the can following condensation of the steam. This process is very energy intensive due to excessive steam requirements. Another available method is “steam-flow” or “steam-vac” closing where live high-pressure steam is injected into the can headspace just prior to closing (approximately 5 to 8 min at 212∞F) (Moulton, 1992). Thus, all of the air is quickly replaced with steam, which will condense and form a vacuum following steaming. Steam-vac closure, when combined with hot fill, usually assures a very high vacuum reading of 200 to 300 mmHg (e.g., processed apple products). Steam-flow closing is relatively efficient, less energy intensive, and less expensive than thermal exhausting. High-speed mechanical vacuum sealing is also commonly used for fruits. In this method, cans filled cold with fruit and syrup are passed into a clincher that clinches the cans (first operation roll seam) but does not form an airtight seal. The cans are then subjected to a vacuum for only a short period of time. This practice will remove only the free headspace air but not all dissolved gases within the product. An advantage of this method is that it eliminates the need for exhausting of cans as a separate unit operation and saves processing space. Vacuum can-closing machines may pose potential problems for fruits packed in syrup. Because of the excess liquid in the can, the vacuum applied may draw with it some of the liquid out of the can because the dissolved air in the can is removed at the same time. This may occur with peach and pear halves packed in syrup, and sliced and diced fruit (Lopez, 1987). In order to prevent such loss of liquid, a prevacuumizing step before vacuum closing is employed, where a vacuum is drawn first on fruit alone in the can, and then, while still under vacuum, the syrup is added (Lopez, 1987). The filled cans are then subjected to the vacuum sealer with practically no dissolved air in the can and no subsequent loss of syrup.

9.5.9 CAN SEAMING

AND

CLOSING

Can container should be closed immediately after filling to prevent excessive cooling of the surface of the product. Modern can seaming machines operate at speeds as high as 300 cans per minute.

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Liquid products may be sealed in cans at speeds of up to 1600 per minute (Lopez, 1987). The double seaming operation is critical for the assurance of an hermetic seal and good keeping quality of the final product during storage. Faulty seaming can result in deformations in the can during processing and eventually recontamination. Glass jars are closed with a screw cap.

9.5.10 CONTAINER CODING All containers should be coded for recall purposes in case of problems in the stored product, such as spoilage, contamination, or consumer complaints. The code should provide necessary information such as the canning plant where packed; the day, year, and hour packed; the packing period; and the line on which the product was packed (Lopez, 1987). This practice should be carefully followed along with the filing of adequate production and shipping records.

9.5.11 RETORT OPERATIONS The simplest pasteurization equipment is a water bath maintained at an appropriately high temperature (Lund, 1975). Because pasteurization temperatures are normally below 100∞C (212∞F), acid food products may be pasteurized in the same type of equipment used for blanching, making the procedure quite convenient. Full crates containing fruits packed in syrup or fruit juice are placed in steam-heated water in large steel tanks. After the required process, cold water is added to the tank for cooling or the crates are lifted up and immersed in a cold-water tank for cooling. Continuous water bath pasteurizers consist of a long tank through which cans move along on a belt. Alternately, the containers can travel through a tunnel along a conveyer belt and are subjected to continuous water sprays. As they travel, the cans pass through several temperature zones from preheating and pasteurization to the final cool. In the pasteurization section, steam at atmospheric pressure (212∞F) is sometimes injected for quicker heating. The moderated temperatures that are possible in these systems make them ideal pasteurizers for fruit products in glass jars that are sensitive to thermal shocks. Another type of unit which is used for pasteurization of fruit, fruit juices, and tomatoes is the continuous, agitating, atmospheric cooker (Lund, 1975). Its operation is similar to that of the more conventional high-pressure continuous cookers, but the operation is limited to atmospheric pressure to accomplish pasteurization. These agitated processes are used to uniformly cook products throughout the container. Product agitation is also possible in batch-type rotary retorts in which cans are often subjected to end-over-end agitation. For small-scale processing of fruit juices, low-cost pasteurizers with a simple design may be suitable (Crandall et al., 1990). Aseptic or ultra-high temperature (UHT) processing has become a success story for fruit beverages, purees, and juices containing small particles. In this process, the food and the packaging material are sterilized separately and then assembled under sterile conditions. The product is first subjected to heat by passing the liquid product through a shell and tube or plate heat exchanger and held for sufficient time in holding tubes to complete the required pasteurization treatment. Following the required treatment, the product is then passed through another heat exchanger where it is cooled. The filling and sealing operations are then performed in presterilized containers (laminated cartons) under aseptic conditions. Plate-type indirect heat exchangers are extensively used for such a purpose. The juice flows through one side of a wall while the heating medium (steam or hot water) flows on the other side. The plate is designed in such a way that it yields extremely high rates of heat transfer. Tubular heat exchangers are also available for pasteurization purposes. For more viscous products such as cream, yogurt, and salad dressings, and for those containing small particulate matters, scraped surface heat exchangers are used so as to prevent surface fouling problems and promote rapid heat transfer. Because the packaging material must be sterilized prior to filling, the material should possess physical properties that permit this application as well as that of hermetic sealing. One type of

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composite package that is extensively used as a packaging material for aseptic processing is a paperboard–foil–plastic laminate also known as “Tetra-Pak®.” This type of package consists of a series of six layers of materials, with polypropylene as the outermost layer, followed by “Surlyn®,” aluminium foil, polyethylene, paperboard, and polyethylene again as the innermost layer. Polypropylene and polyethylene act as heat sealing surfaces, whereas aluminium foil acts as a barrier material, protected from mechanical damage by the paperboard. This composite package in its entirety acts as a barrier to oxygen, light, moisture, and microorganisms, and it meets all of the requirements necessary for successful application of the aseptic process, including heat sealability and strength. Prior to filling, a common sterilization method for this packaging material is the use of hydrogen peroxide employed in combination with heat or ultraviolet radiation treatment. For maximum nutrient retention, high-temperature short-time process (HTST) pasteurization methods are often recommended if heat-resistant enzymes such as peroxidase or pectinesterase are not present in the product. This is because increases in processing temperatures will cause faster destruction rates for the microbial population as compared to nutrients. The presence of heatresistant enzymes, which have a higher heat tolerance than microorganisms, would not always permit fruits, for example, to be subjected to such a process.

9.5.12 COOLING After thermal processing, the contents of the can should be cooled to an average temperature of about 35 to 40∞C (Jackson, 1979). Storage at higher temperatures will cause loss of normal color, and darkening or pink discoloration (stack burn). If cans are cooled too far below the average temperature, they will remain wet and rusting may result due to insufficient surface drying. Water used for cooling should be noncorrosive, low in bacterial and yeast content, and chlorinated for measurable free-chlorine residual detected at the discharge end of the cooler. The cooling water should also be chlorinated with 2 ppm of available chlorine to preclude infection of the can contents by spoilage microorganisms.

9.5.13 LABELING

AND

STORAGE

Following cooling, cans are labeled for identification purposes. Adequately processed cans usually ensure an acceptable canned fruit quality on the retail market for at least 1 year. Storage temperature has been found to be the most important variable in the maintenance of an acceptable product with minimal flavor, color, and textural and nutritive changes. High storage temperatures of 37∞C or higher cause brown discoloration of fruit products, especially canned peaches (Luh and Phithakpal, 1972). The discoloration was accompanied by a decrease in leucoanthocyanin content and may be related to polyphenol oxidase activity. The corrosion of internal lining of canned peaches is also related to storage temperatures. In the warehouse, fruit products should be kept at approximately 20∞C and subjected to good ventilation to protect cans from condensation of moisture. High-temperature storage at approximately 35∞C for orange juice can cause the formation of degradation products leading to malodorous properties (Tatum, 1975). Furthermore, the browning index caused by storage temperature abuse of orange juice may be predicted by CIE tristimulus values (Robertson, 1981). Common storage temperatures for canned fruit seldom average above 70∞F (22∞C) (Woodroof and Luh, 1986). Above 80∞F (27∞C), gradual softening occurs and freezing causes greater changes. Canned fruits should not be allowed to freeze. Freezing may cause distortion of the can seams and eventually lead to microbial spoilage. The most important causes of quality deterioration of canned fruits are very slow chemical changes that take place during storage, resulting in changes in nutritive value, flavor, color, and texture. However, based on estimated kinetic parameters for modeling quality changes, it may be possible to predict color degradation of juices or other liquid foods during processing prior to storage (Cohen et al., 1994).

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Table 9.8 lists commonly canned fruits along with selected operations and their recommended allotted time for processing.

9.6 QUALITY CONTROL OF CANNED FRUITS The color, flavor, and texture of fruits are affected by thermal processing. In the thermal processing of fruits, sacrificing color and flavor are inevitable in order to provide longer shelf life and more convenience. Because heat preservation is the most destructive method of fruit processing, the processor’s goal should be to minimize flavor and color changes while ensuring a safe product. Canned fruits should be regularly examined for four parameters that have the potential to change following a thermal treatment: pH, added color, metallic contamination, and vacuum (Young and How, 1986). Commercial juices such as grapefruit, pear, and orange may be analyzed for bacterial contamination during processing using D-alanine (Gandolfi et al., 1994). Textural properties of fruits are important as they can predict the overall acceptability of the fruits for consumption. Texture loss in thermally processed fruits can be attributed to degradation of pectic substances (Wiley and Thompson, 1960). Fruits selected for thermal processing should be firm enough to withstand high temperatures, although substances such as calcium lactate may be added to grapefruit sections in juice or syrup to decrease firmness loss (Baker, 1993). During processing, the juice of the fruit will dilute the syrup. Consequently, the density of the final syrup (the “cutout” syrup) will be lower than that of the added syrup. For example, a syrup added before processing with a 40∞ Brix will usually “cut out” at 22 to 25∞ Brix. The cutout syrup will depend upon various factors: strength of the added syrup, the type of fruit, its ripeness, soluble solids content, and the ratio of fruit to syrup (Young and How, 1986). In order to preserve the color of processed fruit, selection of fruit with optimal pigmentation is recommended. Undesirable compounds such as melanoidins and melanins are formed in browning reactions. The major pigments in fruits are chlorophylls, carotenoids, and anthocyanins. Chlorophylls are lipid soluble pigments that disappear during fruit ripening. Carotenoids are subdivided into xanthophylls and carotenes. Xanthophylls are yellow (as in Golden Delicious apples and bananas), and carotenes are red (such as lycopene, the major pigment in watermelon and pink grapefruit flesh) and orange (the B-carotene found in orange and apricots). Anthocyanins are water soluble pigments responsible for the red, blue, and purple of many fruits, flowers, and vegetables. All these pigments related to color are destroyed during thermal processing. The chlorophylls are degraded to brown pheophytins (Gold and Weckel, 1959; Schwartz and von Elbe, 1983), the carotenoids are converted to epoxides, and the anthocyanins are rapidly degraded. During heat processing, anthocyanins react quite readily with the metal walls of nonlacquered cans. Thus, it is necessary to lacquer the cans to protect both the product and the can, as the color will usually pass out into the syrup during processing. There are two types of discoloration associated with anthocyanins. The first type occurs when leucoanthocyanins are converted to anthocyanins during canning. This causes the characteristic pink discoloration of pears (Luh et al., 1960) and the excess red color in peaches. Other fruits that undergo pink discoloration include guava, lychee, and banana. The second type involves enzymic browning, which can be prevented by blanching and the addition of ascorbic acid, citric acid, and malic acid. Heat processing may also cause the formation of various zinc complexes such as in canned kiwifruit slices (Cano and Marin, 1992).

9.7 GRADES FOR CANADIAN AND U.S. CANNED FRUITS Most heat-processed fruit in Canada is sold by grade. The standards are established by the Processed Fruit and Vegetable Regulations of the Canada Agricultural Products Standards Act. The products are graded on a variety of quality factors such as flavor and aroma, tenderness and maturity, color, consistency of texture, appearance of packing media, uniformity of size and shape, and freedom from defects and foreign matter (Agriculture Canada, 1982).

E E E E

E

E E E E E E E E

PL PL E* E*

PL

PL PL PL E* E* E* E* EE* MR MR MR

MR MR MR L L L

MR L

MR MR L L

MR

Type Steeld

30–60 30–60 120–180

240–300

20–45

120–480

Blanching Time (S)

3

4 8

3–10 10 4 10

303

4

8–10 12–15 6–8

8 10

15–40

5

401

10

16 12

45–110

8

603

60 93 82

82 88 82

82 71 71 77

88 82 82 82 82 99

88

Fill Temp (∞C)

35 35

7–8

15 15

30–50

10

20

10

303

Include blackberries, blueberries, dewberries, huckleberries, loganberries, and raspberries. PL = plain, E* or EE* = inside side strip of enamel. E = single coat enamel, EE = double coat enamel. Type L = used for strongly corrosive fruits; Type MR = used for mildly corrosive fruits. For No. 2 cans only (307 ¥ 409). Packing media: E = extra heavy syrup, H = heavy syrup, L = light syrup, W = water or artificially sweetened.

E

PL

Coating Endsc

Can Size

Exhaust Time (min)

45 45

20 17–19 10 16–20 8–12 30–55 20 10 12 25 25 7–10 12–15e 20

10

401

80 70

28–35 35

35 35

30–70

35 20–30 20 25–30

10

603

35 35 35

20 20

20 25 10 25

10

45 45 45

25 25

20 25 15 25

10

32 oz

Jar Size 16 oz

Process Time (min) Can Size

E, E, E, E, E, E,

H, L, W H, L, W H, L, W H, L, W H, W H, L, W

E, H, W E, H, L, W L, W

E, H, L, W E, H, L, W E, H, L, W

Packing Mediad E/H/L/W

4.1–4.6 4.1–4.6 3.9–4.4

3.4–4.2 3.8–4.6 3.5–4.1 2.8–3.0 2.5–4.2 3.0–3.9

3.6–4.0 3.0–3.5

3.4–3.5 3.4–4.4 3.0–4.2 3,6–4.0

3.2–4.2

pH Range

Thermal Processing of Fruits

Source: American Can Co. (1949); Hurst (1984); Woodroof, J.G. and Luh, B.S. 1986. Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. pp. 678.

f

e

d

c

b

a

Applesauce Apples In syrup Apricots Berriesa Cherries Currants (Black) Figs Fruit cocktail Grapefruit Lychee Peaches Pears Pineapples Plums (dark) Prunes (dried) Strawberries Tomatoes Cold pack Hot pack Tomato juice

Fruit

Enamel Bodyb

Type of Can

TABLE 9.8 Conditions for Processing of Fruits in Boiling Water

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There are three general grades: Canada Fancy, Canada Choice, and Canada Standard. Canada Fancy fruits are free from blemishes, clean, and of good color and uniform size. Canada Choice fruits have slight variations in size, color, and maturity, but are almost completely free of blemishes. For example, Canada Fancy and Canada Choice are the grades used for apple juice and apple juice from concentrate. Canada Standard fruits are mainly used for sauces and puddings because they may be broken and more or less ripe. The appropriate grade must appear on the main part of the label. The U.S. grade standards for fresh and processed fruits are under the jurisdiction of the U.S. Department of Agriculture, Food Safety and Quality Service (FSQS). Manufacturers and packers frequently employ them in quality control work. The standards for grades usually vary for each commodity. They include the definition of the product, the style of pack (whole, halves, quarters, slices, dices, pieces, or mashed), and the grade (Woodroof, 1986). The U.S. grades include the following: U.S. Grade A or U.S. Fancy, U.S. Grade B or U.S. Choice, U.S. Grade C or U.S. Standard, U.S. Grade D or U.S. Substandard. Kader (1985) published a list of various quality factors for processing fruits listed in the U.S. grade standards and the California Food and Agricultural Code. Examples of quality factors include color uniformity, freedom from decay and defect, maturity based on soluble solids content, firmness, and shape. The above factors vary depending on the type of fruit.

9.8 NOMENCLATURE B = Thermal process time. Ball’s Process time (BB) = Corrected for come-up period (steam on to steam off minus 0.6l) CUT = Come-up period of the retort. D = Decimal reduction time. D value = time required to result in 90% reduction in microbial population. F = Thermal death time. fc = Cooling rate index. It is the time required for the straight-line portion of the cooling curve (Figure 9.5) to pass through one complete log cycle. It is also the negative reciprocal slope of the cooling rate curve. fh = Heating rate index. It is the time required for the straight-line portion of the heating curve (Figure 9.4) to pass through one complete log cycle. It is also the negative reciprocal slope of the heating rate curve. Fo = Process lethality. g = Difference between the retort temperature and food temperature (T) at time t — (Tr – T). gc = The value of g at the end of heating or beginning of cooling period (Tr – Tic). Ic = Difference between the cooling water temperature and food temperature at the start of the cooling process (Tic – Tw). Ih = Difference between the retort temperature and food temperature at the start of the heating process (Tr – Tih). jcc = Cooling rate lag factor. A factor that, when multiplied by Ic, locates the intersection of the extension of the straight-line portion of the semilog cooling curve and the vertical line representing the start of the cooling process — (Tw – Tpic)/(Tw – Tic). jch = Heating rate lag factor. A factor that, when multiplied by Ih, locates the intersection of the extension of the straight-line portion of the semilog heating curve and the vertical line representing the effective beginning of the process (0.6l) = (Tr – Tpih)/(Tr – Tih). l = Come-up period. In batch processing operations, the retort requires some time to reach the operating condition. The time from steam on to when the retort reaches Tr is called the comeup period. 0.6l = Effective beginning of the process. The retort come-up period varies from one process to the other and from one retort to the other. In process evaluation procedures, about 40% of

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this come-up period is generally considered as time at retort temperature because the product temperature increases even during this period. In order to accommodate this, the effective beginning of the process is moved left a distance “0.4l” from the time the retort reached Tr or moved right 0.6l from steam on. Pt = Operator’s Process time (time after come-up period; Pt = B – 0.4l). Tic = Initial food temperature at start of cooling period. Tih = Initial food temperature at the start of heating period. Tpic = Pseudo-initial temperature during cooling. Temperature indicated by the intersection of the extension of the cooling curve and the vertical line representing the start of cooling. Tpih = Pseudo-initial temperature during heating. Temperature indicated by the intersection of the extension of the heating curve and the vertical line representing the effective beginning of the process (0.6l). Tr = Retort temperature. Tw = Cooling water temperature. Z = Temperature sensitivity indicator of D or F. Temperature range in which D or F changes by a factor of 10.

REFERENCES Agriculture Canada. 1982. Canada’s Food Grades. Publication 1720E. Minister of Supply and Services Canada. Almanac of the Canning, Freezing, Preserving Industries, 77th Annual, Vol. 1 and 2. 1993. Edward E. Judge & Sons, Westminster, MD. Alzamora, S.M., Tapia, M.S., Argaiz, A., and Welli, J. 1993. Application of combined methods technology in minimally processed fruits. Food Res. Int. 26: 125–130. Baker, R.A. 1993. Firmness of canned grapefruit sections improved with calcium lactate. J. Food Sci. 58(5): 1107–1110. Ball, C.O. 1923. Thermal Process Time for Canned Food. Bulletin 37, Vol. 7, Part 1. National Research Council, Washington, D.C. Bigelow, W.D., Bohart, G.S., Richardson, A.C., and Ball, C.O. 1920. Heat Penetration in Processing Canned Foods. Canners National Association. Bulletin 16L. Cano, M.P. and Marin, M.A. 1992. Pigment composition and color of frozen and canned kiwi fruit slices. J. Agric. Food Chem. 40: 2141. Cho, S.I., Bellon, V., Eads, T.M., Stroshine, R.L., and Krutz, G.W. 1991. Sugar content measurement in fruit tissue using water peak suppression in high resolution 1H magnetic resonance. J. Food Sci. 56(4): 1091–1094. Cohen, E., Birk, Y., Mannheim, C.H., and Saguy, I.S. 1994. Kinetic parameter estimation for quality change during continuous thermal processing of grapefruit juice. J. Food Sci. 59(1): 155–158. Crandall, P.G., Upadhyaya, J.K., and Davis, K.C. 1990. Portable, low cost equipment for small scale fruit juice processing. Int. J. Food Sci. Technol. 25: 583–589. Desrosier, N.W. 1977. Elements of Food Technology. AVI Publishing, Westport, CT. Downing, D.L. 1989. Processed Apple Products. Van Nostrand Reinhold, New York. Dull, G.G., Birth, G.S., and Magee, J.B. 1980. Nondestructive evaluation of internal quality. Hortic. Sci. 15: 60–63. Eisner, M. 1988. Introduction into the Technique and Technology of Rotary Sterilization. Private Author’s Edition, Milwaukee, WI. Fellows, P. 1988. Food Processing Technology: Principles and Practices. Ellis Horwood, Chichester, U.K. Gandolfi, I., Palla, G., Marchelli, A., Dossena, A., Puelli, S., and Salvadori, C. 1994. D-alanine in fruit juices: a molecular marker of bacterial activity, heat treatments, and shelf life. J. Food Sci. 59(1): 152–154. Gold, H.J. and Weckel, K.G. 1959. Degradation of chlorophyll to pheophytin during sterilization of canned green peas by heat. Food Technol. 13: 281–286. IFT. 1993. Browning of foods: Control by sulfites, antioxidants, and other means. A Publication of the Institute of Food Technologists’ Expert Panel on Food Safety and Nutrition. Scientific Status Summary. Food Technol. 47(10): 75–84.

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Jackson, J.M. 1979. Canning procedures for fruits. In: Fundamentals of Food Canning Technology. Jackson, J.M. and Shinn, B.M., Eds. AVI Publishing, Westport, CT. Jesenka, Z., Pieckova, E., and Sepitkova, J. 1991. Thermoresistant propagules of Neosartorya fischeri; some ecologic implications. J. Food Protect. 54(8): 582–584. Kader, A.A. 1985. Standardization and inspection of fresh fruits and vegetables. In: Postharvest Technology of Horticultural Crops. The Regents of the University of California, Division of Agriculture and Natural Resources, Davis, CA. Kyzlink, V. 1990. Principles of Food Preservation. Elsevier Science, New York. Lopez, A. 1987. A Complete Course in Canning and Related Processes. 12th ed. Book 1, 2 and 3. The Canning Trade, Baltimore, MD. Luh, B.S., Leonard, S.J., and Patel, D.S. 1960. Pink discoloration in canned Bartlett pears. Food Technol. 14: 53–56. Luh, B.S. and Phithakpal, B. 1972. Characteristics of polyphenoloxidase related to browning in cling peaches. J. Food Sci. 37: 264–268. Lund, D.B. 1975. Heat processing. In: Principles of Food Science, Part 2: Physical Principles of Food Preservation. Fennema, O.R. and Lund, D.B., Eds. Marcel Dekker, New York. Madden, J.M. 1992. Microbial pathogens in fresh produce — the regulatory perspective. J. Food Protect. 55(10): 821–823. Merson, R.L., Singh, R.P., and Carroad, P.A. 1978. An evaluation of Ball’s Formula method of thermal process calculations. Food Technol. 32(3): 66. Moulton, K. 1992. Maintaining the Competitive Edge in California’s Canned Fruit Industry. UC Agricultural Issues Center, University of California, Berkeley. Prestamo, G. and Manzano, P. 1993. Peroxidases of selected fruits and vegetables and the possible use of ascorbic acid as an antioxidant. Hortic. Sci. 28(1): 48–50. Prussia, S.E. and Woodroof, J.G. 1986. Harvesting, handling, and holding fruit. In: Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. Ranganna, S. 1986. Handbook of Analysis and Quality Control for Fruit and Vegetable Products. Tata McGrawHill, New Delhi. Richardson, P.S. 1987. Flame sterilization. Int. J. Food Sci. Technol. 22: 3–14. Robertson, G.L. 1981. Relationship between color and brown pigment concentration in orange juices subjected to storage temperature abuse. J. Food Technol. 16: 535–541. Sapers, G.M. 1992. Chitosan enhances control of enzymatic browning in apple and pear juice by filtration. J. Food Sci. 57: 1192–1193. Sapers, G.M., Hicks, K.B., Phillips, J.G., Garzarella, L., Pondish, D.L., Matulaitis, R.M., McCormack, T.J., Sondey, S.M., Seib, P.A., and El-Atawy, Y.S. 1989. Control of enzymatic browning in apple with ascorbic acid derivatives, polyphenol oxidase inhibitors, and complexing agents. J. Food Sci. 54(4): 997–1002, 1112. Schwartz, S.J. and von Elbe, J.H. 1983. Kinetics of chlorophyll degradation to pyropheophytin in vegetables. J. Food Sci. 46: 636–637. Stumbo, C.R. 1973. Thermobacteriology in Food Processing. 2nd ed. Academic Press, Orlando, FL. Stumbo, C.R. and Longley, R.E. 1966. New parameters for process calculation. Food Technol. 20(3): 341. Tatum, J.H. 1975. Degradation products formed in canned single-strength orange juice during storage. J. Food Sci. 40: 707. Voldrich, M. and Kyzlink, V. 1992. Cyanogenesis in canned stone fruits. J. Food Sci. 57(1): 161–162, 189. Vyas, K.K. and Joshi, V.K. 1981. Canning of fruits in natural fruit juices. I. Canning of peaches in apple juice. J. Food Sci. Technol. 18(1): 39–40. Wiley, R.C. and Thompson, A.H. 1960. Influence of variety, storage, and maturity on the quality of canned apple slices. Proc. Am. Soc. Hortic. Sci. 75: 61–84. Wiley, R.C. and Tolby, V. 1960. Factors affecting the quality of canned apple sauce. Proc. Am. Soc. Hortic. Sci. 76: 112–123. Woodroof, J.G. and Luh, B.S. 1986. Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. pp. 678. Young, C.T. and How, J.S.L. 1986. Composition and nutritive value of raw and processed fruits. In: Commercial Fruit Processing. 2nd ed. Woodroof, J.G. and Luh, B.S., Eds. AVI Publishing, Westport, CT.

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Processing 10 Novel Technologies for Food Preservation Hosahalli S. Ramaswamy, Cuiren Chen, and Michele Marcotte CONTENTS 10.1 10.2

Introduction ........................................................................................................................201 Advanced Thermal Processing Technologies ....................................................................202 10.2.1 Thin Profile Processing .......................................................................................202 10.2.2 Agitation Processing............................................................................................204 10.2.3 Aseptic Processing...............................................................................................205 10.2.4 Microwave Heating..............................................................................................208 10.2.5 Radio-Frequency Heating....................................................................................209 10.2.6 Ohmic Heating.....................................................................................................210 10.3 Nonthermal Processing Technologies................................................................................211 10.3.1 High-Pressure Processing (HPP).........................................................................211 10.3.1.1 Principles and Advantages of HPP....................................................211 10.3.1.2 Advantages of HPP............................................................................212 10.3.1.3 Applications of HPP ..........................................................................213 10.3.2 Pulsed Electric Fields (PEF) ...............................................................................214 10.3.2.1 Effects of Process Parameters............................................................215 10.4 Conclusions ........................................................................................................................216 References ......................................................................................................................................217

10.1 INTRODUCTION One of the most important objectives of food processing is to obtain shelf-stable and nutritious food products that can be stored and handled under normal conditions. To ensure the stable shelf life the foods to be processed have to be commercially sterilized to destroy both pathogenic microorganisms that cause the food to become unsafe for consumption as well as the microorganisms responsible for spoilage. So that they can provide valuable nutrients, foods after processing must continue to remain safe and healthy during storage. Thermal processing, one of most widely used sterilization technologies, is still the dominant technique in food industries. As detailed in Chapter 9, thermal processing can be successfully used to produce safe and shelf-stable foods. Usually, the heat resistance of nutrients and other quality components is much larger than that of microorganisms. This fact is taken advantage of in high temperature short time (HTST) processing techniques and in the optimization of several thermal processing technologies, such as retort pouch processing, agitation processing, and aseptic processing. In recent years, advanced processing techniques such as microwave heating and ohmic heating, as well as related computer control 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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technologies, have made it possible for food industries to produce foods with optimal qualities using the idea of deliverable rapid temperature profiles. However, it is a fact that all thermal processing techniques, however advanced, cannot fully eliminate nutrient degradation. Hence, several advanced nonthermal processing technologies such as high-pressure processing and electrical pulse processing have received attention in recent years as alternatives to conventional thermal processing for producing higher quality products. In this chapter, the concepts involved in some novel processing techniques are introduced.

10.2 ADVANCED THERMAL PROCESSING TECHNOLOGIES Advanced thermal processing technologies are aimed at optimizing both food quality and process time. It is well known that the destruction rates (D values) of most common spoilage and pathogenic microorganisms during thermal processing are more sensitive to changes in temperature than to changes of other quality factors; thus, in theory, the use of high temperatures can not only help achieve a shorter process time but can also promote better food quality. However, this theory is based on the assumption that the temperature distribution inside the food during processing is uniform and rapidly changing. For liquid foods, especially for those with low viscosity such as milk and fruit beverages, this assumption can be easily realized with the application of various types of heat exchangers and pumps. However, for solid foods, especially those stored in larger containers, neither rapid change nor uniformity is easy to achieve since heat is transferred by conduction, which has a limited heat transfer rate. Therefore, different thermal processing methods have been developed based on the characteristics of foods and packages. For example, the traditional HTST or ultra high temperature (UHT) techniques, with processing temperatures in the range of 130 to 145∞C, are used for liquid foods. Such techniques are not appropriate for solid foods since they heat slowly and a large temperature gradient exists between the product at the container surface and that at the center; therefore, by the time the required sterility is achieved in the center, the product bulk on the outer regions gets overprocessed, resulting in greater quality destruction. Thus, the product quality is unfavorably affected with HTST or UHT processing techniques. The concept used to improve quality and shorten process time for solid foods is called thin profile processing: reducing package thickness by employing thinner containers such as retortable pouches or thin profile semirigid containers and then applying an optimal temperature profile for a given package size. On the other hand, in agitation retort processing, the product packaged in a given container is agitated during processing to induce mixing of the container liquid and particulate foods. In aseptic processing all three concepts are combined. In the following text, these typical advanced thermal processing techniques are further discussed.

10.2.1 THIN PROFILE PROCESSING Thin profile packages, also called retortable flexible containers, include retort pouches or semirigid containers, and were developed in the U.S. in the early 1950s. Compared with cylindrical containers, the pouch transfers heat faster to the critical point because of its characteristic thinner profile. This fast transfer permits the required amount of heat for sterilization to be applied to the critical point with minimal overcooking of the product bulk near the peripheral container areas. Thus, thin profile containers can potentially provide higher quality retention for solid foods than can conventional cylindrical containers. Retortable flexible containers are laminated structures that are thermally processed like a can; they are shelf-stable and have the convenience of frozen boil-in-the-bag products. The structure of the retortable pouch is usually a 3-ply laminate composed of 0.012 mm polyester film and adhesive laminated to 0.0089 or 0.018 mm aluminum foil that is laminated to a 0.076 mm polypropylene film. The polyester film is used for high temperature resistance, toughness, and printability. The polyester may be reverse-printed by embedding ink between the film and the foil. The aluminum foil provides the barrier properties (light, gas, microorganisms, etc.)

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for extended shelf life. The polypropylene layer facilitates heat sealing. With the development of high-oxygen barrier films such as ethylene-vinyl alcohol copolymer (EVOH) and polyvinylidenechloride (PVDC), a nonfoil retortable pouch that uses a polyester/polyurethane/adhesivemodified high density polyethylene structure has been used for retort packaging items requiring shelf life of 3 months or less, or for products held under refrigeration. The main structure of semirigid containers is a polypropylene/EVOH or PVDC/polypropylene sandwich with appropriate tie layers and fillers. Alternate materials include high-impact polystyrene, crystallized polyester, and polycarbonates. The shelf life of products packaged in semirigid containers can be up to 12 months but the actual life obtained varies with the food product. Retortable flexible containers can be processed in batch retorts or continuous retorts, and in static mode or agitating mode. The heating medium could be saturated steam, steam–air mixtures, or hot water with overriding air pressure. From the heat transfer rate point of view, saturated steam has the highest coefficient, hot water the next highest, and steam–air mixtures have the lowest coefficient. Japanese and European systems tend to favor steam–air mixtures or saturated steam, but water systems are widely used in the U.S. The key reasons for selecting water as the heating medium are familiarity with the system, accessibility, and controllability. The primary consideration in retort operations is the provision of overriding pressure during both heating and cooling. Such pressure is important during heating to provide better heat transfer rates by keeping the heat transfer surface (the pouch walls) in contact with the food material (thereby preventing the expansion of residual gases and swelling of the containers). During cooling, overriding pressure is absolutely necessary to prevent package collapse that can occur due to internal pressure in the container far exceeding the retort pressure. (The latter follows the reduction of steam that accompanies the onset of cooling.) Overriding air pressure is most commonly used in retort operations, hence most retorts for pouch processing are based on either steam–air mixtures or steam-heated water, with air overpressure. Among the major critical factors that affect thermal processing of retort containers are the following: • • • • • • • • • • • • • •

Minimum headspace Product consistency Maximum filling or drained weight Initial temperature Processing temperature Processing time Temperature distribution Container orientation Residual gas in headspace and in food Processing and racking systems Processing medium Product heating rates Materials from which the pouch rack is constructed Divider sheet hole sizes and spacing

Products packed in retortable flexible containers include meats, sauces with or without particulates, soups, fruits and vegetables, specialty items like potato salad, bakery products, and pet foods. Generally, any product currently packaged in cans or glass jars may also be packaged in flexible containers. The major advantages of retortable flexible containers can be summarized as follows (Lopez, 1997): 1. A reduction of 30 to 40% in processing time is possible, with energy savings. 2. The quality losses during processing such as taste, color, flavor, and nutrients can be reduced significantly compared to quality losses in conventional canning processing methods.

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3. Pouches and containers do not corrode externally and there is a minimum of product–container interaction. 4. Use of flexible containers could potentially eliminate or at least greatly reduce the incidence of dented cans, which is the single largest spoilage factor in supermarkets today, accounting for 49% of the total spoilage. 5. The cost to the canner will be less than that with the conventional rigid-type containers. 6. Food can be heated in a microwave oven. 7. Flexible containers are safer in that consumers would not cut themselves with metal cans or be faced with broken glass when jars break.

10.2.2 AGITATION PROCESSING Rotary retort thermal processing makes use of mechanical agitation to increase heat transfer during processing, reducing process time and improving production quality. Clifcorn (1950) reported that heat penetration to the cold spot could be achieved much faster if the container (with a liquid or semiliquid product and normal headspace) was agitated by end-over-end rotation of the sealed container during processing and if higher temperatures were used. It was apparent that sensory characteristics were considerably improved over those of conventionally processed foods. The effects of various parameters (e.g., process temperature, rotational speed, liquid viscosity, particle properties, volume fraction occupied by the particles, etc.) on heat transfer have been studied by many researchers (see Sablani and Ramaswamy, 1995). The rotary retort processing method can be used for liquid, semiviscous, or particulate foods and has many advantages compared to the still retort processing method. These advantages are summarized as follows (Eisner, 1988): 1. When processing products of greater viscosity or heat sensitivity, higher retort temperatures of 126 to 138∞C and precise timing could be used without the problem of overcooking and scorching. 2. Many more products having a liquid-phase component could be improved by a quicker temperature come-up of the container contents than could be improved under the conditions of conventional still retort temperatures. 3. A rapid HTST method for sterilizing products would preserve the color, flavor, and texture as well as the nutritive value of the thermoprocessed foods. 4. Greater flexibility in specifying sterilization times and temperatures would be available for tailoring the sterilization process to fit various products to several container sizes, thus allowing processes with the same sterilization effect (F value) to be selected on the basis of the degree of cooking (C value). 5. Many of the more viscous, semiliquid, or even semifirm heat-sensitive products can be sterilized in large containers without overcooking. 6. Heat-resistant microorganisms are more easily and thoroughly destroyed without damaging the product, even in large container sizes. Based on the principles of rotary retort processing, three types of rotation (axial, end-over-end, and circular) have been developed and applied in both batch and continuous rotary retorts (Figure 10.1). The continuous rotary retort consists of two cylindrical shells: one shell is used for processes such as heating and cooling, and the other is used for storage of hot water to save energy consumption and speed the come-up heating. One of the early retort systems employing the principle of agitation was the FMC continuous pressure cooker (Sterilmatic). Continuous retorts offer additional advantages over batch-type retorts due to increased production throughput and lower labor costs. In the Sterilmatic system, cans are admitted into the retort through a mechanical pressure lock and conveyed through the retort by

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mrw2

mrw2

w

205

mg

mg

w

n r

r mg

mg

mrw2

mg mrw2

FIGURE 10.1 The schematics of different types of can rotation.

means of a horizontal helical spiral. The cans remain at the periphery of the helix, and each can rotates about its own axis for a portion of each rotation. By operating the reel at 5 rpm, each can rotates about 100 rpm around its own axis as it moves through the retort. During the bottom third of the rotational cycle, the can is rolled against the shell of the retort. As the can rolls, the headspace travels along the cylindrical contours of the can, resulting in agitation of the contents. Rotational retorts have also been manufactured in Europe. Hermann Stock (Stock Machinfabrik, Hamburg, Germany) developed rotational retorts as early as 1952. The Stock Rotomat (Stock America) comes in simulator, pilot, and production models, with overhead heating tanks to eliminate temperature come-up times, and offers both axial and end-over-end agitation. The heating medium can be steam, water spray, or water immersion with air overpressure. Lagarde Autoclaves (France) manufactures retorts based on heating using steam–air mixtures. The retort design employs a powerful turbo fan at the back of the retort to create turbulence within the heating medium. While the company’s early retorts were mainly static, it has recently released fully automated rotary systems. Barriquand (France) manufactures retorts based on steam–air and the rapid circulation of reheated water using the water spray principle. A typical retort that can be used for processing thin profile packages as well as for agitation processing is shown in Figure 10.2.

10.2.3 ASEPTIC PROCESSING Aseptic processing and packaging consist of presterilizing of the product before filling it into a presterilized package that is then closed in a sterile atmosphere. The sterilization of the aseptically manufactured food product is accomplished by continuous heating under a HTST or UHT regime. Normally, heating temperatures for UHT sterilization are in the range of 135 to 145∞C, and at these temperatures “residence” times to obtain the desired lethality are only from about 2 to 45 sec. The general process schematic of aseptic processing is shown in Figure 10.3. The concept of aseptic processing originated to solve several problems associated with conventional in-container sterilization of foods: low rate of heat penetration to the slowest heating point in the container, the long processing times required to deliver the required lethality, destruction of the nutritional and sensory characteristics of the food, low productivity, and high energy costs (Smith et al., 1990). Aseptic processing was initiated in 1927 at the American Can Company research department in Maywood, IL, under the direction of C. Olin Ball. The result was the development of the HCF (heat, cool, fill) process for which Ball was granted a patent in 1936. The HCF process was targeted at pumpable liquid and semiliquid foods. In 1938 two HCF units

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FIGURE 10.2 A pilot scale retort for thin-profile packages and agitation processing (Stock Rotomat). Courtesy of Stock America.

Raw material

Heating

Packaging material

Holding

Cooling

Sterilization

Filling

or

Filling

Holding

Cooling

Product

FIGURE 10.3 The general process schematic of aseptic processing.

Packaging material

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were installed for the commercial production of a chocolate-flavored milk beverage. Ball is considered the pioneer of aseptic processing (Mitchell, 1988). The Avoset process, another development in the field of aseptic processing, is credited to George Grindrod at the former Avoset plant located in the San Joaquin Valley in California (Ball and Olson, 1957). This process is unique in that the filling and closing area was treated to eliminate bacteria and further protection was accomplished by ultraviolet (UV) lamps (Mitchell, 1988). The area was also enclosed by a wall with an opening for conveying the finished product. Sterilization was achieved by heating to temperatures of 127 to 138∞C. The Avoset process is no longer in operation but nevertheless was another milestone in the development of aseptic processing. Real progress in the commercial development of aseptic processing technology for foods began with the invention of the Martin–Dole process in the late 1940s in California by the Dole Engineering Company (Lopez, 1987). The process could be used for the sterilization of any low- or high-acid fluid. The technical success of the process was not, however, accompanied by the expected high level of exploitation, the reason being that the package, a metal can, could not be differentiated from conventional in-container sterilized foods (Dennis, 1992). Product heating and cooling were achieved by heat exchangers based on the principles of HTST sterilization, while the containers were sterilized by superheated steam (Mitchell, 1988). The first commercial Dole system was installed in the early 1950s in California for the production of soups (split pea soup) and sauces (white, cheese, and Hollandaise). Another historic development was brought about in the early 1960s by Loelinger and Regez of Switzerland who successfully used hydrogen peroxide (H2O2) to sterilize flexible packaging materials used in the manufacture of milk containers (Lopez, 1987). Obtaining better quality products than those from conventional processing was the main objective of the Martin–Dole process, whereas extending the shelf-life of fresh milk without refrigeration using low-cost containers was the goal of Loelinger and Regez. Compared with conventional canning, the advantages of aseptic packaging of food products are summarized as follows (Lopez, 1987): 1. A higher quality product is obtained by using HTST sterilization especially for largecapacity containers. 2. It is possible to use a wide variety of packaging materials, and many package sizes and shapes are possible. 3. The sterilization and packaging procedures used are continuous. 4. Product heating and cooling equipment is designed with a high ratio of surface area to product volume, thereby providing very efficient heat transfer. 5. The package need not be designed to withstand high temperatures, unlike in product sterilization. However, aseptic processing and packaging also has some limitations such as: 1. It needs large capital investment and a complex control system. 2. The product must be relatively homogeneous fluid that can be pumped. 3. A given system is designed for a limited range of product types. Current aseptic processing and packaging technology in North America are primarily limited to liquid foods but there is considerable interest in the extension of the technology to low-acid liquid foods containing large particulates (Dignan et al., 1989; Heldman, 1989; Lund, 1993; Ramaswamy et al., 1995). There were two problems in applying this technology to particulate foods. The first was to come up with a mechanical means of physical handling that would maintain proper distribution and particle integrity, and ensure commercial sterility with minimal quality loss. This problem was successfully solved with equipment such as scraped-surface heat exchangers (SSHEs) or tubular heat exchangers with a displacement pump (Lee and Singh, 1990). The second

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problem — the real challenge — was the establishment of a microbiologically safe process for particulates without sacrificing the essential quality factors. The key to solving this problem is to develop a reliable method to measure the temperature of moving particulates in aseptic processing systems. So far, several methods of temperature measurement have been studied such as biological validation techniques, moving thermocouple methods, liquid crystal technique, time temperature integrators, melting point indicators, and relative velocity methods (Sastry, 1991).

10.2.4 MICROWAVE HEATING Heating rate remains one of the major limitations for the optimization of conventional thermal processes in which the heat is transferred through both conduction and convection, although advanced equipment such as rotary retorts and scraped-surface exchangers, etc., have been developed. Microwave heating is one of the volumetric heating methods that has the potential to lead to a quantum change in the ability of the food processor to achieve the heating rates necessary to deliver profiles that could improve current UHT process routes (Mullin, 1995). Microwaves used in the food industry for heating are of ISM (industrial, scientific and medical) frequencies (2450 or 900 MHz, corresponding to 12 or 34 cm in wavelength). In this frequency range the dielectric heating mechanism dominates up to moderated temperatures. Polar molecules, the dominant water, try to align themselves with the rapidly changing direction of the electric field. The energy to achieve this alignment is taken from the electric field. When the field changes direction, the molecule “relaxes” and the energy previously absorbed is dissipated into the surroundings, that is, directly inside the food. This means that the water content of the food is an important factor in the microwave heating performance of foods. The penetration ability of microwaves in foods is limited. For normal wet foods the penetration depth from one side is approximately 1 to 2 cm at 2450 MHz. At higher temperatures heating due to the electric resistance of dissolved ions also plays a role in influencing the heating mechanisms, normally further reducing the penetration depth of microwaves. The limited penetration depth of microwaves implies that the distribution of energy within the food can vary. Thus, an important requirement of microwave equipment and microwave energy applications in the food industry is the ability to correctly control heating uniformity (Ohlsson, 1983). It may be expected that a lower frequency system will lead to more uniform heating, However, this has not been the case in the areas of pasteurization and sterilization, where all the commercial systems use a frequency of 2450 MHz (Mullin, 1995). Microwave heating has been widely applied in industrial food applications such as defrosting or thawing of frozen foods, drying, blanching, and pasteurization. Sterilization using microwaves has been investigated for many years but the commercial introduction of this technique has only come about in the last few years in Europe and Japan. Microwave pasteurization and sterilization promise to give very quick heat processing that should lead to small quality changes due to the thermal treatment according to the HTST principle. However, it has turned out that very high requirements of heating uniformity must be met in order to fulfill these quality advantages (Ohlsson, 1991). Pasteurization with microwave heating can also be used with pumpable foods. Microwaves are directed at the tube in which the food is transported, thereby heating the tube directly across the cross section. Again, uniformity of heating must be ensured, which requires selection of the correct dimensions for the tube diameter and proper design of the applicators (Ohlsson, 1983). The destruction kinetics of microorganisms such as Saccharomyces cerevisiae, Lactobacillus plantarum and Escherichia coli as well as the inactivation of enzymes under continuous microwave heating have been reported (Tajchakavit and Ramaswamy, 1998; Koutchma and Ramaswamy, 2000). A further application of microwave heating is drying in combination with conventional hot-air drying or vacuum drying. Often microwaves are primarily used for moving water from the wet interior of solid-food pieces to the surfaces, relying on the preferential heating of water by microwaves. Applications can be found for pasta, vegetables, and various cereal products, where puffing

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up by rapid expansion of the interior of the food matrix can be accomplished using microwave energy (Tempest, 1996). The lethality of microwaves has been debated for over 50 years, on the focus of the debate being the nonthermal effects of microwave heating on microorganisms. However, it is still too early for scientists to reach a definitive conclusion on this subject. Of course, this lack of definitiveness will not adversely affect the application of the microwave heating technique in food processing. Currently, the main barriers to further microwave processing applications are the relatively high cost of microwave production lines, and the lack of knowledge about the dielectric behavior of foods, which is important for the design and optimization of microwave processes.

10.2.5 RADIO-FREQUENCY HEATING Radio Frequency (RF) heating or capacitative dielectric heating is an innovation that stands out among several techniques that are based on electro-technologies such as ohmic heating, radiative or microwave dielectric heating, inductive/ohmic combinations, inductive heating, and radiative magnetic heating. All these inject heat energy from the electromagnetic source directly into the body of the product, initiating volumetric heating throughout. RF heating technology involves applying a high-voltage AC signal to a medium placed between parallel electrodes (one of them being grounded) set up as a capacitor. Heating occurs when an AC displacement current flows. High-frequency heating is accomplished as a combination of dipole heating and electric resistance heating from the movement of the dissolved ions. (When the water polar molecules in the medium try to align themselves with the applied AC electric field and thereby interact with neighboring molecules, the result is lattice and frictional losses as they rotate.) RF heating differs from higher-frequency electromagnetic radiative-microwave heating. Wavelengths in the microwave range (12 cm and 34 cm at 2450 MHz and 915 MHz, respectively) are generally comparable to the dimensions of the sample, and microwave heating occurs in a metal chamber with resonant electromagnetic standing wave modes as in a microwave oven or waveguide. In the microwave frequency range the dielectric heating mechanism dominates up to moderate temperatures. The water content of the foods is an important factor for microwave heating performance. For normal wet foods the penetration depth from one side is approximately from 1 to 2 cm at 2450 MHz. Because RF heating uses longer wavelengths than microwave heating, electromagnetic waves in the RF spectrum can penetrate deeper into the products so that there is no overheating or dominance of hot or cold spots, a common problem with microwave heating. RF heating also offers simple uniform field patterns as opposed to the complex nonuniform standing wave patterns in a microwave oven. Ohmic heating or electric resistant heating relies on direct ohmic conduction losses in a medium and requires the electrodes to be in direct contact with the medium; the current cannot penetrate a plastic film or air gap. Ohmic heating gives direct heating because the product acts as an electrical resistor. The heat generated in the product is due to the loss in electric resistance. RF heating has advantages over low-frequency ohmic heating as the medium — which is enclosed inside an insulating plastic package or surrounded by an air gap — can be heated without the electrodes coming into to direct contact with the medium. The performance of RF heating is also less dependent on smooth contact between the product and the electrodes. RF heating is a very attractive processing method to provide safe and high-quality food products because of the rapid and uniform heating patterns and the large penetration depth. There are several promising applications of RF heating in the food industry due to the potential this technique has to improve the quality of food products. The use of RF heating also can result in reduced energy consumption, which can be considered as a great advantage over the traditional methods of heating. The main applications of RF heating are: heating packaged bread, blanching vegetables, thawing frozen foods, baking and postdrying snacks, cooking, pasteurizing, and sterilizing processed meat products. RF technology in food processing has not been fully explored and offers considerable

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scope for further research. Commercial RF heating systems for food pasteurization or sterilization are not known to be in use, although such systems have been researched over the years (Bengtsson and Green 1970; Houben et al., 1991; Wig et al., 1999). The primary advantage of RF heating is improved uniform heating, which was demonstrated for in-package sterilization of foods in large packages using radio frequency at 27.12 MHz, although enhanced edge heating continued to be an issue (Wig et al., 1999).

10.2.6 OHMIC HEATING Ohmic heating, also called electric resistance heating, is a direct heating method that uses the food itself as a conductor of electricity that is taken from mains that are 50 Hz in Europe and 60 Hz in the U.S. The benefits of ohmic heating are numerous. The most important is that heating is very rapid and uniform. The process is ideal for shear-sensitive products. Ohmic heating can heat the food continuously without needing the hot heat transfer surface of a scraped-surface heat exchanger (SHHE) or a tubular heat exchanger (that may foul). The process is quiet in operation as rotating parts are not used in the system to ensure temperature uniformity. Unlike microwave heating, the depth of heat penetration in the food is virtually unlimited. A high level of control and automation ensure safety during the operation. Finally, it is easier to design a heating time/temperature profile that will ensure sterility because heat is generated within the solids independent of thermal conductivity through the liquid. The concept of ohmic heating technology is not new; early applications date back to 1897. During the last century, attempts were made to use this technique in several food processing applications (de Alwis and Fryer, 1990; Palaniappan and Sastry, 1990). However, several technical problems such as product contamination from improper contact between the electrodes and the food product, adhesion of the product to the electrodes, a suitable control system, etc., became major barriers to the early development of the ohmic heating technique. Recent developments in related sciences and technologies such as materials science, computer control, and aseptic processing have stimulated renewed interest in the ohmic heating method on the part of food scientists and engineers since the 1980s. The most recent industrial achievements in ohmic heating are the development of the ELECSTER process for the pasteurization of milk (which is based on the electropure process) and the APV Baker ohmic heating technology for the sterilization of particulate foods (Skudder, 1991). The latter achievement was recently recognized as a commercial breakthrough at the annual meeting of the Institute of Food Technologies (IFT) in 1996. The ohmic system of APV Baker has been used for the pasteurization and sterilization of a number of food products, and the resulting quality has been excellent. The majority of these installations are for the production of fruit products in Japan (Tempest, 1996). In France the CTCPA (Center Technique de la Conservation des Produits Agricoles) and UTC (Universitè Technologique de Compiègne) jointly installed an APV pilot plant unit in 1995 to help European food companies develop sterilization processes for liquids containing particles (Zubber, 1997). Other cooking operations using electric resistance heating involve rapid cooking of potatoes and vegetables for blanching in industry and preparing foods in institutional kitchens. Recently there has been a great deal of research on the application of this technique to food processing for a variety of purposes. Naveh et al. (1983) reported that ohmic heating was applied to thaw foods. A fully automated prototype for ohmic thawing of fish blocks based on the control of current flow within the product to eliminate the formation of hot spots was designed and tested by Roberts et al. (1998). Huang et al. (1997) investigated the feasibility of using a batch-type ohmic heater to coagulate fish proteins obtained from frozen fish mince wash water. As a rapid method, the ohmic heating principle was also used to design a laboratory apparatus for fat analysis in meat products (Piette and Jacques, 1997). It has been recognized that the ohmic heating system will play an important role in food processing areas such as aseptic processing of food products containing large particulates (Marcotte, 1999).

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10.3 NONTHERMAL PROCESSING TECHNOLOGIES Consumer requirements for foods are constantly changing. Today consumers demand foods that are both fresh and natural. Therefore the steps used to process foods should be designed to preserve their natural quality. Hence nonthermal processing techniques such as high-pressure processing (HPP), pulsed electric field, and ozone treatment have been attracting more attention from food scientists and engineers in recent years not only because of their food preservation capabilities but also because of their potential to achieve some interesting functional effects.

10.3.1 HIGH-PRESSURE PROCESSING (HPP) New and alternative food processing methods, and novel combinations of existing methods are continually being investigated by the industry in pursuit of producing better quality foods more economically. HPP offers one such technique for the pasteurization of foods or for modification of their functional properties and has been highlighted in several recent IFT symposia. In 2002 Avure Technologies, Inc. was awarded the prestigious IFT Industrial Achievement Award for its efforts to develop and commercialize high-pressure processing equipment. High-pressure technology has traditionally been used in nonfood areas on a relatively large scale for the production of ceramic, carbide and steel components, and superalloys, where inert gases or water are used as the pressure medium. As liquid compression results in a small volume change, high-pressure machines using water do not present the same operating hazards as machines using compressed gases. Furthermore, the application of hydrostatic pressure to food results in instantaneous and uniform transmission of the pressure throughout the product independent of the product volume. The hydrostatic treatment is unique in that the effects do not follow a concentration gradient nor do they change as a function of time. Other advantages include the absence of chemical additives and operation at low or ambient temperatures so that the food is essentially raw (Farkas, 1986). Presumably, hydrostatic pressure is a physical treatment that will not cause extensive chemical changes in the food system. Once the desired pressure is reached, it can be maintained without the need for further energy input. Liquid foods can be pumped to treatment pressures, held, and then decompressed aseptically for filling as with other aseptic processes. The application of HPP to food preservation started around 1900 when Hite and his coworkers investigated its effects on food microorganisms by subjecting them to pressures of 650 MPa and found a reduction in the viable numbers of microbes. They also applied HPP to other products such as meat, fruits, and vegetables. The higher resistance of microbial spores to pressure destruction was demonstrated by Timson and Short (1965). The first commercial products processed by HPP were offered in Japan in 1990. Since then, numerous academic and industrial activities on HPP have been conducted, especially in Japan and Europe. Currently, some typical commercial pressure-treated products in Europe or the U.S. are orange juice by UltiFruit, Pernod Ricard Company, France; acidified avocado puree (guacamole) by Avomex Company in the U.S. (Texas and Mexico), and sliced ham (both cure-cooked and raw-cooked) by Espuna Company, Spain. There is no doubt that HPP opens another interesting and promising dimension for food processing because it not only inactivates microorganisms but also provides opportunities for the development of new value-added food products. However, there are two major limitations to the further development of HPP. First, the database of pressure processing kinetic parameters such as D and z values in thermal processing is not comprehensive enough to ensure the reliability of HPP as an alternative to thermal processing. Second, developing a continuous-pressure food processor remains an engineering challenge. 10.3.1.1 Principles and Advantages of HPP There are two principles that underlie the development of an HPP system. The first is the Le Chatelier–Braun principle that governs the influence of high pressure on biomolecules. This

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FIGURE 10.4 The isostatic principle in high pressure processing. From Barbosa-Cánovas, G.V., GongoraNieto, M.M., Pothakamury, U.R., and Swanson, B.G. 1999. Preservation of Foods with Pulsed Electric Fields. Academic Press, London, pp. 1–9, 76–107, 108–155.

principle implies that under equilibrium conditions any event (such as a chemical reaction, phase transformation, molecular transformation, etc.) that is accompanied by a decrease in volume is reinforced by an increase in applied pressure, while an event accompanied by an increase in volume is suppressed by a pressure increase. Generally, high pressure affects noncovalent bonds (such as hydrogen, ionic, and hydrophobic bonds) substantially as such bonds are usually very sensitive to pressure; it follows that low-molecular-weight food components (i.e., those responsible for nutritional and sensory characteristics) are not affected by pressure whereas high-molecular-weight components (whose tertiary structure is important for functionality determination) are pressuresensitive. Some specific covalent bonds, however, are modified by pressure. The second principle that plays a key role in HPP systems is Pascal’s law or the isostatic rule, which states that the transmittance of pressure is uniform and instantaneous (Figure 10.4). This makes HPP independent of the size and geometry of the sample and also shortens processing time. Moreover, overprocessing, which is a major concern for the thermal processing method, is prevented. Compared with conventional thermal processing, HPP has a number of unique advantages. First, it can produce much higher quality “fresh-like” foods because HPP destroys microorganisms at lower temperatures and most of the quality components are not affected. Second, HPP is environmentally friendly since it requires only electrical energy for pressure build up. Moreover, HPP can easily be combined with other treatments such as heating, supercritical CO2, and other methods to increase its efficiency. 10.3.1.2 Advantages of HPP HPP has many advantages: 1. It does not break covalent bonds; therefore, the development of flavors alien to the products is prevented, maintaining the natural qualities of the products. 2. It can be applied at room temperature thereby reducing the amount of thermal energy needed for food products during conventional processing. 3. Because HPP is isostatic (applied uniformly throughout the food), the food is preserved evenly throughout its mass without any particles escaping treatment.

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4. High pressure is not time- or mass-dependent; it reduces processing time by acting instantaneously. 5. HPP is independent of the size and geometry of the food. 6. The process is environment-friendly since it requires only electrical energy and there are no waste products (Thakur and Nelson, 1998). However, much like any other process, HPP also has certain limitations. These are: 1. Food enzymes and bacterial spores are very resistant to pressure and very high pressure is required for their inactivation. 2. Residual enzyme activity and the presence of dissolved oxygen results in enzymatic and oxidative degradation of certain food components. 3. Most pressure-processed foods need low-temperature storage and distribution to retain their sensory qualities (Thakur and Nelson, 1998). 10.3.1.3 Applications of HPP HPP has been applied in various areas of the food industry such as food preservation (pasteurization and sterilization), starch gelatinization, enzyme inactivation (blanching), osmotic drying enhancement, pressure-shift freezing, and thawing enhancement. Generally, HPP has been shown to be more effective against vegetative bacteria (Hoover et al., 1989). However, more recently, by combining higher pressures from 700 to 1000 MPa and higher temperatures from 70 to 90∞C, HPP has been successfully applied to the sterilization of low-acid foods. In Japan, hydrostatic pressure has been used to induce the gelation of different kinds of surimi. Excellent gels have been produced from pollack, sardine, skipjack, and tuna at 4000 kg/cm2 and from squid at 6000 kg/cm2. Pressureinduced gels from marine species were smoother and more elastic than those produced by heat and were considered to be organoleptically superior. In meats, pressures in the range of 1000 to 1500 kg/cm2 have been used to tenderize prerigor beef (Elgasim and Kennick, 1980; MacFarlene, 1985). High pressure has also been used to improve the functionality of foods. For example, pressurization of rice grains at 400 MPa or higher significantly increased their water uptake (Hayashi and Hayashida, 1989). Because high-pressure processing affects only noncovalent bonds, food quality factors such as color, flavor, and nutrients are minimally affected (Knorr 1995). Strawberry jams prepared by high pressurization at pressures from 4000 to 6000 kg/cm2 not only maintain their original, fresh fruit color and flavor but also retain up to 95% of the vitamin C originally present in the fresh fruit. Pressure-processed jams were preferred to heat-processed jams. However, high pressure may cause unwanted browning of fresh foods through activation of enzyme reactions. Knorr (1995) concluded that high pressure accelerated enzymatic browning of potatoes and fruit jams. The texture of several fruits and vegetables have been shown to undergo slight firming as a result of high pressure treatment. This effect can be used to advantage in inducing hardening without using chemical additives to protect texture and form damaged by overcooking and also in shortening the time required for the cell membrane to collapse. (It is this collapse that causes the hardness.) Eshttiaghi et al. (1994) reported that HPP blanching significantly reduced the leaching of nutrients and minerals compared to the leaching that occurred with conventional treatment. They also found that pressure-treated vegetables that were subjected to freezing had higher drying and dehydration rates. Fuchigami et al. (1995) subjected cabbage to pressure-shift freezing using pressures of 100 to 700 MPa and a temperature of –20∞C and found that there was an increase in both textural and histological attributes of pressure-frozen samples when compared to the results from traditional freezing. Seyderhelm et al. (1996) showed that application of a pressure as high as 900 MPa for 2 min at 45∞C was sufficient to completely inactivate commercial pectin methyl esterase (PME) in Tris buffer at pH 7.0. Basak (2001) investigated the pressure-induced inactivation kinetics of PME at

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pH levels of 3.2 and 3.7 in both single-strength and concentrated orange juices. Results showed the biphasic nature of pressure-induced inactivation of PME in both juices, characterized by an instantaneous pulse effect and a first-order rate hold-time effect. By a combination treatment of pressure cycle, hold time, pressure level and pH, 90% inactivation of PME was achieved. Pressurepasteurized single-strength and concentrated orange juice had a shelf-life of several weeks under refrigerated storage conditions (4∞C). Basak and Ramaswamy (1998) investigated the effects of HPP on the texture of fruits (pear, apple, pineapple, and orange) and vegetables (carrot, celery, and green and red pepper) and found that the net effect of pressure on texture was dependent on the magnitude of the pressure, instantaneous loss and pressure-hold time. At low pressure levels the initial loss in texture was recovered mostly during the pressure-hold period, and no such recovery of structure was noted during the standing period (at atmospheric pressure) after pressure treatment. High pressure had no adverse effect on the color of the product immediately following pressurization (Eshtiaghi et al., 1994). Prestamo et al. (1999) reported that the browning of apples due to pressure processing was completely arrested by a combined treatment of high pressure and ascorbic acid. Rastogi and Niranjan (1998) found that water and solute of pressure-pretreated pineapple had a significantly higher diffusion rate than in control during osmotic dehydration.

10.3.2 PULSED ELECTRIC FIELDS (PEF) High-intensity pulsed electric field (PEF) processing involves the application of pulses of high voltage (typically from 20 to 80 kV/cm) to foods placed between two electrodes. PEF treatment is conducted at ambient, subambient, or slightly above ambient temperatures for less than 1 sec, as a result of which the energy loss due to heating of foods is minimized. PEF technology is considered superior to traditional heat treatment of foods because it maintains food quality by avoiding or greatly reducing detrimental changes to the sensory and physical properties of foods. Some important aspects of pulsed electric field technology are: generation of high electric field intensities, design of chambers that ensure foods are uniformly treated with a minimum increase in temperature, and the design of electrodes that minimize the effect of electrolysis. The large field intensities are achieved by storing a large amount of energy in a capacitor bank (a series of capacitors) from a DC power supply, which is then discharged in the form of high voltage pulses (Zhang et al., 1995). Studies on energy requirements have concluded that PEF is an energy-efficient process compared to thermal pasteurization particularly when a continuous system is used (Qin et al., 1995a, 1995b). PEF has been mainly applied to preserve the quality of foods, such as to improve the shelf-life of bread, milk, orange juice, apple juice and liquid eggs. The use of PEFs for inactivating microorganisms is another promising nonthermal processing method that can be used for pasteurization and possibly also sterilization, with the integration of other processing parameters such as pH, ionic strength, temperature, and high-pressure processing (Jeyamkonan et al., 1999). Sale et al. (1970) reported a systematic study on the effect of PEFs on the inactivation of microorganisms. They found that the electric field caused an irreversible loss of the membrane’s ability to function as a semipermeable barrier between the bacterial cell and its environment and that this was the cause of cell death. They developed the following model for the prediction of microbial inactivation under PEFs: Êt ˆ s=Á ˜ Ë tc¯

-

E -E C k

where s is the survival ratio of the numbers of microorganisms after and before treatment; t is the treatment time, which is the product of the number of pulses and pulse width (ms); and Ec, tc, and

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k are constants determined empirically. From this model, it can be seen that for a given process, the inactivation level of microorganisms is only dependent on two parameters: electric strength, E, and treatment time, t. It should be noted that the constants Ec, tc, and k are determined by other process parameters such as properties of the electric pulse, medium and treatment chamber, and microorganism type and growth stage. The results reported on the effects of these factors are reviewed below. 10.3.2.1 Effects of Process Parameters Pulse properties include shape, polarization and frequency. The effects of pulse shapes on the inactivation rate were investigated by Qin et al. (1994). They used square, exponential, and oscillatory voltage pulse shapes for experiments, and the results showed that square wave pulses were more efficient than exponential or oscillatory pulses. They also reported that bipolar pulses were more efficient than monopolar pulses because the alternating stress produced by bipolar pulses resulted in structural fatigue of the membrane and in increased susceptibility of the cell membrane to electrical breakdown. The pulse frequency does not affect the inactivation ratio (Hulsheger et al., 1983). Temperature, electrical conductivity, ionic strength, and pH are usually used to describe medium properties. Pothakamury et al. (1996) reported that increasing medium temperature resulted in a higher inactivation ratio since it decreased the breakdown of the transmembrane potential (TMP) of the cell membrane in addition to causing thermal effects. The effects of medium conductivity on the inactivation rate were studied by Jayaram et al. (1993), who found higher inactivation rates in lower conductivity mediums. This was because lower medium conductivity increased the difference in ionic concentration between the cytoplasm and the medium, thereby facilitating an increased flow of ionic substances across the membrane. The authors believed that this caused a drain on cell energy reserves and eventually weakened the membrane structure, making the membrane susceptible to pulse application. Vega-Mercado et al. (1996) studied the effects of both ionic strength and pH level of the medium and concluded that higher ionic strength reduced the inactivation ratio, while reducing the pH level from neutral increased the inactivation ratio. The sensitivity of different microorganisms to PEF treatment was investigated by Sale et al. (1970). They reported that yeasts were more sensitive than vegetative bacteria to PEF. Qin et al. (1995a, 1995b) investigated the inactivation of E. coli and S. cerevisiae by PEF and also obtained the similar result that yeasts were more sensitive than gram-negative bacteria. In addition, it was found that the growth stage of microorganisms also affected the inactivation rate during PEF processing (Hulsheger et al., 1983; Pothakamury et al., 1996). Results indicated that cells in the logarithmic growth phase were more sensitive to PEF processing than those in the stationary and lag phases. The chamber properties discussed here are electrode configurations and modes of operation. There are five types of electrode configurations used for PEF treatment: parallel plate, wire–cylinder, rod–rod, rod–plate, and coaxial cylinders (Zhang et al., 1995). Parallel plate and coaxial electrode configurations (Figure 10.5) have been used in most of the studies. The former configuration produces a uniform distribution of electric field strength and are simple in design while the latter provides a smooth and uniform product flow and is suitable for industrial applications (Zhang et al., 1995). In terms of operation modes, Zhang et al. (1995) reported that a 9-log reduction of E. coli was obtained using stepwise PEF in a static parallel-plate treatment chamber with 16 pulses at each step. Qin et al. (1995a, 1995b) found that a continuous treatment chamber was more efficient that a static treatment chamber due to the same processing reason mentioned above. However, the results obtained by Martin-Belloso et al. (1997) demonstrated that there was no significant difference between continuous and static treatments.

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Electrode

Filling/withdrawing ports

Treatment region (a) A parallel plate chamber

Product exit

Aluminum support attached to the chamber body with screws

Chamber body

Product intake ports Electrode plastic support attached to the chamber body with screws − +

High voltage electrode High voltage

(b) A coaxial electrode chamber

FIGURE 10.5 Configuration designs of both parallel plate and coaxial electrode chambers.

Pulsed electric field processing has been demonstrated to have the potential to be a food processing alternative to conventional thermal processing. However, most of the reported studies have been conducted at the laboratory level, and differences between results from various researchers still exist. Hence, before this technique can be applied for industrial purposes, further research at the commercial level is necessary.

10.4 CONCLUSIONS New and alternative food processing methods and novel combinations of existing methods are continually being sought by industry in pursuit of producing better quality foods more economically. Hence, new innovations, technologies, and concepts continue to emerge. Over the years, many traditional technologies such as thermal processing have been optimized for producing better quality foods with the use of aseptic processing, microwave, RF, and Ohmic heating. New technologies such as high-pressure processing and pulsed electrical field treatment have emerged as nonthermal alternatives. These new technologies, although emerging as strong alternatives to conventional processing, still need to be rigorously tested and proved to be safe and commercially viable.

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Tempest, P. 1996. Electroheat Technologies for Food Processing. Bulletin of APV processed food sector, PO Box 4, Crawley, West Sussex, U.K. Thakur, B.R. and Nelson, P.E. 1998. High-pressure processing and preservation of food. Food Rev. Int., 14(4): 427–447. Timson, W.J. and Short, A.J. 1965. Resistance of microorganisms to hydrostatic pressure. Biotechnol. Bioeng., 7: 139–159. Vega-Mercado, H., Pothakamury, U.P., Chang, F.J., Barbosa-Canovas, G.V., and Swanson, B.G. 1996. Inactivation of Escherichia coli by combining pH, ionic strength, and pulsed electric fields hurdles. Food Res. Int., 29: 119–129. Wig, T., Tang, J., Younce, F., Hallberg, L., Dunne, C.P., and Koral, T. 1999. Radio Frequency Sterilization of Military Group Rations. Presented at the AIChE Annual Meeting. Zhang, Q., Barbosa-Canovas, G.V., and Swanson, B.G. 1995. Engineering aspects of pulsed electric field pasteurization. J. Food Eng., 25: 261–281. Zubber, F. 1997. Le chauffage ohmique: une nouvelle technolgie pour la stabilization des plats cuisinès. Viandes Prod. Carnès 18(2): 91–95.

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Radiation Processing 11 Ionizing of Fruits and Fruit Products Brendan A. Niemira and Louise Deschênes CONTENTS 11.1 11.2

Introduction ........................................................................................................................222 Ionizing Radiation Physics and Technologies...................................................................223 11.2.1 Gamma Rays........................................................................................................224 11.2.2 Accelerated Electron Beam (E-Beam) ................................................................225 11.2.3 X-Rays .................................................................................................................225 11.2.4 General Mode of Action......................................................................................226 11.3 Chemical Effects of Ionizing Irradiation in Foods ...........................................................227 11.3.1 Macromolecules...................................................................................................227 11.3.2 Small Molecules ..................................................................................................227 11.3.3 Protection in Complex Foods..............................................................................227 11.4 Biological Effects of Ionizing Irradiation in Foods ..........................................................227 11.4.1 Principal Targets ..................................................................................................227 11.4.2 Sensitivity of Organisms .....................................................................................228 11.5 Technical Aspects of Food Irradiation...............................................................................228 11.5.1 Dosimetry.............................................................................................................228 11.5.2 Detection of Prior Irradiation ..............................................................................229 11.6 Irradiation Facilities ...........................................................................................................230 11.7 Controlling Regulations: Commodities, Dose Limits, and Purposes ...............................232 11.8 Applications of Irradiation to Fruit ...................................................................................233 11.8.1 Delay of Ripening ...............................................................................................234 11.8.2 Disinfestation .......................................................................................................234 11.8.3 Reduction of Microbial Load ..............................................................................234 11.9 Response of Fruit Tissue to Radiation Treatment.............................................................235 11.9.1 Wound Response .................................................................................................235 11.9.2 Delay of Ripening ...............................................................................................236 11.9.3 Postharvest Disease and Storage Losses.............................................................236 11.9.4 Physiological Disorders .......................................................................................237 11.10 Quality of Irradiated Fruits ................................................................................................237 11.10.1 Texture, Color, and Sweetness ............................................................................237 11.10.2 Flavor and Aroma ................................................................................................237 11.10.3 Nutritional Quality...............................................................................................238 11.10.4 Influence of Variety/Cultivar ...............................................................................239 11.10.5 Quality Control ....................................................................................................239 11.11 Irradiation of Fruit Juices and Pulps .................................................................................239 11.11.1 Microbiology of Irradiated Juices and Pulps ......................................................239 11.11.2 Sensory Properties of Irradiated Juices and Pulps..............................................240

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11.12 Fruits Currently Being Irradiated ......................................................................................242 11.12.1 Strawberry............................................................................................................242 11.12.2 Mango ..................................................................................................................243 11.12.3 Papaya ..................................................................................................................243 11.13 Combination Treatments ....................................................................................................244 11.13.1 Mild Heat .............................................................................................................244 11.13.2 Modified Atmosphere Packaging (MAP)............................................................244 11.14 Irradiation of Packaging Material......................................................................................245 11.14.1 Types of Plastics Used in Fruit Packaging .........................................................246 11.14.2 Effects of Irradiation on Packaging Plastics .......................................................247 11.14.3 Regulation of Packaging Materials for Use with Irradiated Products ...............248 11.15 Cost-Benefit of Fruit Irradiation ........................................................................................248 11.16 The Future of Irradiation Processing of Fruits..................................................................250 Acknowledgments ..........................................................................................................................251 References ......................................................................................................................................252

11.1 INTRODUCTION Food irradiation is a physical treatment in which food is exposed to ionizing radiation, i.e., radiation of sufficient energy to expel electrons from atoms and to ionize molecules. This radiation may be in the form of high-energy photons (gamma rays or x-rays) or accelerated electrons in the form of an electron beam (e-beam). Foods treated with ionizing radiation have consistently been shown to be wholesome and nutritious [1–9]. Numerous reviews have been written on food irradiation [10–14], including reviews that specifically address the irradiation of fruit and vegetable products [15–17]. Despite the extensive body of evidence and the virtual consensus among researchers, acceptance of irradiated foods by the general public has historically (i.e., pre-2001) been comparatively low [18–20]. This reluctance had been ascribed primarily to a failure on the part of food scientists and food safety experts to adequately educate consumers about the benefits of food irradiation and to dispel persistent fallacies about the process, such as the myth that irradiated food becomes radioactive [21]. Frenzen et al. [22] found that 45.9% of 10,767 U.S. adults surveyed had never heard of food irradiation. Irradiated meats, fruits, and vegetables have been commercially available in the U.S. since 1992, although in limited markets. In 2000, food processors in the U.S. began a major effort to introduce irradiated ground beef and papayas to the marketplace, which led to a steady growth in the capacity of food irradiation facilities in the U.S. In October 2001, following bioterrorist attacks in which spores of Bacillus anthracis were mailed to public figures in the U.S., the U.S. Postal Service began using e-beam technology to irradiate letters and parcels to eliminate bacterial spores. For several weeks, the benefits and technical details of the application of ionizing radiation to a consumer product as an antimicrobial measure were discussed at length in a variety of broadcast, print, and Internet news outlets. As a statement of the safety of the process, the reports typically cited the use of irradiation on food, and usually quoted assessments from the Centers for Disease Control and Prevention, which said that irradiation is a safe and effective technology that can prevent many foodborne diseases [2], and from the U.S. Food and Drug Administration (FDA), which determined that the process is safe and effective in decreasing or eliminating harmful bacteria [7]. This media exposure provided an unprecedented public educational program. A poll of 1008 U.S. adults conducted in November 2001 showed a markedly increased level of support for food irradiation — with 52% of the participants agreeing that the government should require irradiation of foods to ensure a safe food supply — as compared to only 11% of consumers polled in 2000 who stated a willingness to buy irradiated foods [23].

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This shift in consumer attitude toward irradiation as a tool to increase the safety of consumer products is mirrored by a change in the way irradiation is regarded by food processors and researchers. Rather than as a tool to extend product shelf life, much of the latest research on food irradiation, particularly with regard to irradiation of fruit and vegetable products, has been on the elimination or attenuation of food-borne pathogens such as Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella [16, 17, 24, 25]. Early studies with irradiated produce typically used relatively high doses to completely eliminate spoilage bacteria and fungi [26]. The doses employed often exceeded the maximum radiation tolerances of vegetable commodities tested, resulting in loss of quality [27, 28]. Irradiation was, therefore, generally regarded as unsuitable for application to fresh produce [29–31]. However, low dose irradiation, i.e., less than 3 kGy (0.3 Mrad), is currently seen as one of several potential sanitary approaches to fruit and vegetable processing [16, 25, 32]. Recent research has shown that bacteria may be internalized in fruits and vegetables beyond the reach of surface sanitizers [33, 34]. The penetrability and subsurface antimicrobial efficacy of irradiation suggest that it can play an important role in the sanitization of produce. The United Nations Food and Agriculture Organization (FAO) and International Atomic Energy Agency (IAEA) recently initiated a cooperative research program on the use of irradiation to ensure hygienic quality of fresh, precut fruits and vegetables and other minimally processed food of plant origin [35]. This program brings together food scientists from 15 nations, including Brazil, Canada, China, India, the U.K., and the U.S., to share expertise, methodologies, and research data. The various research programs will investigate the effect of irradiation on a variety of human pathogens (e.g., E. coli O157:H7, L. monocytogenes, Salmonella spp., and Shigella spp.) associated with a number of whole and cut fruits, including apple, apricot, blueberry, cantaloupe, jackfruit, mango, peaches, pineapple, pomello, and watermelon. Additional research topics will include other vegetables, modified atmosphere packaging (MAP), sensory response, processing conditions, effect on shelf life and storage, etc. It is expected that the results of this multiyear research program will be used by food processors to design proper protocols for the irradiation treatment of fresh and minimally processed produce, as well as to improve the safety and quality of these fruits and vegetables. This chapter will discuss the use of ionizing radiation in the processing of fruits and fruit products such as juices, ciders, and fruit pulp. While the focus is on fruits and fruit products, valuable insights may be obtained by an examination of the research conducted on irradiation of vegetable products, and these studies will be discussed where appropriate. The key technologies used for irradiation will be compared, and the current state and extent of commercial fruit irradiation will be summarized. Also discussed will be matters of quality control in irradiation processing of fruits, and ways that other treatments may be combined with irradiation in a “hurdle”-type synergistic control program, as well as the regulations and scientific data concerning packaging materials. In addition to a review of the relevant scientific information on radiation microbiology and physiology, this chapter will present summaries of the national and international regulations concerning which fruit commodities may be irradiated, the permitted purposes for irradiation, and the maximum doses allowed by key exporting and importing nations.

11.2 IONIZING RADIATION PHYSICS AND TECHNOLOGIES The level of treatment received by a food product — the radiation dose — is defined as the quantity of energy absorbed during exposure. The international unit of treatment is the gray (Gy). One gray represents one joule of energy absorbed per kilogram of irradiated product, the equivalent of 100 rad or 0.1 Mrad. The energy absorbed depends on the mass, bulk density, and thickness of the food. Each kind of food has to absorb a specific dose of radiation for the desired results to be achieved. An excessive dose may damage the food and make it unacceptable for consumption,

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TABLE 11.1 Technologies for the Production of Ionizing Radiation Factors

Electron Beam

X-Ray

Gamma

Source

Electrons are generated on an emission coil and accelerated to high energy, 5–10 MeVa. Extensive cooling equipment required

Radioactive decay of cobalt-60 (2.5 MeV) or cesium-137 (0.51 MeV). Cooling equipment required

Mechanism

High-energy electrons cleave water molecules, creating oxygen and hydroxyl radicals that damage DNA membranes. Direct cleavage of DNA also occurs During operation, > 2 m concrete or ~0.7 m steel/iron/lead. When source is powered off, no radiation is emitted Seconds

Created when high-energy electrons (up to 5 MeV) strike a metal plate (e.g., tungsten or tantalum alloys); typical conversion efficiency is 5–10%. Extensive cooling equipment required High-energy photons stimulate atoms within target to release high-energy electrons that cleave water molecules into radicals. Direct cleavage of DNA also occurs During operation, > 2 m concrete or ~0.7 m steel/iron/lead. When source is powered off, no radiation is emitted Seconds

Shielding required

Speedb Penetrabilityc

6–8 cm, suitable for relatively thin or low-density products. Passes from multiple angles may be required

30–40 cm, suitable for all products

High-energy photons stimulate atoms within target to release high-energy electrons that cleave water molecules into radicals. Direct cleavage of DNA also occurs > 5 m water or > 2 m concrete or ~0.7 m steel/iron/lead. Source cannot be turned off, shielding of source must be the default position Minutes (depending on source strength) 30–40 cm, suitable for all products

a

MeV = million electron volts. Speed of dose delivery. The desired dose will vary depending on the target organism and commodity irradiated. c Penetrability in food products of average density approximating 1g/cm 3. This figure will vary for individual commodities due to localized variation in density associated with bone, voids, fibrous matter, etc. b

while an inadequate dose will fail to achieve the desired effects. The qualitative terminology used to describe doses is not firmly established, but generally, doses may be characterized as low (< 3 kGy), medium (> 3 and < 10 kGy), or high (> 10 kGy). The three types of ionizing radiation that are used for commercial food processing are gamma rays, e-beam, and x-rays. An overview of the advantages and disadvantages of each is presented in Table 11.1. Each of the technologies, whether high-energy photon or high-energy electron, induces ionization of molecules in the food target, leading to the generation of radicals, breakage of DNA, and other radiochemical effects. While these types of radiation are energetic enough to ionize atoms in the food being treated, it should be noted that they are not energetic enough to induce radioactivity in the target. This process is discussed in greater detail in the Section, “General Mode of Action.” Ozone and heat are generated during the irradiation process for each of the technologies; proper ventilation and temperature control are therefore especially important for the irradiation of fruits to avoid product degradation from these secondary effects [29].

11.2.1 GAMMA RAYS Gamma rays are high-energy photons produced by the disintegration of radioactive isotopes. The isotopes of significance in food irradiation processing are cobalt-60 and, less commonly, cesium-

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137. Gamma radiation is generated when cobalt-60 (half-life of 5.27 years) disintegrates. It first produces an unstable intermediate isotope of nickel-60 with emission of beta-radiation and then disintegrates further into stable nickel-60 with emission of two gamma rays of average energies of 1.17 and 1.33 MeV, respectively. Disintegration of cesium-137 (half-life of 30.17 years) to barium137 produces gamma rays with an energy of 0.66 MeV. The preferred radioisotope for food irradiation has historically been cobalt-60. Gamma rays from cobalt-60 are more energetic than those from cesium-137, but due to its much shorter half-life, cobalt60 irradiators must be replenished with fresh radioisotope (recharged) much more frequently than cesium-137 irradiators. After 10.5 years of operation, a cobalt-60 irradiator will have approximately 25% of its initial strength, while a cesium-137 irradiator will have retained approximately 80% of its initial strength. Despite the need for more frequent recharging, the key factors that have made cobalt60 the preferred radioisotope include the higher initial cost of cesium-137 and the water solubility of the CsCl form of cesium-137 used in irradiators, a point of significant environmental concern. Gamma rays have excellent penetrability and are suitable for irradiating large food items such as pallet- or crate-sized packages of product. The shielding required for a gamma irradiator is roughly comparable to that required for an e-beam or x-ray irradiator (Table 11.1). However, the stigma attached to radioactive material, particularly in a food processing facility, as well as the always-on nature of the source, has led to only limited adoption of gamma sources for food processing. The time required for processing is dependent on the dose desired and the strength of the source. Longer processing times may necessitate some form of temperature control, particularly in temperature-sensitive products such as fruits.

11.2.2 ACCELERATED ELECTRON BEAM (E-BEAM) E-beams are produced by electronic equipment that uses a linear accelerator or a cyclotron accelerator to impart a high velocity, and therefore a high kinetic energy, to a stream of electrons. The accelerators used in commercial irradiators intended to treat food produce a focused beam of electrons with an energy of up to 10 MeV. The processing dose is delivered as a pulse of electrons, and the full dose is delivered quickly, taking typically less than 5 sec. The electrons are aimed at the target with a cone-shaped guide. Higher doses may be delivered by repeated exposure. Despite the intensity of the beam, the short exposure time for product being irradiated generally prevents any significant rise in temperature during processing. The penetrability of the electron beam is lower than that of gamma rays or x-rays (Table 11.1), so product must be packaged in relatively thin cartons treated from multiple sides or some combination of these. The increased handling (e.g., stacking and unstacking pallets of produce, multiple passes, etc.) may result in decreased throughput compared to gamma ray or x-ray processing, as well as increased opportunities for postprocess handling damage to the product. With regard to construction of new irradiation facilities, a significant commercial advantage of e-beam irradiators over gamma irradiators is the electronic nature of the radiation source, which enables it to be completely deactivated — i.e., when it is off, it is off. It is important to note, however, that an electron accelerator does not resemble a home microwave oven; as high-power, industrial-scale electronic devices, electron accelerators require specialized training and maintenance for their operation, and the shielding required for an operating e-beam unit is comparable to that required for a gamma irradiator.

11.2.3 X-RAYS X-rays are high-energy photons and are the form of ionizing irradiation most familiar to the general public due to their widespread medical applications, although at much lower energies. The highenergy x-rays used in food irradiation are generated from high-energy electrons and thus require the same type of accelerator systems used in an e-beam irradiator. The beam of electrons is directed at a high-density metal target. The electrons are absorbed by the metal atoms and the high-energy

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photons (x-rays) are emitted. The efficiency of this process is relatively low, typically 5 to 10%, depending on the specific alloy used in the metal target plate. Alloys of tungsten, tantalum, and various types of stainless steel have been used, and they vary in durability, cost, and efficiency. The energy of the x-ray output is dependent on the energy of the originating e-beam and the conversion efficiency. The vast majority of the energy that is directed at the metal plate is converted into heat, making the proper design of cooling subsystems in x-ray irradiators critical. Also of concern are neutrons that are occasionally knocked from the high-density metal plate by the ebeam. At high energy levels, these neutrons can induce a low level of short-lived radioactivity in the target [36]. This neutron generation is less likely, although not impossible, with the lowerdensity foodstuffs targeted directly in e-beam irradiators. Shielding for an x-ray irradiator is comparable to that required for the other two types of units. X-rays, like gamma rays, are high-energy photons, have a similar penetrability (Table 11.1), and can be used for bulk packages or higher-density foodstuffs. However, x-rays share with e-beam the commercial advantage of being produced electronically and are therefore able to be completely inactivated in a power-off state. The design of some commercial e-beam irradiators includes the ability to convert from e-beam to x-ray operation as needed by installation and removal of the metal target plate. While x-rays would seem to be an attractive middle ground between gamma and ebeam irradiators, the low energy conversion efficiency, the extensive heat build-up in the metal plate, and the low, but measurable, neutron scattering are drawbacks for commercial application.

11.2.4 GENERAL MODE

OF

ACTION

A stream of high-energy photons, either gamma rays or x-rays, can energize electrons within the atoms of the target. These electrons may leave the atom completely (ionization), or the energy of the electrons may rise to a higher level within the atom (excitation). Both processes may yield free radicals, i.e., atoms with unpaired electrons on their outer shell. The stream of high-energy electrons in an e-beam interacts with the atoms directly to create free radicals. These free radicals are very reactive because their unpaired electrons may pair up with the outer shell electrons of the atoms that make up cellular components. Because water makes up the bulk of mass of foods, particularly fresh fruits and vegetables, the water molecule is most frequently affected. The majority of the absorbed energy from the ionizing radiation treatment goes into the creation of hydrogen and hydroxyl radicals from water molecules [26]. The interaction of these free radicals with the organic molecules of the food is the main mode of action of ionizing radiation. Under conditions of limited free water, such as in dried or frozen products, radicals are produced with less efficiency and have reduced mobility. In these products, higher doses become necessary for microbial control [37, 38]. In most discussions of food irradiation, a recurrent question arises regarding the mode of action of high-energy photons (gamma or x-ray) vs. high-energy electrons (e-beam), and the possibility of differential antimicrobial efficacy or effect on product sensory attributes or physiology [39]. It is tempting to make a blanket statement that, based on the physics of energy transfer in the process of generating radical molecules, one would expect to see no appreciable difference in irradiated product based on the means of irradiation. However, direct-comparison experiments to verify this contention with fresh produce are lacking. Papaya treated with gamma radiation tended to be firmer than fruit similarly treated (0.4 or 0.6 kGy) with x-rays; significant, though small, differences in aroma, texture, and color were detectable to one set of triplicate sensory panels but not to another [40]. In the available direct-comparison studies of meats, high-energy photons and high-energy electrons tend to have similar, but not quite identical, effects. E-beam and gamma (1.5 or 3 kGy) were similarly effective at eliminating Salmonella typhimurium from refrigerated beef steaks, but e-beam was not as effective as gamma in the elimination of Pseudomonas fluorescens (a common spoilage bacterium found on meats and produce) [41]. Additional direct comparisons of the various irradiation methods are therefore warranted, especially studies that involve sensitive products such as fruits and vegetables.

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11.3 CHEMICAL EFFECTS OF IONIZING IRRADIATION IN FOODS Although irradiation shows considerable promise for extending shelf life and maintaining microbiological and sensory qualities of fruits, the literature is rich in contradictions about the results of irradiation of specific fruits. This leads to difficulty in standardizing the treatment. The success of the treatment depends on numerous factors. The most important ones are commodity and cultivar, dose of radiation, degree of maturity, physiological status of the fruits, temperature and atmosphere during and after treatment, pre- and postharvest practices, and susceptibility of the microorganisms to be controlled to radiation.

11.3.1 MACROMOLECULES Nucleic acids, because of their large size, are the main targets of free radicals generated by gamma rays. They may be affected in various ways, leading to breaks in one or both strands of DNA or to intra- or extramolecular cross-linking. Polysaccharides, cellulose, pectins, and starches may be partly depolymerized. Proteins are relatively little affected, although reduction of disulfide bonds may lead to inactivation of active sites and conformational changes in enzymes.

11.3.2 SMALL MOLECULES Sugars may be hydrolyzed or oxidized when subjected to gamma radiation. Free amino acids can be deaminated. The small molecules most affected are the polyunsaturated fatty acids. Free radicals react with polyunsaturated fatty acids, producing unstable hydroperoxides and a range of further degradation products. The effect of irradiation on vitamins has been studied extensively. Certain vitamins (A, B12, C, E, K, thiamine), particularly those with anitoxidant activity, are degraded when irradiation is carried out in the presence of oxygen. Loss of vitamin C is commonly overestimated in the literature because ascorbic acid is oxidized to dehydroascorbic acid that is still active as vitamin C. Dehydroascorbic acid is, for the greater part, reduced back to ascorbic acid in living plant tissues. Only when dehydroascorbic acid is further oxidized to diketoglutonic acid is it irreversibly lost as vitamin. In general, small molecules, including vitamins, are little affected by low-level irradiation, and to a lesser extent, by thermal processing.

11.3.3 PROTECTION

IN

COMPLEX FOODS

Antioxidants added to solutions are known to protect sugars, vitamins, and proteins, as well as suspended bacteria from the effects of ionizing radiation [42, 43]. It may therefore be theorized that naturally occurring antioxidants may serve to protect plant tissue from radical molecules. However, the role of natural antioxidants in determining radiation tolerance of fruits and vegetables has not been addressed directly. Kader [15] and Dupont et al. [44] grouped fruits and vegetables according to their tolerance for irradiation. Products that are more radiation tolerant are, in some cases, also reported to be higher in antioxidant concentration, and lower radiation tolerance was similarly associated with low antioxidant concentration [45, 46]. However, methodological differences in how the produce was evaluated in the various studies prevent definitive conclusions. It is important to note that each commodity, and even each cultivar, has its own limits of tolerance. The dose to be applied should be preliminarily tested on a sample.

11.4 BIOLOGICAL EFFECTS OF IONIZING IRRADIATION IN FOODS 11.4.1 PRINCIPAL TARGETS The principal targets of the radical molecules, with respect to biochemical function, are nucleic acids and membrane lipids. Alterations in DNA will affect gene expression and the biosynthesis

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of various enzymes and will interfere with cell division. DNA is generally more sensitive to damage from radicals during active replication or transcription [47]. Bacterial plasmid DNA may also be damaged, disrupting and inactivating plasmid-encoded genes; pathogenic bacteria that carry plasmid-encoded virulence genes may therefore have a reduced virulence after irradiation [48]. Alterations in membrane lipids, particularly polyunsaturated lipids, lead to the perturbation of the membrane and to deleterious effects on various membrane functions — most important, permeability. Proteins are scarcely affected, and enzyme inactivation is rarely observed when irradiated in situ. In pure solution, peptide bonds and sulfhydryl groups, often involved in enzyme activity sites, may hydrolize or oxidize into disulfide intra- or inter-peptide bridges. However, the activity of membrane-associated enzymes may be affected as a secondary effect of membrane lipid degradation.

11.4.2 SENSITIVITY

OF

ORGANISMS

The physiological effects of ionizing radiation on spoilage organisms and human pathogens have been reviewed by Monk et al. [49] and Diehl [26]. Fungi are, typically, more resistant to radiation than bacteria [49, 50]. D10 values, i.e., the amount of radiation necessary to effect a 90% (1-log) reduction, have been reported for yeasts and molds in the range of 1 to 3 kGy [51–53] as opposed to D10 of 0.3 to 0.7 kGy for pathogenic bacteria on produce and fruit juices [54–57]. Human pathogenic viruses are usually more resistant to radiation than bacteria or fungi. The doses required to achieve meaningful virus population reductions, as a rule, result in loss of sensorial quality of the produce [27, 30, 31, 49]. D10 for plant pathogenic fungi has been shown to range from 0.4 to 1.1 kGy [58]. Low dose irradiation has been shown to suppress some plant pathogenic fungi responsible for storage losses [16]. However, the relative resistance of fungi suggests that irradiation is best suited to control of bacterial pathogens, as opposed to viral or fungal pathogens. A concern expressed by various authors [59] is the possibility of increased human or plant pathogen growth in storage following irradiation due to reduced interspecies competition. Although studies of the effect of low radiation doses on the microbial ecology of produce are lacking, studies of irradiated meats have tended to support the position that while irradiation of a product reduces the total population of bacteria on the sample, it does not lead to increased recolonization by pathogens [59, 60, 61]. However, Matches and Liston [62] found that Salmonella grew more rapidly on irradiated vs. nonirradiated fish fillets. Carlin et al. [63] showed that the growth of L. monocytogenes inoculated onto previously disinfected leaves of endive (Cichorium endiva) was significantly enhanced. While this study [63] relied on chemical disinfection rather than irradiation, the result indicates that the interaction of native microflora and bacterial pathogens such as L. monocytogenes is an important subject for future study.

11.5 TECHNICAL ASPECTS OF FOOD IRRADIATION 11.5.1 DOSIMETRY Measurement of the dose of radiation absorbed by the food (dosimetry) is the underlying key to effective use of irradiation. It is important to stress that the quality of the dosimetry will determine the quality of the irradiated product. Material near the outside of a carton or package tends to receive a somewhat higher dose than the product in the center, owing to its proximity to the radiation source. The ratio of the maximum amount absorbed (typically, near the perimeter) to the minimum absorbed (typically, near the center) is referred to as the max/min ratio. In an ideal, perfectly uniform treatment system, this value would be 1; in practicality, max/min ratios of 1.5 to 2 are more typically obtained. The max/min ratio is influenced by the bulk density of the product being treated, the penetrability of the radiation type, the strength of the source, and localized density variations within the product (voids, pits, etc.). A Monte Carlo simulation of a single apple’s dose absorption profile suggests that even within a single piece of fruit, the max/min ratio may be in

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excess of 2.4 due to the topographical complexity of apples [64]. A process that has a high max/min ratio may result in one part of a shipment receiving too high a dose (resulting in violation of regulations, sensory damage to product, etc.) while another part receives too low a dose (resulting in surviving insects or bacteria, inadequately delayed ripening or sprouting, etc.). Thus, design of effective irradiation protocols begins with a preliminary 3-D mapping of the doses absorbed by the product in each part of the package, when it is packaged in the type of carton and packing arrangement expected to be used commercially. Dosimetry is important for establishing the desired dose for each commodity, for obtaining data required for approval of a petition by regulatory agencies, and for developing quality control procedures in the irradiation plant. Dosimetry is also used to determine the configuration of the irradiation field in the facility. Mclaughlin et al. [65] have reviewed the technical aspects of dosimetry in detail. Although the absorbed dose can be calculated, experimental dosimetry is preferred. The energy absorbed is measured by placing dosimeters (radiation sensors) inside the lot being irradiated. In this way, the distribution of the energy absorbed and the minimum and maximum dose can be determined. The choice of dosimeter is influenced by several factors, including environment (humidity, temperature, and light) stability under the process conditions, dose range, and dose rate. It is therefore important to have a quality control procedure to test the dosimeters. An example of a chromatographic dosimeter commercially available for routine measurement of absorbed dose is the Harwell YR (Didcot, U.K.). This is a radiation-sensitive polymethylmethacrylate-based product hat darkens on exposure to ionizing radiation. Chromatographic dosimeters are read with a spectrophotometer, and the extent of color change is calibrated to absorbed dose. Electron paramagnetic resonance (EPR) is a sensitive dosimetry technique suitable for research purposes. The dosimeter consists of an alanine pellet that undergoes a measurable shift in paramagnetic spin state following exposure to ionizing radiation. With proper storage, the dosimeter pellet can hold the new electron spin state for months, providing an opportunity for repeated reference to the same set of processed dosimeters.

11.5.2 DETECTION

OF

PRIOR IRRADIATION

Although irradiation of various food products has been approved by various nations, detection methods are not required for clearance. Development of methods to detect irradiation and to determine the dose absorbed are, nevertheless, important for a number of reasons, e.g., differences in national legislation, control of international trade, quality control, and consumer reassurance. Regulatory authorities and processors are the two groups that are most interested in the development of such methods. Whenever attempts have been made to detect irradiation on an experimental basis, the task has proven most difficult. The changes are not radiation specific. It is probably not possible to develop a detection method for foods in general [66]; however, it is possible to devise such methods for specific food groups. Detection of irradiation in food has been reviewed in detail by Raffi [67] and Delincee [66]. Detection of irradiation has been investigated for a large number of fruits including banana, mango, papaya, and strawberry [Figure 11.1; 68]. These methods were usually based on chemical, physical, and biological changes in fruits. Some methods studied are listed below. After irradiation, carbohydrates may react to give acids and carbonyl compounds. Den Drijver et al. [69] detected glucosone (D-arabinohexos-2-ulose) in an irradiated sugar mixture solution mimicking the sugar composition of the mango, and not in the untreated controls. Studies with apples showed significant changes in starch and pectins [70, 71]. Detection of irradiation based on DNA analysis is promising and widely applicable; however, more research is needed to obtain results that are both specific and reliable [68]. Inhibition of seed germination (half-embryo test) has been used as an indication of exposure to radiation for citrus fruits [72]. It should be noted, however, that alfalfa seeds treated with 5 kGy did not show a significant change in germination rate, suggesting that caution should be exercised with this method of detection [73].

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FIGURE 11.1 Fruits investigated for detection of radiation treatment. (Adapted from IAEA, Analytical Detection Methods for Irradiated Foods, IAEA-TECDOC-587, Vienna, 1991.)

EPR was shown to be reliable with strawberry achenes exposed to a dose of 1 kGy or more [74] and can be used with blueberry, fig, raspberry, and red currant. EPR can also detect prior irradiation of alfalfa seeds (D.W. Thayer, personal communication). The test may not be reliable with all fruits, as shown with plums [75] and with citrus and grapes [68]. Detection of radiationinduced hydrocarbons by gas chromatography or mass spectrometry has been successfully employed in irradiated (1 kGy) perilla (Perilla frutescens) seeds [76]. Rahman et al. [77] showed that the irradiation dose used for quarantine treatment induced a significant decrease of the size of the supraesophageal ganglion of insects. Measurement of luminescence is the first detection method used routinely for inspection purposes. The method, used in Germany, is based on the determination of light emitted by oxidizing substances formed and trapped in irradiated tissue. It can be used with produce containing solid inclusions, such as achenes and adhering minerals. Chemiluminescence, resulting from oxidation reactions after irradiation, was used to detect irradiation of spices, dried herbs, and fruits [78, 79]. Thermoluminescence is released after heating of irradiated products. It is more reliable than chemiluminescence since it is less influenced by interferences from test conditions [68]. This approach gave good results with strawberries [79]. Thermoluminescence has shown promise in detecting irradiation treatment of spices [81] and potatoes [82]. Two methods that showed some potential with meat were tested on fruits with limited success. Changes induced to the microflora by irradiation were too influenced by pre- and postharvest conditions to be reliable. Determination of o-tyrosine in proteins was attempted with strawberries but showed high background values and artifacts [83, 84]. Changes in the content of phenolic compounds [85] may not be specific to radiation because such changes are common under stress conditions. Presently, the more promising irradiation detection methods for fruits seem to be EPR and thermoluminescence. A particular challenge is the low dose of irradiation, 1 kGy or less, with which fruits are treated. Further investigation, standardization, and development of instrumentation will eventually lead to practical inspection methods. It should be noted that ex post facto dosimetry is difficult; while these and other detection methods may be able to indicate that a given product was irradiated, it is unlikely to determine when the product was so treated, or when the dose was delivered [26].

11.6 IRRADIATION FACILITIES Although e-beam, x-ray, and gamma radiation have essentially identical effects on food, irradiation facilities differ greatly with respect to design and physical arrangement. The vast majority of gamma-based facilities use cobalt-60 as the radionucleotide source. The majority of new irradiation

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Product Carrier (with food cartons)

231

Palletized Food products

Product Carrier Tracks

Concrete Radiation Shield (cut-away view)

Robotic De-palletizer Product Carriers

Source Assembly Track (vertical to horizontal)

Cobalt 60 Storage Pool

FIGURE 11.2 Semiautomated cobalt-60 irradiator specially designed for processing food products. (From Nordion International Inc., Kanata, Ontario. With permission.)

facilities under construction in the U.S. are e-beam or dual-use e-beam and x-ray facilities. Several new e-beam facilities have been built in the U.S. in the last decade for irradiation of meat and poultry products, shell eggs, and Hawaiian fruits destined for the U.S. mainland, such as papaya. In 2001, the U.S. Postal Service contracted to purchase eight e-beam systems for irradiation of mail in conjunction with antiterrorism efforts, with options to purchase as many as 12 additional facilities. Thus, in terms of market share and processing capacity, e-beam technology may benefit from increased familiarity with the technology and the economies of scale that come with mass production of the equipment. Figure 11.2 depicts a typical cobalt-60 automatic carrier irradiator specifically designed for processing food products (Nordion International Inc., Kanata, Ontario). This unit is suitable for processing large volumes (up to 8 million ft3 annual throughput) in continuous operation. Produce is loaded into boxes on a conveyer. The boxes move around the cobalt-60 source within a thick concrete enclosure shielding the environment from the gamma rays. The source is lowered into a pool of water when not in use. The dose applied is controlled by regulating the speed of the conveyer. In 2001, a joint venture between an American manufacturer of e-beam/x-ray equipment (SureBeam, San Diego, CA) and a Brazilian irradiation processing corporation (Tech Ion, Manaus, Brazil) was announced [86]. This joint venture is expected to facilitate irradiation of fruit and vegetables produced in Brazil, with the stated goals of reducing postharvest losses in storage and transshipment, and improving disinfestation. A gamma-based irradiation facility is currently in operation in Manaus, Brazil. Fruits irradiated at this facility are destined primarily for the large population centers of Sao Paulo and Rio de Janeiro in the southeast [87]. As part of a significant effort to promote and facilitate food irradiation, the government of Brazil recently established the Food Irradiation Center of Excellence in Rio de Janeiro [86]. Near the city of Rio de Janeiro, a large-scale irradiation facility is under construction, which is designed to incorporate e-beam, x-ray, and gamma-ray processing capacity at a single site [35]. This facility is expected to be operational in 2002 with an accelerator of 15 kW power at 10 MeV, with an expected capacity of 120,000 t/year. The processing capacity of the x-ray processing unit is expected

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to be 80,000 t/year. The cobalt-60 source strength is planned to be 600,000 Ci with a capacity of 120,000 t/year. This facility will process fruits and vegetables intended for domestic consumption, as well as for export. An x-ray facility near Hilo, HI, is currently irradiating fresh fruits intended for shipment to the mainland U.S., with particular focus on the New York, Washington, Oregon, California, and Minnesota markets. The fruits treated include papaya, rambutan, lychee, atemoya, longan, and star fruit [88]. The fruits are treated with 0.25 to 0.40 kGy for disinfestation purposes; an extended shelf life is an additional benefit to the treatment. The projected production capacity of this facility is 12,500 t/year.

11.7 CONTROLLING REGULATIONS: COMMODITIES, DOSE LIMITS, AND PURPOSES Each nation issues its own regulations concerning the commodities that it allows to be irradiated, the maximum (and in some cases, the minimum) dose to be applied, and the purposes for which a product may be irradiated. In many cases, the intended purpose of the irradiation treatment will determine the maximum dose that can be legally applied. Regulations concerning the approval and inspection of food irradiation plants are also nation specific. It is expected, however, that as food processing operations, irradiation facilities will be held to the highest standards of cleanliness and safety. The regulations governing products, dose limits, purposes, etc., can range from very restrictive to very permissive. The International Consultative Group on Food Irradiation (ICGFI) is a group of experts from 46 nations organized under a joint FAO/IAEA/WHO collaboration to review and disseminate scientific information regarding food irradiation. ICGFI maintains an online listing of the regulatory limits governing food irradiation for a variety of developed and developing nations [89]. This section will provide an overview of some key nations with regard to regulations dealing with fruits and vegetables. With changing consumer attitudes toward irradiated foods, the implementation of irradiation on food is increasing worldwide. The controlling regulations in many nations are currently being reviewed and revised, and the current state of affairs may therefore differ somewhat from the information presented herein. U.S. regulations permit the irradiation of dry or dehydrated vegetable-derived spices, seasonings, and flavorings, as well as coloring agents (e.g., paprika) to doses of up to 30 kGy [90]. Approved sources are gamma rays from cobalt-60 or cesium-137; accelerated electrons from a machine source, not exceeding 10 MeV (to eliminate the risk of inducing radioactivity); or x-rays from a machine source, not exceeding 5 MeV. The regulatory limit for fresh fruits and vegetables is 1 kGy, limited to the specific uses of disinfestation and inhibition of produce growth and maturation [90]. For comparison, irradiation to control food-borne pathogens is permitted at higher doses for shell eggs (3 kGy), seeds used for growing sprouts (8 kGy), fresh and frozen poultry (3 kGy), fresh meats (4.5 kGy), and frozen meats (7 kGy). Irradiated foods must bear the internationally recognized radura logo (Figure 11.3) along with the statement “treated with (or by) irradiation.” In 1999, a coalition of U.S. food processors petitioned the U.S. FDA to amend U.S. regulations to allow doses of up to 4.5 kGy for a wide variety of refrigerated and ready-to-eat meat and vegetable products, including juices, and doses up to 10 kGy for frozen meat, vegetable, and juice products. Elimination of human pathogens is the primary goal of these requested dose limits; the potential for extension of shelf life is regarded as a secondary goal [91]. Many European nations (e.g., Austria, Germany, Ireland) have approved only dry herbs, spices, and vegetable seasonings for irradiation, with a maximum dose of 10 kGy. Canada has approved for herbs, spices, and seasonings (max 10 kGy), and allows irradiation of onions and potatoes for sprout suppression (max 0.15 kGy). Mexico will allow any type of fresh fruit to be treated with up to 1 kGy for quarantine treatment or to delay ripening, and up to 2.5 kGy to extend shelf life.

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FIGURE 11.3 The radura — the internationally recognized logo identifying foods that have been irradiated.

Mexican dried fruit may be treated with 1 kGy for disinfestation but with up to 10 kGy for microbial control. The U.K. allows fresh or dried fruit to be treated for disinfestation (max 2 kGy). India has approved irradiation disinfestation of dates and mangoes (max 0.75 kGy). Japan has approved treatment of a single product, potato, for sprout inhibition (max 0.15 kGy). Australia allows irradiation of herbs and spices for disinfestation (max 6 kGy) or microbial control (max 30 kGy). Brazil possesses the most open regulations concerning food irradiation. Prior to 2001, a specified list of fruit commodities were permitted for irradiation [87]. This list included avocado, banana, orange, lemon, and other fruits important for domestic consumption and the export market. The maximum dose permitted was 1 kGy, although the treatment could be applied for a variety of purposes, including disinfestation, delay of ripening, shelf-life extension, or (in combination with a heat treatment) control of microbial load. A joint FAO/IAEA/WHO study [92] examined the wholesomeness of food irradiated above 10 kGy. In that study, it was concluded that food may be safely irradiated to any dose sufficient to achieve the desired physiological or microbial outcome, without appreciable loss of nutritional adequacy. Based on this recommendation, the Brazilian government dramatically revised its regulations governing food irradiation [89]. As of 2001, any food product in Brazil, such as fruit, vegetable, meat, poultry, and fish, etc., may be irradiated to any dose for any purpose. The only limit on the dose applied is based on the product’s physiological tolerance for irradiation from a quality or marketability standpoint. Brazil’s “any product, any dose, any purpose” stance, although based on the scientific recommendations of respected international authorities, is markedly more open than most other nations. Brazilian products that are intended for export must still meet the regulatory requirements of the importing nations, and are therefore, as a matter of practice, subject to dose limits and purpose restrictions. Although efforts are underway to encourage Brazil’s trading partners to lift restrictions on irradiated foods (J.F.B. Medieros, personal communication), dramatic changes to the regulations of importing nations are not expected in the near future.

11.8 APPLICATIONS OF IRRADIATION TO FRUIT Fresh produce may be irradiated for a number of purposes, including inhibition of sprouting in the case of some vegetables, and, of more significance for irradiated fruit, delay of ripening, insect

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disinfestation, and reduction of microbial load. Of these, the most important effect of irradiation in food processing, whether meats and poultry or fresh fruits and vegetables, is reduction of microbial load. While this has traditionally meant elimination of produce spoilage pathogens so as to extend shelf life [93, 94], the elimination of human pathogens is also of increasing importance [16, 17, 25]. Refrigeration and modified-atmosphere packaging are currently the primary means of preserving fresh produce [20]. Fresh produce is generally more sensitive to ionizing radiation than meat or poultry products. Unlike meat, fresh produce is living tissue that respires, maintains water relations with its environment, and, in many cases, may continue synthesis of secondary metabolites [95]. Irradiation can alter these processes, leading to changes in firmness, aroma, color, or taste [25, 30]. Plant physiology and surface-associated microbial ecology can be altered by irradiation; these effects are seen most clearly during subsequent refrigerated handling and storage [27, 82, 96]. In general, ionizing radiation causes less alteration to fresh produce than other preservation techniques with equivalent benefits, e.g., thermal treatment. However, irradiation does not replace these other methods; it supplements them. Irradiated fresh produce, like all irradiated foods, remain subject to the basic rules of good manufacturing practice for preservation of quality and food safety [2].

11.8.1 DELAY

OF

RIPENING

Ripening of fruits can be delayed by doses of 0.2 to 0.5 kGy. The underlying mechanism of the effect is not well understood. It involves interference with biochemical processes that are part of ripening and senescence [97]. Ethylene production by Gala apples was reduced by gamma irradiation (0.44 kGy) [98]. Most studies on fruit irradiation have been carried out with gamma rays. E-beams do not penetrate enough to act on metabolism and increase shelf life of certain fruits, e.g., avocado [99]. Tolerance varies with degree of maturity, as discussed by many authors. Delay in ripening of climacteric fruits requires that the treatment be applied before the onset of the climacteric increase in ethylene production [29, 97]. The need to treat at an early stage of maturity brings about a loss of quality, and may cause abnormal ripening and uneven coloring; however, in the case of tomato, very early treatment may accelerate ripening due to ethylene production in response to wounding [100].

11.8.2 DISINFESTATION Disinfestation, the control of arthropod pests, can be achieved by doses up to 3 kGy. Sterilization to prevent reproduction of the insects during or after storage can be achieved by doses of 0.03 to 0.2 kGy. Doses sufficient to kill insects outright are in the range of 1 to 3 kGy. This application of irradiation is among the most promising since fumigation with chemicals like ethylene dibromide (prevented in the U.S. since 1984) is increasingly questioned. Irradiation is probably the most effective quarantine treatment for control of fruit flies and other insects in a number of commodities, including mango and papaya. Other quarantine treatments exist but are not easily applicable to all fruits. Cold treatment requires several days and is therefore not useful for fruits with short shelf lives. Heat treatment occasionally causes physiological disorders and discoloration.

11.8.3 REDUCTION

OF

MICROBIAL LOAD

The efficacy of ionizing radiation to reduce the microbial load is dependent on a number of factors. Plant fruits and stems typically support 103 to 106 colony-forming units (cfu) per gram of plant tissue [20, 101]. These organisms come from the environment in which the plants are grown, including the soil, water, air, manure, and compost, as well as from postharvest handling, processing, and shipping [24]. It is increasingly recognized that bacteria may become internalized in fresh produce and survive for days or weeks, reducing the efficacy of traditional, surface-oriented antimicrobial measures such as chemical rinses and washes [33, 102–105].

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The vast majority of plant-associated microorganisms are nonpathogenic bacteria. These bacteria often form biofilms that can further reduce the efficacy of antimicrobial measures [106–108]. Human pathogens such as E. coli and Salmonella have been observed to form durable biofilms on industrial surfaces [109, 110]. The extent to which enteric bacteria, as well as the psychotrophs L. monocytogenes and Yersinia entericolitica, participate in preexisting phytoplane biofilms formed by nonpathogenic bacteria is not known [108, 111]. Ionizing radiation can penetrate sheltered areas of fruits, and, in the case of gamma and x-ray, penetrate internal structures to inactivate bacteria. As has been discussed, however, the radiation types differ in penetrability, most significantly in higher-density products. In a study of sliced cantaloupe inoculated with human pathogens (106 cfu/g), after radiation treatment with 3 kGy from a 5-MeV e-beam, Draughon et al. [112] were able to recover Salmonella and Listeria, but not Staphylococcus. The authors concluded that for the 1-cm thick slices examined, an e-beam dose of greater than 3 kGy would be necessary to completely eliminate Salmonella or Listeria. Vegetables have been the object of numerous studies of the elimination and recovery of human pathogenic bacteria following irradiation. Total aerobic plate count and inoculated L. monocytogenes were reduced by ~4 logs on precut bell pepper following a dose of 1 kGy. The pathogen regrew to initial levels within 4 d on peppers subsequently stored at 15 or 10∞C, but remained low on peppers stored at 5∞C [113]. The authors concluded that irradiation followed by refrigeration effectively suppressed pathogen growth throughout the useful shelf life of the produce. E. coli and L. monocytogenes were effectively eliminated (> 5 logs) from diced celery by 1 kGy [28]. Also in that study, total aerobic counts were determined, following irradiation or one of three conventional treatments (i.e., acidification, blanching, or chlorination). Acidification reduced initial counts to a degree comparable to that of 1 kGy, but by the end of the storage period (22 d), the aerobic population following conventional treatments had regrown to equal (acidification) or exceed (blanching and chlorination) the untreated controls, while that of irradiated (0.5 and 1 kGy) celery remained significantly lower throughout the study. The D10 value for human or plant pathogenic bacteria can range from 0.3 to 0.7 kGy [54–57]. Achieving a 5-log reduction, then, would require between 1.5 kGy and 3.5 kGy. Fresh produce will generally tolerate doses of 2 kGy, although variation exists. This suggests, therefore, that irradiation will most likely play a role as one of several options in preserving fruit quality and microbial safety. These other options might include mild thermal treatment, modified atmosphere packaging, ozone treatment, among other processes. A complete discussion of combination treatments is presented later in this chapter.

11.9 RESPONSE OF FRUIT TISSUE TO RADIATION TREATMENT 11.9.1 WOUND RESPONSE Plant tissues are relatively sensitive to ionizing radiation and show a typical wound response, characterized by a transient increase in respiration and ethylene production, even at low doses. Such a wound response was shown after irradiation of strawberries [114]. The rate of respiration increased linearly with increasing dose of gamma irradiation. The effect was transient and the rate of respiration decreased back to preirradiation levels within 24 h for the lowest dose, 0.3 kGy, but slower with an increasing dose. The low dose of 0.3 kGy was insufficient to control mold development, as indicated by the progressive increase in rate of respiration in control and 0.3 kGy irradiated fruits. This progressive mold development could be prevented by treatment with a fungicide and by irradiation at 1 kGy. Ethylene production also increased after irradiation, but in contrast with respiration, it reached a maximum at 1 kGy. The latter observation suggests that irradiation beyond 1 kGy caused membrane damage since ethylene production is membrane associated. Similar results were obtained by Larrigaudiere et al. [115] with cherry tomatoes. Other typical wound (and stress) responses are (1) stimulation of phenol biosynthesis, particularly stim-

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ulation of the first enzyme in the pathway phenylalanine-ammonia lyase (PAL) [116] that predisposes the tissue to enzymatic browning, and (2) an increase in enzymatic defenses against free radicals scavenged by peroxidase, catalase, and superoxide dismutase [117]. Notwithstanding the information presented herein and the extensive body of research on the subject of irradiation of fresh horticultural products (by 1983, more than 1150 publications had been compiled [118]), not enough is known about the reaction of these live tissues to treatment. Of particular interest is the interaction of irradiation with commonly used agricultural practices such as application of hormone or hormone-like chemicals.

11.9.2 DELAY

OF

RIPENING

The mechanisms whereby irradiation delays ripening in several fruits are not fully understood at the present time. The effect is most associated with climacteric fruits, i.e., fruits that show a transient increase in their rate of respiration and ethylene production at the onset of ripening. The majority of fruits of tropical origin are climacteric, and they have, therefore, a short shelf life. They are also sensitive to low temperature, which is the main preservation method for produce. The biochemical basis for the delay in ripening is discussed by Thomas [119]. Irradiation was shown to influence the respitory pattern of the fruits [97] and to cause a shift from glycolysis toward pentose phosphate shunt in bananas and toward the glyoxylate cycle in bananas and mangos [119]. Dubery and coworkers [97] showed that irradiation did not truly delay ripening of mango, but that it interfered with some biochemical processes, including respiration, involved in senescence. Gamma irradiation promoted respiration in Gala apples that had been treated with an ethylene action inhibitor (1methylcyclopropene) and subsequently stored at 20∞C for 3 weeks [120]. An important mechanism whereby the treatment slows down the ripening in climacteric fruits is not fully understood but seems to be the reduction of ethylene biosynthesis and of sensitivity of the tissue to ethylene, probably related to alterations in membrane physical characteristics [29, 100, 121]. The inhibition of ripening of irradiated Bartlett pears was not reversed by subsequent exposure to ethylene, supporting the suggestion of inhibited ethylene action in the fruit tissue [122]. The production of ethylene and a variety of volatile esters and alcohols was inhibited in irradiated Gala apples by doses as low as 0.44 kGy [98].

11.9.3 POSTHARVEST DISEASE

AND

STORAGE LOSSES

According to Dubery et al. [97], the physiological status of the fruits at the time of irradiation is probably the most important factor influencing the response of the tissues. This status is, in turn, influenced by the preirradiation history of the fruits, e.g., mechanical injury, season, and humidity at the time of harvest. The response of each individual batch of fruits is therefore difficult to predict. One important element of success is rapid precooling of the produce and irradiation at the lowest temperature tolerable by the fruits. The ripening inhibition in fruits that have been treated with low doses of ionizing radiation has a notable side effect with regard to plant pathogens. A dose of radiation that is too low to control fungal development directly may, still, indirectly result in reduced fungal disease. Fruits progressively lose their resistance to plant pathogens with ripening. Fruits that have had their ripening delayed retain a higher level of host resistance to fungal plant pathogens. As the spoilage fungi are unable to establish themselves, secondary bacterial plant pathogens such as Erwinia spp. are also less able to become established on the fruit, and overall microbial development will be delayed as an added benefit. As with all applications of irradiation to food products, the treatment functions best as a terminal processing step in a system where postirradiation recontamination is avoided. Also, to maximize the efficacy of the process, preharvest cultural practices should reduce, as much as possible, the initial microbial load on the fruit surface. It is preferable, therefore, that the fruits be packaged before radiation treatment. A brief discussion of the effects of irradiation on packaging materials is presented later in this chapter.

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11.9.4 PHYSIOLOGICAL DISORDERS When exposed to doses beyond their limits of tolerance, fruits suffer physiological disorders that cause undesirable symptoms, mainly tissue softening and enzymatic browning [15, 123]. Tissue softening is caused by partial depolymerization of cell wall polysaccharides, mainly cellulose and pectins, and by damage to cell membranes, which results in loss of intracellular water [30, 124, 125]. Enzymatic browning is an indication of decompartmentation due to damage to membranes, bringing phenolic substrates in contact with polyphenoloxidases [121]. A calcium dip pretreatment prevented loss of firmness and texture in irradiated diced tomatoes [126]. Also, as alteration of the physical characteristics of cell membranes results from oxidative attack on polyunsaturated fatty acids of membrane lipids by oxygen-free radicals, irradiating in an atmosphere with reduced oxygen contact can reduce these effects. However, under these conditions, efficiency of the treatment is also reduced. A high carbon dioxide atmosphere was shown to protect tissues from radiationinduced loss of membrane proteins [127].

11.10 QUALITY OF IRRADIATED FRUITS 11.10.1 TEXTURE, COLOR,

AND

SWEETNESS

Broad statements regarding the effect of ionizing radiation on fruit quality are difficult to make. It is generally true that irradiation tends to make fruits softer and sweeter, primarily through hydrolysis of pectins and release of sugars following depolymerization of carbohydrate polymers [30, 124]. Softer fruit are more susceptible to damage during handling and shipping. This fact, in addition to the desire to reduce the possibility of postirradiation recontamination, reinforces the role of irradiation as a terminal, postpackaging process step. Strawberries softened following a dose of 1 or 2 kGy, and chemical analysis showed an association with an increase in water-soluble pectin and a decrease in oxalate-soluble pectin [30]. Irradiation enhanced the sweetness of strawberries by reducing titratable acidity in comparison with the unirradiated controls [51]. Titratable acidity was similarly decreased in stored apples following irradiation [120]. Gibberellic acid-treated grapefruit tolerated 0.3 kGy with little loss of quality of the fruit, pulp, or juice; however, pitting of the skin, softening of the fruit, and loss of juice quality rose to unacceptable levels following treatment with 0.6 kGy [128]. Firmness of papaya, rambutan, and Kau orange declined following x-ray treatment (0.75 kGy) [129]. Two blueberry cultivars, i.e., Brightwell and Tifblue, did not differ in their response to gamma radiation, with no negative impact after 0.5 kGy; both cultivars showed softening and loss of quality following 1 kGy [130]. The flavor and texture of Sharpblue blueberries were considered acceptable following electron beam irradiation (1 kGy) [131], but the flavor and texture of Climax blueberries similarly irradiated (1 kGy) [132] declined significantly. Color, pH, postharvest decay rate, and other agronomic factors related to the irradiated blueberries were reportedly not affected by dose level in these studies. However, climacteric fruits that have been irradiated when mature, but before the fruits have ripened, may not ripen normally and may develop uneven coloring and skin discoloration [15]. As in the case of grapefruit, effects on skin color and blemishes can be dose dependent [128]. Fresh-cut cantaloupe melons were slightly bleached, softened, and off-flavored following 3 kGy, but these sensory attributes were unaffected by 1 kGy [133].

11.10.2 FLAVOR

AND

AROMA

Radiation-induced alteration of flavor and aroma is variable, and is influenced by the product tested, dose, fruit maturity, cultivar, storage conditions, and other agronomic production factors. With regard to taste, a distinction is made here between sweetness, which results from the balance of sugar content and acidity, and flavor resulting from the complex combinations of volatile, low

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C

Ethylene (log µmol . kg−1 . h−1)

1

5 4 3 2

0.1

1 0.01 B 1

0 kGy 0.5 kGy 1 kGy 1.5 kGy

D

0.8 0.4

0.1

0.01

0 1.2

Total volatile compounds (µmol . kg−1 . h−1)

A

0.0 0 5 10 15 20 Days at 20°C after irradiation

0 5 10 15 20 Days at 20°C after irradiation

FIGURE 11.4 Production of ethylene (A, B) and total volatile compounds (C, D) of Gala apples during ripening. The fruit treated with (B, D) and without (A, C) 0.5 mL MCP were exposed to 0, 0.44, 0.88, and 1.32 kGy gamma irradiation and kept at 20∞C. Vertical bars represent standard deviation. (From Fan, X. and Thayer, D.W., Quality of irradiated alfalfa sprouts, J. Food Prot., 64(10), 1574–1578, 2001. With permission.)

molecular weight compounds. This aspect of fruit quality may more accurately be referred to as the flavor–aroma complex as the two aspects are inseparably linked in fresh fruit. Modifications of taste had been reported by Kader [15] in the case of decreased acidity of strawberries and astringency in persimmons. The odor, color, and overall appearance of irradiated mango proved as acceptable to a sensory panel as nonirradiated controls immediately after treatment; the treated fruit remained acceptable after 50 d of refrigerated storage, while nonirradiated controls became unacceptable after less than 30 d [58]. Aroma and flavor of papaya, rambutan, and Kau orange were generally more intense following x-ray treatment (0.75 kGy) [129], although changes in other sensory characteristics were product specific. Alteration of aroma is to be expected because many fruit aroma volatiles originate from the breakdown of polyunsaturated fatty acids, which are among the main targets of irradiation-generated free radicals. Fan et al. [98] examined the ability of Gala apples to generate volatile compounds after having been treated with gamma radiation, 1-methylcyclopropene (MCP), or both. MCP is an ethylene action inhibitor, and is used agronomically to delay ripening. Figure 11.4 shows the concentrations of ethylene and total volatile compounds generated by the apples in this study. In non-MCP-treated fruit, irradiation inhibited production of most esters and some alcohols in a dosedependent response. In MCP-treated fruit, irradiation had little difference in ester and alcohol production over nonirradiated fruit. The authors conclude that MCP and irradiation result in comparable inhibition of volatiles in the apples.

11.10.3 NUTRITIONAL QUALITY This aspect of food irradiation has been thoroughly investigated [15, 16]. Generally, low doses do not bring about significant losses of vitamins. The natural antioxidants, vitamins A, C, and E, are relatively labile, as are thiamine and vitamin B12. Irradiation is known to oxidize a portion of the total ascorbic acid (vitamin C) to the dehydro form [134]. While both the forms of this vitamin are biologically active, suggesting minimal nutritional impact [135], it is uncommon to see values for the dehydro form co-reported in analyses of ascorbic acid concentration. The impact of irradiation on vitamin C therefore tends to be overstated. Irradiated alfalfa sprouts showed (1) no

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significant decline in total ascorbic acid concentration and (2) increased antioxidant power compared to nonirradiated controls [136].

11.10.4 INFLUENCE

OF

VARIETY/CULTIVAR

An important factor in the irradiation of fruits is consideration of variety- or cultivar-specific responses. Reviews of produce radiation sensitivity often combine different varieties under a single heading, e.g., “leafy vegetables” or “grape” [15, 44], a practice that does not recognize evidence of varietal differences. Plant variety or cultivar can influence the sensory response to irradiation, as well as the sensitivity of associated bacteria. Apple varieties show different degrees of internal browning and changes in firmness [120]. Blueberry varieties have cultivar-specific sensitivity, resulting in variable fruit quality [130–132]. In examining studies of irradiated vegetables, two potato varieties showed differences in the postirradiation (0.20 kGy) viscosity of their starches, whether isolated from irradiated tubers or irradiated in vitro [82]. Differences were seen in the respiration rates, loss of firmness, and in sensitivities associated with aerobic bacteria on iceberg lettuce [137] vs. romaine lettuce [96]. In a direct-comparison study, the radiation sensitivity of E. coli O157:H7 varied significantly when surface inoculated on iceberg or boston lettuce vs. green leaf or red leaf lettuce (B.A. Niemira, unpublished data).

11.10.5 QUALITY CONTROL Because of the possible effects of irradiation on fruit quality at higher doses, it is essential to develop an extensive postirradiation quality control procedure. Basic quality criteria should be measured immediately after treatment and also after a few days of postirradiation storage for delayed effects. Surface color changes and internal discoloration are good indicators of injury. Respiratory rates, ethylene production, ripening, and senescence parameters, e.g., texture and color, give indications on the physiological effects of the treatment. Nutritional effects can be measured through determination of sugars, organic acids, pH, critical vitamins, or carotenoids. Enzyme assays, e.g., as related to enzymatic browning, may monitor biochemical effects. A rigorous sensory evaluation is an important element of treatment optimization and routine quality testing.

11.11 IRRADIATION OF FRUIT JUICES AND PULPS The majority of fruit juices sold in the U.S. receive a conventional heat pasteurization treatment. The pasteurization process results in the loss of essential oils and other juice components, changing the flavor of the resulting juice. Fresh (i.e., nonthermally pasteurized) juices are valued for premium flavor and aroma; however, these products have also been responsible for outbreaks of salmonellosis, enterohemorrhagic E. coli infection, and hemolytic uremic syndrome [138]. Several strains of E. coli O157:H7 were found to survive in fruit pulps during extended refrigerated storage (up to 30 d in the case of grape and up to 20 d in the case of passion fruit pulp [139]). The U.S. FDA has implemented a policy [140] requiring 5-log reductions in human pathogen load in fresh juices, with these regulations to be in full effect for all processors by 2004. A variety of nonthermal means of reducing the microbial load of fresh juices were recently discussed [141]. These include pulsed electric fields, minimal thermal processing, high-pressure processing, and ultraviolet radiation. Regulatory approval for the use of ionizing radiation is currently being sought [91].

11.11.1 MICROBIOLOGY

OF IRRADIATED JUICES AND

PULPS

Preservation and shelf-life extension have been the historical focus of research on irradiation of juices. The key organisms of interest were yeasts and molds, which tend to have a higher D10 value than bacterial pathogens [49]. Juices and pulps, because of their high water content, represent an area that should provide the maximum opportunity for generation and mobility of radical molecules.

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However, the same high water content means that the bulk density of these products is higher than what might be seen in packaged fruits. This must be taken into consideration with regard to the penetrability and max/min ratios attained during the irradiation process. Phytopathogenic fungi were evaluated in irradiated mango pulp, with D10 values ranging from 0.39 to 1.11 kGy [58]. A complete discussion of this study is presented later in this chapter (see “Mango”). Niemira [56] reported a D10 value for Salmonella enteritidis of 0.35 kGy when irradiated in reconstituted orange juice. Buchanan et al. [54] showed that irradiation (1 kGy) effectively inactivated E. coli O157:H7 in inoculated commercial apple juices, with D10 values of 0.12, 0.16 and 0.21 kGy for the three isolates tested. Niemira et al. [57] found that the resistance of four Salmonella isolates irradiated in orange juice also varied, with D10 values ranging from 0.35 kGy to 0.71 kGy. For the most resistant isolate (Salmonella anatum), 3.5 kGy was indicated as the dose required to achieve a 5-log reduction. Similarly, Salmonella enterica serotype Hartford was reduced to undetectable levels in irradiated (3 kGy) orange juice [142]. In that study, the pathogen was able to be recovered through selective enrichment. The antimicrobial efficacy of irradiation is influenced by several factors. Native variation in pathogen resistance among different isolates has been noted [54, 57]. Buchanan et al. [54] showed that the radiation sensitivity of three strains of E. coli O157:H7 irradiated in apple juice was reduced by 54 to 67% by previous growth of the cultures in acid environments. Also, the sensitivity of one test strain decreased in more turbid juices. This difference was ascribed to the antioxidant power of the suspended solids, although data for the antioxidant power of the juices was not presented. L. monocytogenes irradiated in vitro was increasingly protected in solutions of increasing antioxidant power [43]. However, S. enteritidis suspended in commercial citrus juices of varying composition (orange vs. orange/tangerine, extra pulp vs. pulp free, regular vs. calcium enriched, etc.), and varying antioxidant power did not differ in D10 value following gamma irradiation [56]. The influence of natural and artificial antioxidants on radiation sensitivity is therefore an area to be addressed more fully.

11.11.2 SENSORY PROPERTIES

OF IRRADIATED JUICES AND

PULPS

A frequent added benefit of the application of ionizing radiation to fruits is the increase in juice yield during processing due to the softening of internal tissues. As in whole fruits, irradiation oxidizes a portion of juice ascorbic acid to the dehydro form; as in fruits, this conversion is expected to have little nutritional impact [134, 135]. Fetter et al. [143] irradiated a variety of commercially pasteurized juices to a maximum dose of 5 kGy and used taste panels to evaluate juice quality. The irradiated juices of orange, guava, tomato, red currant, black currant, apricot, peach, pear, and apple showed no reduction of flavor quality; the quality of similarly irradiated grape juice was reduced [143]. Chachin and Ogata [144] treated grape, apple, and orange juices with sterilizing (2 to 80 kGy) doses of gamma radiation. Loss of grape juice anthocyanin and orange juice betacarotene was evident after 10 kGy, and was dose dependent up to 80 kGy. A dose of 5 kGy caused browning in apple juice after 5 kGy, and ascorbic acid was reduced in apple and orange juices. Addition of 0.01% propyl gallate reduced the negative effects of high-dose (10 kGy) irradiation on the quality of orange juice, but did not protect apple juice [144]. A later study of high-dose irradiation (10 kGy) of orange juice resulted in a similarly unacceptable degree of flavor degradation and browning; however, the addition of 0.1% sorbic acid before irradiation effectively eliminated loss of flavor and reduced browning [145]. Fan and Thayer [146] determined that irradiation of apple juice caused an initial reduction of the juice’s brownness (A420) and increased the antioxidant power of the juice. These effects were dose dependant up to 8.9 kGy. During subsequent storage of the juice at 5∞C, the rate of browning of the irradiated juice was greater than that of nonirradiated juice, but after 16 d in storage, the irradiated juice was lighter in color. The impact of irradiation was influenced by processing temperature, but not by exclusion of oxygen from the juice or by the level of suspended matter in the juice [146]. Unpasteurized

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0.15

a

a

A a

D

0.15

a b

0.10

241

ab

ab c

0.00 0.15

B a

b

c

0.00 0.15

E

0.10 a

d e

0.05 0.00 0.15

a

C

b

c

d

0.00

0.05 0.00 0.15

F

0.10

0.10 0.05

0.05

Neral (ppm)

Geranial (ppm)

0.05

0.10

0.10

bc

c

a ab 0

ab

ab

b

0.89 2.24 4.23 8.71 Dose (kGy)

a 0

a

a

a

a

0.89 2.24 4.23 8.71 Dose (kGy)

0.05 0.00

FIGURE 11.5 Concentration of geranial (A, B, and C) and neral (D, E, and F) in irradiated and nonirradiated orange juice. Juices were irradiated with 0.0, 0.89, 2.24, 4.23, or 8.71 kGy at 5∞C, and stored at 7∞C for up to 21 d. Volatile compounds were measured with gas chromatography after 1 d (A and D), 7 d (B and E), or 21 d (C and F). Bars with the same letter are not significantly different (LSD, P < 0.05). Comparisons were made within the same storage duration. (From Fan, X. and Gates, R.A., Degradation of monoterpenes in orange juice by gamma radiation, J. Agric. Food Chem., 49(5), 2422–2426, 2001. With permission.)

apple cider irradiated to 3 kGy was identifiable, but not unacceptable, to untrained sensory panelists evaluating aroma; similar results were obtained with reconstituted and fresh orange juices (B.A. Niemira and X. Fan, unpublished data). Niemira et al. [57] found no evident changes in the appearance or aroma of reconstituted orange juice irradiated to 2.5 kGy. Miller and McDonald [128] reported that the juice from irradiated grapefruit was acceptable after 0.3 kGy, but declined in quality at 0.6 kGy. Pickett et al. [142] reported increasing off-flavor in unpasteurized orange juice irradiated to 3 kGy, rendering the juice unpalatable. Spoto et al. [147] irradiated orange juice concentrate and determined the effect of storage time and temperature on juice quality and acceptability. In that study, the highest dose (up to 5 kGy) combined with 25∞C storage resulted in loss of “orange” flavor and increase in “bitter,” “medicinal,” and “cooked” ratings by sensory panelists. Lower dose (2.5 kGy) and cooler storage (0 or 5∞C) were proposed as an acceptable processing regimen. Fan and Gates [136] found that some aroma-related acyclic monoterpenes in orange juice were reduced immediately after irradiation; after refrigerated (7∞C) storage for 21 d, there was no significant difference from the nonirradiated controls (Figure 11.5). A consumer test determined that irradiated apple cider was as acceptable or more acceptable than pasteurized apple cider [149]. The factors that may lead to undesirable flavors, aromas, or changes in appearance of irradiated juices are complex. While extensive studies into the effect of the variety and maturity of the source fruit on the radiation sensitivity of the resulting juice are lacking, it is known that juice composition is variety dependent [150]. In light of the evidence for varietal specificity in irradiated whole fruits and vegetables, the possibility of varietal specificity in juices with regard to negative irradiation effects cannot be discounted. As with whole fresh produce, temperature control during the irradiation process is important. The irradiation process can lead to generation of ozone in the headspace

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Ripening is delayed in

Be n

ef ic

ia

l

Senescence is delayed in

Tangerines Cucumbers Summer Squash Bell peppers Olives

Plums Apples Table grapes Cantaloupes

N

o

e tB

fic ne

ia

l

Lack of tolerance in

Sweet cherries Apricots Papayas

Tomatoes Strawberries Figs

Storage decay is controlled in

Pears Avocados Lemons Grapefruit Oranges

Bananas Mangoes Papayas

Ripening is accelerated in

Peaches

Irradiation tolerance only in

Nectarines

Pineapples Lychees Honeydew melons (?)

FIGURE 11.6 The response of 27 fruits to irradiation. (From Akamine, E.K. and Moy, J.H., Delay in postharvest ripening and senescence of fruits, in Preservation of Food by Ionizing Radiation, Vol. 3, Josephson, S.E. and Peterson, M.S. Eds., CRC Press, Boca Raton, FL, 1983, pp. 129–158. With permission.)

gas of sealed containers; consideration should be given to the possibility of secondary sensory effects from the ozone, rather than from the radiation directly. Another possible source of off-odors or flavors is the plastic used in the processing tests. A bag made of a plastic not suited for irradiation may cause unpalatable migration of radiolysis products. Another example is a fruit juice irradiated in a glass test tube with an unsuitable plastic cap or even a cap liner that is the source of a disagreeable odor or flavor. Any of these situations could lead to a mistaken conclusion regarding the suitability of irradiation for a particular product.

11.12 FRUITS CURRENTLY BEING IRRADIATED A very large volume of information, occasionally contradictory, exists on specific applications of gamma irradiation to fruits [16, 17, 118, 151]. Figure 11.6 shows the response of a number of fruits to irradiation, as summarized by Akamine and Moy [151]. The results are often difficult to compare because of differences in conditions of treatment. A few representative examples are discussed below.

11.12.1 STRAWBERRY Among fruits, strawberry is one of the most studied with respect to application of gamma irradiation. Market tests and commercial applications have been carried out in a number of countries. A dose of 2 kGy seems to be the optimal dose of irradiation in air for precooled, ripe strawberries. Success depends on cultivar: the firmer fruits of “Tioga” tolerated radiation better than the softer fruits of “Brighton” [152]. In 1992, commercial irradiation of strawberries was initiated in Florida. The fruits were irradiated at 0.3 to 1 kGy on pallets in air containing 10% carbon dioxide [153] at 50∞F. The plastic wrapping was removed after 3 d. After 18 d of storage, the strawberries irradiated under

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modified atmosphere were sweet, firm, slightly dark, free of mold, and still saleable, in contrast with control fruits held in air and fruits held under modified atmosphere for 3 d before unwrapping. According to several reports, marketing of the irradiated strawberries was quite successful [154].

11.12.2 MANGO The D10 of 14 phytopathogenic fungi were evaluated in irradiated (0.5 to 1.5 kGy) mango pulp [58]. These ranged from 0.39 for Aspergillus sydowii and Aspergillus ustus to 1.11 for Penicillium oxalicum, confirming the relatively high radiation resistance of fungi. The D10 values were also determined in saline solution, and demonstrated some variance with the data obtained in mango pulp. The more complex medium caused a decrease in D10 for some fungi, e.g., A. ustus (55%), Scopulariopsis brevicaulis (55%), and A. sydowii (76%), and an increased D10 for others, e.g., Pencillium brevicompactium (147%), Aspergillus flavus (159%), and P. oxicalum (179%). The highest dose (1.5 kGy) reduced total aerobic bacteria count by ~1.8 logs, a difference that persisted throughout the study (50 d). Shelf life in irradiated (1 kGy) fruits was extended from 25 to 50 d in refrigerated storage. While the interiors of the fruit used in this study were sampled and found to be free of contamination, the variable effect of fruit pulp on the radiation resistance of resident microorganisms is clearly demonstrated. Pulp represents a combination of plant cell contents with extracellular fluid, found in the intercellular spaces; nevertheless, this observation should be considered in light of evidence of internalization of pathogenic bacteria into intercellular spaces [33]. Numerous reports indicate that mango preservation would greatly benefit from treatment with ionizing radiation. The summary by Akamine and Moy [151] shows the optimal dose to be 0.75 kGy for three-quarter ripe fruits at room temperature. Combination with mild heat treatment by hot water dip or vapor for 5 min at 50 to 55∞C yields even better results [152]. A limiting factor is surface scalding [151]. The effects of irradiation depend on the degree of maturity. Scalding occurred at 0.25 kGy on the mature-green fruits, and tolerance increased with maturity. Refrigeration at 13∞C reduced tolerance to scalding. The susceptibility to radiation injury varied greatly with the origin of the fruits, although, in general, mangoes are considered highly tolerant [15]. These various factors have to be balanced to give maximal shelf life based on delay of ripening and control of disease. A market test for irradiated mangoes in 1986 in Florida was highly successful. Mangos irradiated to 1.5 kGy were as acceptable to a sensory panel as nonirradiated controls immediately after treatment based on odor, color, and overall appearance; irradiated fruit remained acceptable after 50 d of refrigerated storage, while nonirradiated controls became unacceptable after less than 30 d [58]. Mango is one of several fruits being researched as part of an IAEA cooperative research project on the application of irradiation to fruits, vegetables, and vegetablederived products [35].

11.12.3 PAPAYA Extensive research was carried out on gamma irradiation of papaya in Hawaii to overcome the strict quarantine regulations of the mainland against insects, particularly fruit flies. Papaya is considered relatively tolerant of radiation. An effective treatment is 0.75 kGy, in combination with a hot water dip at 48.9∞C for 20 min [151, 152]. The combined treatment delayed ripening, as assessed visually and by measurement of firmness and decay [151]. Hot water dip alone accelerated ripening, while irradiation alone provided only slight control of decay. Little difference was reported between irradiated and fumigated papayas. Irradiated fruits were firmer and had a better flavor than ethylene dibromide-treated fruits. Hot water dip was preferable to vapor-heat treatment. Extension of shelf life was obtained even without refrigeration. The authors concluded in favor of the feasibility of gamma irradiation of papayas. Papayas are one of the fruits currently being irradiated commercially at an x-ray facility in Hilo, HI. These products receive a minimum of 0.25 kGy. Whereas fruit that is steam-treated must be picked green, the irradiated fruit is allowed to tree-ripen longer,

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yielding a higher-quality fruit [88]. Irradiated papayas have quality and nutrition similar to heattreated papayas [155].

11.13 COMBINATION TREATMENTS As has been indicated earlier, achieving a 5-log reduction in bacterial population on fresh fruits and juices would require between 1.5 and 3.5 kGy [54–57]. The higher doses may be more than those many fruits would withstand from a quality standpoint. It is not surprising, then, that much interest has been expressed in combining lower doses of ionizing radiation with other processes to achieve a total reduction of 5 logs. In part, the goal of this research is to determine the extent to which two or more treatments applied together will deliver a greater kill than the sum of each individual treatments, i.e., synergistic effect vs. purely additive effect. The actual physical and chemical mechanisms behind antimicrobial synergy are complex, and vary depending on the treatments being examined, but they generally follow the same underlying rationale. An appreciable percentage of the bacterial population that survives the first treatment is injured or weakened, rendering the surviving population more susceptible to the second treatment than a normal population; thus, the second treatment is more effective than the first. Depending on the treatments being applied, the sequence of treatments may be an important factor in obtaining synergistic kill vs. purely additive. In a study of irradiated Y. entericolitica, Sommers and Bhaduri [48] showed that the plasmid-encoded virulence genes in the surviving population were damaged, rendering the surviving population less virulent. Damage to plasmid-encoded genes is therefore one possible mechanism by which irradiation may predispose a population to greater susceptibility to other antimicrobial treatments.

11.13.1 MILD HEAT The combination of irradiation and mild heat (45 to 55∞C) has been used successfully with papaya and mangoes for delayed ripening and control of anthracnose, even without refrigeration [151]. Heat could be supplied as a hot water dip or as water vapor. Storage under modified atmosphere brought additional benefits. Combination of a 12-min steam treatment and gamma irradiation (1 to 2 kGy) increased the shelf life of mango pulp to 270 d vs. 90 d for irradiated or nonsteamed, or 15 d for untreated mango pulp (nonsteamed and nonirradiated) [156]. The combination treatment had no undesirable effect on chemical or sensory properties of the mango pulp. Fan et al. [157] determined that a combination of a warm water dip (47∞C, 2 min) and lowdose irradiation (0.5 or 1 kGy) could effectively enhance the safety and quality of cut lettuce. Caution may be warranted, however, as the population dynamics of pathogens in storage, posttreatment, are not fully understood. Lettuce was treated with a combination of warm (47∞C) water and chlorine (100 mg/ml), and inoculated either before or after these treatments with L. monocytogenes and E. coli O157:H7. After an initial reduction, these pathogens rebounded and grew extensively on lettuce stored at abusive temperatures (10∞C). Growth was not observed in lettuce treated with cold (1∞C) water plus chlorine [158]. Though this study did not include irradiation as a treatment, it illustrates the importance of proper controls over every processing step, particularly when the effect of interacting treatments is not fully understood.

11.13.2 MODIFIED ATMOSPHERE PACKAGING (MAP) Low-dose irradiation combined with modified atmosphere is increasingly considered for control of microorganisms and delayed ripening. As stated above, reduced oxygen content in the atmosphere limits the efficiency of irradiation because less activated oxygen is available. An interesting alternative is irradiation in air enriched with carbon dioxide. Carbon dioxide is fungistatic at levels above 12%, a concentration beyond the tolerance of most plant tissues (strawberries are an excep-

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tion). Couture and Willemot [159] showed the synergistic action of the combined stresses of gamma radiation and high carbon dioxide for control of mold development on strawberries. Irradiation at the low dose of 0.3 kGy, combined with storage in air containing 10% carbon dioxide, delayed ripening, as assessed by anthocyanin content and mold development. The method has since been adapted commercially in Florida [153]. MAP of fruits involves equilibrating the respiratory carbon dioxide production and oxygen depletion by the plant material with the film permeability in order to maintain an adequate composition of the atmosphere (oxygen and carbon dioxide concentrations) within the limits of tolerance of the fruits inside the package. With irradiated fruits, the temporary increase in the rate of respiration due to the initial wound response has to be taken into account. The technology can be applied in several ways, e.g., irradiation of a wrapped pallet, of small individual containers, or of small perforated containers under a common wrapping on a pallet. Accumulation of carbon dioxide and depletion of oxygen may lead to fermentation and development of off-odors and off-flavors. Perforated, microporous, or semipermeable films may resolve this problem. Preirradiation flushing of the package with a low-oxygen gas mixture would reduce the undesirable effects of irradiation on the produce, e.g., loss of vitamin C, and on the packaging material, e.g., development of undesirable flavor and odor; however, this reduces the efficacy of the process. Sealed packaging acts as a water barrier and reduces weight loss. The integrity of the seal should resist the irradiation treatment in order to maintain the internal modified atmosphere and to prevent microbial recontamination. A study of lettuce and cabbage packed in a high-nitrogen atmosphere showed an extension of shelf life but no inhibitory effect on resident microflora, prompting the authors to caution against the potential for growth of pathogens on vegetables so packaged [160]. Irradiation of this type of packaged product would seem to be a valid means of addressing this concern. Combination of irradiation with MAP has shown success with irradiated packaged meats [59], and is an area of active research on fruits and vegetables [35, X. Fan, personal communication). Couture and Willemot [159] and Brecht et al. [161] showed beneficial effects of low-dose irradiation and storage under modified atmosphere for strawberries. Brecht et al. [161] obtained reduced decay incidence of Rhizopus stolonifer and Botrytis cinerea on strawberries packaged under 7% oxygen and 20% carbon dioxide and irradiated at 1 kGy. Grandison [99] reported a reduced rate of respiration and a delay of up to 3 d in ripening for bananas exposed to 0.1 to 0.3 kGy in an electron accelerator. Higher doses caused browning of the skin, splitting of the fruits, and loss of peel texture and vitamin C. Combination of irradiation with MAP under a 25-µm low density polyethylene (LDPE) film extended the shelf life, although not synergistically. MAP did not alleviate the disorders caused by electron beam irradiation to avocados. After irradiation under MAP, total aerobic counts were reduced by ~3 logs on iceberg lettuce [137] and by ~1.5 logs on romaine lettuce [96] by doses of 0.19 kGy and 0.35 kGy, respectively. Rate of spoilage and shelf life was reported to be unchanged for iceberg lettuce, but the shelf life of romaine lettuce was extended from 14 to 18 d to the length of the study, 22 d. MAP reduced total aerobic plate counts by ~2 logs on shredded carrots irradiated to 0.45 kGy, a difference in microbial population that persisted throughout a 9-d storage evaluation [162]. Many minimally processed fruits and vegetables, such as prepared salad mixes, incorporate MAP as a means to control spoilage and extend shelf life. The precise gas permeabilities of the plastics used in the packages are of critical importance for these products. Especially with regard to fruits and vegetables, which respire during storage, studies of MAP plus irradiation must take into account the effects of irradiation on the material used to hold the product.

11.14 IRRADIATION OF PACKAGING MATERIAL Like other foods treated by irradiation, fruits are prepackaged to facilitate handling and to prevent recontamination. Irradiation of fruits involves constraints on the types of packaging materials

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suitable for this technology. The combination of irradiation and MAP brings additional constraints on the suitability of the packaging material. Irradiation may affect the characteristics of materials and packaging performance. It is therefore important to know these effects as they will influence the choice of a packaging compatible with the requirements of the agri-food industry. The functional properties of the material should be preserved. The toxicological and sensory aspects should also be addressed. Although the effects of irradiation are generally minimal, i.e., below 10 kGy, volatiles may be generated during the treatment. They may be transferred to the food and may produce unpleasant flavors. Migration of other compounds through contact while packaging food must be monitored to insure that the food is chemically harmless. Selection criteria for packaging materials to be used with irradiation include sealability, resistance to cracking and delamination, protection against recontamination, barrier to oxygen and humidity, and chemical inertness. Traditional packaging materials such as metal, glass, and paper are relatively resistant to ionizing radiation. Metal and glass are essentially impervious to the effects of irradiation, although with high doses, glass can become somewhat browned. Paper and paper products such as pressboard, cardboard, etc., can lose mechanical strength due to the breakdown in cellulose at the microscopic level. However, the low water content of these products greatly reduces the sensitivity of the cellulose in paper vs. that in fruit or vegetables. Paper and cardboard are used mainly for support packaging. Plastics are the type of packaging that most frequently comes into contact with the food treated with ionizing radiation. Plastics have historically been basic resin polymers to which additives (e.g., stabilizers, antistatic and antislip agents, and plasticizers) have been added. Radiation acts on both the polymers and additives. Increasingly, the plastic films used for packaging are laminates, with two or more layers of dissimilar plastics combined to provide the desired mixture of strength, durability, gas permeability, ability to take inks, etc. Determining the effects on the packaging material is thus made more difficult as each plastic component of the laminate has a unique response to irradiation, and this response may or may not be the same for the plastic as part of a laminate as it would be for the plastic standing alone. Also, some specialty laminates can incorporate a metal foil layer, adding another layer of complexity. A relatively recent development in packaging technology has been the incorporation of preservatives and antimicrobial compounds within the plastic, designed to suppress the growth of spoilage organisms or pathogens [163]. These are of increasing interest to food processors and food safety professionals. For use in food products to be sold in the U.S., these compounds must be cleared by the U.S. FDA.

11.14.1 TYPES

OF

PLASTICS USED

IN

FRUIT PACKAGING

Fresh fruits are usually packaged in plastics of high or intermediate permeability (Table 11.2). The most commonly used materials are LDPE, polypropylene (PP), polyvinyl chloride (PVC), and ethylene vinylacetate (EVA). Seasonal trade requires punnets made of woven chip or thermoformed polystyrene (PS) with LDPE or PVC cling wrap. Small polyethylene terephthalate PET trays are also becoming popular. Juice-absorbing pads can be used to soak residual liquid from the fruit or prepackaging rinses. Properties of PET are well preserved during irradiation [164]. In the U.S., PS films have been approved for irradiation; however, the PS foam trays have to be PE-coated, which increases packaging cost. Single-layer plastic films commonly used for fruit packaging, including LDPE, PP, PVC, polyvinylidene cholride (PVDC), and PET, released odors after irradiation at the low doses used for fruits [165]. At high doses, cellulosic materials, such as paper and board, lose mechanical strength. At the doses typically used with fruits, this effect is minimal and may be offset by using PE or foils as a laminate for support [166]. Adhesives used in absorbing pads are not permitted for irradiation in the U.S. and Canada. Pads with a mechanical link between paper and polyethylene (PE) layers would be a solution. PP and PVC are not recommended for use with irradiated fruits for the reasons discussed earlier. Several suitable materials have not yet received

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TABLE 11.2 Suitability of Plastic Materials for Fruit Irradiation Polymer

Use

Polyethylene (PE) Polypropylene (PP) Ethylene vinyl alcohol (EVA) Polyvinyl chloride (PVC) Polyethylene tetraphtalate (PET) Polystyrene (PS) Polyvinylidene chloride (PVDC) Nylon-8 Nylon-11 Cellulose

Pallet shrink packaging, bags, lids coatings, wrap films Bags Coatings, MAP, frozen fruits, sealing MAP, overwrapping Bags Trays, films Barrier layer Bags Bags Bags, overwrapping

Suitability Good Mediocre Good Poor Good Excellent Good Excellent Good Mediocre

Source: Willemot, C.M., Marcotte, M., and Deschenes, L., Ionizing radiation for preservation of fruits. in Processing Fruits: Science and Technology, Vol 1: Biology, Principles and Applications, Somogyi, L.P. and Ramaswamy, H.P., Eds., Technomic Publishing Lancaster, PA, 1996, pp. 221–260, chap. 9.

clearance for irradiation. Because of the complexity of the problem, the applications have to be evaluated individually. Dried fruits irradiated for insect disinfestation are stored for long periods. The durability of the irradiated material has to be tested. Barrier films used with dried fruits are typically multilayers of polyolefins, EVA, PVDC, or nylon. Oxygen barrier films reduce radiation-induced vitamin C loss [166]; however, films laminated with adhesives have not yet been cleared for irradiation. At the present time, no packaging material can be considered suitable for irradiation of fruits unless appropriate migration tests have been performed. Even with approved plastic films, off-odors may be generated; however, as off-odors are typically the result of radiation-induced rancidity of fats, this risk is low in most fruits because of the low fat content [2]. The main parameters for the choice of a packaging material before fruit irradiation are the FDA list and the specific requirements of the fruits.

11.14.2 EFFECTS

OF IRRADIATION ON

PACKAGING PLASTICS

The effects of the ionizing radiation on polymers are mainly cross-linking and cleavage. The two effects can occur simultaneously, and the relative importance of each depends on the material and on the conditions under which the treatment is carried out [167]. Cross-linkage, the formation of bonds between adjacent chains, generates a very resistant three-dimensional network. The extent to which cross-linkage occurs between the layers of plastic laminates will depend on the plastics involved. Cleavage refers to reduction in chain length and, thus, to a loss of mechanical strength and an increase in porosity of the material. In general, the low doses applied to fruits cause only insignificant modifications of the physical properties of plastics; however, some materials are more susceptible to ionizing radiation and are not recommended for irradiation of prepackaged food. Mass transfer from packaging material to food after irradiation may involve migration, absorption, and permeation. Research has demonstrated that these three phenomena can be affected by irradiation. Migration refers to the transfer of low molecular weight molecules to the food and is of importance for toxicology and organoleptic quality. The plastics used for fruit packaging are mostly very high molecular weight thermoplastics derived from ethylene. They are chemically inert and have a low solubility in water and oil; therefore, migration concerns mainly low molecular weight additives and their products generated by irradiation. Migration rates have been shown to be affected

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by radiation [164]. The effect is more important with gamma rays than with accelerated electrons. Irradiation of plasticized PVC film causes migration of HCl and tainting of food by other lowmolecular-weight molecules [166]. Kilcast also showed migration from irradiated PET and high-impact polystyrene (HPS) trays. He suggested the use of expanded PS. No taint was observed from PE-based films, while PP is known to be very susceptible to irradiation [168]. Bourges et al. [168] showed that oxidation products of antioxidant additives produced at doses below 10 kGy could migrate into food-simulating solvents. A study (gamma and e-beam) of two semirigid copolyesters showed that 5 kGy had no effect, and that 25 or 50 kGy had no significant effect on migration of nonvolatiles into food-simulating solvents [169]. A later study (gamma) of an amorphous nylon polymer using the same doses (5, 25, and 50 kGy) showed the generation of volatiles but not of nonvolatile compounds [170]. No new compounds were detected in that study. Comparatively little is known at the present time about toxicity of degradation products of many other additives. The extent to which the concentration, efficacy, or migration of these antimicrobial compounds (or any of their radiolytic products) are affected by irradiation is essentially unexplored [163]. Permeation, the transfer of molecules through the packaging material, is only slightly affected by irradiation below 10 kGy [165]. This aspect should, however, be considered in the context of MAP [171]. Absorption of volatiles and other constituents of food by packaging material can be important. Scalping of terpenes that contribute to the aroma of citrus fruits by packaging material [172] is a serious problem for the fruit juice industry. Absorption has been only slightly studied in the context of food irradiation.

11.14.3 REGULATION PRODUCTS

OF

PACKAGING MATERIALS

FOR

USE

WITH IRRADIATED

The Codex General Standard for Irradiated Foods [173] established worldwide standards for packaging of irradiated food. These guidelines are very general and offer little precision with respect to the types of materials and additives suitable for prepackaging of food to be irradiated. The U.S. is the only country with a list of packaging materials approved for irradiation (Table 11.3). Some of these plastics may be amended with various adjuvants, including preservatives, etc. (Table 11.4). The FDA list draws on data from research carried out by the U.S. Army on irradiation-sterilized food packs, as well as data from more recent research. The regulation covers only plain films or laminates, presently used in food packaging. Juice-absorbing pads in film-covered trays often contain adhesives between plastic and paperboard layers. These pads and trays such as the popular uncoated polystyrene foam trays are not approved for irrradiation, although polystyrene films may be irradiated. In Canada, no such list of packaging materials approved for irradiation has been established. The standards are intended to serve only as guidelines. Whatever may be the material or the process, it must not constitute a health hazard. Until August 1988, there were no specific approvals for packaging materials. Since then, permission has been granted for Cryovac films and for bags used in the shrink- and vacuum-packaging of refrigerated products [174]. In Canada and the U.S., a clearance petition should be presented to the authorities for plastic materials that are not mentioned in the FDA list and that come in contact with the food during irradiation. Several materials that are good candidates for packaging of irradiated fruits cannot currently be used. These materials await a concerted effort of industry groups to petition the appropriate governmental regulatory bodies for approval.

11.15 COST-BENEFIT OF FRUIT IRRADIATION The beneficial effects of irradiation for fruit disinfestation, reduction of spoilage, and extension of shelf life offer potential for significant cost savings to the food industry. One of the major concerns

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TABLE 11.3 U.S. Code of Federal Regulations 21CFR179.45: Packaging Materials Approved for Irradiated Foods Material

Maximum Dose

Nitrocellulose-coated or vinylidene chloride copolymer-coated cellophane Glassine paper Wax-coated paperboard Films of polyolefin or polyethylene terephthalate. These may contain: 1. Sodium citrate, sodium lauryl sulfate, polyvinyl chloridea 2. Coatings comprising a vinylidene chloride copolymer containing a minimum of 85% vinylidene chloride with one or more of the following comonomers: Acrylic acid, acrylonitrile, itaconic acid, methyl acrylate, and methyl methacrylate Kraft paper (only as a container for flour) Polystyrene film Rubber hydrochloride film Vinylidene chloride–vinyl chloride copolymer film Nylon 11 Ethylene–vinyl acetate copolymers Vegetable parchments Polyethylene filma Polyethylene terephthalate filma Nylon 6 filmsa Vinyl chloride–vinyl acetate copolymer filma Acrylonitrile copolymersa

10 kGy 10 kGy 10 kGy 10 kGy

a

0.5 kGy 10 kGy 10 kGy 10 kGy 10 kGy 30 kGy 60 kGy 60 kGy 60 kGy 60 kGy 60 kGy 60 kGy

This material may be amended with additional materials, listed in Table 11.4.

TABLE 11.4 U.S. Code of Federal Regulations 21CFR179.45: Adjuvants and Amendments Approved for Incorporation into Certain Packaging Materials Approved for Irradiated Foods Adjuvant/Amendment

Limit (By Weight of Polymer)

Amides of erucic, linoleic, oleic, palmitic, and stearic acid BHA (butylated hydroxyanisole) BHT (butylated hydroxytoluene) Calcium and sodium propionates Petroleum wax Mineral oil Stearates of aluminum, calcium, magnesium, potassium, and sodium Triethylene glycol Polypropylene, noncrystalline

1% 1% 1% 1% 1% 1% 1% 1% 2%

of any potential user is whether the process is economically viable. Although irradiation plant capacity and processor commitment to the technology are growing at perhaps the fastest rate since irradiation was first developed over 40 years ago, the economic feasibility of irradiation has to be examined for each individual application. Also, the cost of this innovative technology has to be compared with other existing processing technologies [175].

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The cost of gamma irradiation is similar to irradiation using an electron accelerator. The latter source is not likely to be used with fruits because of the low penetration capacity of the electron beam; however, electron accelerator-generated x rays, with significant improvement in the x-ray yield, may become an interesting alternative for bulk irradiation of fruits. Irradiators are costly to install, and a substantial capital investment is needed [131]. The main factors contributing to the initial cost of any irradiation facility are the source (radionucleotides or electron accelerator equipment), the shielding, and the product conveyer system. This important capital investment has delayed the development of food irradiation. Lack of food irradiation capacity means that irradiated food cannot be in the marketplace, and it has no opportunity to gain market share. Without market share, there is insufficient economic incentive for the capital outlay of a new irradiation plant. The main operating costs of a gamma irradiator include depreciation and replacement of the source, while maintenance of the electronics and the electricity consumption are more significant factors for e-beam and x-ray irradiators. Urbain [13] concluded that the operating costs for food irradiation were in the range of commercial feasibility. The cost of irradiating foods was estimated between U.S. $0.02 and $0.04 per kg [176]. Giddings [177] estimated the processing cost at $0.05 for a throughput of 10 million kg a year. A study by Fundacion Chili in 1985 evaluated the cost of disinfesting fruits by gamma irradiation at $0.025 per kilogram for $1 million capital investment in a batch-type cobalt-60 facility [178]. Cost estimates are dependent on the throughput capacity of the irradiation plant, the dose delivered, the distance the produce must travel to and from the irradiator, the distance between the plant and the markets being served, etc. Ancillary market benefits of irradiation (reduction of storage losses, premium prices commanded by “specialty” markets, etc.), may be offset by ancillary market drawbacks (necessity for increased public education or outreach spending, potential for increased regulatory oversight, etc.). These complex factors make an accurate cost-benefit analysis of food irradiation difficult. There are few examples of large-scale commercialization from which to gain insight, but the recent increase in irradiation of ground meats and Hawaiian fruits is expected to allow for real-world data to be applied to these calculations. Kunstadt and Steeves [179] discussed in detail the economics of fruit disinfestation by irradiation and the effects of various parameters on the unit-processing cost. They presented a detailed analysis of the costs incurred for different types of cobalt-60 irradiators; this analysis remains instructive as x-ray treatment of fruits becomes more prominent in the marketplace. The influence of dose, packing density, and volume of operation was calculated. As expected, unit-processing costs decrease rapidly with increased throughput. Therefore, an economically successful irradiator has to operate at a level exceeding the minimum economic volume of production. It has to be designed for a realistic throughput. The unit-processing cost increases linearly with increasing dose of irradiation; it also depends on processing time. The linearly increasing cost with decreasing packing density is related to the efficiency of utilization of the source. Similar trends were observed with different types of irradiators. According to these calculations, the unit-processing cost of an irradiator designed for throughput of 60 million kg/year at a density of 0.4 g/cm3 and a dose of 0.15 kGy would vary by less than $0.01 for a throughput of 250,000 t/year to $0.107 per kg for 10,000 t.

11.16 THE FUTURE OF IRRADIATION PROCESSING OF FRUITS Treatment of fruits by irradiation is increasingly being recognized as an effective method for reducing postharvest losses, ensuring hygienic quality of produce and facilitating international trade of specific commodities of tropical origin; however, worldwide, more than 80% of food irradiation is applied for the disinfection of spices, herbs, and dehydrated seasonings. The industry has shown

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widespread interest in this technology, has provided quality control through the necessary permissions that were granted, the quality of the treated produce was high enough to offset the price increase, the cost per unit was acceptable to the consumer, and no mandatory negative labeling would discourage purchasing. Loaharanu [154] reported on several successful market trials, mainly for fruits, in the 1980s. In 1986, 2 t of mangos were irradiated at 1 kGy in Puerto Rico and flown to Miami. In 1987, Hawaiian papayas were shipped to Los Angeles and irradiated at a low dose. In both trials, the fruits were well received by consumers. In Lyon, France, in 1988, strawberries irradiated at 2 kGy sold at a slightly higher price than nonirradiated fruits. The consumers preferred the irradiated strawberries for their better quality. Since the opening of the $6.8 million Vindicator irradiator at Tampa, FL, in January 1992, various fruits have been irradiated and sold successfully in Florida and Chicago. The Hawaiian fruits now being treated with x-rays are reportedly selling well in their target markets. These fruits include papaya, avocado, rambutan, lychee, and star fruit. The increasing capacity of food irradiation facilities in the mainland U.S. suggests that as the processors seek to maintain their plants at or near full capacity, the range of products being treated may expand from ground and frozen meats to include more fruits and vegetables. Ongoing research and progress concerning the technical aspects of irradiation of fruits should further develop the use of this technology. Choice of cultivars, harvesting conditions, degree of maturity, combination treatments, suitable packaging, and minimum doses required for the desired benefits are among the parameters that will lead toward better quality and longer shelf life of irradiated fruits. The phasing out of several fumigants used for disinfestation is likely to encourage the use of irradiation. Following the U.S. ban on ethylene dibromide, many producers turned to hot water dips to disinfest mangoes and papayas; however, this treatment may cause physiological disorders to the fruits, particularly discoloration and internal browning. Irradiation is an alternative process of considerable interest because it controls microbial development and sterilizes or kills insect pests without affecting fruit “freshness.” It may become a major technique for meeting the quarantine requirements of several countries [180], including the U.S., with respect to disinfestation of fruits against insects such as the Mediterranean fruit fly. Developing countries will particularly benefit from the technology. Their often warm and humid climates accelerate ripening and decay of produce, often causing extensive losses. Most produce of tropical origin, e.g., mango and papaya, are chill sensitive. Irradiation is an alternative to low-temperature storage for preservation of some of these products. Developing countries are at great distances from their main industrialized markets, and irradiation would help maintain quality and safety of produce during transportation. The increase in consumption of fruits of tropical origin from developing countries, combined with the demand for safe, nutritious, and convenient food in industrialized countries will likely contribute to the expansion of the use of this technology. This can only serve to increase the trade in tropical fruits, thus benefiting the exporting and importing nations. Food-borne illness resulting from contaminated produce and juices has encouraged research into the use of irradiation of fruits for the purpose of improving the safety of these products. Sanitization, a goal that has a resonance with consumers, has joined the traditional goals of disinfestation, delay of ripening, and shelf-life extension as stated aims of produce irradiation. Ultimately, the consumer will benefit from the presence in the marketplace of a greater variety of fruits, including tropical fruits, year-round, that are fresher, longer-lived, healthier, and safer than before.

ACKNOWLEDGMENTS The authors would like to acknowledge the thoughtful reviews of J.L. Alford and J.S. Novak, and express grateful appreciation to K. Lonczynski for technical assistance in the preparation of this manuscript.

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77. Rahman, R., Rigney, C.J., and Bush-Peterson, E., Irradiation as quarantine treatment against Ceratistis capitata (Diptera: Tephritidiae): Anatomical and cytognetic changes in mature larvae following irradiation, J. Econ. Entomol., 83, 1449–1454, 1990. 78. Delincee, H., Ehlermann, D.A.E., and Bogl, K.W., The feasibility of an identification of radiation processed food, in Health Impact, Identification and Dosimetry of Irradiated Foods, Bogl, K.W., Regulla, D.F., and Suess, M.J., Eds., Neuerberg, ISH-Heft, 125, 58–138, 1988. 79. Heide, L. and Bogl, K.W., Detection methods for irradiated food — luminescence and viscosity measurements, Int. J. Radiat. Biol., 57, 201–209, 1990. 80. Heide, L., Guggenberg, R., and Bogl, K.W., Application of thermoluminescence measurements to detect irradiated strawberries, J. Agric. Food Chem., 38, 2160–2163, 1990. 81. Schreiber, G.A., Ziegelwann, B., Guitzch, G., Helle, N., and Bögel, K.W., Luminescence techniques to identify the treatment of food by ionizing radiation, Food Struct., 12, 385–396, 1993. 82. Al-Kahtani, H.A., Abu-Tarboush, H.M., Abou-Arab, A.A., Bajaber, A.S., Ahmed, M.A., and ElMojahadidi, M.A., Irradiation and storage effects on some properties of potato starch and use of thermoluminescence for identification of irradiated tubers, Amer. J. Potato Res., 77, 245–259, 2000. 83. Hart, R.I., White, I.A., and Reid, W.I., Technical note: Occurrence of o-tyrosine in non-irradiated foods. Int. J. Food Sci. Technol., 23, 643–647, 1988. 84. Willemot, C., Chayer, D., Gesselin, C. Couture, R., Drouin, S., and Castalgne, F., La methode de detection de L= irradiation par la presence d-orthotyrosine dans les proteines n=est pas aplicable aux fraises, Can. Inst. Food Sci. Technol., 23, 350–352, 1989. 85. Grootveld, M. and Jain, R., Methods for the detection of irradiated foodstuffs: aromatic hydroxylation and degradation of polyunsaturated fatty acids, Radiat. Phys. Chem., 34, 336–338, 1989. 86. SureBeam, SureBeam Brazil joint venture to support state of Rio de Janeiro’s Center of Excellence. http://www.surebeam.com/pressreleases/2001/071301_surebeam.html, 2001. 87. Medieros, J.F.B. and Ross, R.T., Irradiation comes of age: the Brazilian experience, in Proc. Food Irradiation 2000, April 3–5, 2000, Arlington, VA, 2000, chap. 5. 88. Mermelstein, N.H., E-beam irradiated beef reaches the market, papaya and gamma-irradiated beef to follow, Food Technol., 54(7), 88–92, 2000. 89. ICGFI, Inventory of product clearances and governing regulations. http://www.iaea.or.at/icgfi/, 2002. 90. Anon., Irradiation in the production, processing, and handling of food, in Code of Federal Regulations, 21CFR179, 2000. 91. NFPA, National Food Processors Association — Petition to amend 21CFR179 (Irradiation in the production, processing, and handling of food). http://www.nfpa-food.org/petition/petition.pdf, 2000. 92. FAO/IAEA/WHO, High-Dose Irradiation: Wholesomeness of Food Irradiated with Doses above 10 kGy, Joint FAO/IAEA/WHO Study Group on High-Dose Irradiation, Geneva, 15–20 September 1997, in WHO Technical Report Series 890, 1997. 93. Maxie, E.C., Sommer, N.F., and Mitchell, F.G., Infeasibility of irradiating fresh fruit and vegetables, Hortic. Sci., 6, 202–204, 1971. 94. Willemot, C.M., Marcotte, M., and Deschenes, L., Ionizing radiation for preservation of fruits, in Processing Fruits: Science and Technology, Vol 1: Biology, Principles and Applications, Somogyi, L.P. and Ramaswamy, H.P., Eds., Technomic Publishing Lancaster, PA, 1996, pp. 221–260, chap. 9. 95. Salisbury, F.B. and Ross, C.W., Plant Physiology, 3rd ed. Wadsworth, Belmont, CA, 1984. 96. Prakash, A., Guner, A.R., Caporaso, F., and Foley, D.M., Effects of low-dose gamma irradiation on the shelf life and quality characteristics of cut Romaine lettuce packaged under modified atmosphere, J. Food Sci., 65(3), 549–553, 2000a. 97. Dubery, I.A., van Rensburg, L.J., and Schabort, J.C., Malie enzyme activity and related biochemical aspects during ripening of g-irradiated mango fruit, Phytochemistry, 23, 1383–1386, 1984. 98. Fan, X., Argenta, L., and Mattheis, J., Impacts of ionizing radiation on volatile production by ripening Gala apple fruit, J. Agric. Food Chem., 49(1), 254–262, 2001. 99. Grandison, A.S., Combination treatments including electron beam irradiation for extending the shelflife of fresh fruits, Communication at the 2nd FAO/IAEA Coordination Meeting on Irradiation in Combination with Other Processes for Improving Food Quality, June 28 to July 2, 1993, FRDC, SaintHyacinthe, QC, Canada 1993.

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147. Spoto, M.H.F., Domarco, R.E., Walder, J.M.M., Scarminio, I.S., and Bruns, R.E., Sensory evaluation of orange juice concentrate as affected by irradiation and storage, J. Food Process. Preserv., 21(3), 179–191, 1997. 148. Fan, X. and Thayer, D.W., Quality of irradiated alfalfa sprouts, J. Food Prot., 64(10), 1574–1578, 2001. 149. Yulianti, F., Boylston, T.D., and Reitmeier, C.A., Consumer evaluation of irradiated and pasteurized apple cider. Abstr. 30G–10, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 150. Huggart, R.L., Rouse, A.H., and Moore, E.L., Effect of maturity, variety and processing on color, cloud, pectin and water-insoluble solids of orange juice, In Florida State Horticultural Society Proceedings, 88, 342, 1975. 151. Akamine, E.K. and Moy, J.H., Delay in postharvest ripening and senescence of fruits, in Preservation of Food by Ionizing Radiation, Vol. 3, Josephson, S.E. and Peterson, M.S. Eds., CRC Press, Boca Raton, FL, 1983 pp. 129–158. 152. Baccaunaud, M. 1991. Effets de Ionisation sur les fruits et legumes destines a la consommation en frais, In Ionisation des Produits Alimentaires, Tech & Doc-Lavosir, Paris. 153. Marcotte, M., Irradiated strawberries enter the U.S. market, Food Technol., 46(5) 80–86, 1992. 154. Loaharanu, P., International trade in irradiated foods: regional status and outlook, Food Technol., 43(7), 77–83, 1989. 155. Moy, J.H., Bian, X., Pang, W.Q., and Kawabata., O., The effects of gamma-radiation vs. heat on the quality of papayas. Abstr. 96–1, IFT Annual Meeting, New Orleans, LA, June 23–27, 2001. 156. Youssef, B.M., Asker, A.A., El-Samahay, S.K., and Swailam, H.M., Food Res. Int., 35(1), 1–13, 2002. 157. Fan, X., Toivonen, P.M., and Sokorai, K.B., Combination of ionizing radiation and heat shock on quality of fresh-cut iceberg lettuce. Abstr. 30G–34, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 158. Delaquis, P.J., Toivonen, P.M. and Stewart, S., Growth of Listeria monocytogenes and Escherichia coli 157:H7 is enhanced in ready-to-eat lettuce washed in warm water. Abstr. T44. IAFP Annual Meeting. Minneapolis, MN. Aug 5–8, 2001. 159. Couture, R. and Willemot, C., Combinaison d’une faible dose d’irradiation avec l’atmosphere controlee pour ralentir le murissement des fraises, in Proceedings of the International Conference on Technological Innovation in Freezing and Refrigeration of Fruits and Vegetables, Reid, D.S., Ed., University of California, Davis, CA, 1989, pp.40–45. 160. Koseki, S. and Itoh, K., Effect of nitrogen gas packaging on the quality and microbial growth of freshcut vegetables under low temperatures, J. Food Prot., 65(2)326–332, 2002. 161. Brecht, J.K., Sargent, S.A., Bartz, J.A., Chau, K.V., and Edmond, J.P., Irradiation plus modified atmosphere for storage of strawberries, Proceedings of the Florida State Horticultural Society, 105, 97–100, 1992. 162. Hagenmaier, R.D. and Baker, R.A., Microbial population of shredded carrot in modified atmosphere packaging as related to irradiation treatment, J. Food Sci., 63(1), 162–164, 1998. 163. Bussey, R., Troubleshooting migration issues in packaging, Food Safety Magazine, 7(6), 58–60, 2001. 164. Kiloran, J.J., Chemical and physical changes in food packaging materials exposed to ionizing radiation, Rad. Res. Rev., 3, 369–388, 1972. 165. Varsanyl, I., Investigation of polymer membranes of food packaging quality to gases and water vapor after radiation treatment with radurizing doses, Acta. Alim., 4, 251–269, 1975. 166. Kilcast, D., Irradiation of packaged food, in Food Irradiation and the Chemist, Johnston, D.E. and Stevenson, M.H., Eds., The Royal Society of Chemistry, Cambridge, U.K. 1990, pp. 140–152. 167. Wilson, J.E., Irradiation of polymers: crosslinking versus scission, in Radiation Chemistry of Monomers, Polymers, and Plastics, Marcel Dekker, New York, 1974, chap. 7. 168. Bourges, F., Bureau, G., Dumonceasu, J., and Pascat, B., Effects of electron beam irradiation on antioxidants in commercial polyolefins: determination and quantification of products formed. Packag. Technol. Sci., 5, 205–209, 1992. 169. Komolprasert, V., Truong, P., Begley, T.H., and McNeal, T.P., The suitability of semi-rigid amorphous copolyesters for prepackaged irradiated foods: migration study. Abstr. 73D-15, IFT Annual Meeting, New Orleans, LA, June 23–27, 2001. 170. Komolprasert, V., McNeal, T.P., Adhikari, C., and Begley, T.H., The chemical changes in an morphous nylon polymer upon irradiation. Abstr. 100B-10, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002.

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12 Microbiology of Fruit Products Randy W. Worobo and Don F. Splittstoesser CONTENTS 12.1 12.2

Important Groups of Microorganisms ...............................................................................262 Yeasts..................................................................................................................................263 12.2.1 General Properties ...............................................................................................263 12.2.2 Detection and Enumeration .................................................................................264 12.2.3 Identification ........................................................................................................264 12.2.4 Incidence ..............................................................................................................264 12.2.5 Spoilage................................................................................................................265 12.3 Molds..................................................................................................................................265 12.3.1 General Properties ...............................................................................................265 12.3.2 Mycotoxins ..........................................................................................................265 12.3.3 Detection and Enumeration .................................................................................266 12.3.4 Microscopic Methods ..........................................................................................267 12.3.5 Identification ........................................................................................................267 12.3.6 Distribution ..........................................................................................................267 12.3.7 Spoilage................................................................................................................268 12.4 Lactic Acid Bacteria...........................................................................................................268 12.4.1 General Properties ...............................................................................................268 12.4.2 Detection and Enumeration .................................................................................268 12.4.3 Incidence ..............................................................................................................268 12.4.4 Spoilage................................................................................................................269 12.5 Acetic Acid Bacteria ..........................................................................................................269 12.5.1 General Properties ...............................................................................................269 12.5.2 Detection and Enumeration .................................................................................269 12.5.3 Incidence ..............................................................................................................269 12.5.4 Spoilage................................................................................................................269 12.6 Coliforms............................................................................................................................270 12.7 Spore-Forming Bacteria .....................................................................................................270 12.8 Pathogenic Bacteria............................................................................................................271 12.9 Preservation ........................................................................................................................271 12.9.1 Pasteurization .......................................................................................................271 12.9.2 Hot Fill.................................................................................................................272 12.9.3 Aseptic Processes ................................................................................................272 12.9.4 Canning ................................................................................................................272 12.9.5 Microbiology........................................................................................................272 12.9.6 Heat-Resistant Molds ..........................................................................................272 12.9.7 Composition of the Fruit Product .......................................................................274 12.9.8 Frozen Fruit .........................................................................................................275 12.10 Low Water Activity ............................................................................................................275 12.10.1 Fruit Juice Concentrates ......................................................................................275 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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12.10.2 Preserves ..............................................................................................................275 12.10.3 Dried Fruits..........................................................................................................276 12.10.4 Osmotolerant Fungi .............................................................................................276 12.11 Refrigeration.......................................................................................................................276 12.12 Antimicrobials ....................................................................................................................277 12.12.1 Sulfur Dioxide .....................................................................................................277 12.12.2 Benzoic and Sorbic Acids ...................................................................................277 12.12.3 Dimethyl Dicarbonate and Diethyl Dicarbonate.................................................278 References ......................................................................................................................................278

12.1 IMPORTANT GROUPS OF MICROORGANISMS Many fruits and fruit products are high-acid foods that have a pH under 4 (Table 12.1). Although the organic acids will differ with the kind of fruit, the predominant acids are often citric, malic, and tartaric. The amount of acid and the pH is influenced by the cultivar, fruit maturity, climate, and makeup of the soils. Fruits produced in the regions having a cool climate usually are more acidic than those grown in warmer regions. Grapes harvested after a cool, cloudy growing season, for example, may have a pH under 3 and a titratable acidity of over 1%, while the same vines may yield fruit with a pH of 3.4 or higher and a titratable acidity of under 0.7% when the summer is hot and sunny. The low pH of many fruits is the major factor that influences the composition of their microflora. In general, most yeasts and molds grow well under acid conditions, and, thus, fungi are often the predominant microorganisms in fruit products. Only a few bacteria are sufficiently aciduric to be important.

TABLE 12.1 Principal Acids and pH of Various Fruits Fruit

pH

Nonvolatile Acids

Apple Banana Blackberry Cantaloupe Cherry Cranberry Grapefruit Grape Guava Lemon Mango Orange Papaya Passion fruit Peach Pear Pineapple Plum Watermelon

3.3–4.1 4.5–5.2 3.0–4.2 6.2–6.5 3.2–4.7 2.5–2.7 2.9–3.4 3.0–4.5 3.0–3.2 2.2–2.4 3.3–3.7 3.0–4.0 4.5–6.0 2.6–3.3 3.1–4.2 3.4–4.7 3.2–4.0 2.8–4.6 5.8–6.0

Malic, citric, lactic Citric, malic, tartaric Malic, citric, isocitric Citric, malic Malic, citric, isocitric Citric, malic, quinic Citric Tartaric, malic Citric, malic, lactic Citric Citric, tartaric Citric, malic Citric, malic ketoglutaric Citric, malic Malic, citric Malic, citric Citric, malic Malic, quinic Citric, malic

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TABLE 12.2 Some Yeast Genera Associated with Fruit Products Genus

Sexual Spores

Fermentative

Other Properties

Brettanomyces Candida Citeromyces Clavispora Cryptoccus Debaromyces Dekkera Endomyces Endomycopsella Filobasidiella Geotrichum Hanseniaspora Hansenula Issatchenkia Leucosporidium Lodderomyces Metschnikowia Nadsonia Pichia Rhodosporidium Rhodotorula Saccharomyces Saccharomycopsis Schizosaccharomycs Sporidioabolus Sporobolomyces Torulaspora Trichosporon Wickerhamiella Williopsis Yarrowia Zygosaccharomyces

— — + + — + + + + + — + + + + + + + + + — + + + + — + — + + + +

+ ± + + — ± + ± — ± ± + ± + ± + + + ± — — + ± + — — + — + + — +

Acetic acid produced Pseudo and septate hyphae Warty ascospores Clavate ascospores Capsules, sometimes hyphae Warty ascospores Ogival, acetic acid produced Blasidiospores Clavate conidia Basodiospores Septate hyphae, arthrospores Lemon-shaped cells Hat-shaped ascospores Pseudohyphae Teliospores, hyphae Pseudohyphae Club-shaped ascospores Bipolar budding Hat/saturn-shaped ascospores Carotenoid pigments Carotenoid pigments Strong fermenters Septate hyphae Cell fission, no budding Dark teliospores Cartenoid pigments No hyphae Arthroconidia One rough ascospore Ascospore saturn shaped Septate hyphae Preservative-resistant

Source: Barnett, J.A., Payne, R.W., and Yarrow, D. 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge, U.K.; Deak, T. and Beuchat, L.R. 1987. Identification of foodborne yeasts. J. Food Prot., 50: 243–264.

12.2 YEASTS 12.2.1 GENERAL PROPERTIES Yeasts are fungi whose usual growth form is unicellular; many reproduce by budding. They do not represent a homogeneous taxonomic group. At least 215 species are important in foods (Deak and Beuchat, 1987), and it is estimated that 32 genera are associated with fruits and fruit products (Table 12.2). Only a few species of yeasts are pathogenic for man and other animals (Pfaff et al., 1978). None of the pathogenic species are common contaminants of fruits and fruit products.

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ENUMERATION

Fruit samples are homogenized in a blender or StomacherTM, then further diluted in a protective solution such as 0.1% peptone water. The desired decimal dilutions are surface cultured on agars that suppress the growth of bacteria. Recommended media are potato dextrose agar, which is acidified to pH 3.5 with 10% tartaric acid, or plate count agar to which has been added to 100 mg/l chloramphenicol (Mislivec et al., 1992). The antibiotic-containing agar often will yield higher recoveries of yeasts than the acidified agar (Koburger, 1970), although the differences are minimal when high-acid foods such as many fruit products are being cultured (King, 1992). The plates are incubated at 20 to 25∞C for 5 d before counting. When culturing low water activity foods such as fruit juice concentrates, special plating techniques may be required (Tilbury, 1980; Corry, 1987). To avoid osmotic shock, dilutions are often performed in 0.1% peptone water containing 20% w/w sucrose. Appropriate dilutions are then inoculated onto a suitable yeast agar that contains 50% w/w glucose, fructose, or sucrose. The plates are incubated at 25 to 30∞C and counted at 5 d. Species of osmotolerant yeasts include Zygosaccharomyces rouxii, Torulaspora delbrueckii, and Debaromyces hansenii (Jermini and Schmidt-Lorenz, 1987b). A number of more rapid methods have been developed for the detection of yeasts in different fruit products. The application of gas chromatography to measure trace amounts of ethanol under 0.2% has long been used to monitor yeast growth in bulk-stored grape juice. Another procedure is impedimetric detection, which depends upon changes in resistance to the flow of an alternating current when the food is cultured in a medium selective for yeasts. The hours required to produce a measurable change in impedance (growth to 106 to 107 yeasts/ml) can be correlated with the initial yeast population in the fruit product (Schaertel et al., 1987). A very rapid and sensitive method is a bioluminescent procedure that measures the quantity of adenosine triphosphate (ATP) in living yeasts. The luciferin–luciferase–ATP reaction permits the detection of as few as 10 yeast cells in only 3 to 5 min. The procedure has considerable application for beverages that contain fruit juice (Little and LaRocco, 1986).

12.2.3 IDENTIFICATION Traditional methods for the identification of different species of yeasts rely on a very large number of tests and observations, which include cell morphology, spore formation, fermentation of sugars, assimilation of other compounds as a carbon source, utilization of nitrate as a nitrogen source, requirements for various growth factors, and resistance to cycloheximide (Lodder, 1970; Barnett et al., 1983). In addition to being laborious, some tests can be very difficult. For example, although ascospore formation is an important criterion, some yeast strains may lose their ability to produce spores or form so few that detection is very difficult. Various schemes have been proposed to simplify the identification of yeasts that are important in foods. Beech et al. (1968) devised criteria that did not utilize spore formation. Their procedure was based on colony and cell morphology, pigmentation, various sugar fermentations, and assimilation tests. More recently, Deak and Beuchat (1987) published a procedure for the identification of 215 species of yeasts from foods that involved only 10 to 15 tests that were largely assimilation and fermentation reactions. Several commercial kits have been developed for the identification of yeasts of clinical importance. When tested against 166 yeasts isolated from fruit juices, the API 20C kit (Analytab Products, Plainview, NY) correctly identified 86% of the cultures, while the ID 32C kit (BioMeriux Vitek Inc., Hazelwood, MO) correctly identified 76% (Deak and Beuchat, 1993).

12.2.4 INCIDENCE Yeasts are widely distributed in the environment. In the plant world, they can be isolated from leaves, flowers, and tree exudates, as well as from the surface of fruits (Pfaff et al., 1978). Fruits, as received

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at the processing plant, often are contaminated with large numbers of yeasts. Sound apples, for example, may yield viable populations of 103 to 104 per gram of fruit (Marshall and Walkley, 1951; Swanson et al., 1985). Grapes, which possess a greater surface area, are more fragile in texture than apples and have contained as many as 107 yeasts per gram (Splittstoesser and Mattick, 1981). In the latter study that involved 135 samples of grapes, 43% yielded counts of 106 per gram or higher. Fruit that has been damaged by birds, insects, or pathogenic fungi may contain very high yeast populations. The yeasts are introduced into the exposed tissue, often via insects, and are able to use the sugars and other nutrients to support their growth. Other sources of contamination include the surfaces of mechanical harvesters (Moyer et al., 1969) and the bins and lug boxes used in the transport of fruit to the processing plant. In the factory, many of the yeasts are removed by early processing steps that may include washing and peeling of the fruit. Recontamination can subsequently occur at points where fruit solubles collect and when there is opportunity for yeast growth. Conveyor belts, equipment that slices or dices fruit, presses, and the ports of filling machines are all potential sources of contamination (Berry, 1979).

12.2.5 SPOILAGE Manifestations of yeast growth will depend upon the nature of the fruit product and the strain of yeast. Growth of a strongly fermentative type such as certain strains of Saccharomyces cerevisiae may produce sufficient CO2 to burst the container, 90 lb/in.2 or more. Growth of some species in a clear fruit juice may produce only slight haze and sediment. Yeast populations of 105 per milliliter or less usually can be detected with the naked eye. While CO2 and ethanol are usually the predominant metabolic products of yeasts, glycerol, acetaldehyde, pyruvic acid, and a-ketoglutaric acid are also formed (Rankine, 1968). Certain oxidative yeasts such as species of Brettanomyces will generate acetic acid in wines and other fruit products. Although yeasts may possess hydrolytic enzymes that degrade pectins, starch, and certain proteins (Pfaff et al., 1978; Biely and Slavikova, 1990), enzymatic activity is usually much less than that exhibited by other aciduric microorganisms, molds in particular.

12.3 MOLDS 12.3.1 GENERAL PROPERTIES Filamentous fungi represent an important group within the microflora of fruit products for a number of reasons: 1. Some species are xerophilic and, thus, are potential spoilage agents of foods of low water activity such as dried fruits and fruit juice concentrates. 2. Growth of molds on processing equipment such as wooden tanks can result in the generation of off-flavors in wines, juices, and other fruit products. 3. The ascospores of some species are very heat-resistant and will survive the commercial pasteurization treatments that are given to most fruit products. 4. Mold-infected raw fruit may become soft after processing because pectinases were not inactivated by the thermal treatment (Ogawa et al., 1974). 5. The metabolic products of many molds are toxic to various animals and presumably man (Liewen and Bullerman, 1992).

12.3.2 MYCOTOXINS Over 100 different toxic compounds are produced by some 200 mold species (Krogh, 1989). Many are associated with foods other than fruits and fruit products. For example, aflatoxins, the

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TABLE 12.3 Mycotoxins Associated with Fruit Products Fungus Aspergillus flavus Byssochlamys

Eurotium Fusarium moniliforme Neosartorya

Penicillium expansum Talaromyces flavus

Mycotoxin

Fruit

Aflatoxins Patulin Byssochlamic acid Byssotoxin Malformins Patulin Physicon Trichothecenes Zearalenone Fumitremorgans Mevinolins Terrein Trypacidin Tryptoquivalins Verruculogen Patulin Dehydroaltenusin Mitorubrins Vermicelline Vermistatine Wortmannin Wortmannolone

Figs Canned juices, etc.

Reference Sharman et al., 1991 Davis and Diener, 1987 Frisvad and Samson, 1991

Preserves Banana

Frisvad and Samson, 1991 Chakrabarti and Ghosal, 1986

Canned juices, etc.

Frisvad and Samson, 1991

Apple juice Canned juices, etc.

Liewen and Bullerman, 1992 Frisvad and Samson, 1991

most thoroughly studied mycotoxins, are found primarily on cereals, including corn, cottonseed, and peanuts. Patulin appears to be the most common mycotoxin in processed fruits (Table 12.3). It is produced by a number of mold species, included Penicillium expansum, the most usual cause of rots of the apple (Harwig et al., 1973). Patulin is resistant to thermal destruction between pH 3.5 and 5.5 (Lovett and Peeler, 1973), and, thus, traces of the compound are commonly present in canned apple juice and other heated apple products. Recently, the U.S. Food and Drug Administration (FDA) implemented finished product patulin levels of less than 50 ppb for apple juice (Food and Drug Administration, 2001). Aflatoxins have been found in dried figs and fig paste (Sharman et al., 1991). About 11% of the fig paste and 9% of the whole dried figs yielded detectable levels, with 40 mg/kg being the highest concentration. The heat-resistant molds, Byssochlamys, Eurotium, Neosartorya, and Talaromyces, produce a number of mycotoxins when growth occurs in a thermally processed fruit product (Table 12.3). Growth by these species usually would not present a serious public health problem, however, because the spoiled food would be avoided by most consumers.

12.3.3 DETECTION

AND

ENUMERATION

Many of the plating procedures that were described previously for yeasts are also applicable for molds. Interpretation of the results may be difficult, however, because the number of colonyforming units will be influenced by the presence of fruiting structures that contain large numbers of spores, and also if the mycelial mass is broken into many viable fragments by the homogenization treatment.

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Certain specialized media have been recommended for the enumeration of filamentous fungi. Dichloran rose bengal chloramphenicol agar (DRBC) is useful when the fruit product contains molds whose colonies may spread over the entire surface of the plate (King et al., 1979). Dichloran 18% glycerol agar (DG18) permits the enumeration of xerophilic molds that might be present in dried fruits, fruit juice concentrates, and other foods of low water activity such as fruit cakes (Hocking and Pitt, 1980). A number of more rapid methods for the detection of molds in foods have been studied (Williams, 1989; Mislivec et al., 1992). One chemical procedure has been to quantitatively measure chitin, which is a component of the fungal cell wall (Lin and Cousin, 1985). The analysis is somewhat tedious and, thus, has not been widely adopted by the food industry. Immunoassays represent another improved method for detecting fungi (Lin et al., 1986; Notermans et al., 1986). An immunoassay specific for Aspergillus and Penicillium is based on the detection of a heat stable extracellular polysaccharide (Kamphuis and Notermans, 1992). Another approach is to analyze for metabolic products of molds. For example, the degree of moldiness in mechanically harvested grapes may be related to glycerol content of the must (Ravji et al., 1988). A difficulty with this method is that the procedure is only semiquantitative because different species and strains of fungi produce varying amounts of glycerol.

12.3.4 MICROSCOPIC METHODS Several microscopic procedures are used for the enumeration of molds (Splittstoesser, 1992). Perhaps the oldest is the Howard mold count that originally was developed to monitor the soundness of tomatoes used in the production of catsup (Howard, 1911). In the Howard method, the incidence of molds is determined by examining a number of fields under the microscope using a special counting chamber and carefully prescribed techniques. A positive field contains one or more mold filaments that exceed a certain length. The FDA has established mold count criteria (Defect Action Levels) for a number of fruit products, including apple butter; apricot, peach, and pear nectars and purees; frozen and canned berries; canned citrus juices; cranberry sauce; and pineapple products. A Geotrichum count has been developed to assess the cleanliness of equipment used in the processing of fruits for canning and freezing (Cichowicz and Eisenberg, 1974). The mold Geotrichum candidum forms a characteristic brush-like morphology when growing on the surfaces of processing equipment. Typical filaments are counted using a rot fragment slide and a stereoscopic microscope at 30 to 45 ¥ power. An advantage to the method is that it permits the evaluation of canned fruits as dead filaments can be counted. With frozen foods, a disadvantage is that Geotrichum counts do not always correlate well with viable counts when the two procedures are compared (Splittstoesser et al., 1977).

12.3.5 IDENTIFICATION Identification of filamentous fungi to the species level is based largely on colonial and microscopic morphology (Pitt and Hocking, 1985; Samson and van Reenen-Hoekstra, 1988). Unknown isolates are cultured on a number of different media to assure that mycelial morphology and sporulation are typical. Important characteristics include colony pigmentation and physical appearance, the morphology of fruiting structures, whether mycelia are septate, and the presence of ascospores. Successful identification depends upon the ability to recognize the different specialized structures.

12.3.6 DISTRIBUTION Molds are widely distributed in the environment. They are present in soils, in the air, on plants, and in water. They play a major role in the mineralization of organic materials in the soil. Growth occurs wherever conditions are favorable, which includes the walls and ceilings of fruit processing factories when a high relative humidity persists. Conidia and other mold spores, which are released

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TABLE 12.4 Major Lactic Acid Bacteria That Spoil Fruit Products Genus

Morphology

Fermentation

Species L. plantarum L. brevis L. collinoides L. fermentum L. fructivorans L. hilgardii L. mesenteroides L. paramesenteroides L. oenos P. pentosaceus

Lactobacillus Lactobacillus

Rods Rods

Homofermentative Heterofermentative

Leuconostoc

Cocci

Heterofermentative

Pediococcus

Cocci-tetrad

Homofermentative

in high numbers, survive desiccation and, thus, are common inhabitants of the air, including the atmospheres of food processing factories.

12.3.7 SPOILAGE Although molds are aerobic microorganisms, many are very efficient scavengers of oxygen, and, thus, processed fruits, including those hermetically sealed in cans or glass, are susceptible to spoilage. When the amount of vegetative growth is very limited, evidence of spoilage may be the changes produced by fungal enzymes such as the breakdown of starch or pectins. When more growth is permitted, colonies may develop in the headspace or as strands throughout a beverage or similar product.

12.4 LACTIC ACID BACTERIA 12.4.1 GENERAL PROPERTIES The lactic acid bacteria are gram positive, catalase negative organisms that grow very well under anaerobic conditions. The lactobacilli are rod-shaped, while the pediococci and leuconostocs are spherical (Table 12.4). The homofermentative species produce mainly lactic acid from hexose sugars; the heterofermenters produce one molecule of lactic acid, one molecule of carbon dioxide, and one two-carbon compound. The latter is usually acetic acid or ethanol or a combination of the two. The streptococci (Lactococcus and Enterococcus species) are not included in Table 12.4 because they rarely have been involved in the spoilage of fruit products (Carr, 1958). Perhaps this is because they are less tolerant to high acidity and a low pH.

12.4.2 DETECTION

AND

ENUMERATION

The lactic acid bacteria are very fastidious with respect to their nutritional requirements. MRS agar (deMan et al., 1960) is a rich medium that supports growth of most lactic acid bacteria. It can be made selective for this group with the addition of 0.02% sodium azide (Mundt et al., 1967). Another medium that is useful for lactic acid bacteria is plate count agar supplemented with 10% tomato juice (Stamer et al., 1964).

12.4.3 INCIDENCE Lactic acid bacteria can be isolated from the plant world, including grains, fruits, and vegetables (Splittstoesser and Gadjo, 1966). The numbers on fruit as received from orchards and vineyards is usually very low. Buildup occurs on processing equipment where fruit solubles can accumulate

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and during the fermentation of juices. Lactic acid bacteria are common contaminants of ciders, wines, and citrus juices (Luthi, 1959; Hatcher et al., 1992).

12.4.4 SPOILAGE Growth of lactic acid bacteria in juices and other fruit products may result in the formation of haze, gas, acid, and a number of other changes. Certain heterofermentative lactobacilli have been responsible for slime in cider (Carr, 1970). The lactobacilli and leuconostocs that are present in citrus juices generate acetylmethylcarbinol and diacetyl, compounds that give the juices an undesirable, buttermilk-like flavor (Parish and Higgins, 1988; Hatcher et al., 1992). Some strains are extremely tolerant of ethanol and, thus, can grow in wines. Lactobacillus fructivorans (formerly L. trichodes) has grown in appetizer and dessert wines containing as much as 20% ethanol (Fornachon et al., 1949). Growth has resulted in the formation of filaments, and in California, spoilage has been referred to as Fresno mold (Vaughn et al., 1949). Most lactic acid bacteria have the ability to decarboxylate malic acid to lactic acid. This malolactic fermentation is often desirable in high-acid wines because the acidity is reduced, and desirable flavors are produced (Wibowo et al., 1985). Oenococcus oenos (formerly Leyconostoc oenos) appears to be the most acid and alcohol-tolerant species and often is isolated from wines that are undergoing a malo-lactic fermentation. Cultures of O. oenos are available commercially and are used by many winemakers to inoculate their wines during or after the ethanol fermentation but before bottling. (Because of haze and gas, growth of malo-lactic bacteria in a wine after bottling would be considered a type of spoilage.)

12.5 ACETIC ACID BACTERIA 12.5.1 GENERAL PROPERTIES The acetic acid bacteria are gram negative, aerobic rods that are separated into two genera, Acetobacter and Gluconobacter (DeLey et al., 1984; DeLey and Swings, 1984). While both oxidize ethanol to acetic acid under acid condition, Acetobacter species can oxidize acetic acid to carbon dioxide, and, thus, the genus is termed an overoxidizer. Utilization of lactate by the acetobacters is another test for separating the two genera (Carr, 1968).

12.5.2 DETECTION

AND

ENUMERATION

Various media have been used for the isolation of acetic acid bacteria from fruits and fruit products (Blackwood, 1969; Carr and Passmore, 1979; Drysdale and Fleet, 1988). A typical agar might contain 2% yeast extract, 2% ethanol, 2% agar plus brom cresol green to signal acid formation. The addition of 50 to 100 mg/l cycloheximide is recommended to suppress growth of yeasts when they outnumber the acetic acid bacteria (Drysdale and Fleet, 1988; Passmore and Carr, 1975).

12.5.3 INCIDENCE Damaged fruit often undergo an alcoholic fermentation by yeasts, followed by growth of acetic acid bacteria. Grapes infected by Botrytis cinerea may exhibit a sour rot that results in the buildup of over 106 acetobacters per gram; Acetobacter aceti and A. pasteurianus have been the predominant species (Drysdale and Fleet, 1988). In studies on sound grapes, about 50% of the samples yielded acetic acid bacteria, with the populations ranging from 10 to 104 per gram. All of the isolates were strains of Gluconobacter oxydans (Splittstoesser and Churey, 1992).

12.5.4 SPOILAGE Because the bacteria are obligate aerobes, juices, wines, and cider are most susceptible to spoilage while held in tanks prior to bottling (Joyeux et al., 1984). Relatively small amounts of acetic acid

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have a marked effect on wine quality. In the U.S., the Bureau of Alcohol, Tobacco, and Firearms limits the amount of volatile acid to 0.12 g of acetic acid per 100 ml in white wines and 0.14 g per 100 ml in red wines. Growth in juices that do not contain ethanol results in hexose sugars being oxidized to gluconic and ketogluconic acids (Drysdale and Fleet, 1988). Other products of carbohydrate metabolism are dihydroxyacetone, 2,3-butanediol, and acetoin. Some strains of Acetobacter pasteurianus and Gluconobacter oxydans produce microfibrils composed of cellulose, which results in the formation of flocs in different fruit juice beverages (Juven and Shomer, 1985).

12.6 COLIFORMS Coliforms are often found in juices and other fruit products. They usually are species other than Escherichia coli. They can be recovered from the surface of fruit (Vaughn et al., 1952; Goverd et al., 1979), and their presence usually does not indicate direct fecal contamination. Because of the low pH of juices, most coliforms die off rapidly in single-strength products (Hahn and Appleman, 1952); however, in frozen juices, such as concentrated orange juice, coliforms may survive for months or longer (Wolford, 1956; Fuentes et al., 1985).

12.7 SPORE-FORMING BACTERIA Low numbers of viable bacterial spores can be recovered from many fruit products. The spores are widely distributed in the environment, and they are not destroyed by preservation processes such as fermentation, canning, or freezing. Most bacterial spores are of little concern to the processor of fruits because the low pH of fruit products prevents spore germination or outgrowth (Blocher and Busta, 1983). There are, however, a few reports that indicate that some relatively rare bacterial spore-forming species are sufficiently aciduric to be a potential problem. Townsend (1939) studied anaerobes responsible for the spoilage of canned fruits. He found that a vegetative cell inoculum of Clostridium pasteurianum could initiate growth in pear and apricot juices having a pH of 3.6. Bacillus macerans and B. polymyxa are gas-producing species that have been isolated from acidified vegetables and from canned peaches and pears that had an original pH of 3.8 to 4 (Vaughn and Stadtman, 1946; Vaughn et al., 1952). Several reports indicate that certain spore-forming bacilli can grow in wine. Gini and Vaughn (1962) isolated strains of Bacillus coagulans, B. subtilis, B. macerans, B. pumilis, B. sphaericus, and B. pantothenticus from dessert wines that contained as much as 20% ethanol by volume. Growth was obtained when the isolates were inoculated into a Grillo wine having a pH of 3.6 and 19% ethanol. More recently, aciduric spore formers have been obtained from wine corks (Daly, 1982; Lee et al., 1983). The cultures, identified as strains of B. coagulans, B. badius, and B. sphaericus, initiated limited growth when inoculated into red and white wines having an initial pH of 3.5 and 3.3, respectively. Spore-forming bacilli that actually prefer a low pH have been responsible for spoilage of apple juice (Cerny et al., 1984) and a blend of fruit juices (Haglund, 1993). The organisms have a pH optimum between 2.5 and 5.5 and, based on their cyclohexane fatty acids and hoppanoid content, are believed to be similar to Bacillus acidocaldarius (Cerny et al., 1984). However, B. acidocaldarius, which was originally isolated from acidic hot springs (Darland and Borck, 1971), grows at 75 to 80∞C, while the apple juice isolate had a maximum growth temperature of 50∞C and an optimum of 40 to 42∞C (Cerny et al., 1984). Subsequently in 1992, these organisms were reclassified to a new genus, Alicyclobacillus, based on their different comparative rDNA sequence analyses. The genus has expanded to include several species such as Alicyclobacillus acidocaldarius, A. acidoterrestris, A. cycloheptanicus, and A. hesperidium (Wisotzkey et al., 1992). Since the original

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observation, numerous reports of beverage and acid products due to Alicyclobacillus spp. has been identified (Splittstoesser et al., 1994; Walls, 1994; Wisse et al., 1998; Yamazaki et al., 1996). A report has found the spores of similar cultures in commercial pasteurized apple, grape, and cherry juices (Splittstoesser et al., 1994, unpublished data). These findings agree with Cerny et al. (1984), in that the organism does not grow in the usual rich bacteriological media such as trypticase soy agar, plate count agar, and brain–heart infusion agar. Good growth and sporulation, however, have been obtained in potato dextrose agar, either at pH 3.5 or 5.6, as well as K-media. Studies on heat resistance showed D-values for the spores in apple juice to be 23 min at 90∞C, with a zvalue of 7.7∞C. Thus, the spores are sufficiently heat resistant to survive the thermal process applied to many fruit products.

12.8 PATHOGENIC BACTERIA In general, the different bacteria that cause disease in humans are not associated with fruit products. Most originate from animal reservoirs, and many do not tolerate the low pH of fruits. There are, however, a few exceptions. Fruit such as melons may contain enteric pathogens on their rind, which then may be introduced onto the flesh when the fruit is cut. The original source of the contamination can be polluted irrigation water. Precut wrapped watermelon was responsible for 18 foodborne illnesses involving Salmonella oranienbert (Anon., 1979). Growth of Shigella sonnei, S. flexneri, S. dysenteriae, Salmonella derby, and S. typhi occurred when jicama (pH 5.97), papaya (pH 5.67), and watermelon (pH 6.81) were held at room temperature (Escartin et al., 1989). Storage of melons and cantaloupe at 5∞C prevented growth of salmonellae during a 24-h incubation although the viable populations did not decrease (Golden et al., 1993). Thus, refrigeration may improve the safety of melons when the incidence of pathogens is low. In New Jersey, 296 people became ill after consuming nonpasteurized cider that had been contaminated with Salmonella enterica serovar Typhimurium (Anon., 1975). The orchard had been fertilized with animal manure, which was thought to be the source of contamination because cider is commonly made from fruit collected from the orchard floor. Subsequent studies showed that S. enterica serovar Typhimurium could survive in apple juice of pH 3.6 for at least 30 d (Goverd et al., 1979). Another pathogen associated with fruit products is Escherichia coli O157:H7, an organism that produces bloody diarrhea and hemolytic uremic syndrome. Contaminated apple cider has been the cause of several outbreaks, including one in Massachusetts in which 23 people became ill (Besser et al., 1993). The bacteria will survive in cider held at 8∞C for 20 d or longer but often will die off in 3 d when the cider contains 0.1% sodium benzoate (Zhao et al., 1993). Although most producers of cider do not wish to pasteurize, our unpublished data show the organism to be quite heat sensitive. Thus, D-values in pure cider were 12 min at 52∞C and only 0.37 min when the cider contained 0.1% sodium benzoate.

12.9 PRESERVATION Various methods are used for extending the shelf life of fruit products. As will be shown in this section, the microorganisms of importance will vary with the preservation methods.

12.9.1 PASTEURIZATION The common thermal processes applied to fruit products include hot fill, aseptic systems, and traditional canning in which the food is heated after being filled and closed into a container (Lopez, 1987). The physical properties of the food and the nature of the container often dictate the process to be used.

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12.9.2 HOT FILL A fruit juice or similar product is heated to 88 to 95∞C, then filled into a glass or metal container. After closing, the container may be inverted for several minutes before cooling to kill any spoilage microorganisms that might be present on the cap. Applesauce that is packed in metal containers may require a higher closing temperature, 93∞C or above, to prevent detinning. Certain plastic containers become distorted by high fill temperatures. This can be circumvented by heating the juice to 90 to 95∞C, then cooling to a temperature of about 70∞C for filling. The lower temperature is still lethal for most aciduric spoilage microorganisms.

12.9.3 ASEPTIC PROCESSES Aseptic technology is widely used for the preservation of fruit juices and juice beverages in consumer-size packages. The juice and package are sterilized separately in a sterile chamber. The juice is often flashed to 93∞C or higher temperature in a heat exchanger, held for 30 sec and then cooled before filling into a sterile carton. The carton, often a paper–plastic–aluminum laminate, is presterilized by exposure to 30% hydrogen peroxide at an elevated temperature. Aseptic systems are also used for the preservation of large volumes of fruit products that will later be reprocessed. The method is applicable to 50-gal drums, as well as large tanks that hold thousands of gallons.

12.9.4 CANNING This process is used to preserve a large variety of fruit products, including berries, whole fruits, and fruit sections. After filling the fruit into cans or jars, a syrup is often added, and then air is removed by subjecting the container to a steam exhaust treatment. Following closing, the containers are held in a boiling water bath until fruit in the slowest heating region in the can or jar reaches a temperature of 88 to 91∞C.

12.9.5 MICROBIOLOGY The thermal processes cited above are much less severe than those used for low-acid foods such as vegetables and meats. The milder processes are usually adequate because the most heat-resistant microorganisms, bacterial spores, do not grow in a high-acid fruit medium, and most acid-tolerant microorganisms possess little heat resistance (Table 12.5). The data in Table 12.5 illustrate a number of principles regarding canning of fruits. First of all, of the three groups of microorganisms, only ascospores of certain molds have the potential to survive commercial pasteurization treatments. Although many molds produce asexual spores such as conidia, as illustrated with Aspergillus flavus, these structures possess little heat resistance. The ascospores of yeasts are relatively sensitive to heat although they are considerably more resistant than are the vegetative cells. Non-spore-forming bacteria are also very heat sensitive. The spoilage of pasteurized fruits by yeasts and bacteria, therefore, usually results either from gross underprocessing or because contamination occurred following the heat treatment. Cooling water, which usually is recycled, may permit the buildup of bacteria and yeasts and, thus, increase the opportunities for contamination when small volumes of water are introduced into the container during cooling.

12.9.6 HEAT-RESISTANT MOLDS Most fruit products are not given a thermal process that is sufficient to destroy the ascospores of Byssochlamys, Neosartorya, and Talaromyces. The reason that spoilage is not more common is not completely understood. Although the counts are usually low on sound fruit, less than one per gram,

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TABLE 12.5 Heat Resistance of Various Aciduric Microorganisms Microorganism

Medium

Aspergillus flavus conidia Byssochlamys fulva ascospores Eurotium herbariorum ascospores Neosartorya fischeri ascospores

Molds PO4 Glucose-tartrate Grape juice Apple juice

D-value

D55 D90 D70 D85

= = = =

3.1 10 2.5 18

Reference

Doyle and Marth, 1975 Bayne and Michener, 1979 Splittstoesser et al., 1989 Splittstoesser and Churey, 1989

Yeasts Saccharomyces cerevisiae ascospores vegetative Kluyveromyces bulgaricus ascospores vegetative cells

Escherichia coli O157:H7 Oenococcus oeni Lactobacillus plantarum Lactobacillus fructivorans

Apple juice

D55 = 106 D55 = 0.90

Splittstoesser et al., 1978

PO4-citrate

D60 = 40 D60 = 0.20

Put and DeJong, 1982

D52 D43 D45 D60 D60

McLellan et al., 1995 Splittstoesser, unpublished data

Bacteria Apple juice Wine Wine Wine Grape juice

= = = = =

12 0.33 0.35 1.7 10

ascospores can be recovered from a variety of fruits, including grapes, strawberries, and cherries, when larger samples of 100 g or more are cultured (King et al., 1969; Splittstoesser et al., 1971, 1974a). Fruit in contact with soil, such as strawberries and pineapple, often are the most heavily contaminated because soil seems to be the normal spore habitat. There is little data to indicate that growth of the molds on processing equipment is a significant source of ascospores. Various processing treatments such as washing, pressing, and filtration will remove many of the ascospores before pasteurization. The author has recovered ascospores from grape pomace after pressing and from diatomaceous earth used in the filtration of cherry juice. Thus, low initial contamination coupled with the removal of spores during processing may explain why more spoilage does not occur. A procedure for detecting ascospores is to heat the sample for 30 to 60 min at 70∞C and then to culture the homogenate or juice on a selective medium such as acidified potato dextrose agar at pH 3.5 or plate count agar containing 100 mg/l chloramphenicol. If the sample is a clear juice, a large volume may be cultured by passing the material through a membrane filter having a pore size that will retain the spores. The purpose of the heat treatment is to destroy heat-sensitive yeasts and molds and to activate dormant ascospores. Although activation greatly improves spore recovery, we have observed that only a fraction may develop into visible colonies even when incubations as long as 30 d have been used (Splittstoesser et al., 1993). Byssochlamys fulva and B. nivea appear to be the most important spoilers of pasteurized fruit products. Their anamorphs (no ascospores produced or observed) are Paecilomyces fulva and P. niveus. The colonies of B. fulva range from yellow-brown to olive-brown, while those of B. nivea are usually white to tan. The colonies of neither species are ever green. The smooth-walled ascospores of B. fulva are somewhat larger than those of B. nivea, while the latter also produces chlamydospores. The asci, which contain eight ascospores, are not formed in an ascocarp (Samson and van Reenen-Hoekstra, 1988).

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Neosartorya fischeri (anamorph: Aspergillus fischeri) is likely the most important food spoilage species within this genus. The asci are produced in white cleistothecia that give the cream to white colonies a granular appearance. The ascospores, which commonly are present in a free state, are readily recognized by the presence of two equatorial crests. In Talaromyces flavus (anamorph: Penicillium dangeardii), both the mycelia and gymnothecia are usually pigmented yellow, although reddish hues may also develop. The elliptical ascospores have spiny walls. Conidia are borne on penicillia-like structures.

12.9.7 COMPOSITION

OF THE

FRUIT PRODUCT

The composition of the food can have a marked effect on thermal resistance. This is illustrated by Lactobacillus fructivorans in which D-values were much lower in wine than in grape juice (Table 12.5). Other factors that have an effect are the concentration of sugars, the pH and types of organic acids, and the presence of certain antimicrobials. Numerous studies have shown that microorganisms become more heat-resistant as the concentration of sugars is increased (Beuchat, 1988a; Corry, 1976; King and Whitehand, 1990; Splittstoesser et al., 1974b). The ascospores of Zygosaccharomyces bailii, for example, gave a D-value of 9.4 min when heated at 58∞C in a medium containing 30% glucose (aw = 0.963), while in 60% glucose (aw = 0.858) the D-value was increased to 59 min (Jermini and Schmidt-Lorenz, 1987a). Similar responses have been observed for the spores of Byssochlamys (Beuchat, 1981a; Splittstoesser et al., 1974b) and Talaromyces flavus (Beuchat, 1988a; King and Whitehand, 1990). Some organic acids seem to sensitize microorganisms to heat while others afford protection. The response appears to differ depending upon the microbial species. Citric acid increased the resistance of Candida lambica, Saccharomyces chevalieri, and Torulopsis magnoliae when these yeasts were heated in orange juice concentrate (Juven et al., 1978). The ascospores of Byssochlamys fulva survived better in 0.05 M, pH 3 solutions of tartaric, malic, and citric acid than they did in distilled water (Splittstoesser et al., 1974b). The ascospores of Aspergillus (Neosartorya), however, were unaffected (Splittstoesser and Splittstoesser, 1977). Fumaric acid, and to a lesser extent, acetic and lactic acids sensitized the ascospores of Byssochlamys fulva to heat (Splittstoesser et al., 1974b). Fumaric acid also decreased the heat resistance of Talaromyces flavus ascospores (Beuchat, 1988b) while citric, lactic, malic, and tartaric acids had no effect (King and Whitehand, 1990). Gillespy (1940) was the first to observe that sulfur dioxide sensitized the ascospores of Byssochlamys fulva to heat. Thus, spores heated 10 min at 85∞C in pH 3 citrate buffer yielded 19% survival in the absence of SO2 and less than 0.5% survival in the presence of 1.0%. King et al. (1969) showed that 90 ppm SO2 reduced D-values of B. fulva and B. nivea about 50% when added to grape juice. The spores of Neosartorya fischeri were also greatly sensitized to heat: in 0.05 M, pH 3.3 tartaric acid solution, D80 = 123 min compared to 6.1 min when the solution contained 100 mg/l SO2 (Splittstoesser and Churey, 1991). Various studies have shown that sorbic and benzoic acids reduce the heat resistance of a variety of fungi. Thus, the presence of 500 and 1000 mg/l sorbic or benzoic acid reduced the heat resistance of conidia of Aspergillus flavus and Penicillium puberulum, ascospores of Byssochlamys nivea, and vegetative cells of Geotrichum candidum (Beuchat, 1981b). Research on 12 yeast genera indicated that benzoic and sorbic acids not only sensitized the yeasts to heat but also had a marked effect on the viable recovery of cells that had been injured by heat. Potassium sorbate was found to be more effective than sodium benzoate (Beuchat, 1981a). Our unpublished studies with Escherichia coli O157:H7 gave the opposite results, in that the average D50 in apple juice containing 600 mg/l benzoic acid was 5.9 min compared to 12 min in the same concentration of sorbic acid. Other evidence that the response to heat and preservative differs with the microorganism has been obtained with the ascospores of Neosartorya fischeri. With this organism, D85-values were about the same when the concentration of sorbic acid ranged from 0 to 1000 mg/l. Concentrations over 60 mg/l

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in the plating medium, however, produced a marked reduction in the recovery of heat-injured spores (Splittstoesser and Churey, 1989).

12.9.8 FROZEN FRUIT Numerous fruits and fruit products are preserved by freezing. These include whole berries and cherries, fruit juice concentrates and purees having a Brix of 45∞ or higher, and various sliced fruits. The products may be packed in retail-size cartons or in large containers for reprocessing, such as 50-gal and 30-lb tins. Dry sugar or syrup is often added to the fruit prior to freezing. The predominant microorganisms are usually yeasts and molds although lactic acid bacteria can be numerous in certain products such as orange juice concentrate. The numbers will vary, depending upon sanitary conditions and the type of process. Juice concentrates and pasteurized products will usually yield low counts because most of the microorganisms will have been killed by the heat treatment. Treating fruit products with sulfur dioxide will also destroy many of the contaminating microorganisms. Often, the counts in the frozen product will reflect the sanitary conditions of belts and other processing equipment. Many of the contaminating microorganisms, especially vegetative cells, are killed by the freezing process. A significant percentage, however, will survive freezing, and, thus, frozen fruit products can yield relatively high counts. During storage in the frozen state, additional death of microorganisms occurs, but the rate is slow. Considerable variation in the yeast and mold counts on fruits or pieces of fruit is a common phenomenon. As the fruit is transported across the various unit operations, there is an opportunity for each piece to contact a different contaminated surface and thus pick up a different load of microorganisms. It is important, therefore, that a representative sample be cultured. It has been my experience, for example, that a sample of frozen apple slices should weight at least 100 g.

12.10 LOW WATER ACTIVITY A variety of fruit products depend upon a low water activity for their preservation (Tilbury, 1980; Troller and Christian, 1978). These include fruit juice concentrates, jams and jellies, and dried fruits. The water activity (aw) of these foods will vary, depending upon the concentration of sugars and other water binding compounds and, in the case of dried fruits, upon the final water content. The fruit product usually will not spoil when the aw is below 0.65, even when high populations of contaminating microorganisms are present (Corry, 1987). At water activities above 0.70, spoilage may occur although at the lower aw values, factors such as numbers and kinds of microorganisms, storage temperature, pH, and the presence of preservatives all play a role. Often, growth of spoilage organisms is slow, and, thus, a desired shelf life can be achieved.

12.10.1 FRUIT JUICE CONCENTRATES Those of 70∞ Brix or higher are usually biologically stable. Water activities for three concentrates are: apricot 40∞ Brix = 0.93, grape 30∞ = 0.962, and orange 72∞ = 0.735 (Chen, 1987). Retail packs of juice concentrates such as orange and apple are generally at 45∞ Brix and have an aw of about 0.90. Because this is not sufficiently low to inhibit many yeasts and molds, these foods are marketed as frozen concentrates.

12.10.2 PRESERVES Jams, jellies, and related products have a soluble solids content of 65%, mainly sugar, and a water activity of 0.75 to 0.86. After cooking the fruit-sugar-pectin mixture to form a gel, the product is put into containers at a temperature of 88∞C or higher. The containers are then tipped to sterilize the cap or the top is sprayed with hot water.

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Most osmotrophic yeasts and molds possess little heat resistance, and, thus, the preserves are commercially sterile until the seal is broken. After opening, growth of chance contaminants is usually prevented by storing the jam or jelly at a refrigeration temperature. Dietetic products have higher water activities and thus permit growth of a wider range of microorganisms. Their spoilage is prevented by including a preservative such as benzoic acid or sorbic acid.

12.10.3 DRIED FRUITS In the U.S., the more widely dried fruits are raisins, prunes, dates, apples, figs, apricots, and peaches. Many are sun-dried to yield a product with less than 25% water. Dehydrated fruits, which often are utilized for reprocessing, have received a heat process that reduces the final water content to 1 to 5%. The aw of most sun-dried fruits, including raisins, is less than 0.65, and thus they do not support the growth of microorganisms. It is a common practice to treat fruits with 1000 to 4000 mg/l sulfur dioxide prior to drying to inhibit browning (Somogyi and Luh, 1986). This treatment also destroys many of the microorganisms that were originally present on the surface of the fruit. High-moisture dried prunes, which typically contain 35% water and have an aw of 0.94, would be classified as an intermediate moisture food. Growth of yeasts and molds in these foods is prevented by the addition of up to 0.1% sodium benzoate or potassium sorbate (Schade et al., 1973).

12.10.4 OSMOTOLERANT FUNGI Yeasts and molds are usually responsible for the spoilage of low aw fruit products. Although some bacteria can grow at high osmotic pressures, they are not acid-tolerant and thus are more likely to spoil more neutral foods such as salted fish. Zygosaccharomyces rouxii is perhaps the most osmotolerant yeast. Some strains can grow at an aw as low as 0.62 (Corry, 1987). It is not a true osmophile, in that most strains grow best at water activities above 0.90. Other species of Zygosaccharomyces, Torulopsis, and Hansenula are also osmotrophic (Tilbury, 1980). Some of the more osmotolerant molds are Xeromyces bisporus (minimum aw = 0.605), Chrysoporium xerophilum (aw = 0.686), and members of the Aspergillus glaucus group (aw = 0.71–0.77). Dried prunes were a source of some of these isolates (Pitt and Christian, 1968). Eurotium herbariorum (teleomorph: Aspergillus glaucus) is a potentially important mold in fruit preservation because, in addition to its osmotic tolerance, it produces ascospores that have some degree of heat resistance (Stadler, 1985). A culture isolated from spoiled grape jelly yielded optimal growth in media containing 40 to 60% sucrose, and its ascospores gave a D-value of 5.2 min when heated at 70∞C in 65∞ Brix Concord grape juice (Splittstoesser et al., 1989). Because the ascospores would have been killed by the hot-fill process, it was believed that contaminated lids were the cause of the spoilage outbreak. At the time of the trouble, the hot water sprays used to sterilize the lids were not functioning properly.

12.11 REFRIGERATION Although many species of yeasts and molds will grow at low temperatures, refrigerated storage is a practical means for extending the shelf life of fruit juice beverages, melon sections, and mixtures of fruit that may include citrus sections, berries, pineapple, and pieces of melon. Low temperatures lengthen the microbial lag phase, the dormant period before cell multiplication is initiated, and also retard the growth rate. Generally, the length of the lag phase is inversely related to the size of the microbial population. Refrigerated apple cider that has an initial yeast count of 104 to 105/ml may have a shelf life of only several weeks, compared to several months for a previously pasteurized, low-count orange juice that had been repackaged under very clean

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conditions. Thus, refrigeration is most successful when the initial population of spoilage organisms is low. The shelf life of refrigerated juices can be extended by the addition of preservatives, usually benzoic or sorbic acids. This will be discussed in the next section.

12.12 ANTIMICROBIALS Various compounds are used to extend the shelf life of fruit products. As will be discussed in this section, many are used in conjunction with other preservation procedures.

12.12.1 SULFUR DIOXIDE One of our earliest preservatives, SO2, is used in fruit products to prevent enzymatic and nonenzymatic browning reactions, to destroy microorganisms, and as a bleach for specialties such as maraschino cherries (Joslyn and Braverman, 1954; Roberts and McWeeney, 1972; Taylor et al., 1986). It is its undissociated, molecular form that is germicidal. With a pK of 1.81, this means that very little is active in fruit products having a pH above 3.5. Yeasts are believed to be more resistant to SO2 than most species of molds and bacteria. Some reports indicate that certain yeasts can grow in the presence of 1000 mg/l sulfur dioxide although it has not always been clear in the publications as to the concentration of the free SO2 and the amount that was in the undissociated form. Carr et al. (1976) observed that as little as 1.5 mg/l undissociated SO2 inhibited the growth of lactic acid bacteria in fermented cider. High levels of SO2 have been used for the preservation of grape juice at ambient temperature (Potter, 1979). The “brimstone process” involves the addition of 1200 to 2000 mg/l SO2. When the juice is to be fermented following storage, most of the SO2 is removed by flash heating and spraying with nitrogen. The effluent SO2 is trapped in a 10% milk of lime solution. It is claimed that the process yields a high-quality grape juice that can be fermented into wine. Sweet cherries and other fruits are preserved in solutions containing 7,500 to 15,000 mg/l SO2 (Payne et al., 1969). When they are to be made into maraschino cherries, calcium salts are added to promote firmness and to prevent cracking. There is a synergism between SO2 and low-temperature storage of grape juice. Studies on 135 juices showed that as little as 200 mg/l SO2 prevented spoilage of juice stored at 5∞C, providing the initial yeast population did not exceed 106/ml (Splittstoesser and Mattick, 1981). At the end of the 30-week storage period, the juices that had not fermented were completely free of viable yeasts.

12.12.2 BENZOIC

AND

SORBIC ACIDS

These compounds are widely used for the preservation of a variety of fruit products, including juices and juice beverages, cider, fresh fruit cocktails, dried fruit, jams and jellies, and wines. The two preservatives are discussed together because their mode of action appears to be similar, and thus they can substitute for each other in food systems. It is believed that their inhibitory mechanism involves cell membranes and the transport of substrates. Depletion of ATP appears to be responsible for the inhibition of Zygosaccharomyces cerevisiae (Warth, 1991). Both compounds are lipophilic acids with the undissociated molecule being the active form. At pH 3.5, a common value for fruit products, 83% of the benzoic acid is undissociated, compared to 95% undissociated sorbic acid. At pH 5.5, only 4.7% of the benzoic acid is active, compared to 15% of the sorbic acid. These differences are not always appreciated when the effectiveness of the preservatives is compared. In addition to pH, other properties of a food may influence the effectiveness of these preservatives. The percentage of ethanol in a table wine affects the concentration of sorbic acid needed to prevent refermentation by yeasts. A wine containing 10% ethanol required over 250 mg/l sorbic

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acid for stability, compared to only 100 mg/l when the ethanol content was 12% (Splittstoesser et al., 1978). In general, fungi are more sensitive to these preservations than are bacteria. Bell et al. (1959) observed that 66 species of fungi were inhibited by 0.1% sorbate in a pH 4.5 broth, while 20 out of 39 lactic acid bacteria grew under these conditions. Gluconobacter oxydans is another resistant bacterium. Some strains can grow in the presence of 1000 mg/l sorbic acid in a medium of pH 3.5 to 3.8 (Eyles and Warth, 1989; Splittstoesser and Churey, 1992). Some of the most resistant yeasts (strains that can grow in the presence of over 1000 mg/l sorbic acid) are Zygosaccharomyces bailli, Candida parapsilosis, and Pichia membranaefaciens (Deak et al., 1992). Yeasts and gluconobacters that possess resistance to sorbic acid are not uncommon on grapes (Splittstoesser and Churey, 1992). About 50% of 111 grape samples yielded counts that ranged from 10 to 104/g. Although there is little published information, it is likely that these microorganisms are also common contaminants of other fruits. Some of the microorganisms that spoil fruit products can degrade these preservatives. Lactic acid bacteria reduce sorbic acid to form 2-ethoxyhexa-3,5 diene, a compound that imports a geranium-like off-odor to wines (Crowell and Guymon, 1975; Edinger and Splittstoesser, 1986). Certain Penicillium species degrade potassium sorbate (Marth et al., 1966), while Aspergillus niger metabolizes benzoate (Shailubhai et al., 1982).

12.12.3 DIMETHYL DICARBONATE

AND

DIETHYL DICARBONATE

These compounds have been proposed as sterilants for wines, fruit juices, and related fruit products (Ough, 1993); however, because diethyl dicarbonate reacts with ammonia to form ethyl carbamate, a carcinogen, its use in foods was banned in 1972. Currently, 200 mg/l dimethyl dicarbonate (DMDC) can be added to wines and fruit juices to prevent their spoilage by bacteria and yeasts. The compound has a potential for use in fruit juices and other beverages. Vegetative cells and mold conidia are very sensitive to the dicarbonates. On the other hand, heat-resistant structures such as the ascospores of Byssochlamys fulva are quite resistant and, thus, are likely to survive an exposure to 200 mg/l DMDC (Splittstoesser and Wilkinson, 1973; Van der Riet et al., 1989). When added to a beverage, the dicarbonates rapidly hydrolyze to the corresponding alcohol and carbon dioxide. The challenge is to destroy the contaminating microorganisms before hydrolysis is completed. Effective metering and mixing equipment is required, and the numbers of contaminating microorganisms must be low.

REFERENCES Anon. 1975. Salmonella typhimurium outbreak traced to commercial apple cider. Morbidity and Mortality Weekly Report, 24: 87. Anon. 1979. Salmonella oranienburg gastroenteritis associated with consumption of precut watermelons. Morbidity and Mortality Weekly Report, 28: 522–523. Barnett, J.A., Payne, R.W., and Yarrow, D. 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge, U.K. Bayne, H.G. and Michener, H.D. 1979. Heat resistance of Byssochlamys ascospores. Appl. Environ. Microbiol., 37: 449–453. Beech, F.W., Davenport, R.R., Goswell, R.W., and Burnett, J.K. 1968. Two simplified schemes for identifying yeast cultures. In Identification Methods for Microbiologists, Part B, B.M. Gibbs and D.A. Shapton (Eds.), Academic press, London, pp. 151–175. Bell, T.A., Etchells, J.L., and Borg, A.F. 1959. Influence of sorbic acid on the growth of certain species of bacteria, yeasts, and filamentous fungi. J. Bacteriol., 77: 573–580.

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Berry, J.M. 1979. Yeast problems in the food and beverage industry. In Food Mycology, M.E. Rhodes (Ed.), G.K. Hall, Boston, pp. 82–90. Besser, R.E., Lett, S.M., Weber, J.T., Doyle, M.P., Barrett, T.J., Wells, J.G., and Griffin, P.M. 1993. An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA, 269: 2217–2220. Beuchat, L.R. 1981a. Combined effects of solutes and food preservatives on rates of inactivation of and colony formation by heated spores and vegetative cells of molds. Appl. Environ. Microbiol., 41: 472–477. Beuchat, L.R. 1981b. Synergistic effects of potassium sorbate and sodium benzoate on thermal inactivation of yeasts. J. Food Sci., 46: 771–777. Beuchat, L.R. 1988a. Thermal tolerance of Talaromyces flavus ascospores as affected by growth medium and temperature, age, and sugar content in the inactivation medium. Trans. Br. Mycol. Soc., 90: 359–364. Beuchat, L.R. 1988b. Influence of organic acids on heat resistance characteristics of Talaromyces flavus ascospores. Intl. J. Food Microbiol., 6: 97–105. Biely, P. and Slavikova, E. 1990. Distribution of pectin-degrading activities among yeasts. Zentralblatt f. Mikrobiologie, 145: 345–351. Blackwood, M. 1969. Etude des bacteries acetiques isolees de raisins et de vins. I. Identification de souches isolees. Connaiss. Vigne. Vin, 3: 227–241. Blocher, J.C. and Busta, F.F. 1983. Bacterial spore resistance to acid. Food Technol., 37: 87–99. Carr, J.G. 1958. Lactic acid bacteria as spoilage organisms of fruit juice products. J. Appl. Bacteriol., 21: 267–271. Carr, J.G. 1968. Methods for identifying acetic acid bacteria. In Identification Methods for Microbiologists. B.M. Gibbs and D.A. Shapton (Eds.), Academic Press, London, pp. 1–8. Carr, J.G. 1970. Tetrad-forming cocci in ciders. J. Appl. Bacteriol., 33: 371–379. Carr, J.G., Davies, P.A., and Sparks, A.H. 1976. The toxicity of sulfur dioxide towards certain lactic acid bacteria from fermented apple juice. J. Appl. Bacteriol., 40: 201–212. Carr, J.G. and Passmore, S.M. 1979. Methods for identifying acetic acid bacteria. In Identification Methods for Microbiologists, 2nd ed., F.A. Skinner and D.W. Lovelock (Eds.), Society for Applied Bacteriology, Tech Series 14, Academic press, London, pp. 33–47. Cerny, G., Hennlich, W., and Poralla, K. 1984. Spoilage of fruit juice by bacilli: isolation and characterization of the spoilage organism. Z. Lebensm. Unters. For., 179: 224–227. Chakrabarti, D.K. and Ghosal, S. 1986. Occurrence of free and conjugated 12, 13-epoxytrichothecenes and zearalenone in banana fruits infected with Fusarium moniliforme. Appl. Environ. Microbiol., 51: 217–219. Chen, C.S. 1987. Calculation of water activity and activity coefficient of sugar solutions and some liquid foods. Lebensm. Wiss. Technol., 20: 64–67. Cichowicz, S.M. and Eisenberg, W.V. 1974. Collaborative study of the determination of Geotrichum mold in selected canned fruits and vegetables. J. Assoc. Offic. Anal. Chem., 57: 957–960. Corry, J.E.L. 1976. The effects of sugars and polyols on the heat resistance and morphology of osmophilic yeasts. J. Appl. Bacteriol., 40: 269–276. Corry, J.E.L. 1987. Relationships of water activity to fungal growth. In Food and Beverage Mycology, 2nd ed., L.R. Beuchat (Ed.), Van Nostrand Reinhold, New York, pp. 51–99. Crowell, E.A. and Guymon, J.F. 1975. Wine constituents arising from sorbic acid addition, and identification of 2-ethoxyhexa-3, 5-diene as source of geranium-like off-odor. Am. J. Enol. Vitic., 26: 97–102. Daly, N.M. 1982. The microbiology of wine corks. B.B. thesis, University New South Wales, Kensington, NSW. Darland, G. and Brock, T.D. 1971. Bacillus acidocaldarius sp. Nov., an acidophilic, thermophilic sporeforming bacterium. J. Gen Microbiol., 67: 9–15. Davis, N.D. and Diener, U.L. 1987. Mycotoxins. In Food and Beverage Mycology, 2nd ed., L.R. Beuchat (Ed.), Van Nostrand Reinhold, New York, pp. 517–570. Deak, T. and Beuchat, L.R. 1987. Identification of foodborne yeasts. J. Food Prot., 50: 243–264. Deak, T. and Beuchat, L.R. 1993. Comparison of the SIM, API 20C and ID 32C systems for identification of yeasts isolated from fruit juice concentrates and beverages. J. Food Prot., 56: 585–592. Deak, T., Reichart, O., Szakmar, K., and Peter, G. 1992. Spoilage yeasts of unusual sorbate resistance. In Modern Methods in Food Mycology, R.A. Samson, A.D. Hocking, J.I. Pitt, and A.D. King (Eds.), Elsevier, Amsterdam, pp. 55–59.

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Food Additives in Fruit 13 Direct Processing Laszlo P. Somogyi CONTENTS 13.1 13.2 13.3 13.4

13.5

13.6 13.7 13.8 13.9

13.10

13.11

13.12

13.13

Introduction ........................................................................................................................286 13.1.1 Definition and Functions .....................................................................................286 Food Additive Categories...................................................................................................287 Industry Segments..............................................................................................................287 Functions of Direct Food Additives ..................................................................................288 13.4.1 Preservation..........................................................................................................288 13.4.2 Processing ............................................................................................................288 13.4.3 Appeal and Convenience .....................................................................................288 13.4.4 Nutrition...............................................................................................................289 Government Regulations....................................................................................................289 13.5.1 U.S. ......................................................................................................................289 13.5.2 Europe ..................................................................................................................290 13.5.3 Japan ....................................................................................................................291 Trends, Issues .....................................................................................................................291 Food Additive Industry Structure ......................................................................................292 Utilization of Food Additives ............................................................................................292 Sweeteners..........................................................................................................................293 13.9.1 Functions..............................................................................................................293 13.9.2 Products, Applications, and Regulatory Status ...................................................294 13.9.3 High-Intensity Sweeteners...................................................................................294 13.9.4 Polyols .................................................................................................................298 13.9.5 Trends...................................................................................................................300 Acidulants...........................................................................................................................300 13.10.1 Functions..............................................................................................................300 13.10.2 Products and Applications ...................................................................................301 13.10.3 Regulatory Status.................................................................................................304 Thickeners and Stabilizers .................................................................................................305 13.11.1 Functions..............................................................................................................305 13.11.2 Products................................................................................................................305 13.11.3 Regulatory Status.................................................................................................311 13.11.4 Application of Gums in Fruit Products...............................................................311 Emulsifiers..........................................................................................................................313 13.12.1 Functions..............................................................................................................313 13.12.2 Products and Applications ...................................................................................313 13.12.3 Regulatory Status.................................................................................................314 Flavors ................................................................................................................................315 13.13.1 Products and Function .........................................................................................315

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13.13.2 Technology of Manufacturing .............................................................................315 13.13.3 Regulatory Status.................................................................................................317 13.13.4 Use of Flavors in Processed Fruits .....................................................................318 13.14 Colors .................................................................................................................................318 13.14.1 Functions..............................................................................................................318 13.14.2 Products................................................................................................................318 13.14.3 Use of Color Additives in Fruit Products ...........................................................321 13.14.4 Trends and Regulatory Status..............................................................................321 13.15 Vitamins .............................................................................................................................323 13.15.1 Functions..............................................................................................................323 13.15.2 Products................................................................................................................324 13.15.3 Applications .........................................................................................................325 13.15.4 Regulatory Status.................................................................................................326 13.16 Preservatives.......................................................................................................................326 13.16.1 Functions..............................................................................................................326 13.16.2 Products................................................................................................................327 13.16.3 Regulatory Status.................................................................................................328 13.16.4 Trends and Issues ................................................................................................328 13.17 Antioxidants .......................................................................................................................329 13.17.1 Functions..............................................................................................................329 13.17.2 Products and Applications ...................................................................................329 13.17.3 Technology and Manufacture ..............................................................................333 13.17.4 Regulatory Status.................................................................................................333 13.17.5 Trends and Issues ................................................................................................334 13.18 Enzymes .............................................................................................................................334 13.18.1 Products and Functions .......................................................................................334 13.18.2 Applications in Fruit Processing .........................................................................335 13.18.3 Technology and Manufacture ..............................................................................336 13.18.4 Trends, Issues, and Developments ......................................................................337 References ......................................................................................................................................337

13.1 INTRODUCTION 13.1.1 DEFINITION

AND

FUNCTIONS

The broadest practical definition of food additive is any substance that becomes part of a food product either directly or indirectly during some phase of processing, storage, or packaging. Direct food additives that are discussed in this chapter are those that have been intentionally added to food for a functional purpose in controlled amounts, usually at low levels (from parts per million to 1 to 2% by weight). Indirect additives, on the other hand, are those entering into food products in small quantities as a result of growing, processing, or packaging. Examples of these are lubricating oils from processing equipment or components of packaging material that migrate into food before consumption. Direct food additives are used in foods for six main reasons: • • • •

To To To To

ensure microbial safety (e.g., against botulism and listeria) maintain product consistency improve or maintain nutritional value extend shelf life (e.g., retard the onset of rancidity)

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• •

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To facilitate food processing (e.g., provide leavening or control acidity) To enhance flavor or texture, or impart desirable color

The differences between food ingredients and additives are mainly that of quantity used in any given formulation. Food ingredients can be consumed alone as food (e.g., sucrose and butter), whereas food additives are used in small quantities relative to the total food consumption but which nonetheless play a large part in the production of desirable and safe food products.

13.2 FOOD ADDITIVE CATEGORIES In the U.S., over 2500 chemicals, including more than 1500 aroma chemicals, are permitted for direct use. When intentionally added to food at very low levels, additives perform specialized functions that favorably impact the economics of processing, formulating, and distributing food products. Included in the overall direct food additive category are: • • • •

Inorganic chemicals — phosphates, sulfites, salt, etc. Synthetic chemicals — dyes, silicones, benzoates, vitamin A, etc. Extraction products from natural sources — gums, essential oils, vitamin E, etc. Fermentation-derived products — enzymes, yeast, citric acid, biogums, etc.

Fruits are important raw material sources for the manufacture of many food additives. Examples include citrus oil and essence as flavor; color from red grape skin; tartaric acid from grape juice; pectin from apple and citrus pulp; and enzymes from pineapple, papaya, and figs. Most food additives have a long history of use; others are the result of recent research and development to fill particular requirements of modern food processing. Some are common chemicals of industry that are upgraded in terms of purity to allow their use in food. There is much discussion about whether a food additive is natural or synthetic. The fact is that this classification, in many instances, has become somewhat arbitrary. Many food additives synthesized in chemical laboratories are also found occurring naturally in normal food. Vitamin C (ascorbic acid) produced by a chemical process is the same chemical that is found in oranges. Similarly, citric acid that is today produced commercially by enzymatic fermentation of sugars is the same chemically as the naturally occurring chemical that has been found to make lemons and limes tart. The practice of adding chemicals (e.g., spices, herbs, vinegar, and smoke) to food dates back many centuries. In recent years, however, the ubiquitous presence of chemical additives in processed foods has attracted much attention and public concern over the long-term safety of additives to man. Although the safety issue is far from subsiding, there is scientific consensus that food additives are indispensable in the production, processing, and marketing of many fruit products. The judicious use of chemical additives — typically in the range of a few parts per million to less than 2% by weight of the finished product — contributes to the abundance, variety, stability, microbiological safety, flavor, and appearance of the food supply. While direct food additives offer a major contribution to the palatability and appeal of a wide variety of foods, their level of use is relatively insignificant in the total human diet.

13.3 INDUSTRY SEGMENTS The food additive business comprises more than 20 segments. Ten major product categories of food additives and their functions are presented in Table 13.1 (Somogyi, et al., 1996). Compounds in these food additive categories that are utilized in fruit processing will be covered in this chapter.

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TABLE 13.1 Food Additives and Their Functions in Processed Fruit Products Food Additive

Nutritional Value

Preservation, Protection

Production Improvement

Appeal Modification

X X

X

X

X X

X X

X X X X X X

Acidulants Antioxidants Colors Emulsifiers Enzymes Flavors Nutrients Preservatives Sweeteners Thickeners/stabilizers

X X

X X X

X X

Within the same category, products may belong to several chemical classes and offer specialized functionality (e.g., water- and oil-soluble antioxidants that include ascorbic acid and hindered phenols; water-soluble azo-dyes, and water-dispersible carotenoid as food colors).

13.4 FUNCTIONS OF DIRECT FOOD ADDITIVES Direct food additives serve several major functions in foods. Many additives, in fact, are multifunctional. The basic functions are preservation, processing, appeal and convenience, and nutrition.

13.4.1 PRESERVATION Food preservation techniques have advanced in the past 100 years and now include thermal processing, concentration and drying, refrigeration and freezing, modified atmosphere, and irradiation. However, the use of chemical preservatives frequently augment these basic preservation techniques and represent the most economical way for food manufacturers to ensure a reasonable shelf life for the product. Antioxidants and antimicrobial agents perform some of these functions as well.

13.4.2 PROCESSING Food processors are increasingly using food additives to insure the integrity and appeal of finished products. Emulsifiers maintain mixtures and improve texture in breads, dressings, and other foods. They are used in ice cream when smoothness is desired, in breads to increase shelf life and volume and to distribute the shortening, and in cake mixes to achieve batter consistency. Stabilizers and thickeners assist in presenting an appealing, consistently-textured product. Sorbitol, a humectant and sweetener, is used to retain moisture and enhance flavor. With the removal of sugar from many foods for dietetic reasons, a bulking agent substitute such as polydextrose is growing in importance.

13.4.3 APPEAL

AND

CONVENIENCE

The changing eating habits of consumers, partly brought about by the large increase in the percentage of women who work outside the home, is creating a growing need for convenience foods. In many of these types of foods, it is essential that a variety of additives be used to provide the taste, color, texture, body, and general acceptability that are required. This need for convenience, while maintaining aesthetic appeal and taste, is becoming extremely important. Most food additives

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such as gums, flavoring agents, colorants, and sweeteners are included by food processors because consumers in the developed countries demand that food look and taste good as well as being easy and safe to serve.

13.4.4 NUTRITION There have been tremendous advances in knowledge of human nutrition, and consumers are increasingly aware of the value of good nutrition. Vitamins, antioxidants, proteins, and minerals are added to foods and beverages as supplements in an attempt to ensure proper nutrition for those who do not eat a well-balanced diet. In addition, additives such as antioxidants are often used to prevent deterioration of natural nutrients during processing. Recently there is more importance attributed to disease prevention through proper nutrition, as well as to increasing performance through sport nutrition products. On the other hand the medically based desire for good nutrition through a balanced diet may adversely affect consumer demand for some food additives such as fat substitutes.

13.5 GOVERNMENT REGULATIONS The application of food additives is highly regulated worldwide although regulatory philosophy, the approval of specific products, and the level of enforcement differ from country to country.

13.5.1 U.S. In the U.S., the Food and Drug Administration (FDA) has the primary jurisdiction over food additives, although clearance for use of food additives in certain products must be obtained from other government agencies as well. For example, pesticides used on raw agricultural products are covered by the Federal Insecticide, Fungicide, and Rodenticide Act that was issued in 1972 and amended in 1988. In the U.S. (Code of Federal Regulations, 2002), the only additives that can be marketed for food are: • •

Included in an approved list and comply with existing specifications for food grade material Received pre-marketing clearance by the FDA

For regulatory purposes, all food additives fall into one of three categories: • • •

Prior sanctioned substances Generally Recognized As Safe, known as GRAS substances Regulated direct and indirect additives

Prior sanctioned substances (approximately 1400) are products that already were in use in foods prior to the 1958 Food Additives Amendment of the Federal Food, Drug and Cosmetic Act and therefore are considered exempt from the approval process. Some prior sanctioned substances also appear in the GRAS list. This is the “grandfather clause” of the amendment. GRAS substances (approximately 700 compounds) are a group of additives regarded by qualified experts as safe because their extensive use in the past has not shown any harmful effects. The FDA is involved in an ongoing review of GRAS and prior-sanctioned lists to ensure that these substances are tested by the latest scientific methods. Likewise, the FDA also reviews substances that are not currently included on the GRAS list to determine whether they should be added.

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All other additives are regulated, meaning that a specific food additive petition must be filed with the FDA, requesting approval for use of the additive in any application not previously approved. A food or color additive petition must provide convincing evidence that the proposed additive performs as it is intended. Animal studies using large doses of the additive for long periods are necessary to show that the substance would not cause harmful effects at expected levels of human consumption. In deciding whether an additive should be approved, the agency considers the composition and properties of the substance, the amount likely to be consumed, its probable long-term effects, and various other safety factors. Long delays (>5 years) and the high cost of regulatory clearance limit new introductions to only a handful of products that have been successfully petitioned in the past decade. In addition, the FDA operates the Adverse Additive Monitoring System to help as an ongoing safety check for all additives. The system monitors and investigates all complaints by individuals or their physicians that are believed to be related to specific foods, food additives, or dietary supplements. All natural and synthetic color additives are subject to the Color Additive Amendment of 1960 and are not included in the food additive regulations. Flavor substances are regulated somewhat differently, and rules are less restrictive. In the U.S., there is a comprehensive list of flavor substances that are deemed GRAS, based on history of their use, review of available toxicology, and the opinion of experts. These GRAS lists (through GRAS list 21 by Smith et al., 2003) have been compiled since 1970 by the expert panel of Flavor Extracts Manufacturers Association of the U.S. (FEMA). Over the years, over 1800 compounds were deemed GRAS by the FEMA panel. Founded in 1909, FEMA is an industry association that originally started pursuing voluntary self-regulation and later was granted quasi-official status on regulatory matters regarding flavor chemicals by FDA. The FEMA expert panel was formed in 1960. This independent panel, composed of eminently qualified experts recruited from outside the flavor industry, has expertise in human nutrition, physiology, metabolism, toxicology, and chemical structure–activity relationships.

13.5.2 EUROPE Food additives are regulated by the European Parliament and Council Directive 89/107/EEC of December 21, 1988. The Directive requires that all permitted food additives be assessed by the European Scientific Committee for Food (SCF) for their safety against the criteria that are listed in the Annex of the Directive (Goodburn, 2001). The Directive provides for the adoption of specific directives outlining positive lists of authorized food additives and conditions for their use. The three specific directives were adopted in 1994 and 1995. Since then all regulations relating to the use of additives have been the same in all 15 EU member states. Previously, each country had widely different rules. The three specific directives are: • • •

Sweeteners: Directive 94/35/EC of June 1994 Food colors: Directive 94/36EC of June 1994 Food additives other than colors and sweeteners: Directive 95/2/EC of February 1995

Since 1988, several amendments to, and adaptations of, Directive 89/107/EEC have been introduced or proposed. These efforts have been toward a uniform registration process so that a registration obtained in one country would be valid in all EU member countries. The new EU food additive law, however, will not prevent individual countries from asking for additional or country-specific requirements

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for new product registrations. At the EU level, several institutions and groups are involved in the development of food additives law, including the Scientific Committee for Food (SCF), one of the institutions of the European Commission (the Union’s executive body) which deals with safety issues; representatives from different national professional organizations, representatives from the food industry, and retailers, etc. The Standing Committee on Foodstuffs ensures close cooperation between the Commission and the member states. The EU rules for the evaluation, marketing, and labeling of novel food such as genetically modified foods are also being developed. Labels need to be carried by foods containing genetically modified living organisms such as yogurt containing an altered culture and any food whose modification might raise moral or health worries to consumers. A tomato, for example, containing a strawberry protein would have to carry a label to alert consumers allergic to strawberries. However, novel foods that are identical to conventional foods (e.g., sugar from beet genetically modified to resist disease) would not have to be labeled.

13.5.3 JAPAN In Japan, the Food Chemistry Division of the Ministry of Health and Welfare (MHW) has jurisdiction over food additives through the Food Sanitation Law that was enacted in January 1948, with several amendments adopted since then. Amendments to the regulations as well as additions or deletions to Kohetisho (the Japanese Codex of Food Additives) were mostly influenced by two major objectives: (1) protection of food sanitation and customer safety, and (2) harmonization with international regulatory requirements (Anon., 1994a). Most discussions on regulating food additives in Japan have been related to defining what food additives should be under legal restriction, and on labeling requirements. Very often in these discussions, differentiating “synthetic” and “natural” food additives had been at issue. In Japan, those two generally used terms have often misled customers into a blind belief in natural food additives. However, regulatory bodies as well as the food additive industry no longer distinguish additives with these terms. On the other hand, the problem of environmental pollution, particularly by chemical companies, was brought into public consciousness and criticism of chemical substances became severe. A negative image of chemical substances spread over the public, including chemicals used in food. Because the term chemical caused a negative reaction, industry began to use “non-chemical” or “natural” for substances that exist in natural products. The public also preferred “natural” as an opposite to “synthetic,” believing that “natural” meant “better.” Industry put great effort into development of natural substances instead of new chemicals because there was little legal restriction on natural food additives while there were very strict legal requirements on chemical additives. Since May 1996 natural food additives have been regulated under the Food Sanitation Law (Somogyi et al., 1996). However, a large number of “natural” food additives that are listed as “existing food additives” remain outside of legislation. The list of existing “nonchemically synthesized” food additives that was finally confirmed by the MHW on April 16, 1996 includes 489 “existing natural” food additives. Today both synthetic and natural food additives need to be declared on the labeled.

13.6 TRENDS, ISSUES While there are many differences in food tastes and preferences among the different regions of the world, the major trends driving the food additive utilization appear to be very similar in all regions: • • •

Concern over safety of processed food Health consciousness; desire for low-calorie foods Desire for convenience

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• • • • •

Desire by consumers, as well as food manufacturers, for value-added products Increasing regulatory constraints Shift from synthetic to natural- or semi-natural-based products Increasing costs of new products in terms of R&D and product commercialization Expansion of food service products (fast-food, airline meals, etc.)

New and improved technologies for the most part aim at processing and preserving fruits with minimal use of chemical additives. Aseptic packaging, controlled atmosphere packaging, irradiation, and membrane separation/microfiltration processes are typical examples of this trend. Conversely, microwave heating for both industrial and home use and extrusion represent technologies that are likely to stimulate expanded use of additives such as less volatile flavors more compatible with microwave heating and specialized blends of gums/emulsifiers for the reformulation of innovative extruded food products. A shift away from commodity to more processed, higher value food products will favor increased use of food additives for processing. Increasingly, sales of ingredient-and-additive blends will dominate in the future. The synergistic effects that enhance the functionality of these materials while reducing the quantity needed will play an ever more significant role in formulated foods. Information on these blends will be scarce because they will be developed in-house by food additive suppliers as well as food manufacturers wishing to maintain confidentiality in order to optimize exclusive commercial benefit. China’s increasing ability to produce and export acceptable-quality food additives pushed prices and profitability downward for some previously attractive additives like vitamin C, sodium erythorbate, and benzoates. The Republic of Korea and India may prove to be factors in the near future as well. Other issues affecting the food additive industry include increasing government regulatory activity, increasing R&D and legal expenses, and the great length of time needed to perfect, gain approval, and market new food additive products.

13.7 FOOD ADDITIVE INDUSTRY STRUCTURE Food additive suppliers are an important part of the food manufacturing system, marketing products to both commodity processors and food processors. Figure 13.1 depicts the food additive industry structure and flow of products. Some 60 to 70% of food additives are used in the manufacturing of food; 20 to 30% are used in commodity processing operations such as fruit juice processing, vegetable packing, oil seed crushing, flour milling, meat packing, etc.; and the remaining 5 to 10% in other uses such as pharmaceutical and personal care products.

13.8 UTILIZATION OF FOOD ADDITIVES The technical need and level of utilization of food additives vary within each food manufacturing sector served, depending on the functional requirements of the application (e.g., microbial preservation, preventing oxidation, improving color, flavor, nutrition qualities, etc.) and the strength and performance characteristics of the additive (e.g., solubility, heat and light stability, chemical compatibility). Fruit processing operations that include processing, compounding, packaging, and preservation require the use of a variety of food additives. In general, the level of direct food additive use in processed fruits is small in comparison with other food industry sectors. In particular, formulation of dairy products, baked goods, snack foods, or confections require greater use and a wider variety of food additives. In Table 13.2 the extent of food additive utilization in various fruit processing operations is illustrated.

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COMMODITY PROCESSOR • Sugar refining • Grain milling • Slaughtering • Oil seed processing

20–30%

293

FOOD ADDITIVE MANUFACTURERS

5–10%

• Acidulants • Emulsiltors • Flavors, colors, etc. 60–70%

DISTRIBUTORS BLENDERS

FOOD MANUFACTURERS

REFINED INGREDIENTS • Flour • Sugar • Fats & oils • Fruit juice, etc.

• Canned food • Snack food • Dairy products • Bread, cookies, etc.

30%

70% RETAIL FOOD STORES • Produce • Processed food

OTHER USES • Pharmaceutical • Cosmetics • Animal feed, etc.

FOOD SERVICE • Fast food meals • Hospital food • Airline meals, etc.

FIGURE 13.1 Food additives: pattern of use in food and other applications.

TABLE 13.2 Unit Operations in Fruit Processing and the Need for Food Additives Unit Operation

Need for Additives

Preparation of Fruits for Processing Crushing Low Shelling Low Washing/cleaning Low Extraction/refining Medium Clarification Medium Concentration Medium Cutting/sizing Low Drying Low Controlled atmospheric storage Low Manufacturing Fruit-Based Compounding/formulating Extruding Canning/aseptic pack Freezing Dehydrating Fermentation Irradiation

Products High High Low Medium Medium Medium–high Low

13.9 SWEETENERS 13.9.1 FUNCTIONS Sweeteners are used in fruit processing for many functional reasons, as well as to impart sweetness. They render certain fruits palatable, and add flavor, body, bulk, and texture. They change the freezing

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point, control viscosity (which contributes to texture), and prevent spoilage. Certain sweeteners bind moisture that is required by detrimental microorganisms in fruits. Alternatively, some sweeteners can serve as food for fermenting organisms that produce acids that preserve the fruit. These auxiliary functions must be kept in mind when considering applications for artificial sweeteners. Sweeteners may be classified in a variety of ways: • • •

•

Nutritive or nonnutritive: Materials either are metabolized and provide calories or are not metabolized and thus are noncaloric. Natural or synthetic: Commercial products that are modifications of a natural product, e.g., honey or crystalline fructose, are considered natural. Regular or low-calorie/dietetic/high-intensity: Although two sweeteners may have the same number of calories per gram, one may be considered low-calorie or high-intensity if less material is used for equivalent sweetness. As foods: Fruit juice concentrates or low moisture fruit powders, for example, can impart substantial sweetness.

13.9.2 PRODUCTS, APPLICATIONS,

AND

REGULATORY STATUS

Sucrose, commonly known as table sugar (or refined sugar) is the standard against which all sweeteners are measured in terms of quality taste and taste profile. It is consumed in the greatest volume of all sweeteners; however, sucrose, high fructose corn syrup, dextrose, and other natural sweeteners such as molasses, honey, maple syrup, and lactose sweeteners are normal ingredients (e.g., they can be consumed alone as food) and are not considered additives (Wodicka, 1980) and, therefore, will not be covered in this chapter. The discussion that follows concentrates on the highintensity sweeteners (e.g., saccharin, aspartame, acesulfame K, etc.) and polyol alternative sweeteners (e.g., xylitol, sorbitol, etc.).

13.9.3 HIGH-INTENSITY SWEETENERS High-intensity sweeteners are manifoldly sweeter than sucrose (30 times greater or more) and closely mimic its sweetness profile (Anon., 1992). Because of the very low use levels, however, high-intensity sweeteners cannot perform other auxiliary functions in food and often must be used in conjunction with other additives such as bulking agents. High-intensity sweeteners, once used mainly for dietetic purposes, are now used as food additives in a wide variety of fruit products such as fruit drinks, dry fruit beverage bases, jams, jellies, and other fruit desserts (O’Brien Nabors and Galardi, 1991). With the obesity epidemic and the public attention it currently receives, sweeteners that allow for consumer-appealing labeling are of particular interest. These sweeteners when used alone or in combination may permit such labeling as “low-calorie,” “reduced calorie,” “light,” “sugar-free,” and “does not promote tooth decay” (O’Brien Nabors, 2002). Approval of high-intensity sweeteners differs from country to country. At present, four highintensity sweeteners are in use in the U.S.: aspartame, acesulfameK, sucralose, and saccharin. In addition, tagatose received approval by the FDA in 2002. Cyclamates are also permitted in Canada. Inside the European Community, six sweeteners have been generally approved: acesulfameK, aspartame, cyclamate, neohesperidin, saccharin, and thaumatin. In Japan, saccharin, aspartame, and the plant extracts glycyrrhizine and stevioside are widely used certified food additives. The relative sweetness and regulatory status potency of several important, current and proposed sweeteners are given in Table 13.3. 13.9.3.1 Alitame Alitame is approved in Australia, New Zealand, Chile, Colombia, China, Indonesia and Mexico for use in food, beverages, and as table top sweeteners. Approval is pending in the U.S., Japan, Brazil, and Canada.

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TABLE 13.3 Sweeteners and Regulatory Status of Sweeteners in Selected Regions Regulatory Statusa

Sweetener

Sweetnessb Relative to Sucrose

U.S.

Canada

European Community

Japan

Sugar (sucrose) High-fructose corn syrup Crystalline fructose Dextrose Xylitol Mannitol Sorbitol Lactitol Sodium Cyclamate Aspartame Acesulfame K Saccharin Sucralosec Alitamed Thaumatine Neohesperidine Stevioside Glycyrrhizine Neotame Tagatose Erythritol Isomalt Lactitol Maltitol Mannitol Sorbitol Xylitol

1 1–1.5 1.2–1.7 0.75–0.85 1 0.7 0.5–0.7 0.3–0.4 30 200 200 300 600 2000 3000 2000 300 50 8000 0.9 0.6–0.7 0.4 0.3–0.4 0.9 0.5 0.6 1

A A A A A A A P P A A A A P A N N N P A A A A A A A A

A A A A A A A P A A A N A P A N N N N N A — A A A A A

A A A A A A A A A A A A A P A A N N N N P A A A A A A

A A A A A A A A N A P A A P A N A A N N A A A A A A A

a

A = approved; P = petition filed; N = not approved. Sweeteness is dependent on several factors, including the concentration of sweetener, temperature, pH, and type of medium used. c Sucralose is approved in Australia and Russia. d Alitame is approved in Australia, Mexico, New Zealand, China, Indonesia, and Mexico. e Glycyrrhizin, Thaumatin, and Neohesperidin are approved as flavorant and flavor enhancer but not as a sweetener in the U.S. b

Source: Sugar and Sweetener Situation and Outlook Report, U.S. Department of Agriculture, Economic Research Service, December 1991, and updated by the author.

13.9.3.2 Acesulfame K (ace-K) In 1998, the U.S. FDA approved the use of Hoechst AG’s acesulfame K (SunetteTM) in nonalcoholic beverages, following a string of earlier approvals for use in: • •

Chewing gum, dry beverage mixes, instant coffee and tea, gelatins, puddings, nondairy creamers, and table sweeteners (1988) Candies and hard and soft caramels (1992)

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• •

Bakery goods, dairy products, syrups, and bulk sweeteners (1994) Alcoholic beverages (1995)

Presently, it is approved for use in foods and beverages in 34 countries. Acesulfame K has a rapidly perceptible sweet taste 200 times as potent as sucrose. It has a good shelf life and is relatively stable across temperatures and pH ranges associated with preparation and processing of foods. One major advantage of ace-K is its synergy with other sweeteners, including nutritive and nonnutritive types. It is being used in canned fruits and sugar-reduced jams and some dry beverage bases. 13.9.3.3 Aspartame Aspartame was approved in the U.S. in 1981 for use in prepared foods, dry beverage mixes, as a table top sweetener, and, in 1983, in liquid soft drinks. Presently, it is approved for use in 75 countries as well. Aspartame is about 200 times sweeter than a 4% sugar solution, and despite the shortcomings of its short shelf life (in beverages), it is the most successful high-intensity sweetener currently used. Worldwide, there are more than 4200 products containing aspartame in 46 product categories. Its role as a food ingredient that enhances fruit flavors makes it highly suitable for soft drinks and yogurt. Aspartame is also used in chilled and frozen desserts, fruit juice beverages, and sugar-free jelly and whips. Two major disadvantages of aspartame are its instability in acidic conditions and its loss of sweetness during prolonged heating. 13.9.3.4 Cyclamates Cyclamates were discovered in the U.S. in 1937, and they started being used as a sweetener in the mid-1950s. In 1970, they were banned from use in all foods and beverages in the U.S., the U.K., and many other countries. But on the basis of recent data, cyclamates have been reapproved in over 50 countries including Canada and most European countries. In the U.S., a petition for reapproval is still pending. Cyclamates are 30 times sweeter than sucrose and have been particularly useful in fruit products because they enhance fruit flavors, even in low concentrations, and can mask the natural tartness of some citrus fruits. Moreover, the cyclamate solutions used for canned fruits have a lower specific gravity and osmotic pressure than sucrose syrups and, hence, do not draw water out of the fruit. Thus, fruit packed in cyclamate solutions tend to have a greater drained weight than those packed in sucrose. Thickening and consistency represent the major problems with jams and jellies sweetened with cyclamate. Low-methoxy pectin is usually employed as a gelling agent in jams and jellies because it does not require sugar for gel formation. Due to the lower concentrations of osmotically active compounds, jams and jellies containing cyclamates may require a preservative to extend their shelf life. 13.9.3.5 Glycyrrhizin The FDA had granted GRAS status to ammoniated glycyrrhizin for use as a flavor enhancer and a natural flavoring agent. Applications include use as a flavorant, flavor modifier, and foaming agent. In Japan, glycyrrhizin is most commonly used as a sweetener. 13.9.3.6 Neohesperidin DC Currently used in Belgium, Holland, and Germany, neohesperidin DC is extracted from the peel of bitter Spanish orange (Citrus aurantium). It offers an outstanding sweetness, being thousands of times sweeter than sucrose; however, its taste is quite different from that of sugar, but neohesperidin works well in low concentrations, coupled with other sweeteners such as aspartame and acesulfame K. Its somewhat limited applications hinge mainly on beverages, confectionery, and desserts. It is especially recommended for fruit juices and nectars.

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13.9.3.7 Saccharin Saccharin has been used as a food additive since the early 1900s and is the most widely used nonnutritive sweetener worldwide. Saccharin is an organic petroleum compound. It is 200 to 700 times sweeter than sucrose in its various forms. It is both inexpensive and very stable in processing and storage but, due to its bitter aftertaste, has limited potential. Because of its less-than-perfect sweetness profile, saccharin tends to be regarded as only a partial replacement for sugars or is used in combination with other high-intensity sweeteners that mask the aftertaste. The market for saccharin in diet drinks has grown alongside the rise to fame of aspartame, but this is now diminishing as manufacturers reformulate drinks to include other blends of high-intensity sweeteners. Other uses include pickles, jams, jellies, stewed fruits, and canned fruits. The FDA took saccharin off the GRAS list in the early 1970s as a result of a study suggesting it caused cancer in rats. A ban on saccharin use in the U.S. was proposed, but stayed by Congress in 1977. In 1991, the FDA withdrew the proposed ban on saccharin. It was reapproved for tabletop, personal care, and pharmaceutical use in Canada in 1992 and a petition has been filed for reapproval in beverages and foods. However, saccharin is sold only in pharmacies in Canada. Saccharin is approved for use in more than 100 countries. 13.9.3.8 Sucralose Sucralose is a highly stable compound derived by the selective chlorination of sucrose and is 600 times sweeter than sugar. In April 1998, sucralose received the broadest initial FDA approval ever given a low-calorie sweetener. Sucralose was approved for use in 15 food and beverage categories including baked goods and mixes, beverages, frozen dairy desserts, jams and jellies, and processed fruits and fruit juices. In August 1999 approval for the use of sucralose as a general-purpose sweetener was granted. Petitions for approval to use it in multiple food and beverage categories are also in file for the EU and Japan. Sucralose is now approved in 34 countries and has been in use since 1991. Marketed under the brand name SplendaTM, it is used in a wide range of products, including jams, jellies, and canned fruits. 13.9.3.9 Stevioside An extract from the South American plant Stevia rebaudiana is approved for sweetening purposes in Japan, the Republic of Korea, and Brazil. It can be used in dietary supplements in the U.S. Pure stevioside is about 300 times as sweet as a 0.4% sucrose solution. This high-intensity sweetener has been increasingly popular in Japan because of its taste and the perception that, as a “natural” product, it is healthier than synthetic sweeteners. Stevioside is commonly added to Japanese-style vegetables, dried seafood, soy sauce, and miso products, but it has only limited use for sweetening fruit products. 13.9.3.10 Thaumatin Thaumatin is approved in the U.S., Canada, Mexico, Taiwan, Japan, the European Union, Australia, New Zealand, South Africa, Switzerland, Israel, the Republic of Korea, and Vietnam. Although it is approved for use as a sweetener in many countries, thaumatin is used primarily as a flavor enhancer. In the U.S., thaumatin was granted GRAS status as a flavor enhancer by the FDA. 13.9.3.11 Tagatose Tagatose occurs naturally in dairy products, but the commercial product is made via a patented process. It has the bulk of sugar, is almost as sweet, but provides only 1.5 kcal/g. It has the potential for use in many products where sucrose is currently used. Tagatose was declared as GRAS by the manufacturer’s self-determination process, and can be used in the U.S. food supply.

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13.9.4 POLYOLS Polyols (sugar alcohols or polyalcohols) are chemically reduced carbohydrates. These compounds are important sugar substitutes that are utilized when their different sensory, special dietary, and functional properties make them desirable (Giese, 1993). Yet, because polyols are absorbed more slowly from the digestive tract than is sucrose, they are useful in certain special diets. When consumed in large quantities (25 to 50 g/d), however, they have a laxative effect, apparently because of the comparatively slow intestinal absorption. In most European countries, sorbitol, xylitol, mannitol, and lactitol are utilized in low-calorie food formulations because the European Community Nutritional Labeling Directive of 1991 has assigned an average caloric value of 2.4 kcal/g to all polyols (Giese, 1993). In the U.S., polyols are currently used mainly in sugar-free confections and in foods for diabetics. The recent approval of low calorie claims, however, should make them attractive for new applications. The caloric values of polyols for labeling purposes are as follows: • • • • • • • •

Erythritol, 0.2 kcal/g Lactitol, 2.0 kcal/g Maltitol, 3.0 kcal/g Mannitol, 1.6 kcal/g Sorbitol, 2.6 kcal/g Xylitol, 2.4 kcal/g Isomalt, 3.0 kcal/g Hydrogenated starch hydrolysates, 3.0 kcal/g

Food products sweetened with polyols and containing no sucrose can be labeled as “sugarless,” “sugar-free,” or “no sugar” but must also bear the statement “Not a reduced calorie food,” “Not a low calorie food,” or “Useful only in not promoting tooth decay.” FDA recently approved a health claim for polyols as reducing or not promoting the risk of tooth decay. 13.9.4.1 Sorbitol Sorbitol occurs naturally in many edible fruits and berries, including pears, apples, cherries, prunes, and peaches. Its nontoxic nature has long been recognized, and in 1974, the FDA named sorbitol as one of the first four chemicals on its revised list of substances, of which that sanctioned for use in foods is GRAS. Sorbitol is only 60% as sweet as sucrose; however, it has many functional properties desirable in a sweetener (Dwivedi, 1991). The useful functional properties of sorbitol include high viscosity (contributing to body and texture), hygroscopicity (resulting in its humectant, as well as its softening, nature), sweetness or cool taste, sequestering ability, crystallization and modification (retardation), and bulking agent ability. Because sorbitol can be digested without insulin and is also noncariogenic (not promoting tooth decay), in Europe, it is used as a sugar substitute in diabetic food products (marmalades, jams, and fruit juices, etc.). In general, sorbitol is used in fruits either to aid in the retention of the original quality of the products on aging and shipment to the end user or to provide a texture or other product quality that would otherwise not be present in the original formula. Application of sorbitol in fruit products includes its use as humectant and/or bodying agent in shredded coconut, glazed, and dried fruits, and gelatin products. In these applications (some of which are suggested and may not be in actual use), smaller concentrations of sorbitol are required than in its use in sugarless gums and candies. 13.9.4.2 Mannitol Mannitol is only about 70% as sweet as sucrose and is also noncariogenic. It occurs in fungi and is produced by bacterial reduction of fructose in some fermentation reactions. Applications include

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as a dusting agent for chewing gum and as a bulking agent in powdered food. Mannitol has a more serious laxative effect than sorbitol, and a warning label is required against consumption exceeding 20 g/d. Amounts consumed as a fruit additive sweetener are believed to be minor. 13.9.4.3 Xylitol Xylitol is a five-carbon polyol with a sweetness similar to sucrose. It is found in small amounts in a variety of fruits and vegetables and is formed as a normal intermediate in the human body during glucose metabolism. Xylitol has good solubility, blends well with foods, and has a lower melting point than sucrose, an advantage in the manufacture of confectionery products. There is also evidence that xylitol is not only noncariogenic but reduces dental cavities when used as a replacement for sucrose. In the U.S., it is mainly used in compressed candies, chewing gum, and dental care products. Xylitol is very expensive; therefore, it is usually used in small amounts in combination with other sweeteners. In a blend with aspartame, the two compounds have an excellent synergistic effect. Also, xylitol is blended with other polyols to minimize undesirable properties, such as hygroscopicity or the laxative effect of sorbitol, or to improve the solubility of mannitol. 13.9.4.4 Lactitol Lactitol monohydrate, a sugar alcohol, has physicochemical properties different from those of sugars. It is derived from milk sugar and is used as a sweetener in Japan, Israel, and Switzerland. In the U.S., lactitol may be used in bakery products, chocolate, ice cream, chewing gum, and other confections. Lactitol has a sweetness value approximately one third that of sucrose and is therefore suitable where bulking with low sweetness is required. A special grade lactitol DC is used for direct compression in mints, tabletop sweeteners, and other sugar-free compressed products. 13.9.4.5 Erythritol Erythritol is about 70% as sweet as sucrose. Like other polyols it does not promote tooth decay and is safe for diabetics. However, it is distinctive for its caloric content (the lowest of the polyols: 0.2 cal/g) and its high digestive tolerance. Studies have shown that due to its small molecular size and structure, more than 90% of ingested erythritol is absorbed and excreted unchanged through the kidneys within 24 h, so that laxative effects sometimes associated with polyol consumption are unlikely. In 1997, the FDA accepted for filing a GRAS affirmation petition allowing manufacturers to produce and sell erythritol-containing foods in the U.S. According to the petition, erythritol is intended for use as a flavor enhancer, formulation aid, humectant, nutritive sweetener, stabilizer and thickener, sequesterant, and texturizer. 13.9.4.6 Maltitol Maltitol, approximately 0.9 times a sweet as sucrose with similar sweetness and body, has application in products such as chewing gum, dry bakery mixes, and chocolate. Because of its heat stability and good handling properties, it is suitable for use in sugar-free baked products. 13.9.4.7 Isomalt Approximately 0.45 to 0.65 times sweet as sugar, isomalt can be used in candies, gums, ice creams, jams and jellies, fillings and frostings, beverages, and baked products. As a sweetener/bulking agent, it has no off-flavors and works well in combination with other sweeteners. Isomalt has been available in the U.S. since 1990 and is used in more than 40 countries worldwide.

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13.9.4.8 Hydrogenated Starch Hydrolysates (HSH) A mixture of sorbitol, maltitol, and hydrogenated oligosaccharides, HSH is also known as maltitol syrup and hydrogenated glucose syrup. Depending on the sorbitol and maltitol content, the sweetness of HSH can vary from 0.25 to 0.5 times that of sucrose. HSH serve a number of functional roles including use as bulk sweeteners, viscosity or bodying agents, humectants, crystallization modifiers, and rehydration aids. They also can serve as sugar-free carriers for flavors, colors, and enzymes. HSH has been used by the food industry for many years, especially in confectionery products.

13.9.5 TRENDS The rapid growth of aspartame since its 1981 approval, plus the ban on cyclamate and the partial ban on saccharin, spurred extensive research and development on low-calorie and no-calorie sweeteners. The FDA’s approvals of acesulphame-K and sucralose, its acceptance of GRAS petition for erythritol, and the changes sanctioned in the caloric value labeling of polyols promise significant change in the future sweetener use. Price, safety, taste, solubility, and stability are expected to be the significant factors determining which of the approved and available dietetic sweeteners will be used.

13.10 ACIDULANTS 13.10.1 FUNCTIONS Acidulants are acids that either occur naturally in fruits and vegetables or are used as additives in food processing. Acidulants serve many functions; the following compounds are utilized in fruit processing: • • • • • • • •

Acetic acid Adipic acid Citric acid Fumaric acid Lactic acid Malic acid Phosphoric acid Tartaric acid

These compounds perform a variety of functions in fruit processing. The primary functions of acidulants in fruit processing include: • • • • • • • •

Acidifier pH regulator Preservative and curing agent Flavoring agent Chelating agent Buffer Gelling/coagulating agent Antioxidant synergist

Acidulants are used to add a desired tartness to many food products. Because of the low pH created when used, food acids can also prevent growth of microorganisms that might cause food to spoil. Through chelation of trace metal ions and low pH, acidulants also prevent rancidity and discoloration of foods by functioning as synergists to antioxidants such as butylated hydroxyanisole

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TABLE 13.4 Typical Food Uses of Acidulants in Fruit Processing

Acid

Carbonated and Noncarbonated Soft Drinks

Citric acid Phosphoric acid Malic acid Fumaric acid Adipic acid Lactic acid Tartaric acid

X X X X X X X

Dry Beverage Bases X

Wine

Jams, Jellies, Preserves

Gelatin Desserts

Fruit Snacks

Frozen Foods

Canning

X

X

X

X

X

X

X X X

X X X

X

X X

X

X X

Pickles and Olives

X

X X

(BHA), butylated hydroxytoluene (BHT), and ascorbic acid. They are also used as buffers during various stages of food processing, as well as in finished products. The versatility of acidulants will certainly be a factor in their continued and increased use as food additives in the future (Table 13.4). At equal concentrations, the acidulants vary in their ability to depress pH and in the degree of acidic taste, or intensity of tartness, produced. The following replacement percentages using anhydrous citric acid as equal to 100% tartness are approximate guides.

13.10.2 PRODUCTS

AND

Acid

% Required to Replace Anhydrous Citric Acid

Citric, anhydrous Fumaric Tartaric Malic Adipic Phosphoric (85% soln.)

100 67–72 80–85 78–83 110–115 55–60

APPLICATIONS

13.10.2.1 Citric Acid Citric acid is the most versatile and widely used food acidulant. It has been used in foods for more than 100 years (Gardner, 1980). As a result, it is often employed as the standard for comparison in evaluating the effects of other acidulants. Citric acid is found in numerous natural products, and it is one of the important acids involved in plant respiration. Several fresh fruits such as lemons and limes owe their tangy taste to the presence of citrate ions. Its useful characteristics include excellent solubility, extremely low toxicity, chelating ability, and pleasantly sour taste. The traditional method of obtaining citric acid at the beginning of the 20th century was extraction from the juice of citrus fruits, especially lemons and limes, and later from pineapple wastes. These processes became practically nonexistent, however, with the development of fermentation technology. Today, most citric acid is produced commercially by mold fermentation of sugar solutions (most commonly, dextrose derived from enzyme-treated cornstarch and beet molasses) using strains of Aspergillus niger. Fermentation can be carried out either in shallow pans (surface fermentation) or in deep tanks (submerged fermentation). The technology and commercial processes to produce citric acid by candida yeast fermentation on paraffins also exists. Citric acid is used as a flavor enhancer, as a preservative (e.g., in beverages and syrups), as an antioxidant synergist with ascorbic or erythorbic acid (e.g., for fresh and frozen fruits and

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vegetables), and as a pH regulator (e.g., in gelatin desserts and jellies). Sodium and potassium citrates are widely used as buffers, often in combination with the parent acid in fruit butters, fruit jellies, and preserves. The FDA classifies citric acid and its sodium, potassium, and calcium salts as GRAS for sequestering, miscellaneous, and/or general purpose uses in foods. Beverages are the major food use for citric acid, accounting for an estimated 65% of citric acid’s total food acidulant consumption. Citric acid and its sodium salt are used extensively in carbonated beverages. It is also used as a flavor enhancer and preservative. Sodium citrate often is used in carbonated beverages as a buffer to regulate tartness if the acid level is high. Aseptically packaged or canned fruit drinks (fruit-juice-added drinks and sodas) are large markets for citric acid. Wine coolers, cocktail mixers, ice tea mixes, thirst quenchers, and other beverages are additional citric acid beverage markets. In candy, citric acid is used primarily to enhance the flavor of fruits and berries, but also to invert the sucrose and prevent crystallization of the sugar and to prevent oxidation of the ingredients. Citric acid imparts the desired degree of tartness that is especially important in dried-fruit snacks. Citric acid is used in jams, jellies, preserves, and gelatin desserts to adjust the pH for maximum gel formation and to enhance the flavor. In jams and jellies, citric acid is added in amounts sufficient to compensate for a deficiency of fruit acidity. In the processing of frozen foods, citric acid is used for several reasons. Through its chelating and pH adjustment properties, citric acid is able to optimize the stability of frozen food products by enhancing the activity of antioxidants and inactivating enzymes. In processing frozen fruits, citric acid solutions are used to neutralize the residual lye from lye peeling operations. Citric acid is also used in canning fruits to: • • •

Lower pH, thereby reducing heat-processing requirements Optimize flavor in canned fruits such as prunes and grapefruit Enhance activity of antioxidants to prevent color and flavor degradation (by chelating trace metal ions that might otherwise catalyze enzymatic oxidation)

Citric acid and its salts are also used in a variety of other applications: to prevent crystallization in honey, to stabilize spices, and to act as a synergist for antioxidants used in retarding rancidity in foods containing fats and oils. Other uses of this versatile acid and its salts include their incorporation in enzyme preparations for clarifying fruit juices. 13.10.2.2 Malic Acid Malic acid is similar to citric acid both in its acidifying character and taste effects but does not exhibit the “burst” effect of citric. It is found widely in fruits and is strongly associated with apples, although it predominates in several fruits and vegetables. It is the second major acid next to citric in citrus fruits and is present in most berries, beans, and tomatoes. Malic acid is one of the miscellaneous or general purpose food additives in the FDA list of GRAS substances. It is an optional ingredient in the FDA standards for fruit butters, preserves, jams, and jellies. Malic acid is prepared by hydrolyzing maleic anhydride to maleic acid and, at elevated temperatures and pressures, forming an equilibrium mixture of maleic acid, fumaric acid, and malic acid. Malic acid is isolated from the other two acids, which are recycled if not wanted. Malic acid is used in a variety of products, but mostly in fruit-flavored sodas such as those with apple and berry flavor. Malic acid is the preferred acidulant in low-calorie drinks, and in cider and apple drinks, it enhances flavor and stabilizes the color of carbonated and noncarbonated fruitflavored drinks and cream sodas. In sugar-free drinks, malic acid masks the off-taste produced by sugar substitutes. Recent and projected future good growth is chiefly the result of use in dietetic

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fruit-flavored sodas containing aspartame because of its synergism and ideal blending properties. Blends of malic acid and citric acid are now available that exhibit some better taste characteristics than either separate acidulant. 13.10.2.3 Phosphoric Acid Phosphoric acid has a characteristic flavor and tartness and is used almost entirely in cola-flavored carbonated beverages. (A small quantity is also used in some root beer brands.) It is the only inorganic acid extensively employed as a food acidulant. Occasionally, phosphoric acid is utilized as a buffering agent in jams and jellies to adjust acidity for maximum gel formation. It is the least costly of all the food-grade acidulants; it is also the strongest, giving the lowest attainable pH. Phosphoric acid is produced primarily by the furnace process. Elemental phosphorus (P4) is burned at about 1650 to 2750∞C, yielding phosphorus pentoxide (P2O5), which reacts with water to produce phosphoric acid (H3PO4). Although the product is relatively pure, food-grade material requires additional purification to meet standards. Food-grade phosphoric acid is supplied mostly as 75% aqueous solutions, but 80% and 85% solutions are also available. 13.10.2.4 Fumaric Acid Fumaric acid is principally used in fruit juices and gelatin desserts, pie fillings, maraschino cherries, and wines. Fumaric acid increases the strength of gelatin gels and acts as a calcium liberator when incorporated in alginate preparations. In many of these applications, fumaric acid competes with other acidulants such as citric acid, tartaric acid, and malic acid. Although it is less costly than some alternatives, its relatively strong acid taste and low solubility make it less appropriate for certain food uses. It is currently manufactured in the U.S. by acid catalyzed isomerization of maleic acid. To overcome the slow solubility rate of fumaric acid in water at low temperatures, the acid is mixed with 0.3% dioctyl sodium sulfosuccinate (DOSS) and 0.5% calcium carbonate. This is sold as a modified acid for use in dry beverage powders, frozen fruit concentrates, and similar other applications. Rapid rates of solution are claimed for the mixture, even in ice water, and the fumaric acid remains as a homogeneous suspension in frozen concentrates for long periods of time. Furthermore, fumaric acid and its salts have a tendency to stabilize the suspended matter in both flash-pasteurized and frozen concentrates and to inhibit the development of off-odors and darkening. 13.10.2.5 Adipic Acid In fruit products, it is principally used as an acidulant and gel-inducing agent in bottled beverages and as powdered concentrates for fruit-flavored beverages, imitation jellies, jams, and gelatin desserts. In grape-flavored products, it adds a lingering supplementary flavor and provides an excellent set to food powders containing gelatin. It is often used in dry food products because it is nonhygroscopic and offers longer storage life under humid conditions. Adipic acid is currently produced commercially in the U.S. by nitric acid oxidation by cyclohexanol. 13.10.2.6 Lactic Acid Lactic acid is one of the most widely distributed acids in nature and one of the earliest used in foods. Because of its mild taste relative to other food acids, it is also used to enhance flavor in fruit drinks and confectionery. In addition, lactic acid is used as a preservative in brine-preserved foods such as pickles and Spanish-style olives where it insures clarity of the brine by inhibiting spoilage and further fermentation. It is also used in certain frozen desserts (to provide a mildly tart flavor without masking that of the natural fruit) and in some jams and jellies. Lactic acid derivatives

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are consumed by the food industry; for example, ethyl lactate is used in flavorings, and sodium lactate is used as a preservative and seasoning. Calcium lactate serves as a firming salt for fruits and vegetables and is also used as a gelling agent for demethylated pectins. Unlike the other acids, it is a viscous, nonvolatile liquid, food-grade lactic acid sold as 88% and 50% aqueous solutions, both of which are colorless and odorless. Until 1963, lactic acid was produced solely by fermentation of carbohydrates such as corn, potato, or rice starch; whey, cane, or beet sugar; or beet molasses. Now it is manufactured also by synthetic means in the U.S. as a by-product of acrylonitrile manufacture. Lactonitrile is produced from acetaldehyde and hydrogen cyanide. It is then esterified with methanol and hydrolyzed to produce lactic acid. 13.10.2.7 Tartaric Acid Tartaric acid has a strong, tart taste and augments natural and synthetic fruit flavors, especially grape and cranberry. Tartaric acid is a natural component of numerous fruits including the currant, raspberry, cranberry, and grape. As a food acidulant, tartaric acid is widely used in cranberry and grape-flavored foods and beverages and in candies (in conjunction with citric acid) to produce the sour apple, wild cherry, and other especially tart flavors. For similar reasons, it is selected as the acidulant for grapeflavored and for tart-tasting jams and jellies. Both tartaric acid and its mono potassium salt (cream of tartar) are used in baking powders and leavening systems. Specialized uses, high prices, and limited availability inhibit tartaric acid from widespread use as a food acidulant. Food grade tartaric acid is obtained from waste products of the wine industry such as press cakes, unfermented grape juice, or argols. In the manufacture of the acid, the by-products are first extracted with hot water and treated with hydrochloric acid and then lime. The calcium tartarate is then crystallized and decomposed with sulfuric acid to obtain the acid. 13.10.2.8 Vinegar In the U.S., the use of the term vinegar without qualifying adjectives implies only cider vinegar. There are several popular types of vinegar produced commercially for the use as an additive in food products. Although a 4 to 8% solution of pure acetic acid would have the same taste characteristics as cider vinegar, it could not qualify as vinegar. In the U.K., malt vinegar is supplied by trade agreement. In Europe, wine vinegar is the most common variety. Vinegar is produced from apple cider, grapes, or wine, sucrose, glucose, or malt by successive alcoholic and acetous fermentation. The resulting vinegar is then pasteurized and filtered and often concentrated for convenience of handling and storage. 13.10.2.9 Acetic Acid Very little pure acetic acid, as such, is used in fruit products. But it is the principal component of vinegar, one of the first food acidulants. It is found in unprocessed figs. It has a pungent odor and is utilized in pickled fruits. Pure acetic acid is produced synthetically by the oxidation of acetaldehyde and butane and as the reaction product of methanol and carbon monoxide. Smaller quantities are obtained from the pyroligneous acid liquors acquired in the destructive distillation of hard wood.

13.10.3 REGULATORY STATUS In the U.S., the major food acidulants are listed by the FDA as GRAS when used in accordance with good manufacturing practice. Fumaric acid is also on the FEMA GRAS list as a flavor component and is a food additive in FDA regulations. In Western Europe, the major acidulants are widely approved; the new EC guidelines, however, indicate tartaric acid should not be used for sparkling water. In Japan, major food-use acidulants

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are considered conventional (i.e., chemically synthesized) food additives; the exception is phosphoric, which is not recognized as an acidulant in Japan.

13.11 THICKENERS AND STABILIZERS 13.11.1 FUNCTIONS Hydrocolloids are the main products used as thickeners and stabilizers in fruit processing. Thickening and stabilization agents provide a number of useful effects to fruit products (Table 13.5). The technical base for these effects results from the ability of these materials to modify the physical properties of water. Most fruit products are composed largely of water. Hydrocolloids (also called gums, water-soluble materials) are long-chain polymers that function as (Dziezak, 1991): • • • • •

Rheology modifiers, affecting the flow and feel (mouth) of food and beverage products Suspension agents for food products containing particulate matter Stabilizers for oil/water mixtures Binders in dry and semidry food products Agents able to create both hard and soft gels in food products that require this physical form

Hydrocolloids are obtained from a variety of sources. Most hydrocolloids are derived from plant materials such as seaweed, seeds, and tree exudates; others are products of microbial fermentation, and still others are produced by chemical modification of natural polysaccharides (Glicksman, 1982). They can be categorized as: • •

•

Natural gums — extracted from plant- and animal-based products Modified or semi-synthetic gums — chemically derivatized from natural organic materials such as cellulose and starch or produced via microbial fermentation of natural materials Synthetic gums — obtained by polymerization of monomers synthesized from petroleum or natural gas precursor (This group, which includes such hydrocolloids as polyacrylic acid, polyvinylpyrolidone [PVP], associative thickeners, etc., will not be discussed here because they are not utilized in fruit products.)

13.11.2 PRODUCTS 13.11.2.1 Unmodified Starches Unmodified starches, produced by the wet milling of field corn, supply the major amount of thickening material for the American food and beverage market. Unmodified cornstarch is produced by the wet milling of field corn, which involves treatment with a mild acid, grinding it to a fine paste, and removing the starch by centrifuging. Other natural starches of significance include potato and wheat starches used in Europe, and tapioca, arrowroot, and sago used in other areas. Unmodified cornstarch, commonly called pearl starch, is used in the fruit processing industry in the preparation of pie fillings. It is the base ingredient in many formulated thickening products because of its modest price. In addition to its thickening function, it is also a significant source of nutritional value. It is modestly priced in comparison to modified starch but even more so compared with such exotic materials as cellulose ethers, guar, xanthan, and alginates. But in recent years, unmodified starch consumption has suffered a decline in popularity because of the shift to lowcalorie foods, the shift to prepared foods, and the availability of alternative modified and

X X X X X X X X X X

Unmodified starches Modified starches Casein Gelatin Carboxy methylcellulose Methylcellulose Guar gum Alginates Xanthan gum Pectin Locust bean gum Gum arabic Carrageenan Agar

X X

X X

X X

X X

X X X X X X X X X X X X X

X

X

X

X X

X

X X

Water Binding

X

X X X

X

X

Mouthfeel

X

X

X

X X

Foam Stabillization

X X

Flavor Fixation

X

X

X

X X X X

Protective Film Forming

Source: Newman H. Giragosian, Outlook for Food Thickeness, paper presented at the Chemical Marketing Research Association Meeting, Houston, TX, February 1983.

X X

X

X

X

X

Gelation

Crystalization Control

X

X X X

X

Synergistic Effect

306

X

Thickening

Hydrocolloid

Emulsion Stabilization

Suspending Properties

TABLE 13.5 Major Food Thickeners and Stabilizers and Their Principal Functions

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semi-synthetic thickeners. Unmodified starches are used mainly for their thickening, gelling, adhesive, and film-forming properties. 13.11.2.2 Modified Starch Modified starch is starch that has undergone one of a variety of treatments to alter its physical properties and/or functionality. Possible treatments include roasting; treatment with acids, alkalis, or enzymes, bleaching, esterification, and oxidation. Modified starches used in food products or food processing have been modified for several reasons to: • • • • • •

Extend the bodying or gelling effect of normal starches Improve resistance to acid or heat degradability and to low temperature and freeze-thaw (eliminating aggregation) Improve texture Modify gelling tendencies as desired Increase viscosities at high temperature without gelling on cooling Improve instant solubility and gelling in cold water

Modified or derivatized starches are generally designed for more selective food applications than is unmodified starch. They are used in a wide variety of fruit products including baby foods, purees, candy (e.g., bonbons), jellies, cake mixes, doughs, puddings, pie fillings, batter mixes, dairy desserts, snack foods, and canned fruits. 13.11.2.3 Casein Casein is mostly imported from New Zealand and Ireland, although some is obtained domestically. It is a protein occurring naturally in and obtained from milk; it is the main ingredient in cheese. Casein is usually precipitated from skim milk by an acid or by rennin. The commercial sodium caseinate is obtained by evaporating a solution of casein in sodium hydroxide. Casein is marketed as sodium, calcium, potassium, or magnesium caseinate and is used in confections, puddings, bakery fillings, and frostings. Its use in fruit products depends mainly on its excellent cohesive and adhesive dried film properties. Researchers are working with a casein derivative to develop an edible film that keeps cut fruits and other produce fresh in the refrigerated case (McHugh and Krochta, 1994). In 1970, only five protein products were available from milk. In the last 15 years, numerous advances have been made in the methods to isolate and modify proteins. As a result, we now have hundreds of specialty milk proteins available. Most suppliers offer multitudes of specialty casein products. 13.11.2.4 Gelatin Derived from collagen, gelatin is the only food hydrocolloid extracted from animal source. Gelatin is an insoluble fibrous protein that occurs in vertebrates and is the principal constituent of the connective tissues and bones. It is obtained from pork skin and bones (Type A) or beef skin and bones (Type B). Gelatin is about 97% protein and can absorb up to ten times its weight in water; it can be chemically modified to render it insoluble. Type A is used mostly for confectionery products and Type B for dairy applications. The largest food applications for gelatin are dairy products such as ice cream and yogurts, confectionery products such as the gummy animal chewable, and gelatin fruit desserts. Gelatin is not heat stable.

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13.11.2.5 Guar Gum Guar gum is one of the most economical and widely used gums in the U.S., with extensive use in a variety of industrial and food applications. Along with natural starch and gelatin, guar gum is close to being a commodity product. Major uses for guar gum include ice cream, imitation bakery jellies and dry mix bakery formulations, beverages, and water-based frozen desserts. Guar gum is derived from a soy-like legume native to the semi-arid regions of the Asian subcontinent (India and Pakistan). It has been used in these regions as a vegetable, as well as livestock fodder since prehistory. In spite of several attempts by major domestic guar concerns to encourage domestic production of guar in Texas and other arid agricultural areas of the American Southwest, most of the guar gum consumed in the U.S. is derived from imported degummed guar beans (splits) sourced primarily from India. Because of changing supplies due to weather and harvest conditions, as well as the demand for guar gum for industrial applications, the price of guar gum tends to shift dramatically and therefore is one of the most difficult to forecast. 13.11.2.6 Locust Bean Gum Locust bean gum is obtained from the carob tree, and the major source of locust bean gum is the Mediterranean countries. The size and quality of the crop is directly related to climatic conditions; therefore, there are periodic shortages and great fluctuations of supplies. Chemically, locust bean gum is similar to guar gum. Anionic, cationic, and hydroxyalkyl derivatives are also produced commercially. Locust bean gum swells in cold water, but heating is necessary for maximum solubility. Locust bean gum is widely used in frozen dairy products, in conjunction with guar gum and carrageenan, and is used for preventing syneresis in cream cheese. In addition, locust bean gum is used in many nonemulsified sauces and dressings as a thickener and, in bakery products, as a moisture retention aid. 13.11.2.7 Agar Agar is produced in Portugal, Spain, and Morocco; a significant amount of material is produced in Japan as well. Methods of harvesting seaweed for agar production are similar to those used for carrageenan production. Other collection methods include the use of skin-divers to harvest seaweed from the sea floor. The product is usually sun dried locally than shipped for the location of the final production sites. Agar is used primarily in baked foods (icings, toppings, and meringues) and in confectionery products. Because agar is the most expensive of the seaweed extracts, there have been continual efforts to substitute it with other gums such as carrageenan. 13.11.2.8 Gum Arabic Gum arabic is the most water-soluble of the natural gums (up to 50%), and its solutions are of relatively low viscosity. Other advantages of gum arabic as a food additive include nontoxicity and its lack of odor, color, and taste. Gum arabic is obtained by hand-picking dried exudate from various trees of the genus Acacia, primarily from Acacia senegal. It is also known as gum acacia, gum senegal, mimosa, kordofan, verek, and gum egyptian. The Sudan accounts for about 85% of the output, the remainder coming principally from West Africa. Some upgrading may be undertaken by the use of air flotation or other mechanical means for removal of foreign matter. A premium grade of gum arabic is produced by dissolving, filtering, and spray drying the exudate. Gum arabic is used as an emulsifier in beverages for citrus oil and flavors, as a crystallization retarder and emulsifier in confectioneries, and as a stabilizer in dairy and bakery products. As the source of supply has sometimes been unreliable because of political and social events in the Middle East, many U.S. users have turned to substitutes including starch derivatives.

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13.11.2.9 Carrageenan Carrageenan is readily soluble in water to form an inelastic gel and is commonly used in combination with other gums. Its most unique property is a high degree of reactivity with certain proteins such as casein. The largest application of carrageenan food use is in dairy products (frozen desserts, flavored milk powder, and nondairy creamers). For example, cocoa can be suspended in milk with the use of about 0.025% carrageenan. Carrageenan is extracted from Irish moss, a common name applied chiefly to various Chondrus and Gigartina species, which are harvested off the Atlantic shores of New England, the Canadian Maritime Provinces, and several Western European countries. The Philippines supply a carrageenan type called “Philippine carrageenan” that does not correspond to the grade used in Western Europe for food applications and should only be used for pet food. 13.11.2.10 Pectin Pectin is a fruit extract obtained mainly from apple, orange, and lemon wastes. Whereas pectin obtained primarily from citrus fruits is used in the U.S., pectin from apples represents a major part of Western European pectin production. The main commercial types used in foods are pectin itself and potassium pectinate, sodium pectinate, and amidated pectin. Pectic substances are extracted with hot water that has been acidified (pH 1.5 to 3), usually with hydrochloric or nitric acid. Some de-esterification normally occurs in this step. Following separation from the residue, the extract may be concentrated and marketed as liquid pectin. Solid pectin may be produced by spray drying or precipitation with alcohol, or it may be produced as an insoluble salt. The supply of pectin is limited due to the availability of raw material for processing. In the U.S., pectin processing has been discontinued and production facilities were relocated to Mexico because of constraints in the regulatory approvals required by the EPA to operate a plant with the large volume of waste generated in processing. Pectin is the most frequently used hydrocolloid in processed fruits. Traditional uses of pectins are in jellies and jams. Newer applications include gummy candies, fruit snacks, and fruit-flavored juices and carbonated drinks. In gummy candies and jellies, it is continuing to replace starch for the fruit flavor aspect. It is also easier to work with, and the texture is better for cuttables and chewables. In fruit-flavored drinks, it stabilizes the constituents and makes the product more appealing. Pectin is well known and perceived by consumers as a natural product. Because of this, it is unlikely to suffer the same negative antiadditive trend as some other food additives. 13.11.2.11 Alginates The alginate category includes the various salts of alginic acid and propylene glycol alginate (PGA). The term algin is usually applied to sodium alginate. The gum is extracted principally from Macrocystis pyrifera, a giant brown algae or kelp of the Pacific Ocean that is commercially harvested, mainly off the coasts of Southern California and Australia. A smaller brown kelp comprising various Laminaria species is harvested from the North Atlantic, principally off the coasts of Maine, Nova Scotia, and the British Isles. The gum is commonly extracted at about 80∞C at pH 5 to 6 for 8 to 9 h. Isolation is based on disintegration of the plants in sodium carbonate, filtering, and precipitation of alginic acid with mineral acid. A unique feature of the usual isolation process for agar involves a freeze–melt extraction type of purification of the gellied extract prior to final drying. Alginic acid may be marketed as such or converted to sodium alginate. Sodium alginate is used primarily as a binder in frozen desserts, instant pudding mixes, and fabricated puddings. Sodium alginate has been used in a cold-processed lemon pie filling to provide freeze-thaw stability.

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Major suppliers are devoting substantial R&D efforts to developing novel applications, such as production of structured “imitation” fruit pieces that do not break down during baking. The nonsodium light metal salts of alginic acid are used as sodium alginate alternatives in low-sodium and dietary food specialties. 13.11.2.12 Xanthan Gum The FDA approved xanthan gum for food use in 1969. Since then, its scope of approved applications has greatly increased. Xanthan gum is produced by fermentation of carbohydrates using cultures of Xanthomonas campestris. The fermentation broth may be concentrated and sold as such for technical uses, or the polysaccharide may be recovered and purified by precipitation with alcohols. Xanthan gum is used in a wide range of relishes, syrups, sauces, bakery fillings, prepared puddings, glazes and toppings, beverage mixes, and fruit and carbonated beverages. A significant amount of food use is in dairy industry products, where it prevents the separation of the contained whey from the rest of the food product. Xanthan gum is showing some interesting synergies with other natural water-soluble polymers. Physical synergies can be observed between xanthan and guar gum, and xanthan and locust bean gum. Combinations of xanthan and locust bean gum can form gels whereas mixtures of guar and xanthan have a higher viscosity than each of these products alone (whereas xanthan is higher than either of the galactomannanes). 13.11.2.13 Gellan Gum Gellan gum is the latest hydrocolloid approved for food use. The product received FDA approval in 1990 for limited use in icings, frostings, bakery fillings, and low-solids jams and jellies, and in 1993, it was approved for use in all processed foods (Duxbury, 1993). It is also approved for food use in Japan, but it is not yet approved in European countries. Gellan gum is produced with a fermentation process (such as that used for the fermentation of xanthan gum), by the organism Auromonas elodea. There are two forms of gellan gum. The first is a high-acetyl gum that is partly acetylated and provides thermo-reversible gels. The second is a low-acetyl gum forming a firmer and more brittle gel. It forms gels at use levels as low as 0.05% and is stable in a pH range of 3.5 to 8. 13.11.2.14 Cellulose Ethers Cellulose ethers include the carboxymethyl, hydroxyalkyl, methyl, and ethyl derivaties. These ethers are produced by reacting alkali cellulose with the appropriate halide, sulfate, or epoxide. 13.11.2.14.1 Carboxymethylcellulose (CMC) CMC is the primary cellulose ether consumed in food and beverage applications. In fruit-related products, CMC is used in the preparation of frozen dairy products, beverages, bakers’ goods, dry drink mixes, syrups, glazes, and icings and toppings. The current search for microwavecompatible food additives makes CMC a candidate for this rapidly growing formulated food market area. CMC, a nonnutritive substance, is also popular in diet food formulations requiring thickeners and stabilizers. 13.11.2.14.2 Other Cellulose Ethers Methylcellulose (MC) and hydroxypropylcellulose (HPC) are also used in specialized food and beverage applications, but their relatively high market prices preclude them from volume applications in processed fruit.

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13.11.2.15 Other Hydrocolloids A number of other thickening agents have been used by the U.S. food industry. They are currently used infrequently commercially and represent a very minor portion of the U.S. food additives market. Most are higher priced, in erratic supply, and increasingly face competition from the principally used thickeners. Such other thickening and stabilizing agents and their principal uses include the following: • • • • •

Ghatti gum is an exudate from the tree Anogeissus latifolia, obtained from India and Sri Lanka; no functional properties other than thickening and emulsion stabilization exist. Tragacanth gum is the exudate from several species of Astragalus, obtained from the Middle East and used in salad dressings and sauces. Karaya gum is the exudate of Sterculia urens, a tree native in India and used for extreme thickening to pastelike gels. Tara gum is permitted for use in the U.S. It originates from the tara bush, indigenous in South America. This gum is structurally similar to guar and locust bean gum. Konjac flour, obtained by grinding the root of the Amorphophallus konjac plant, konjac is GRAS and is widely used in Asia in noodles and similar products. Clinical studies have shown that konjac flour added to some foods can cut serum cholesterol by 10% and blood glucose levels by 75%.

13.11.3 REGULATORY STATUS In the U.S., all hydrocolloids listed here are approved and classified as either food additives or GRAS compounds by the FDA (Code of Federal Regulations, Title 21, Parts 172.580–172.874 and Parts 182.1480–184.1724, respectively). Worldwide, most hydrocolloids are approved as having established food grade specifications in the Food Chemical Codex. Their safety has been confirmed by the Food Agriculture Organization/World Health Organization Joint Expert Committee on Food Additives and have European Economic Community (EEC) specifications as well. In general, natural hydrocolloids face increasing environmental regulation due to the large volumes of aqueous effluents and solids generated in processing.

13.11.4 APPLICATION

OF

GUMS

IN

FRUIT PRODUCTS

13.11.4.1 Water Dessert Gels In the U.S., gelatin is by far the most commonly used hydrocolloid for the preparation of water dessert gels. Solutions containing 1 to 2% gelatin form thermally reversible, elastic, clear gels when prepared with boiling water and cooled below room temperature. Dessert gels based on seaweed extracts are also used, primarily because of their ability to set up gels without refrigeration at room temperature. In countries where refrigeration is not common, particularly in tropical countries, this is an important advantage. The soluble salts of algin have the ability to enter into controlled chemical reactions with calcium or other polyvalent ions to form edible gels. This important and unique property has been utilized in formulating dry powdered mixes that are cold-water-soluble and set up gels at room temperature. The gels formed are chemically set, nonthermally reversible, and therefore will not melt at room temperature. Algin concentrations of 0.4 to 1% of the finished dessert are used. Carrageenan water solutions of 0.5 to 1% concentration form gels on heating and cooling, which set up at room temperature.

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13.11.4.2 Fruit Drinks, Juices, and Nectars CMC is used at approximately 0.5% to thicken fruit juices and to prevent floating or settling of fruit during preparation, as well as impart a clearer, brighter appearance, produce a desirable gel texture, and reduce syneresis. Alginates, particularly propylene glycol alginate is used at 0.1 to 0.2% concentration to suspend pulp in fruit drinks. Guar is used as a thickening and viscosity control agent in fruit nectars. Gum Arabic is used as an emulsifier in fruit flavors. Gelatin is used extensively for clarification or fining of cider, fruit juices, and wine. Among the products available for clarification, when applied correctly, gelatin does not modify the organoleptic quality of the finished product. Amount of gelatin used is in the range of 50 to 300 g/1000 l juice to be treated. 13.11.4.3 Pie Fillings and Jellies Starch is used as a primary thickener in most pie fillings; however, all-starch fillings have a firm starchy appearance that is not always desirable and may be subject to syneresis. By the selection of the proper starch, usually a modified one, and the proper gum, a variety of textures ranging from a semifluid to firm consistency can be prepared. In lemon pie fillings containing about 4% starch, the addition of 0.5% high-viscosity CMC gives firm body and prevents cracking, shrinkage, and syneresis. In canned or frozen fruit pie fillings, gums are used as a partial or complete replacement for starch because starches tend to retrograde in liquid media. The use of CMC has been recommended in canned berry, frozen peach, or cherry pie fillings at 0.2 to 0.5% alone or in combination with modified waxy maize starch to give a less opaque appearance on storage. MC is used in pie fillings to reduce absorption of water by the crust during baking, to protect flavor during baking, and to stabilize the gel after baking. The incorporation of 0.1% algin in bakery jellies reduces syneresis and improves spreading qualities and storage life. Algin, at approximately 0.3% reduces boil-off of juices during the preparation of fruit pies and controls weeping and spreading after preparation. Locust bean gum has been used as a stabilizer for canned berry pie fillings and in frozen pie fillings in combination with starch. Guar-starch combinations also have been used in frozen fruit pie fillings to prevent dehydration, shrinking, and cracking, as have tragacanth–starch combinations to provide thickening, clarity, and brilliance. 13.11.4.4 Jams, Jellies, and Preserves Most jellies, jams, and preserves produced in the U.S. utilize pectin as the gelling agent; however, at one time or another, all the hydrocolloids have been used or proposed for use in these products. Marmalade, jams, and jellies are one of the important uses of furcellaran gum, where it has the advantage of not requiring the prolonged boiling necessary with pectin, thus retaining many of the volatile flavor compounds in the fruit, giving a fresh flavor product. 13.11.4.5 Flavor Fixation Gum arabic is almost exclusively used as the fixative in spray-dried fruit flavors, which are extensively used in dry packaged mixes such as beverage powders, puddings, gelatin desserts, cake mixes, etc., where flavor stability and long shelf life are important. These spray-dried powdered flavors are produced by preparing an oil-in-water emulsion of citrus oils or other fruit flavors, with gum arabic as the emulsifier. The emulsion is then dried by spraying a fine mist of the liquid into a current of hot air. The flavor material is encapsulated in the gum arabic, resulting in a dry,

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free-flowing powder, and preventing evaporation of volatile constituents or oxidative deterioration of the flavor. 13.11.4.6 Dehydrated Fruits The rehydration properties of dehydrated fruit juices can be improved by the addition of gum prior to drying. CMC has been used for the preparation of dehydrated fruit juices to improve the resuspension and reconstitution properties. The use of MC as a drying aid in the preparation of spray-dried grapefruit and orange juice at 0.1 to 4 solids basis can facilitate the production of free-flowing powders. 13.11.4.7 Low-Calorie Food The market for low-calorie foods has had a rapid growth in recent years. In most of these products, sucrose and other sugars are replaced by high-intensity sweeteners such as saccharin or aspartame. The role of gums in these products is primarily to replace the viscosity and “mouth feel” lost when sugar or oil is removed. In still other products, such as puddings and pie fillings, gums may be used to replace all or part of the starch normally used (Hegenbart, 1993). The gum may also function as a bulking agent, a flavor-blending media, a masking agent for the aftertaste caused by artificial sweeteners, or to provide a feeling of fullness or satiety. The use of CMC has been reported to improve stability and inhibit syneresis in dietetic jams and jellies. A combination of 0.4% highviscosity CMC and 0.8% low-methoxy pectin has been recommended.

13.12 EMULSIFIERS 13.12.1 FUNCTIONS Emulsifiers are additives that allow normally immiscible liquids, such as oil and water, to form a stable mixture. Emulsifiers possess both hydrophilic and lipophilic groups within the same molecule; the ratio of hydrophilic to lipophilic groups, known as the HLB value, is a characteristic indicator for emulsifiers, which allows assessment of their action and performance. Emulsifiers are widely used in foods in order to perform one or more of the following functions (Stauffer, 1996): • • •

Increase stability and prevent phase separation in food emulsions (e.g., mayonnaise, salad dressings, etc.) Improve the shelf life of flavors and retard the onset of rancidity in fats and oils containing food emulsions Improve texture, reduce crumb firmness, and complex with starches (baked goods)

Recent interest in this category of food additives has been encouraged by consumers’ health consciousness. As most emulsifiers can decrease the amount of fat or oil in the product or add bulkiness or soften texture by increasing the amount of air suspended throughout the product, emulsifiers can serve to some degree as substitutes for fat and can decrease calories (MentzerMorrison, 1992). In the case of lecithin, the growth of its market is supported by some functions of lecithin itself, such as improvement in and/or prevention of adult diseases such as hypertension, arteriosclerosis, and hyperlipidemia.

13.12.2 PRODUCTS

AND

APPLICATIONS

The most common and commercially important emulsifiers are (Somogyi et al., 1996):

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• • • • • •

Monoglycerides and diglycerides of fatty acids and their esters (e.g., glyceryl monostearate) Lactylated esters (e.g., sodium stearoyl lactylate) Propylene glycol esters (e.g., propylene glycol monostearate) Lecithin Sorbitan esters (e.g., sorbitan monostearate) Polysorbates (e.g., polyoxyethylene sorbitan monolaurate)

A great number of other emulsifiers are also used by the food industry, but in smaller volumes. With the possible exception of lecithin, few emulsifiers are used as a single additive. Mostly, emulsifiers are offered as blends of emulsifiers, emulsifiers and water, emulsifiers and fats, or as blends with other functional additives (e.g., colors, flavors, gums for specialized applications, i.e., ice cream, baked goods, etc.). These products are formulated for specific applications or specific customers so that the combination provides both enhanced performance and ease of use. Emulsifiers may be divided into natural and synthetic. This division is significant because of consumers’ concerns related to use of artificial food additives. Within the natural emulsifiers, the lecithins are by far the main products. Raw lecithin is produced mainly from soybeans. Lecithin is the most widely used chocolate emulsifier. Other important applications include baked goods and dairy products. The more prominent food uses of synthetic emulsifiers are baking, shortenings, ice cream, imitation dairy products, prepared mixes, margarine, salad dressings, and miscellaneous prepared foods (e.g., confectionery, pasta, potatoes, and peanut butter). Application of emulsifiers in processed fruit is in fruit drinks, flavor emulsions, pie fillings, and fruit snacks. The addition of dill oil to processed pickle products requires an emulsifier to solubilize the oil in the brine. A hydrophilic emulsifier is used at a level of 2 parts to 80 parts of oil to produce a clear solution of the spice oil in the brine. Similarly, orange oil used to flavor orange drink or soda is solubilized with a hydrophilic emulsifier. The manufacturing method for glycerin fatty acid esters is either esterification of glycerin and fatty acid or ester exchange between fat and glycerin. The latter method is mainly used, yielding a mixture of monoglycerides, diglycerides, triglycerides, free fatty acids, and free glycerin. Monoglyceride is obtained by distillation, with a yield of at least 90%. About 80% of the total consumption of glyceride is distilled glycerides.

13.12.3 REGULATORY STATUS Classes of permitted emulsifiers, worldwide, include: • • • • • • • • • • • •

Mono- and diglycerides Acetylated monoglycerides Stearyl-2-lactilates Lactilated esters of fatty acids Diacetyl tartaric acid esters (DATEM) Polyglycerol esters and ethoxylates Sorbitan esters Citric acid esters Succinylated monoglycerides Lecithin Propylene glycol esters Sucrose fatty acid esters

In the U.S., lecithin, mono- and diglycerides, DATEM, and triethyl citrate are classified as GRAS by the FDA. The other emulsifiers are approved under multipurpose additive regulations that permit their use in specific products at set levels.

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In Europe, food emulsifiers are classified under EEC Council Directive reference numbers. Examples are mono- and diglyceride of fatty acids–E471; acetic, citric lactic, and phosphoric esters of mono- and diglycerides–E472 a–f; and polyglycerol esters of fatty acids–E475. Sorbitan esters are not approved for food use in Western Europe (only in cosmetics and pharmaceuticals). Propylene glycol esters are not approved in Germany. In Japan, glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, sucrose fatty acid esters, and lecithin are listed in the positive list of food additives. Also, saponins, phytosterol, and other natural products are permitted.

13.13 FLAVORS 13.13.1 PRODUCTS

AND

FUNCTION

Flavors are concentrated preparations used to impart a specific aroma to a food or beverage. This group of additives comprises more than 3000 individual products. Fruits are frequently utilized as sources of raw material for the preparation of many flavor compounds (e.g., lemon oil and berry concentrates, etc.). In the U.S., natural fruit flavors represent over one third of the flavor market. Annual consumption of natural flavors are estimated at $400 million in 2000 (Somogyi and Kishi, 2001). Flavors may be added to food products for the following reasons (Giese, 1994): • • • • •

To create a totally new taste (e.g., Coca-ColaTM) To enhance, extend, round out, or increase the potency of flavors already present To supplement or replace flavors to compensate for losses during processing To simulate other more expensive flavors or replace unavailable flavors To mask less desirable flavors, i.e., to cover harsh or undesirable tastes naturally present in some food (rather than to hide spoilage)

Some of the large specific products are as follows (Somogyi and Kishi, 2001): •

•

•

Essential oils and natural extracts: aromatic raw materials isolated or concentrated from botanical or animal sources by processes such as distillation, expression, solvent extraction, and maceration. Included are vanilla, cocoa, cola, and spice oleoresins, etc. They are used to compound flavors and fragrances to impart flavor, color, and aroma to products. Aroma chemicals: aromatic chemically defined structures prepared by synthesis or isolated from natural sources (e.g., menthol from cornmint oil). Included are anethole, vanillin, citronellol, and esters, geraniol/nerol and esters, Linalool and esters, etc. Compounded flavors: complex blends/mixtures of various components containing from few to 100 or more constituents. Compounded flavors may contain aroma chemicals, natural extracts, essential oils, solvents, and in some cases other functional ingredients (e.g., antioxidants and emulsifiers, etc.). They may be concentrated, diluted with solvents, or bound to carriers (e.g., free-flowing powder). These are the flavors of commerce utilized by the food, tobacco, cosmetics, and pharmaceutical industry. They are usually customized for a specific application and/or specific customers.

13.13.2 TECHNOLOGY

OF

MANUFACTURING

A flowsheet depicting the preparation of flavors from starting materials to the end product is presented in Figure 13.2. 13.13.2.1 Artificial Flavors Artificial flavors are substances that are chemically synthesized for flavor applications. They originate from other than natural sources. They are available in liquid and dry form. The production

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PLANTS, ANIMALS

PETROCHEMICAL RAW MATERIALS

Agriculture

• Steam Distillation • Extraction • Expression Synthetic Chemistry

Chemical Industry

Separation Distillation Modification AROMA CHEMICALS Flavor and Fragrance Industry

Also: Specialized Blenders for Semimanufactured Flavors

ESSENTIAL OILS, EXUDATES, SECRETIONS • Processing • Concentrating • Dissolving NATURAL RAW MATERIALS

• Compounding • Mixing • Blending

Essential Oil and Extract Industry

Flavor and Fragrance Industry

FLAVOR COMPOSITIONS

CUSTOMERS: MANUFACTURERS OF: FOOD AND BEVERAGES

FIGURE 13.2 The flavor industry: flow of manufacturing processes for flavors.

process of artificial flavors consists of compounding or mixing together the various aroma chemicals specified in the formula. In the case of dried flavors, a spray-drying process is added after mixing. Compounds having characteristic functional groups are incorporated to artificial flavor compositions. For example, esters give the characteristic fruity aroma, a-undecalactone is usually included in a typical peach flavor composition, and ethyl methyl phenyl glycidate imparts strawberry flavor character. For the production of synthetic aroma chemicals, a chemical plant is needed, consisting primarily of multipurpose reactors and some specialized equipment for special reactions (nitration), as well as crystallization devices and drying chambers for solid products. The most important equipment is for efficient distillation (solvent stripping, purification of raw products) and fractional distillations (separation of products from mixtures). These purification procedures determine the quality of the aroma chemical. Most artificial flavors are produced on short, customized, batch runs. 13.13.2.2 Natural Flavors Natural flavors include essential oil, oleoresin, essence or extractive, distillate, or any product of roasting, heating, or enzymolysis that contains the flavoring constituents derived from a spice; fruit or fruit juice; vegetable or vegetable juice; edible yeast, herb, bark, bud, root, leaf or similar plant material; meat, seafood, poultry, egg, dairy products; or fermentation product thereof, whose significant function in food is flavoring rather than nutritional. In practice, natural flavors are essential oils, oleoresins, and true fruit extracts. The other items are raw materials for compounding flavors. The following aromatic compounds are responsible for the distinct flavor of fruits (Reineccius, 1994): • • • •

Alcohols in the homologous series C1 to C12 series Carbonyls (benzaldehyde, furfural, etc.) Volatile acids (formic and acetic) Esters

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317

Lactones Phenols

Essential oils and essences are aromatic volatile fractions obtained from plant material through various methods. In processing essential oils and natural extracts, distillation (stream, vacuum, and fractional, etc.), solvent extraction, expression (or pressing), and enzymolysis are the major methods (Fischetti, 1980). Purification of these oils and extracts is achieved mainly by various types of distillation (normal, steam, vacuum, molecular, and hydrodiffusion). Natural flavors can be compounded from a wide range of ingredients to produce flavor types in concentrations sufficiently high to satisfy the requirements of processed foods. Compounding is a technically simple operation. In a kind of open pot (usually stainless steel, capacity from 4 l to 10,000 l) equipped with a mixing device and possibly a heating coil, the ingredients are added and thoroughly mixed, and then packed into appropriate containers. In a compounding plant system, pipes deliver solvents and large-volume liquid ingredients from a tank farm. Small-volume constituents are weighed in and added manually. In view of this and the large number of different composition produced in a given plant, the possibilities for automation are limited. A special type of natural flavor is fruit flavor concentrate. This type is defined as a fruit concentrate or puree where some of the water is removed and natural flavors are added, as well as possibly gums, starches, and colors. Fruit flavor concentrates are used in yogurt, baked goods, snacks, and beverages. The most common fruit-flavor concentrates include apple, berry, grape, and citrus products (Giese, 1994). Compositions prepared by compounding are always liquid. For some flavor applications, liquid flavors are encapsulated, primarily by spray-drying an emulsion containing the liquid flavor and a carrier (gum arabic, starch, etc.). Powdered flavors usually contain 10 to 25% of the liquid flavor composition.

13.13.3 REGULATORY STATUS Safe use of flavoring materials was outlined by the 1958 Food Additives Amendment. The law permits that an expert panel, qualified to determine the safety of flavoring materials, provides GRAS flavor status to a material that could then be used without further testing. An expert panel sponsored by the Flavor and Extract Manufacturers Association (FEMA) issues lists of GRAS flavor materials. All of the FEMA GRAS lists have been published by Food Technology; the most recent list, the 21st, was published in 2003 (Smith et al., 2003). The flavor-labeling regulations (Federal Code of Regulations, Title 21 and 101.22) describe how flavors must be declared in food labels. Spices and natural and artificial flavors may be declared as such in the ingredient listing of the product without naming specific flavoring ingredients. This inclusive labeling of flavors may be done as long as no flavor representation is made on the package of food. If a flavor representation is made on the package of the food (e.g., “lemon drink”), the name of the food must indicate the nature of the flavor (e.g., “artificially flavored”). U.S. labeling laws distinguish as natural only those flavor compositions containing chemicals extracted from natural sources produced by fermentation or made enzymatically from naturally occurring ingredients. Western European legislation considers as natural those flavors compounded with natural and/or nature-identical ingredients. The narrower definition curtails the latitude of food processors to label the food natural in the U.S. and serves to stimulate the U.S. demand at premium prices for natural ingredients (Somogyi and Kishi, 2001). In addition, federal “Standards of Identity” regulations (CFR Title 21: Part 145 for canned fruit and fruit juice products, Part 150 for fruit butters, jellies, preserves, and related products) covering many canned and frozen fruit products and jams allow only the inclusion of natural flavoring substances in these products.

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Following the lead of the U.S., inclusion in a positive list (compiled by a federal agency) that spells out which chemicals are permitted for food use has become the prevalent basis for legislation for regulating flavor chemicals worldwide. Most industrial countries have their own regulations and control over flavoring use, and legislation is frequently changing Bauer (1994). Salzer and Jones (1998) compiled an update on worldwide flavor legislation.

13.13.4 USE

OF

FLAVORS

IN

PROCESSED FRUITS

Fruit flavors, both natural and synthetic products, represent the largest product category of the flavor industry, encompassing about 48% of the sales value. Flavors are utilized in a wide range of products. Fruit beverages (including alcoholic beverages) are the largest user of fruit flavors, followed by desserts, dairy products, confectioneries, baked goods, snacks, jellies, and jams. The most frequent usage of fruit flavors is the citrus varieties by the beverage industry.

13.14 COLORS 13.14.1 FUNCTIONS Colors are used in foods to improve appearance and thereby influence the perception of texture and taste. For centuries, natural color materials have been mixed with foods in an attempt to improve appearance. When synthetic colors were introduced in the late 19th century, they were immediately adopted by the food industry. These were certified FD&C color additives that were allowed for use in foods according to the Federal Food, Drug and Cosmetic Act of 1938 and its amendments, and food color additives (mostly natural materials) that were exempt from FD&C certification. In order to market their products, U.S. producers of certified colors must submit product samples from each batch of material to the FDA. The U.S. Food and Drug Administration analyses these materials to see that they meet specific purity classifications. In other parts of the world, only selfcertification exists. Food colors may be added to food for the following reasons (Francis, 1998): • •

•

• • • •

To restore the original appearance of the food where the natural colors have been destroyed by heat processing (only occasionally in canned fruits) and with subsequent storage To ensure uniformity of color due to natural variations in color intensity, e.g., fruits obtained at different times during the season, thereby assuring uniformity in appearance and acceptability To intensify colors naturally occurring in foods where the color is weaker than that which the consumer associates with a food of that type of flavor (e.g., fruit yogurts, sauces, soft drinks) To help protect flavor and light-sensitive vitamins during shelf storage by a sunscreen effect To give attractive appearance to foods that would otherwise look unattractive or unappetizing (e.g., colorless gelatin-based jelly) and thus enhance enjoyment To help preserve the identity or character by which foods are recognized, i.e., product identification To serve as visual indication of quality — thus, in addition to enhancing the acceptability of foods, colors aid in food manufacture, storage, and quality control

13.14.2 PRODUCTS 13.14.2.1 Certified Food Colors Certified food colors can be divided into dyes and lakes. Of the eight colors certified for use in food in the U.S. (Table 13.6), eight can be used as dyes (one, Citrus Red No. 2, is limited to use

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TABLE 13.6 FD&C Certified Dye and Lake Colors Approved for Use in the U.S. Color

FD&C Dye

FD&C Lake

Blue No. 1 Blue No. 2 Red No. 3a Red No. 40 Yellow No. 5 Yellow No. 6 Green No. 3 Citrus Red No. 2

X X X X X X X X

X X X X

Notes: The color additive Orange B (which is listed for use in casings of Frankfurters and sausages) has not been manufactured in recent years. a

A proposal by FDA to ban the permanently listed uses of Red No. 3 is still pending. Source: Report of Certification, U.S. Food and Drug Administration.

in coloring orange skins) and five as lakes. Dyes are water-soluble compounds that impart color to a substance through dissolution. Color regulators specify a minimum of 85% pure dye for primary colors, but most dye lots contain from 90 to 93% pure dye. Certified dyes fall into several chemical classes: azo-dyes (Yellow No. 5, Yellow No. 6, Red No. 40, Citrus No. 2), triphenyl-methane dyes (Blue No. 1, Green No. 3), xanthine type (Red No. 3), and sulfonated indigo (Blue No. 2). Table 13.7 shows the physical properties of these eight certified food colorants. Certified food colorants, both primary and blends, are produced in a variety of forms including powder, liquid, granules, paste, and dispersion. FD&C dyes are also used in the production of lakes, which are pigments prepared by combining a certified dye with an insoluble alumina hydrate substratum. Lakes are both water- and oil-insoluble and impart color through dispersion in food. Thus, they are suitable for coloring foods that cannot tolerate water and products in which the presence of water is undesirable, such as dry beverage bases and dessert powders. The FD&C lakes do not have a legal specified minimum dye content; manufacturers use formulations of from 11% (standard) to 42% pure dye (concentrated). Certified food colors are used in most products in the 50 to 300 ppm ranges. 13.14.2.2 Noncertified Food Colors Noncertified colors can be from either natural origins (primary sources), such as vegetables (carrot oil, red beet juice, paprika, etc.) and fruits (grape skin, cranberry juice concentrate), or produced synthetically (e.g., ferrous gluconate, apo-carotenal, riboflavin, etc.) (Linden and Lorient, 1999). Food color additives exempt from certification in the U.S., their colors are listed in Table 13.8. In general, the traditional markets for noncertified food colors have been the lipid-based, highfat food systems such as butter, margarine, shortening, popcorn oil, processed cheeses and spreads, salad dressing, and snack foods (Somogyi et al, 1996). Water-soluble forms are often also available, and other applications include baked foods, fruit juice drinks, confections, and dairy products. Certain noncertified food colors are important in the beverage industry. For example, caramel is used in beverages, primarily in cola drinks, and synthetic carotenoids are important in orange drinks (Borenstein and Bunnel, 1967). No doubt technological advances will lead to new applications for the noncertified colors, as well as wider use in existing markets.

a

N/A Fair

Very poor

Fair

Fair Very good Fair Good

N/A Poor

Poor

Poor

Fair Fair Fair Fair

N/A Good

Good (unstable in alkali) Poor

Poor Good Good Good

Stability to pH Change

N/A Good

Very poor

Good

Poor Very good Good Good

Compatibility with Food Components

Food use restricted to coloring skins of oranges at a level not to exceed 2 ppm by weight (not used in processing).

N/A = not applicable.

Citrus Red Fast Green FCF

Sodium Indigo Disulfonates

Blue No. 2

Citrus Red No. 2 Green No. 3

Brilliant Blue FCF

Blue No. 1

a

Erythrosine Allura Red AC Sunset Yellow FCF Tartrazine

Common Name

Stability to Oxidation

Good Excellent

Poor

Excellent

Very good Very good Good Good

Tinctorial Strength

Orange–red Blue

Deep–blue

Blue Yellow Red Lemon yellow Green-blue

Hue

Insoluble 20

1.6

20

9 25 19 20

Water Solubility

320

Red No. 3 Red No. 40 Yellow No. 6 Yellow No. 5

FD&C Name

Stability to Light

TABLE 13.7 Physical and Chemical Properties of Certified U.S. Colorants

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TABLE 13.8 Food Color Additives Exempt from Certification Color Additive

Color

Source

Annatto extract Beet juice Dehydrated beets (beet powder) Canthaxanthina Caramel Apocarotenalb Beta-carotene Carrot oil Cochineal extract (carmine) Cooked cottonseed flour Ferrous gluconatec Fruit juice (grape and cranberry) Grape skin extractd (enocianina) Paprika Paprika oleoresin Riboflavin Saffron Titanium dioxidee Turmeric Turmeric oleoresin Vegetable juice

Yellow Red Purple Red Brown Orange Yellow Yellow Red Brown Yellow Red Red Red Red Yellow Yellow White Yellow Yellow Red

Vegetable Vegetable Vegetable Synthetic Semisynthetic Synthetic Synthetic Vegetable Insect Semisynthetic Synthetic Fruit Fruit Vegetable Vegetable Synthetic Vegetable Synthetic Vegetable Vegetable Beet and red cabbage juice

Notes: Under the Code of Federal Regulations, Title 21. No color additive may be used in foods for which standards of identity have been promulgated under Section 401 of the Federal Food, Drug & Cosmetic Act, unless the use of added color is authorized by such standards. a b c d e

May not exceed 66 mg/kg of solid, or 1 pint of liquid, food. May not exceed 33 mg/kg of solid, or 1 pint of liquid, food. Coloring ripe olives only. Used only in beverages. May not exceed 1% by weight of the food.

Although a great number of noncertified food colorants are used by the U.S. food processing industry, demand is concentrated in a few products, namely paprika, caramel (principal use is in cola beverages), annatto, and synthetic carotenoids.

13.14.3 USE

OF

COLOR ADDITIVES

IN

FRUIT PRODUCTS

In the U.S., most canned fruit products, jams, and pure juice (canned, frozen) are regulated by Standard of Identity that prohibits the use of food colorants. Fruit products that are colored by synthetic or natural colorants are fruit drinks, fruit beverage powders, pie fillings, fruit desserts, maraschino cherries, apple rings, crab apples, flavored apple sauces, and certain jellies.

13.14.4 TRENDS

AND

REGULATORY STATUS

As a general guideline, in the U.S., FD&C dyes and lakes can be used in any food product unless otherwise prohibited by special regulations such as the Standard of Identity.

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TABLE 13.9 Major Western European Synthetic Dyes for Food Applications Color Tarazine Yellow Orange S Amaranth Cochineal Red A Erythrosine Patent Blue Indigotin

EC No. E E E E E E E

102 110 123 124 127 131 132

Application Desserts, lemonades, pasta, ice cream, syrup Fruit juices, fruit preserves Candies, ice cream, jams, fruit preserves Candies, ice cream, jams, beverages, desserts Fruits, maraschino cherries, ice cream Candies, beverages Candies, beverages

In the years since 1970, food colors, especially the synthetic organic colors, have received tremendous publicity — nearly all of it bad. Food colors have been the prime target of consumer activists. As a result of this, the food industry prefers the use of natural colorants in deference to synthetic colorants. Due to inadequate heat stability of most natural colorants, however, the FD&C certified synthetic colorants are expected to continue to be used by the U.S. food industry. A number of formerly certified FD&C colors have been “delisted” under the provisions of the Delaney Clause of the Food, Drug and Cosmetic Act, either because they were found to be carcinogenic or because there was no assurance that they could be made free from carcinogenic impurities. These actions have steadily reduced the number of certified colors available to the U.S. food industry (Table 13.6). The technical, safety, and legal issues of whether some synthetic colorants are carcinogenic is expected to continue. Presently, all eight certified colors are permanently listed as color additives by the FDA regulations, while all lake colors except Red No. 40 are provisionally listed. In 1990, the FDA banned FD&C Red No. 3 lake colors and imposed restrictions on the use of Red No. 3 dye. Some replacement colors have been suggested such as Red No. 40, carmine, cochineal extract, and beet concentrates, but no equivalent viable alternative colors are yet available for many food products. Opportunities for inventions of new color additives are highly limited because of the strict regulations for colors required for FDA approval. New product possibilities include colors produced by fermentation. As an example, an alga that has high beta-carotene content is being developed by biotechnology. Food application approval of a synthetic azo-dye carmazine (Yellow No. 10) has been petitioned, and presently, the compound is undergoing animal testing. Carmazine would be more stable and similar in color to FD&C Yellow No. 5. The Nutrition Education and Labeling Act of 1990 requires that all certified colors be specifically declared by name. Prior to this regulation, food processors were required only to put “food color” on the label with the exception of Yellow No. 5 (tartazine), which has been shown to cause an allergic reaction in 2% of the population. There is some increasing consumer concern about the use of colorants in food preparations in Western Europe. In line with this, a distinct trend toward the use of natural colors can be observed. The principal synthetic colorings in Western Europe are shown in Table 13.9. As for the natural colorings, caramel color accounts for over 90% of all coloring used in food. Natural colors approved in Western Europe include annatto, cochineal, carmine, curcumin (extracted from turmeric), chlorophyll derivatives, spray-dried beet root juice, or spices with a secondary coloring effect. In Japan, color additives are under regulation by the Food Sanitation Law. The legal definition is somewhat different from the general notion about synthetic (Table 13.10) and natural food colors (Table 13.11). In general, the term natural color additives refers to a substance that satisfies one of the conditions in the following list:

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TABLE 13.10 Synthetic Colors Approved for Food Use in Japan

• • • • • • •

Color

Common Name

Dye

Food Red No. 2 Food Red No. 3 Food Red No. 40 Food Red No. 102 Food Red No. 104 Food Red No. 105 Food Red No. 106 Food Yellow No. 5 (U.S. FD&C No. 6) Food Yellow No. 4 (U.S. FD&C No. 5) Food Blue No. 1 Food Blue No. 2

Amaranth Erythrosine Allura Red AC New Coccine Phloxine Rose Bengal Acid Red Sunset Yellow FCF Tartrazine Brilliant Blue FCF Indigo Carmine

X X X X X X X X X X X

Lake

X

X X X X

Color agents obtained from living organisms Chemically modified color agents originally derived from natural resources Chemically synthesized color substances that are exactly the same structure as naturally existing color agents Colors obtained by crushing dry plants or animals The squeezed juice of plants or animals that contain color agents Colors isolated from a microbial medium Colors obtained by enzymatic or microbial treatment of a natural substance

Since 1987, certificates from the government for dye colors are no longer required in Japan, having been replaced by self-imposed inspection.

13.15 VITAMINS 13.15.1 FUNCTIONS Vitamins are nutritive substances required for normal growth and maintenance of life. They play an essential role in regulating metabolism, converting fat and carbohydrates into energy, and forming tissues and bones. A total of 13 vitamins are recognized as essential for human health, and deficiency diseases occur if any one is lacking. Because the human body cannot synthesize most vitamins, they must be added to the diet. Most vitamins are currently consumed as pharmaceutical preparations or over-the-counter vitamin supplements. Some, like vitamins A, C, D, and E and folic acid, however, are directly added to food products. They are added for several related reasons to: • • • • •

Restore vital nutrients lost during processing — important with canned fruits, fruit juice, and refined and processed foods Standardize nutrient levels in foods when these fluctuate due to seasonal variations, soil differences, and methods of preparation Fortify fabricated foods that are low in nutrients and promoted as substitutes for traditional products, including complete fruit drinks and imitation products Fortify a major staple, such as bread, with a nutrient known to be in short supply Prepare functional foods (nutraceuticals) containing vitamins that are shown to be useful in preventing chronic diseases

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TABLE 13.11 Japanese Natural Color Additives for Food Processing Name

Color

Alkanet color Annatto extract Cacao color Carob germ color Cowberry color Elderberry color Gooseberry color Japanese persimmon Kusagi color Licorice color Madder color Orange color Red cabbage color Turmeric oleoresin/curcumin Uguisukagura color

Red–red purple Yellow–orange Brown Yellow Red–blue Red–blue Red–blue Dark red Blue Yellow Yellow–red purple Yellow Red–purple red Yellow Red–blue

Colorant Alkanin (naphthoquinone) Bixin, norbixin (carotenoid) Flavonoid Flavonoid Anthocyanin Anthocyanin Anthocyanin Flavonoid Trichotomin Isoliqueritigenin (flavonoid) Rubrtorin (anthraquinone) Xantophil (carotenoid), carotene Anthocyanin Curcumin Anthocyanin

Japanese Foodstuffs Also Used as Nonsynthetic Color Additives Fruit Juice Uguisukagura juice Elderberry juice Orange juice Cowberry juice Gooseberry juice Salmonberry juice Thimbleberry juice Strawberry juice Dark sweet cherry juice Cherry juice Dewberry juice Huckleberry juice Grape Juice

Blueberry juice Blackberry juice Black current juice Plum juice Berry juice Boysenberry juice Whortleberry juice Mulberry juice Morello cherry juice Raspberry juice Red currant juice Loganberry juice

Vegetable Juice Red cabbage juice Beet red juice Beefsteak plant juice Onion juice Tomato juice Carrot juice Rennet casein

Others Paprika Saffron

Source: Food Chemistry Division, Japanese Ministry of Health and Welfare, 1996.

While the addition of vitamins is practiced primarily for nutritional purposes, vitamins may also be used as functional ingredients in foods. The antioxidant vitamins C and E frequently are utilized to prevent undesirable color changes and to retard the development of rancid flavor. Riboflavin (vitamin B2: provides a greenish lemon-yellow color) and beta-carotene (pro-vitamin A: provides an orange color) are utilized as natural coloring substances.

13.15.2 PRODUCTS Vitamins are usually divided into two groups: fat-soluble and water-soluble. The fat-soluble group consists of vitamins A, D, E, and K. The water-soluble group consists of vitamin C (ascorbic acid), as well as the B vitamins. Eight nutritive B complex vitamins are required for health reasons by humans. These are niacin, riboflavin, pantothenic acid, pyridoxine, folic acid, thiamin, biotin, and vitamin B12.

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TABLE 13.12 Major Vitamins and Their Marketed Forms Consumed as Food Additives Principal Synonyms

Vitamin Vitamin A Vitamin B Niacin

Retinol Vitamin B3

Thiamine Riboflavin Pantothenic acid Pyridoxine Cyanocobalamin Biotin Folic acid Vitamin C

Vitamin B1 Vitamin B2 Vitamin B5 Vitamin B6 Vitamin B12 Vitamin H or B8 Vitamin Bc or Bg Ascorbic acid

Vitamin D D2 D3 Vitamin E Tocopherols

DL-alpha tocopherol

Vitamin K K1 K3

Phytonadione Menadione

Major Market Forms Vitamin A acetate Vitamin A palmitate Nicotinic acid Niacinamide Thiamine hydrochloride Riboflavin Calcium pantothenate Pyridoxine hydrochloride

Folic acid Ascorbic acid Sodium ascorbate Calcium ascorbate Ascorbyl palmitate

Ergocalciferol Cholecalciferol DL-alpha tocopherol acetate DL-alpha tocopherol D-alpha tocopherol Phylloquinone

Menadione sodium bisulfite

The most commonly marketed forms of the major vitamins consumed in the U.S. as food additives are listed in Table 13.12. Vitamin C (ascorbic acid) is the most commercially important vitamin used as a food additive in terms of volume. Approximately 30 to 40% of total U.S. demand for this product is as a nutritional additive for foods and beverages. Vitamin C occurs naturally in citrus fruits, berries, and most other fruits. It is commercially produced exclusively from glucose through hydrogenation to sorbitol, followed by fermentation of sorbitol to L-sorbose, and several steps of chemical synthesis and purification procedures.

13.15.3 APPLICATIONS The most important applications for vitamin C include fruit juices, fruit-flavored drinks, juiceadded sodas, and dry cocktail or beverage powder mixes. As an antioxidant, this vitamin is frequently added to fruit juice to preserve and protect against color change of the fruit ingredients. By doing so, these products are also promoted as the high vitamin sources. In fruit processing operations, primarily following blanching, thermal processing, and freezing, significant losses in ascorbic acid and other nutrients were observed. Further losses amounting to 26 to 47% of ascorbic acid occurred in canned fruits during two-years’ of storage at 80∞F. Therefore, an excess of all vitamins must be added to insure that the food contains the amount declared on

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the label after the intended storage period (Borenstein, 1975). It is necessary to add as much as 30% higher an amount over the nutrition claim. Besides vitamin C, most essential vitamins are largely added to milk, breakfast cereal, and meal replacement products; however, beta-carotene is utilized in orange flavor fruit drinks and fruit snacks, also called “energy bar” products aimed at health-conscious customers and often fortified with several essential vitamins.

13.15.4 REGULATORY STATUS In the U.S., vitamins are included in the GRAS list of the FDA under dietary supplements (21 CFR 182, Subpart F) or under nutrients (subpart I). The exception is calcium pantothenate, which is cleared as a food additive for special dietary uses and requires a label statement of its appropriate concentration, expressed as pantothenate. In Western Europe, vitamins are not regarded as food additives unless they are fulfilling such additional functions as: • •

Antioxidants: ascorbic acid (vitamin C), tocopherols (vitamin E) Colors: riboflavin-5-phosphate (vitamin B2)

In Japan, vitamins used as food additives are classified as dietary supplements under Japanese regulations. No maximum-use or labeling obligations are legislated for vitamins. Vitamin E (D g tocopherol) is not included in this category because its use is approved only as an antioxidant. Vitamin C is used as an antioxidant (see the antioxidant section). Riboflavin and its derivatives are used as vitamin supplements, as well as color additives.

13.16 PRESERVATIVES 13.16.1 FUNCTIONS In the broadest sense, a chemical preservative may be defined as any additive substance that tends to prevent or retard deterioration when added to foods. It may prevent or retard changes in odor, flavor, nutritive value, or appearance; therefore, chemical preservatives fall into three main categories: antimicrobial agents, antioxidants, and sequesterants. In this section, only the antimicrobial agents utilized in fruit preservation are discussed. Economic loss can be considerable without adequate control of undesirable microorganisms in production, transit, storage, and distribution of fruits. Antimicrobial agents inhibit the contamination of foods by microorganisms such as yeasts, bacteria, molds, or fungi. The principal mechanisms are reduced water availability and increased acidity. They may also preserve, to some degree, other important food characteristics such as flavor, color, texture, and nutritional value. The number of antimicrobial preservatives approved for use in food is remarkably limited. The primary food additives used in the U.S. for this function are: • • • •

Sorbic acid and its potassium salt Calcium and sodium propionates Sodium benzoate Parabens

Only sorbates, benzoates, propionates, and sulfites are used broadly in fruit processing. Although these preservatives are sometimes applied alone, usually they are used in conjunction with other methods of preservation such as refrigeration, freezing, and dehydration to obtain more thorough control of deleterious microorganisms. Also, several organic acids such as citric, malic,

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TABLE 13.13 Factors Influencing the Effectiveness of Antimicrobial Preservatives in Processed Fruits Food Composition Moisture content Presence of other inhibitors Salt Sugar Spices Hydrogen ion concentration (pH) Contamination Sanitary condition of ingredients and equipment Processing methods and control Temperature (time vs. temperature) Filtration Radiation Types of microorganisms present Handling of Processed Product Packaging Length of storage Storage environment (temperature, humidity, etc.)

lactic, ascorbic, erythorbic acid, and tartaric function as preservatives, but because of their greater use as acidulants or antioxidants, they are covered elsewhere in this chapter. While sulfites have an antimicrobial effect as well and are frequently utilized in dried fruits, usually their primary role is to function as an antioxidant (sulfites are covered in the antioxidant section). Chemical preservatives play a very important role in the fruit processing industries, from manufacture through distribution to the ultimate consumer. The choice of a preservative takes into consideration the product to be preserved, the type of spoilage organism endemic to it, the pH of the product, period of shelf life, ease of application, and so on (Table 13.13). No one preservative can be used in every product to control all organisms, and therefore, combinations are often used. In certain foods, specific preservatives have very little competition. In the concentrations used in practice, none of the preservatives discussed here is lethal to microorganisms in foods. They therefore will not reduce existing contamination but, instead, retard further growth of organisms already present, provided the degree of contamination is not too high; therefore, products that have already spoiled or fruits prepared under poor sanitary conditions will not benefit from the use of these preservatives.

13.16.2 PRODUCTS 13.16.2.1 Benzoic Acid Benzoic acid, usually in the form of the potassium or sodium benzoate salt, is most effective in the pH range of 2.5 to 4. The food processors, because of the low solubility of the free acid prefer the salt forms. Important uses of benzoates are in fruit juices and drinks, jams and jellies, pie fillings, fresh fruit cocktails, pickles, and condiments. In carbonated drinks, 0.03 to 0.05% is used;

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in noncarbonated drinks, up to 0.1% is used. The potassium benzoate salt form is utilized preferentially over sodium benzoate in many food products. The potassium salt form was developed specifically for use in reduced or low-sodium food products. Benzoates are more active against yeasts and molds and less effective against bacteria. It is not recommended for bacterial control because the use level is restricted to 0.1%, and the activity is poor above pH 4 where bacteria is the greatest problem. It is of interest that benzoic acid occurs naturally in some fruits such as cranberries, plums, and olives. 13.16.2.2 Sorbic Acid Sorbic acid is an unsaturated organic acid. Since 1955, it has been used as a food preservative. The free acid and its potassium salt forms, collectively referred to as sorbates, are used in food systems. They exhibit a broad spectrum of antimicrobial activity; they are more effective against yeast and mold than against bacteria. Sorbates function in an inhibitory microbial activity to a maximum pH of 6.5, which is sufficient range for most fruit products. With its neutral taste character, it is a choice preservative. Potassium sorbate salt is used where high water solubility is desired. Sorbates are frequently used in dried fruits, fruit salads, and carbonated and noncarbonated beverages. Usage rates of sorbates in fruits are low, being 0.025 to 0.075% in fruit drinks, 0.1% in beverage syrups, 0.05 to 0.1% in pie fillings, and 0.02 to 0.05% in dried fruits. 13.16.2.3 Propionic Acid Propionic acid and its sodium and calcium salts are highly effective mold inhibitors but have essentially no effect against yeast and bacteria. Because of its corrosive nature, propionic acid, a liquid, is rarely used in the food industry. The sodium and calcium salts are used in its place, yielding the free acid within the food at low pH. Their effectiveness against yeast extends up to pH 6; therefore, the main market for propionate salts is in bakery products, chiefly because these salts do not inhibit yeast action. In fruit product applications of 0.2 to 0.4%, propionates are recommended to retard mold growth on blanched apple slices, figs, cherries, blackberries, and dried plums.

13.16.3 REGULATORY STATUS Sorbates, benzonates, and propionates are included in the FDA GRAS list. Sodium and potassium benzoates are restricted to a maximum level of 0.1% in foods that are not covered by Standards of Identity. There is presently no upper limit imposed on the use of sorbates and propionates in foods. Labels of foods in which these preservatives are used must bear a statement of the common name of the compound and its purpose, such as “added to retard mold growth” or “added as preservative.” These preservatives are also approved in the western European countries and in Japan.

13.16.4 TRENDS

AND ISSUES

The chemical synthesis methods of producing chemical preservatives are well established. No new novel processes for these compounds have been commercialized in recent years. In general, increased demand by consumers for processed and prepared foods, as people tend to do less cooking at home, has stimulated use of antimicrobial preservatives over the past several years. At the same time, however, media and consumer reaction to chemical preservatives has stymied or limited the growth of several preservatives in favor of “all-natural” and “no preservatives added” food products. The irradiation of foods to prevent microbial growth, instead of the use of chemical preservative additives, has been under discussion for a number of years. However, it does not appear that this

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approach to preventing growth of molds and bacteria will have a significant detrimental impact on demand for chemical preservatives in the U.S. in the near future. Most chemical preservatives in use today have specialized uses and established niche markets in the food industry. A great amount of interchangeability does not exist because of specific inhibitory actions toward bacteria or mold or yeast. Blends of antioxidants and preservatives (some natural and some synthetic chemicals) can provide multiple functions for fruit products. One such combination of ingredients is a blend of erythorbic acid, citric acid, and potassium sorbate as an antioxidant and antimicrobial substitute for sulfites on fresh-cut fruits. With the high cost of developing a new preservative (or other food additive) and the time required to obtain FDA approval, it is unlikely that new preservative chemicals will be developed in the future. Rather, blends of existing preservatives and other available inhibiting food additive products will be used.

13.17 ANTIOXIDANTS 13.17.1 FUNCTIONS Antioxidants are food additives that retard atmospheric oxidation and its degrading effects, thus extending the shelf life of foods. Examples of food oxidative degradation include products that contain fats and oils in which oxidation would produce objectionable rancid odors and flavors, some of which might even be harmful. Antioxidants are also used to scavenge oxygen and prevent the discoloration of cut or bruised fruits and vegetables. Oxidative browning, which is caused by the action of polyphenol oxidase with catechol tannins, is a very important problem in handling most fruits, especially peaches, nectarines, apples, and cherries. Discoloration is encountered in all methods of processing, but mainly with methods that do not involve cooking, such as freezing, drying, and making juices. Changes in flavor accompany changes in color. Browning may take place before peeling, due to bruising, but is accelerated once the skin is broken or tissue cells are ruptured. One of the important methods of controlling browning of fruits is by using water-soluble antioxidants. To improve the performance of antioxidants, two other types of food additives, sequesterants (e.g., EDTA, citric acid) and synergists (e.g., mixtures of antioxidants and lecithin), are frequently used with them. Antioxidants may also be added to food packaging, but such use is not covered in this chapter. Food antioxidants are effective in very low concentrations (0.01% or less of the fat content of foods) and not only retard rancidity, but also protect the nutritional value of the food by minimizing the breakdown of vitamins and essential fatty acids. The maximum concentration of food antioxidants approved by the FDA is 0.02%. At one time, safety questions were raised about several synthetic antioxidants. These have largely been resolved, although the impact on the market for synthetic antioxidants still exists.

13.17.2 PRODUCTS

AND

APPLICATIONS

Food antioxidants are generally classified as natural or synthetic. The most commonly used natural antioxidants are ascorbic acid (vitamin C) and erythorbic acid and their sodium salts, plus the mixed a- and g-tocopherols. Vitamin C finds more major use as a nutritive supplement or in pharmaceutical preparations. Small amounts, however, are intentionally used for antioxidant purposes. Citric acid and tartaric acid are also natural antioxidants (and antioxidant synergists) but most frequently are added to foods as acidulants and are covered in the acidulant section of this chapter. Synthetic antioxidants used as direct food antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butyl hydroquinone (TBHQ), and propyl gallate (PG). Most food-grade antioxidants are sold as either solids or solutions.

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TABLE 13.14 Food Antioxidants Oil-Soluble Products: Butylated hydroxyanisol (BHA)a Butylated hydroxytoluene (BHT)a tert-Butyl hydroquinone (TBHQ)a Propyl gallate (PG)a Tocopherolsa Thiodipropionic acid Dilauryl thiodipropionate Ascorbyl palmitate Ethoxyquin Water-Soluble Products: Ascorbic acida Erythorbic acida Glucose oxidase/catalase Gum guaiac Sulfitesa a

Major products.

Also, antioxidants may be divided into water-soluble compounds and oil-soluble ones (Table 13.14). The major use of oil-soluble antioxidants is in edible fats and oils. They are also used to prevent rancidity of shelled nuts used as snacks and as an ingredient in bakery products, citrus oils and other fruit flavor emulsions, confectioneries, and so on. Water-soluble antioxidants are widely used in fresh prepared (peeled, cut), frozen, dehydrated and thermally processed fruits (especially those packaged in nonmetallic containers), fruit juices and drinks, juice concentrates, flavoring compositions, and fruit snacks. The increased use of lightly processed foods containing fruits and fruit juice drinks is utilizing large quantities of water-soluble antioxidants, mainly ascorbic acid. 13.17.2.1 BHA BHA was introduced commercially in foods in 1948. Commercial preparations of BHA contain two isomers: 3-tertiary-butyl-4-hydroxyanisole and 2-tertiary-butyl-4-hydroxyanisole. The major applications for BHA are in edible rendered fats, edible frying fats and oils, salad oils, and shortenings. It has also been found to be an effective antioxidant for nut meats, orange-flavored fruit drinks, and processed fruits containing carotene pigment. While BHA can and is used alone as an antioxidant, it has also been used in combination with other antioxidants and synergists. BHA is frequently blended with BHT, TBHQ, or PG to optimize performance. 13.17.2.2 BHT BHT, a hindered phenolic-type compound was approved for use as a food antioxidant in 1954. BHT is used in blends with BHA or BHA/propyl gallate in vegetable oils and in edible animal fats. The major use of BHT, however, is as an antioxidant for waxes used in the manufacture of food packages and wrappers. Like BHA, BHT is effective at low concentrations. Concerns over the safety of chemical additives in food caused a decrease in demand for BHT. Also, currently, the availability of oxygen absorbent products has reduced the necessity for antioxidants such as BHT and BHA.

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TABLE 13.15 European Community Usage Levels for Ascorbic Acid in Processed Fruits and Beverages Application

Quantities Used

Prevention of Browning in: Frozen fruit 300–1000 mg/kg Fruit juices 120–200 mg/kg Nectars 300–500 mg/kg Canned fruit 1000–3000 mg/kg

Maximum Levels Recommended

2000 200 500 2000

mg/kg mg/kg mg/kg mg/kg

Prevention of Discoloration in: Olives 100–500 mg/kg Flavor Protection in: Citrus beverages 150–200 mg/l Wine 25–100 mg/l

200 mg/kg 200 mg/kg

13.17.2.3 TBHQ TBHQ was first introduced for food applications in 1972. TBHQ shows an exceptional activity in protecting unsaturated vegetable oils and animal fats from rancidity. One of its largest applications is in soybean oils. Although mostly used by itself, TBHQ can be used in combination with BHT and BHA. TBHQ is often used as a replacement product for propyl gallate. 13.17.2.4 Propyl Gallate (PG) PG has been used as a food antioxidant since the 1950s. Its current primary use is more as a synergist in combination with BHA and BHT. This antioxidant is a highly effective product for animal fats. 13.17.2.5 Ascorbic Acid Ascorbic acid (vitamin C) and sodium ascorbate is widely used as antioxidants and vitamin supplements. As an antioxidant, ascorbic acid is used primarily in prepared foods such as frozen fruits, fruit drinks, canned fruits, and applesauce. Its use as a direct food additive has increased in the recent past. 13.17.2.6 Erythorbic Acid Erythorbic acid (iso-ascorbic acid–a stereo isomer of ascorbic acid) and sodium erythorbate are used primarily as antioxidants in cured meats (e.g., bacon, sausages) and in salad bars as an oxygen scavenger. They are also used in canned and frozen fruits, fruit juices and nectars, olives, and refrigerated guacamole products to retard discoloration and off-flavor development (Table 13.15). The principal difference between ascorbic and erythorbic acid is that ascorbic acid has vitamin C activity while erythorbic acid does not. Vitamin activity is not necessary for antioxidant application, and in some instances, regulatory constraint makes it undesirable to add a nutrient to processed fruit product. Replacement of ascorbic acid with less expensive erythorbic acid provides comparable results in respect to antioxidant activity on an equal weight basis, or 1.23 kg of sodium erythorbate can replace 1 kg of ascorbic acid in various frozen or canned fruit products.

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Erythorbic acid (and salt) has benefited significantly from the FDA’s 1986 ban on the use of sulfites for fresh or uncooked vegetables in salads. 13.17.2.7 Tocopherols Tocopherols are naturally occurring antioxidants found in seed oils, fruits, vegetables, and animal tissues and are receiving considerable attention as a possible replacement for synthetic antioxidants. Tocopherols are of considerable interest both as an antioxidant and as a nutrient. Vitamin E is atocopherol. Although all isometric forms (a, g, and d) of tocopherol show antioxidant activity, the 80% gamma and 20% delta mixture of natural tocopherols has the best antioxidant activity (DLalpha-tocopherol, which is actually a D,L-racemic mix, is primarily noted as a source of nutritional vitamin E). The mixed natural tocopherol products can be used to protect a variety of fruit products, including dehydrated and processed fruits and citrus fruit drinks. 13.17.2.8 Sulfites Sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite are very effective to retard the browning of fruits in which the enzymes have not been inactivated by sufficiently high heat (Sapers, 1993). In addition to preventing enzymatic browning, sulfur compounds will reduce destruction of carotene and ascorbic acid, which are the important nutrients of fruits. Sun-dried fruits (e.g., apricots, peaches, and pears) are usually exposed to the fumes of burning elemental sulfur before being put out to dry in the sun. Apples are often treated with solutions of sulfite before dehydration. Solutions range from 0.2 to 0.5% (as SO2) and are made up of sodium sulfite and sodium bisulfite in about equal portion. Sulfite solutions are less suitable than burning sulfur because the solutions penetrate the fruit poorly and leach out natural sugar, acid, and flavor components. Treatment of fruits with sulfites is the most effective means available today to control browning; however, because sulfites have been banned in certain categories of products and their regulatory status for other categories is in question, the food industry is looking for alternative means of controlling browning. For years, restaurants and other food service outlets had prevented the browning of fresh produce by the use of sulfites. But because of allergic reactions of some consumers (especially asthmatics) to sulfites, regulations were issued and alternatives sought (Taylor and Bush, 1986). One of the most promising antioxidant/preservative alternatives is a blend of erythorbic and citric acids. The ascorbic acid derivatives, ascorbic acid 2-phosphate and ascorbic acid-6-fatty acid esters, are also reportedly effective. Another suggested substitute (which functions in water but not with fats and oils) is the sequesterant and chelating agent ethylene diamine tetraacetic acid (EDTA), which has been widely used in processed potatoes, salad dressings, sauces, and beverages. Cyclodextrin is another sulfite alternative that can be used to prevent browning. Finding a complete substitute for sulfites, however, has not yet been realized. This is because the sulfites not only acted as antioxidants to prevent browning, but also performed preservative functions in preventing microbial spoilage. 13.17.2.9 Gum Guaiac Gum guaiac is a resinous secretion of a tropical evergreen, Guaiacum officinalis. This resin contains complex phenolic compounds chemically related to guaiacol, guiaretic acid, and guaiaconic acid. Like other phenolic compounds, gum guaiac is more effective in animal fats than vegetable oils. Gum guaiac is an approved antioxidant for natural flavoring substances and other natural substances used in conjunction with fruit flavors. It is also approved for addition to food packaging materials.

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13.17.2.10 Spice Extracts Rosemary-based antioxidants have been of continuing interest to some food processing segments because they appear to offer superior antioxidant performance in certain applications and are considered label-friendly. Rosemary extracts is used primarily in processed meats and poultry, as well as in snack foods, dehydrated products, beverages, and flavoring oils. While enjoying GRAS status as a natural flavoring, spice extracts do not have specific FDA approval for the use as antioxidants and thus cannot be promoted as such.

13.17.3 TECHNOLOGY

AND

MANUFACTURE

Natural antioxidants are extracted from certain plants and plant seeds; however, the most widely used natural-based antioxidants are currently produced commercially by fermentation processes of natural raw materials followed by synthetic chemical operations (e.g., ascorbic acid production from sorbitol). Tocopherols can either be manufactured from vegetable oil sources or produced by fermentation from the intermediate phytol. Erythorbic acid is obtained as a coproduct in the manufacture of ascorbic acid, or by chemical synthesis.

13.17.4 REGULATORY STATUS Past developments have had a detrimental effect on the demand for BHA. In June 1982, the Japanese government reported that, according to feeding studies done in Japan with BHA at 2% of the entire diet, BHA was found to be carcinogenic. Consequently, BHA would not be allowed in food products sold in Japan after July 1, 1982. Various governments, including those of the U.S., Canada, and the U.K., requested a delay in the implementation date until further studies could be done. The date was then deferred to February 1, 1983, and the ban was never implemented. Subsequent to the initial deadline, the World Health Organization’s Food and Agriculture Organization studies showed that BHA dosage levels would have to be high (e.g., about 2% of the oil or fat content of the food) before any carcinogenic effects would become apparent. (Normal BHA content level is 200 ppm of the fat or oil content of the food.) However, due to Japan’s announcement of its initial study, BHA was removed from some of the food and food packaging sold in the U.S. and Japan. The findings of the Japanese study relative to BHA were surprising because several other studies conducted worldwide had found BHA to be anticarcinogenic. The original Japanese researcher has now agreed that BHA is not carcinogenic; however, irreparable damage to BHA use as a food antioxidant has no doubt already occurred (Anon., 1994b). Although BHT was never removed from the FDA’s GRAS list, demand for BHT as a direct food additive dropped significantly in the 1970s, largely due to an FDA proposal to restrict the use of BHT as a food additive throughout the 1980s. That proposal, as well as the general trend toward the use of all-natural ingredients in foods, has negatively impacted BHT use in foods. In 1986, six sulfiting agents — sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite — were banned by the FDA for use in raw vegetables and fruits in salad bars. In 1987, the FDA ruled that all packaged foods that contain 10 ppm or more of sulfur dioxide equivalents disclose on the label that sulfiting agents are present. GRAS status is not recognized for use of sulfates in food recognized as a source of vitamin B1 and of fruits and vegetables intended to be served raw to customers or sold raw to consumers or to be presented to the customer as fresh. Also in 1988, FDA established limitations of the maximum residual sulfur dioxide equivalent levels in many food categories. Those limits, applying to fruit products are as follows: jams and jellies, 30 ppm; nut products, 25 ppm; dried fruits, 2000 ppm; fruit juice concentrates, 1000 ppm; single-strength fruit juice, 300 ppm; glace fruits, 150 ppm; maraschino cherries, 150 ppm; wine, 275 ppm; and vinegar, 75 ppm.

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13.17.5 TRENDS

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Opportunities exist for manufacturers of food antioxidants and other preservatives to increase the shelf life of fresh and processed foods. (The sulfite ban has created one such opportunity.) As people incorporate larger amounts of polyunsaturated fats in their diets and decrease the perceived unhealthy saturated fats, the need for food antioxidants also increases. This is because many of the unsaturated fats, such as that found in olives and nuts, are relatively unstable and become rancid more quickly. With total American consumption of fats and oils increasing due to snack, convenience, and processed foods, more antioxidants are needed. Common table salt and sugar are known to be natural antioxidants and preservatives; use of these products is declining also. With the trend toward lowered sodium and more dietetic foods and beverages, a higher level of protection is needed from substitute or replacement chemicals. In addition, shelf life is a major consideration in the development and marketing of snack foods that are proliferating in the U.S. In these products, consumer acceptance depends on maintaining the original taste, appearance, freshness, flavor, odor, and texture. Antioxidants preserve many of these qualities. The food industry has a preference for natural antioxidants. In this industry, it is desirable to have label designations on food products identifying the food antioxidants as naturally occurring or derived from naturally occurring materials. Once the natural-based antioxidants can be made as cost-effective as the synthetic types, their impact on this market will be greater. The use of synthetic antioxidants is tightly regulated in western Europe. TBHQ is not approved in EC countries. The existence of different legislation for synthetic antioxidants (e.g., no BHA is allowed in Germany; no BHT is allowed in France) means that food companies and suppliers of antioxidants must be aware of the differences and adapt their strategies accordingly. Because synthetic antioxidants are paid unfavorable attention, research on and development of effective natural antioxidants is actively carried out. Some examples of development are the extract of Glycyrrhiza glabra, polyphenol extracted from tea leaves, lignans from sesame, and extracts of spices such as rosemary, cinnamon, clove, and allspice. In the case of rosemary (Rosmarinus officinalis, L.), a standardized extract of its oleoresin has the active antioxidant components (Six, 1994). Rosemary extracts at very low, about 200 to 400 ppm, concentrations have particularly excellent antioxidant properties. Specific application include pickling brines and snack food nuts. Because of the disadvantage of conferring the particular rosemary taste and some color to food, its use is limited to food types where this is not a problem. Another development that could impact the use of current food and beverage antioxidants is the use of specialty silicate powders that are capable of filtering oxygen from liquids. Currently, research is in progress to commercially develop an insoluble organic powder that could be used to remove oxygen from beverage products such as fruit juices, wines, and carbonated soft drinks before packaging. The antioxidant material could also be coated or implanted in individual product packages. Such a product could have broad commercial use in the food industry.

13.18 ENZYMES 13.18.1 PRODUCTS

AND

FUNCTIONS

Enzymes are catalysts used during food processing to make chemical changes to the food. They are biological catalysts that make possible or greatly speed up chemical reaction by combining with the reacting chemicals and bringing them into the proper configuration for the reaction to take place. Enzymes are not affected by the reaction themselves. All enzymes are proteins and become inactive at temperatures above around 40∞C or in unfavorable conditions of acidity or alkalinity. Enzymes are important in food technology because of the roles they play in the composition, processing, and spoilage of foods. Sometimes, the presence of natural enzymes is advantageous; for example, cellulases and pectic enzymes assist in the softening of fruits during ripening to give

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desirable texture. In other cases, natural enzymes may produce undesirable reactions such as the browning reactions of cut fruits due to polyphenol oxidases. Recognition of the function of enzymes and their usefulness in bringing about desirable changes has led to their large-scale use as modifiers of food ingredients (Blenford, 1993). Enzymes are extremely specific and can act only on a single class of chemicals such as proteins, carbohydrates, or fats. Some of the specific functions food enzymes perform are to: • • • • • • • •

Speed up reactions Reduce viscosity Improve extractions Carry out bioconversions Enhance separations Develop functionality Create/intensify flavor Synthesize chemicals

Today, many fruit processes utilize enzymes. The food-grade enzymes encompass a wide variety of commercial products that are employed in the production, conversion, and modification of foods because of their highly efficient and selective catalytic function.

13.18.2 APPLICATIONS

IN

FRUIT PROCESSING

Many of the advances in fruit juice technology have been closely linked to developments in enzyme technology. The introduction of pectic enzymes made clarification of fruit juices possible, and the reduction in viscosity by enzymes permitted the preparation of concentrates. Treatment of fruit pulp with pectic enzymes made it possible to press soft fruits such as strawberries, pears, and soft cold-storage apples. A more recent development is enzymatic liquefaction where combinations of pectinases and cellulases can liquefy fruit with the resulting advantages of increased yields (Wrolstad et al., 1994). 13.18.2.1 Pectic Enzymes Pectic enzymes are used to catalyze the hydrolysis of pectinaceous materials. Complete hydrolysis leads to the production of pectic acids. Pectic substances are found widely in plant tissues, particularly in fruits. While pectic enzymes occur in many fruits, the commercial enzyme products are derived from strains of the fungus Aspergillus niger. The commercial enzymes are mixtures of several of the pectic enzymes, particularly pectin methylesterases and polygalacturonases. Differences of the commercial preparations are due to variations in the kinds and amounts of particular pectic enzymes present. Pectic enzymes have many uses in fruit processing. The importance of enzymes in fruit juice and wine processing is emphasized by their separate coverage in Chapter 4 of this book. Briefly, through the uses of these enzymes, juices of clarity and high yield may be produced. Also, pectic enzymes are indispensable for making fruit juice concentrates or purees. Fruit juices for jelly manufacturing are usually depectinized completely by pectic enzymes in order to obtain clear juices and make uniform jelly possible by adding back a standard amount of pectins when the jelly is made. Pectic enzymes are also used for the recovery and stabilization of citrus oils from lemon and orange peels. 13.18.2.2 Amylases These enzymes act on starch and starch-containing plant materials. Of all the commercial enzymes, amylases have the most numerous applications, primarily in the hydrolysis of starch to sugars. The use of amylases in fruit processing is minor, compared to industrial starch processing, but important.

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Fungal a-amylases produced from Aspergillus oryzae by fermentation are used for the starch removal from fruit juices and extracts, from flavoring extracts, and in the preparation of starch-free pectin. For the latter use, special amylase preparations completely free from pectinase are requisite. 13.18.2.3 Cellulases Cellulose, the most abundant of all naturally occurring organic compounds are present in all fruit tissues. Multienzyme cellulase complexes are utilized commercially for conversion of cellulose to glucose. The commercial cellulases are derived from Aspergillus niger, or other microorganisms, such as Trichoderma viride by controlled fermentation. Commercial applications for cellulases in fruit processing is limited. It includes usage in fruit juice manufacture to aid extraction and clarification of citrus juice and extraction of flavoring material from citrus pulp. Cellulase offers improved production of fruit juices by destroying the cellulosic components of the plant and allowing more free flow of the juice. An application in coffee manufacture results in the hydrolysis of coffee gums to prevent liquid coffee concentrates to gel. Crude cellulase preparations are used to treat organic wastes from fruit processing operations. 13.18.2.4 Glucose Oxidase and Catalase Commercial glucose oxidase preparation, usually combined with catalase, is designed to keep oxygen out of products that might be sensitive to it by catalyzing the reaction of glucose to gluconic acid and absorbing oxygen in the process. The presence of catalase in the enzyme preparation is needed to provide oxygen for the reaction of glucose to gluconic acid. The commercial glucose oxidase preparations are derived from Aspergillus niger. Oxygen is responsible for a wide range of deterioration in processed fruits, most importantly those of undesirable flavor and color changes. Light, particularly sunlight and fluorescent light, results in a deterioration of citrus beverages in a manner that is referred to as forming “sunstruck flavor.” Oxygen is necessary for the formation of peroxides and the resultant unwanted changes in flavor and color of citrus oils. By enzymatic removal of oxygen, successful applications for glucose oxidase were reported with citrus concentrates and drinks in preventing the development of offflavor (Scott, 1975) and the prevention of enzymatic browning of fresh frozen fruits. Also, glucose oxidase is effective in protecting cans of beverages against oxidative corrosion, and it has been recommended for the prevention of iron pickup in canned fruit drinks. 13.18.2.5 Bromelain, Ficin, and Papain Enzyme systems for the tenderization of muscle protein are usually made up from proteases extracted from fruit tissues. Thus, certain fruits are utilized to produce protease enzymes, which are important functional food ingredients as they are capable of hydrolyzing both plant and animal proteins to peptides and amino acids. One of the potent proteases is extracted from the latex of a tropical fig tree. Papain is derived from the latex of papaya (Carica papaya); and bromelain from pineapple. These enzymes can hydrolyze a variety of proteins.

13.18.3 TECHNOLOGY

AND

MANUFACTURE

The majority of today’s food processing (and other industrial) enzymes are produced by submerged fermentation using bacteria, yeast, and fungi (Ashie, 2003). In the vast majority of cases, the fermentation culture is batch processed using a carbon-containing feedstock such as glucose, molasses, or starch hydrolysates on which the yeast, fungi, or bacterial microbe can grow. Other components of the broth include a nitrogen source for the proteinaceous enzyme’s growth, plus mineral salts and yeast extracts. Fermentation normally takes 2 to 3 days with aerobic stirring (to

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provide the oxygen for the microbe’s growth). Most exudate (enzyme) is extracellular. Recent upgrades of enzyme production, however, include intracellular production of the enzyme followed by rupture of the cell wall and collection of the product enzyme. Animal glands and plant tissue also provide enzymes, but to a much smaller extent today. As compared to animal- or plant-derived enzymes, microbial enzymes offer several advantages such as purity, economy, consistency, and availability. The microbial enzymes can be modified to suit the needs of a reaction. For example, properties such as the thermostability or pH tolerance of an enzyme can be enhanced by routine mutagenesis of more sophisticated techniques such as protein engineering.

13.18.4 TRENDS, ISSUES,

AND

DEVELOPMENTS

Current research and development work continues to look for more applications and more specialized functioning enzymes (some genetically engineered), but the commercial use of enzymes in food processing and food modification has not advanced tremendously since the development of an enzymatic process for high-fructose corn syrup in the 1960s. New commercial applications that could significantly expand the current market for food-grade enzymes are slow to appear on the horizon, and few appear to be of economic value. Enzyme infusion into intact plant tissues has received recent attention to achieve specific changes in fruits. To date, this technique has focused primarily on manipulation of cell wall components (McArdle and Culver, 1994). For example, enzymes can be applied to intact citrus products to selectively alter albedo structure and facilitate peeling and segmentation. In another promising work, pectin methyl esterase was infused into blanched peach halves. After thermal processing, infused fruit were significantly firmer than the untreated fruits, presumably because the enzyme-induced removal of polyuronide methoxyl groups increased calcium binding of the cellwall tissues. Although many food and agricultural applications are amenable to treatment via enzyme technology, the projected returns often do not defray the high cost of developing new enzymes for small-volume applications. The impetus to use enzymes in these situations will depend on a price differential sufficiently large and stable to justify the development costs. Much of the scientific basis appears to be available now and ready to be refined.

REFERENCES Anon. Sweetness with calorie reduction. Food Ingr. Process. Int., (3): 15–19, 1992. Anon. Japanese Standards for Food Additives. 6th ed., English version, Japan’s Ministry of Health & Welfare, Tokyo, 1994a. Anon. FASEB panel: BHA is not a cancer risk. Inform, 5(10): 1167, 1994b. Ashie, I. N. A. Bioprocess engineering of enzymes. Food Technol., 57(1): 44–51, 2003. Bauer, K. International flavor legislation, in Source Book of Flavors, G. Reineccius, Ed., Chapman & Hall, New York, 1994. pp. 876–901. Blenford, D. Enzymes–food engineering’s catalytic converters. Food Ingr. Proc. Int., (1): 12–13, 1993. Borenstein, B. Vitamin fortification technology, in Technology of Fortification of Foods, National Academy of Sciences, Washington, D.C., 1975. pp. 1–7. Borenstein, B. and Bunnel, R. H. Carotenoids: Properties, occurrence, and utilization in foods. Adv. Food Res., 15: 195–276, 1967. Code of Federal Regulations, Food and Drugs. Title 21, Subchapter B — Food for Human Consumption. Parts 100–199. U.S. Government Printing Office, Washington, D.C., 2002. Duxbury, D. D. Multi-functional gum gets final approval. Food Process., 54(2): 62, 1993. Dwivedi, B. K. Sorbitol and mannitol, in Alternative Sweeteners, 2nd ed., O’Brien Nabors, L., and Geraldi, R. C., Eds., Marcel Dekker, New York, 1991, chap. 18. Dziezak, J. D. Gellan gum receives FDA approval. Food Technol., 44(11): 88–90, 1990.

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Dziezak, J. D. A focus on gums. Food Technol., 45(3): 117–132, 1991. Fischetti, F., Jr. Natural and artificial flavors, in Handbook of Food Additives, 2nd ed., Furia, T. E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 11. Francis, F. J. Colorants. American Association of Cereal Chemists. St. Paul, MN, 1998. Gardner, W. H. Acidulants in food processing, in Handbook of Food Additives, 2nd ed., Furia, T.E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 5. Giese, J. H. Alternative sweeteners and bulking agents. Food Technol., 47(1): 114–126, 1993. Giese, J. H. Modern alchemy: use of flavors in food. Food Technol., 48(2): 106–116, 1994. Glicksman, M. Origins and classification of hydrocolloids, in Food Hydrocolloids, Vol. 1, Glicksman, M. Ed., CRC Press, Boca Raton, FL, 1982, pp. 4–17. Goodburn, K. EU Food Law. CRC Press, Boca Raton, FL, 2001. Hegenbart, S. Bind for glory: Designing foods using gums. Food Prod. Des., 2(10): 21–42, 1993. Linden, G. and Lorient, D. Pigments and aromas, in New Ingredients in Food Processing, CRC Press, Boca Raton, FL, 1999, chap. 15. McArdle, R. N. and Culver, C. A. Enzyme infusion: a developing technology. Food Technol., 48(11): 85–89, 1994. McHugh, T. H. and Krochta, J. M. Milk-protein-based edible films and coatings. Food Technol., 48(1): 97–103, 1994. Mentzer-Morrison, R. Fat replacers. Inform, 3(12): 1270–1282, 1992. O’Brien Nabors, L. Sweet choices: Sugar replacements for food and beverages. Food Technol., 56(7): 28–34, 2002. O’Brien Nabors, L. and Gelardi, R. C. 1991. Alternative sweeteners: an overview. In Alternative Sweeteners, 2nd ed., O’Brien Nabors, L. and Gelardi, R. G., Eds., Marcel Dekker, New York, 1991, chap. 1. Reineccius, G. Source Book of Flavors. Chapman & Hall, New York, 1994. Salzer, U. J. and Jones, K. Legislation, Toxicology, in Flavourings, Ziegler, E. and Ziegler, H. Eds., WileyVCH, New York, 1998, chap. 7. Sapers, G. M. Browning of foods: control by sulfites, antioxidants and other means. Food Technol., 47(10): 75–84, 1993. Scott, D. Applications of glucose oxidase, in Enzymes in Food Processing, 2nd ed., Reed G. Ed., Academic Press, New York, 1975, pp. 519–549. Six, P. Current research in natural food antioxidants. Inform, 5(6): 679–688, 1994. Smith, R. L. et al. GRAS flavoring substances. Food Technol., 57(5): 46–59, 2003 Somogyi, L. P., et al. Food Additives. Specialty Chemicals Strategy for Success. SRI Consulting, Menlo Park, CA, 1996. Somogyi, L. P. and Kishi, A. Aroma chemicals and the flavor and fragrance industry. Chemical Economics Handbook, SRI Consulting, Menlo Park, CA, 2001. Stauffer, C.E. Emulsifiers for the food industry, in Y.H. Hui, Ed., Bailey’s Industrial Oil & Fat Products, 5th ed., Vol. 3, Wiley-Interscience, New York, 1996. Taylor, S. L. and Bush, R. K. Sulfites as food ingredients. Food Technol., 40(6): 47–50, 1986. Wodicka, V. O. Legal considerations on food additives, in Handbook of Food Additives, 2nd ed., Furia, T.E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 1. Wrolstad, R. E., Wightman, J. D., and Durst, R. W. Glycosidase activity of enzyme preparations used in fruit juice processing. Food Technol., 48(11): 90–98, 1994.

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Assurance, 14 Quality Quality Control, Inspection, and Sanitation Conrad O. Perera and Anne D. Perera CONTENTS 14.1 14.2 14.3 14.4

Introduction ........................................................................................................................339 Quality Control (QC) .........................................................................................................340 Quality Assurance ..............................................................................................................341 Quality Systems .................................................................................................................342 14.4.1 Hazard Analysis Critical Control Points (HACCP)............................................342 14.4.1.1 HACCP Principles .............................................................................343 14.4.1.2 Prerequisite Programs for HACCP....................................................344 14.5 Quality Control, Inspection, and Testing...........................................................................344 14.5.1 Moisture Content .................................................................................................345 14.5.2 pH.........................................................................................................................345 14.5.3 Respiration of Fruits............................................................................................345 14.5.4 Time of Harvesting..............................................................................................346 14.5.5 Postharvest Handling ...........................................................................................346 14.5.6 General.................................................................................................................346 14.6 Inspection and Test Procedures .........................................................................................347 14.6.1 Sampling ..............................................................................................................348 14.6.2 Sensory Evaluation ..............................................................................................348 16.6.3 Textural Characteristics .......................................................................................348 14.6.4 Determination of Drained Weight .......................................................................348 14.6.5 Frozen and Canned Fruits ...................................................................................349 14.7 Sanitation............................................................................................................................349 14.7.1 Microbiological Sampling Plans .........................................................................349 14.7.2 Sanitation Procedures ..........................................................................................351 14.8 Summary ............................................................................................................................352 References ......................................................................................................................................352

14.1 INTRODUCTION The need to address both food safety and food quality concerns has become increasingly important to the food industry in general. There have been several contributing factors: increasing awareness of consumers to the importance of nutrition in prevention of certain diseases, health risks associated with residues in certain foods, the availability of a wide variety of foods to consumers and the resulting level of competitiveness in the food industry, stringent government regulations relating 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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to the composition of food products and food additives, nutritional labeling requirements, international competitive factors relating to regional trade agreements, and environmental issues. Additionally, there is a danger of losing customer confidence and future demand for products, thus affecting the success of businesses, if quality and safety of their products are compromised. As a result, food manufacturers must now address concerns that traditionally were only peripheral to the scope of their operations, in addition to their traditional issues. This applies almost universally to all sectors of the food processing and manufacturing industry. The importance of the fruit sector of the fresh produce food industry is based on consumer dependence on fruit as a source of vitamins, minerals, carbohydrates, and dietary fiber and on their use as beverages, as specialty products, and as ingredients in many processed products. Consequently, fruits find use as freshly harvested produce, as well as processed products. In general, fresh fruits are high-moisture products, with moisture levels ranging from 50% up to 96% and are thus extremely susceptible to spoilage and microbial contamination. In this respect, the preservation of their quality and safety is a particular challenge to the food industry. Increasingly, the food industry is becoming dependent on the use of formal quality assurance (QA) programs for ensuring the safety and quality of foods. These quality assurance programs have evolved from their status as inspection-oriented programs to encompass a much broader scope involving all areas of management and production personnel. A food quality assurance program can be viewed broadly as a program designed to ensure the achievement of both safety and quality criteria or specifications of a product during all stages of handling, processing, preparation, packaging, storage, and distribution. It is critical, therefore, to establish specifications or at least certain criteria at all stages of these operations. Because of the wide variety of fruits and their uses and applications, the criteria for evaluating safety and quality vary considerably. For sale as fresh produce, it is of major importance that fruit should possess sensory attributes such as firmness, surface color, flavor, and spoilage-free appearance. Of greater importance for fruit intended for processing are attributes such as wholeness, good drained weight, and the absence of insects, residues, and microorganisms. Consequently, the safety and quality requirements vary, and there is the accompanying diversity in activities directed at achieving these specified requirements. Traditionally, achievement of safety and quality requirements was heavily dependent on inspections, by which materials and products were screened for defects and lack of conformance. With the growing trend in competitiveness and shrinking profit margins, food industries have had to take a much more proactive and preventative approach to ensure that their products meet the current stringent market demands for safety and quality. This chapter covers the principles of total quality assurance as it relates to the fruit industry, along with specific details on sanitation and inspection procedures necessary to achieve the requirements of safety and quality desired in this industry.

14.2 QUALITY CONTROL (QC) The fruit processing industry knows what good quality food production is, but often does little to maintain standards unless motivated by economics, regulations, and adverse publicity because of the lack of effective management, lack of apparent increased profits, high cost of quality control programs, or due to lack of knowledge of effective control programs. Fruit processors need QA and QC systems because of (1) liability claims — there are about 500 deaths per year due to Listeria outbreaks alone in the U.S. (ERS-USDA 2001), (2) consumer demand and loss of sales due to poor quality, (3) investigative costs for testing, experts, and future QC work; (4) cleanup costs and plant closings; and (5) food handler illnesses that affect efficiency. The benefits of good quality programs are (1) increased yield or productivity, (2) reduced distribution costs, (3) reduced cost of consumer relations and services, (4) more effective sales, (5) reduced recall costs, (6) decreased operating costs, (7) decreased marketing costs, (8) pride of workforce, (9) job security, and (10) profits.

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QC involves (1) conforming to standards and specifications of a product, (2) issuing manufacturing instructions, (3) setting in-process specifications, (4) sample collection and testing, (5) record keeping and reporting, (6) inspections, and (7) managing consumer complaints. A company’s objective should be to make a profit. Its objective in manufacturing a product is to satisfy consumer needs and expectations. Consumer complaints fall into many different categories. They may be concerned with the flavor, odor, color, texture, appearance, net volume or weight, and deterioration, etc. A procedure for handling consumer complaints should be developed and instituted.

14.3 QUALITY ASSURANCE QA oversees and evaluates quality control, wholesomeness, and integrity. It involves (1) reviewing quality control sampling protocols, (2) developing recall protocols, (3) developing managementreporting systems, (4) implementing HACCP and food safety programs, (5) helping to develop QC methods, and (6) development and improvement of laboratory methodologies. Functions of QA will include some of the following: 1. Managing Good Manufacturing Practices (GMP) including: • Documenting and developing processing procedures, equipment cleaning and sanitation schedules, a preventative maintenance program for equipment, and a pest and rodent control program • Providing employee training on GMP, sanitation, and proper product handling procedures • Conducting monthly audits for adherence to GMP and quality assurance practices • Inspecting plant and equipment sanitation daily prior to production 2. Managing Good Laboratory Practices (GLP) including: • Ensuring adherence to GLP • Documenting all testing methods, record keeping, and calibration processes • Evaluating data generated by the laboratory • Conducting monthly internal GLP audits 3. Managing a safety program including: • Ensuring employee safety • Adhering to Occupational Health and Safety Act (OSHA) or similar and other regulatory agency requirements 4. Assuring the quality of raw materials by: • Implementing the Vendor Certification Program to evaluate the microbiological, chemical, and biological quality of raw materials • Assuring appropriate raw material storage practices • Documenting procedures for out-of-specification raw materials 5. Assuring the quality and traceability of finished products by: • Initiating a microbiological and chemical testing program for finished products • Assuring appropriate finished product storage practices • Implementing systems for lot-identification and tracking all stages of distribution • Documenting procedures for out-of-specification finished products • Developing a product recall program 6. Evaluating the plant environment by: • Conducting an environmental microbiological sampling program • Documenting the environmental sampling program • Evaluating microbiological data and suggesting corrective actions if necessary 7. Developing an HACCP plan including: • Documenting of process flow charts

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• Identifying of key process control points and limits • Preparing of detailed process control procedures and corrective actions 8. Managing employee training including: • GMP training • HACCP training • Job training QA is based on a deliberate, conscious planning of all safety and quality-related activities, with a continuous monitoring and evaluation of all programs and processes related to the production and use of a given product. The success of this type of approach depends on the involvement and commitment of all personnel, including management personnel who are in any way responsible for the product at any given stage of processing. QC is viewed as the assessment of the quality and safety of raw materials in process products and finished products by means of an examination or inspection in relation to the established criteria or specification; this activity covers only one aspect of activities within the scope of quality assurance. An effective QA program can be developed around QC activities with a preventative approach to ensure finished product quality and safety.

14.4 QUALITY SYSTEMS A quality system may be defined as “The organization, structure, responsibilities, procedures, processes, and resources for implementing quality management” (Burrows, 2001). During the past decade, quality experts have proposed approaches, concepts, and systems for addressing quality during manufacturing. Total Quality Management (TQM) and ISO 9000 have become firmly established in large manufacturing and service industries worldwide. Many food manufacturing organizations have developed and implemented TQM or ISO 9000 programs in order to enhance their quality objectives. Details on applications of these quality programs in the food industry have been reviewed recently (Golomski, 1993; Battaglia, 1993; Surak, 1992) and are beyond the scope of this chapter.

14.4.1 HAZARD ANALYSIS CRITICAL CONTROL POINTS (HACCP) HACCP is a management tool developed in the late 1960s to ensure the safety of foods in space flight programs (Ropkins and Beck, 2000). This technique for ensuring food safety has become widely accepted in the food industry, both by government regulatory agencies and food manufacturers, as well as world organizations such as WHO and FAO (Ropkins and Beck, 2000; Perera and De Silva, 1999). It has been implemented in North America, the European Union, Australia, New Zealand, Singapore, and many other countries today. HACCP is now considered to be an effective means for ensuring the safety of foods and food products with respect to known hazards and has become the basis for controlling and ensuring food safety in food manufacturing processes; it has become part of total quality assurance programs in the food industry and can be incorporated within an ISO 9000 quality system (BSI Quality Assurance, 1991; Ropkins and Beck, 2000; Perera and De Silva, 1999). In the fruit industry, the implementation of an HACCP program can lead to the identification of known hazards that affect the safety of the final product, with the view of eliminating these hazards at the earliest possible stages and at any subsequent stage of the manufacturing or production system (Alli, 1993; Perera and De Silva, 1999). A hazard analysis determines the significance of a hazard associated with a product for the safety of the consumer of the product with which the hazard is associated. This results in the identification of critical control points, which are points in a process (location, step operation, and raw material) that, if not effectively controlled, may cause, allow, or contribute to the presence of the hazard in the final product. There is a wide variety of known hazards in the fruit industry. Depending on the type of product, whether fresh, frozen, or canned fruit, the detriments associated with the product can vary in significance. These hazards

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are grouped into the following three broad categories: biological, chemical, and physical. In the biological category, bacteria such as Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Salmonella, and Clostridium botulinum lead the list of hazards in terms of their widespread occurrence and the dangers that they pose (Stauffer, 1988; Mazzotta, 2001). Certain molds (e.g., Aspergillus flavus, Aspergillus ochraceous, Penicillium articae) are also included in this category because they produce mycotoxins like aflatoxins, ochratoxins, and patulins that can be very poisonous (Wogan and Marletta, 1985; Anklam and Battaglia, 2001). Insects and their fragments in fruits and fruit products also fall in this category. Chemical hazards include compounds that present a threat to the consumer’s health; they include agricultural residues (e.g., insecticides and fungicides used on farms and during storage), heavy metals, and regulated chemicals, as well as food additives. Physical hazards include the presence of extraneous material such as metal fragments, wooden splinters, and stones, introduced primarily during harvesting or processing. In certain processes, where the size of fruit is critical for adequate thermal processing, such as in the case of canned fruits, lack of uniformity in the size of the raw fruits could also act as a physical hazard, and this needs to be identified and controlled. 14.4.1.1 HACCP Principles HACCP is based on seven accepted principles; the successful implementation of an HACCP program demands that each of these principles be well understood and strictly adhered to. In addition, a successful HACCP program requires that certain prerequisite programs be present to support the HACCP program (Perera and De Silva, 1999). In the fruit industry, an effective program would demand an initial detailed knowledge of all fruits and raw materials coming in, including packaging materials and all finished products, as well as their intended use, shelf life stability, and conditions for storage and distribution, such as temperature and labeling instructions. A detailed step-by-step flow diagram of the process is also required. This should cover all processes from the point of entry of raw materials, through manufacturing, packaging, and all postmanufacturing steps, including storage distribution, retailing, and consumer handling. Once the developed flowchart has been verified as accurately reflecting the actual operations used for preparation of the product on the factory floor, the following seven principles of the HACCP program can be applied: 1. Hazard assessment: All steps in the manufacturing process, including the use of raw materials and ingredients, processing, distribution, and marketing, through to use by the consumer should be evaluated for identification of known biological, chemical, or physical hazards. 2. Critical control point determination: This is the process to determine which of the identified hazards require control such that loss of control results in an unacceptable safety risk. The identification of the critical control points requires a thorough familiarity with the food process, including each step in the operation from the receiving of fresh fruits and other raw materials to the shipping of the finished product. 3. Establishment of specification: Appropriate target levels and associated tolerances need to be established so that they can be used as criteria for determining whether the steps that are identified as critical points are actually in control. 4. Development of monitoring and testing procedures: Testing procedures for monitoring each critical control point will have to be developed to ensure that each critical control point is consistently monitored. 5. Establishment of corrective action: This involves the development of procedures that must be followed whenever the monitoring process demonstrates that an identified critical point is out of control on the basis of the established target levels and tolerances, including the obligatory corrective action to be taken in the event that this deviation from control occurs.

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6. Record keeping: A system of documentation needs to be developed that will describe all aspects of the HACCP program and give evidence of its functioning based on all data obtained from testing and analysis, any deviation, and corrective actions. 7. Verification procedures: Procedures need to be established as part of the HACCP program to verify that the operating HACCP system consistently conforms to the established procedures. A successfully implemented HACCP program, when properly monitored, will ensure effective control of hazards and public health risks and will lead to improved food safety. As the HACCP program takes care of all known hazards in any production line, it improves product efficiency, reduces waste, and reduces the need for rework. This should cut down on cost, improve sales performance, and enhance consumer confidence in the product. The quality and safety of processed fruits are directly related to the state of the incoming fresh fruits used in production. Microbial contaminations, impurities, and the general appearance of the incoming fruits must therefore meet certain minimum specifications. Producers of processed fruits such as canned, frozen, and modified atmosphere packaged (MAP) fruit products can demand that suppliers follow an HACCP program to ensure that all raw materials coming in meet the desired specifications. This will not only cut down costs but will also ensure a safer and better quality product. 14.4.1.2 Prerequisite Programs for HACCP The effectiveness of an HACCP program in any fruit processing establishment is dependent upon the existence of adequate procedures that control the operational conditions within the establishment, so that the environmental conditions are favorable to the production of a safe product. Good Manufacturing Practices (GMP) are a prerequisite for effective implementation of an HACCP program. This means control of the premises and facilities, sanitation, receiving and warehousing of fruits and any raw materials, handling and storage of products, maintenance of equipment, training of personnel, and developing a recall program. These are required to be in operation before a successful HACCP program can be implemented.

14.5 QUALITY CONTROL, INSPECTION, AND TESTING In order to address the quality characteristics of fruits, the entire production chain from planting through harvesting, postharvest handling, storage, and at every stage of fruit processing until the final product is obtained requires control. QC should be exercised during the production of the crop, with a view to the intended use of the produce obtained at harvesting (Kramer, 1973). The quality of harvested fruits is dependent on the quality of seeds, use of fertilizers and pesticides, and general field management practices. For example, seeds must be tested for extraneous matter and weed seeds, in addition to percentage of germination. Certain micronutrients may have a deleterious effect on the end product; for example, lime applied to oranges whose peel is to be used for marmalade should not contain magnesium, which will cause the marmalade to be lumpy (Kramer, 1973). Pesticides must be free of any substances not approved for use, and the concentration of the active ingredient must conform to specifications. Chemicals applied to growing crops may exert profound effects on the appearance of fresh and processed food products, and these must be monitored. For example, copper sprays have been found to reduce the size of cherries (Hartz and Lawver, 1965), certain herbicides cause lemon and other citrus fruits to turn yellow and then to brown (Johnson, 1945), and other chemicals have been found to cause off-flavors in both fresh and processed products (Mahoney, 1962). Chemicals used during crop production must therefore be chosen to protect or enhance product quality and must be used at appropriate concentrations and times so that harvested fruits are free of pests and contain pesticide residues below the approved limit. Prior to harvesting, fruits intended for the fresh market should be supplied with more water, especially toward the end

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of the growing season in order to maximize total yield and improve appearance. If the crop is intended for use as a concentrated or dehydrated product, the later irrigations should be reduced or entirely suspended to cut down on dehydration costs (Kramer, 1973). Control of postharvest biochemical and physiological processes presents a special challenge. These processes increase the susceptibility of fruits to spoilage after harvesting and affect their quality and shelf life. The shelf life of fruits after harvesting is affected by various intrinsic and extrinsic properties of the harvested fruit. The intrinsic properties of fruits that affect their shelf life and quality include the moisture content of the fruit, pH, rate of respiration, biological structure, and ethylene production, whereas extrinsic factors include time and method of harvesting, handling, temperature, humidity, and general hygiene, as well as the methods used in packaging (Haard and Chism, 1996; Day, 1993). These are considered below.

14.5.1 MOISTURE CONTENT The high moisture content of most fresh fruits, between 50 and 96% (Kramer, 1973), make them generally susceptible to spoilage and they act as media for the growth of food poisoning microorganisms. Thus, much precaution needs to be taken after harvesting to limit the exposure of the harvested fruits to any of these food spoilage microorganisms so as to ensure safety and preserve the quality of the fruits. Water loss in fruits often results in wilting and weight loss, which could be desirable or undesirable depending on how the fruits are to be used. For example, water loss in apples reduces succulence and textural quality, which is undesirable. Loss of water in dates to be sold as dried dates is, however, very desirable. The surrounding atmosphere of harvested fruits should therefore be carefully controlled to prevent any undesirable loss in quality.

14.5.2

PH

The pH of harvested fruits is an important parameter in determining whether potential food spoilage microorganisms are liable to grow during postharvest handling and storage. Most fruits such as lemons, oranges, pineapples, apples, and peaches are high-acid products, having pH values below 4.5 (Holdsworth, 1983), which do not favor the growth of C. botulinum. They may, however, support growth of some acid-tolerant yeasts and molds (Kramer, 1973). Fruits that have pH values above 4.5 are, however, susceptible to the growth of C. botulinum, and a rigid temperature control must be maintained (below 3∞C) (Day, 1993) to safeguard against the growth of these microorganisms. It is also important to control the acidity of fruits after harvesting because changes in the pH of the fruit affect both the flavor and ultimate wholesomeness of the product, whether intended for the fresh market or for further processing. As most fruits mature, the acidity decreases, and the sugar content increases; thus, a relatively high sugar content and low acidity indicate full maturity. If pH of the fruit is low, it is an indication that the fruit is somewhat immature (Khattak, 1973). For example, the pH of citrus varies from 2.9 to 3.1 for immature valencia oranges and from 4.1 to 4.4 for very mature fruit of the same kind (Harding et al., 1945).

14.5.3 RESPIRATION

OF

FRUITS

Most fleshy fruits show a characteristic rise in respiratory rate coincident with the obvious changes in color, flavor, and texture that typify ripening (Haard and Chism, 1996). The rate of respiration is indicative of the rapidity with which compositional changes are taking place within the plant material and, hence, gives an indication of the potential shelf life of the fruit (Day, 1988). Respiration rates of fruits are markedly influenced by temperature, maturity, type of fruit tissue, size, and variety. For example, the respiration rate of apples at 25∞C is 10 times that at 5∞C (Haard and Chism, 1996). Fruits of the same variety with a larger surface area have higher respiration rates than smaller ones (Day, 1993). Bruised or cut fruits tend to have increased respiration rates and,

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therefore, have a lower shelf life. In the handling of fruits after harvesting, it is essential to avoid cutting and bruising in order to keep the respiration rate at an optimum.

14.5.4 TIME

OF

HARVESTING

The quality and subsequent shelf life of harvested fruit is dependent on the maturity of fruits at the time of harvesting. It is therefore critical that fruits be harvested at optimal maturity (Zomorodi, 1990). Most fruits are firmer prior to their peak maturity, and when harvested at this stage, are less susceptible to mechanical damage during handling and processing. Harvesting, when done too early, often gives poor quality fruits and erratic ripening, whereas late harvesting may increase susceptibility to decay and premature softening (Day, 1993), which subsequently leads to shorter shelf life of most fruits, increasing their risk of contamination by dangerous microorganisms. It is important that the timing of harvesting be carefully controlled to reduce these potential setbacks after harvesting.

14.5.5 POSTHARVEST HANDLING Handling of fruits during harvesting and after, if not done with care, causes damage to the plant cellular structure, destroying the natural protective tissue and textural integrity of the fruits. Such injuries increase moisture loss, enzymatic activities (e.g., browning of bruised fruits, which is catalyzed by the enzyme polyphenoloxidase) (Haard and Chism, 1996), susceptibility to microbial spoilage, discoloration, and respiration. These adverse effects can be minimized by eliminating bruising at all stages of harvesting and postharvesting. It is important that fruits be stored at adequate temperatures and humidity after harvesting to ensure their safety and quality. Respiration rates of fruits usually increase with increasing temperature. Moreover, other tissue attributes that indirectly influence biochemical transformations, such as membrane integrity, are influenced by temperature. Lowering of temperature tends to retard undesirable physiological reactions, as well as spoilage caused by growth of microorganisms (Haard and Chism, 1996). Fruits that have relatively fast respiratory rates respond best to lower temperatures, and with such fruits, precooling prior to shipping or storage can contribute greatly to an extended shelf life (Haard and Chism, 1996). Certain fruits, particularly of tropical origins, are susceptible to chilling injury by exposure to temperatures below 5 to 15∞C. Visible effects of chilling injury include localized tissue necrosis (e.g., apple), failure to ripen (e.g., banana), development of areas of tissue that do not soften on cooking, surface pitting (e.g., grapefruit), and wooliness in texture (e.g., peach) (Haard and Chism, 1996). Susceptible produce such as bananas, papayas, avocados, melons, and pineapples should be stored at recommended temperatures so as to eliminate such temperature-induced injury. (Because most fruits are 80 to 95% water, they lose moisture rapidly whenever the relative humidity is less than 80 to 95 % below saturation. Moisture losses of 3 to 6% are usually enough to cause marked deterioration of quality, and, thus, a balance between temperature and relative humidity must be maintained to preserve the quality of fruits after harvesting. It is generally recommended that fruits be stored at relative humidities sufficiently high to minimize water loss and to maintain cell turgor, but not so high as to cause condensation and accompanying microbial growth (Haard and Chism, 1996).

14.5.6 GENERAL Lack of adequate hygienic practices at all stages of harvesting, postharvesting, and processing, drastically increases the risk of contamination with food poisoning bacteria. This jeopardizes the safety and quality of the fruit for consumption. All equipment used in the handling of harvested fruits must be cleaned regularly and personnel should be adequately trained to observe personal hygiene, good sanitation, and good manufacturing practices. The quality characteristics of processed fruit products can be adversely affected by processing. Most processed fruit products are thermally processed (e.g., jams, syrups), dehydrated (e.g., raisins), or frozen. These processes affect the texture, flavor, and color, as well as the nutritive value of the

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product, and must be carefully controlled if a high-quality product is desired. Processing variables that may influence chemical changes during processing include time, temperature, mode of heat transfer, type of container used, water quality, and pH (Haard and Chism, 1996). Proper preparation and pretreatment of fruits prior to processing is vital to achieving the desired quality. Overheating during thermal processing may result in excessive softening of fruits, browning, off-flavors, and loss of nutritive value, while insufficient heating may result in inadequate inactivation of enzymes and destruction of microorganisms. Frozen products must be kept at temperatures well below the freezing point ( -18∞C or below) (Haard and Chism, 1996) to prevent off-flavors and color changes from developing. The texture of dried fruits is generally dependent on the method of moisture removal, and precautions must be taken during dehydration to minimize oxidation, browning reactions, case hardening, and nutrient loss. It is quite clear therefore that the way in which fruits are handled after harvesting will generally determine their shelf life, safety, and quality at the time of consumption. The implementation of effective and efficient quality control techniques must, therefore, follow immediately after harvesting to maintain the standards specified for each product. Monitoring of the desired quality characteristics by use of appropriate inspection and test procedures becomes necessary to ensure that effective control is exercised.

14.6 INSPECTION AND TEST PROCEDURES An effective quality control system includes an organized receiving department where all incoming merchandise is thoroughly checked for conformance to desired specifications. Inspection and test procedures are used routinely to confirm that raw materials and products have the desired quality characteristics. In this section, the quality characteristics of fruits and the inspection and test methods are discussed. All fruits intended for the fresh market must be inspected for bruises, size, color, ripeness, or level of maturity. Fruits should be handled carefully in order to reduce bruising and decomposition. Where washing is required, wash water must be potable and under enough pressure to remove all adhering dirt. Sieving, sifting, or hand-picking must be done immediately to sort out all decomposed or unclean fruits, as well as under- or oversized fruits. Unfit products should be set aside and disposed of in such a place and manner that they cannot contaminate food and water supplies (FAO, 1984). Selected fruits for the fresh market must be stored at the appropriate temperature and humidity. During storage, care must be taken to prevent any form of infestation from rodents and insects and contamination by bacteria that affect the safety and shelf life of the fruits. Fruits received for further processing into products such as canned fruit syrups, concentrated fruit juices, and jams must be handled with equal care. All arrivals of delivery must be anticipated and adequate freezer and cooler space made available prior to arrival of merchandise. Trucks supplying fresh fruits and other raw materials must be inspected for cleanliness, freedom from foreign odors, and other contamination. All packaging should be inspected for imperfections. If the interior temperature of the truck does not conform to that specified for storage of the incoming fruits, they should be rejected or special precautions must be taken in their handling and a note made of these. Incoming fruits should be checked for soundness, ripeness, fungal rot, bacterial rot, fly eggs, and maggots before being accepted for processing. If the fruits are acceptable, unloading must be accomplished quickly to minimize exposure to exterior temperatures (Thorner and Manning, 1983). Proper records must be kept of the date and time goods were received, descriptions of goods and conditions under which they were received for future referencing, or whether they need to be traced in the event of a recall. During processing, written procedures for every stage of production must be critically followed and continuously monitored. Any deviations must be noted and recorded. Temperatures, product consistencies, flavors, textures, and so on must be continuously inspected during processing because they can have a great effect on the safety and quality of the finished product.

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Fruits that require washing before further processing should be properly cleaned with a stream of water under enough pressure to remove all adhering dirt. Conveyors and containers for transporting raw materials should be of a material and construction that will permit thorough cleaning. Extreme care must be taken in transporting perishable products to prevent bruising and product contamination (FAO, 1984). Although inspection procedures during processing will vary from one processed fruit to the other, routine quality control inspection procedures for the fruit industry will involve proper sampling techniques; sensory evaluation (appearance of product-uniformity of size and color, texture, and flavor characteristics such as taste and aroma); determination of syrup density; sugar content; fill container and maximum headspace; drained weight; viscosity and flow properties; presence of insect filth (maggots, rodent hair, and mites); and condition of container, cartons, cases, vacuum cans, plastic films, or any other product used for a product’s wrapping (Thorner and Manning, 1983).

14.6.1 SAMPLING Sampling is the selection of a certain portion or number of units from a particular lot of the same product for testing. It is extremely important that the sample selected be as representative as possible of the entire batch from which it is sampled. The sample size should be large enough to allow repeat analysis, and the condition of the sample analyzed at the laboratory should reflect conditions at the time of sampling (FAO, 1984). A sampling plan must be followed in sample collection during processing to allow the data collected to be easily analyzed statistically.

14.6.2 SENSORY EVALUATION Sensory evaluation of processed fruit products should be done by well-trained personnel who can make fine distinctions between the senses of taste, smell, touch, and sight. It is sometimes preferable in certain instances that sensory evaluation be done by untrained personnel, and this decision must be made by the quality control department. Odor, taste, and appearance play an almost indispensable role in the acceptability of the finished product. Specifications should be set up as part of the quality control program by which all samples taken during processing and at the end of production will be compared with each other. Samples should be observed for color, density, viscosity, and visual evidence of spoilage. A taste panel should be used to test the product for its aroma, taste, “mouth feel,” and aftertaste, in accordance with predetermined procedures and the results compared with desired expectations to determine if the product is acceptable for sale or not.

16.6.3 TEXTURAL CHARACTERISTICS Textural characteristics of the product such as density and viscosity measurements can be measured by any standard procedure, and equipment designed for these measurements can be purchased in the market. It is important that results be reliable and reproducible. Sugar content of syrups that affect mouth feel of canned fruit syrups and juices can be measured using standard hydrometers.

14.6.4 DETERMINATION

OF

DRAINED WEIGHT

For most fruits packed in liquid, the drained weight of the solid product is more important than the total net weight. A standard drained weight must be specified for the finished product on the basis of which inspections and tests will be done. An example of a test procedure for drained weight (Thorner and Manning, 1983) involves emptying contents of the container onto a previously weighed specified screen, allowing it to drain for 2 min, and reweighing the product on the screen. The weight of the screen is subtracted, and the difference is the drained weight of the product. This is a fast and easy procedure that can be routinely used to test for the drained weight of finished produce.

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14.6.5 FROZEN

AND

349

CANNED FRUITS

Freezing for fruit preservation utilizes low temperatures to prevent the multiplication of microorganisms. The quality of raw materials and sanitary conditions of equipment are therefore critical. Materials should be handled and prepared as quickly as possible under the best sanitary conditions to minimize the number of bacteria present at the time of freezing (FAO, 1984). The time and temperature combination for heating and the pH of fruits must be carefully controlled in all canned produce manufacture to effectively control microbial spoilage. A heat treatment process (time and temperature combination) that has maximum destructive effect on microorganisms and minimum effect on the quality of the food must be used. These process parameters must be properly documented and consistently monitored and inspected for compliance. The pH of the fruit produce must be inspected at all stages of production using a reliable pH meter or by titration. The pH of the final product should not deviate significantly from the desired pH.

14.7 SANITATION Even though the health benefits associated with regular consumption of fresh fruits have been clearly demonstrated, an increasing proportion of reported outbreaks of foodborne illnesses are traced to fresh produce (Zepp et al., 1998). This was further highlighted in the recent outbreaks of foodborne illnesses due to consumption of minimally processed fruits and fruit juices (Anon., 2001). Salmonella grows extremely well on any low acid fruit or vegetable with sugar in it at temperatures above 15∞C. The realities of postprocess distribution and consumer handling make it highly likely that temperature abuse may occur. Ingestion of as few as 10 live cells of the most aggressive strains can be fully infectious to susceptible individuals. These strains often harbor genetic elements that give them multiple resistance to medical antibiotics and therefore infections are very difficult to treat (IFT Expert Report, 2002). Therefore, improved surface decontamination methods are needed to increase our confidence in the microbial safety of fresh-cut and non-heatprocessed juice products or minimally processed produce.

14.7.1 MICROBIOLOGICAL SAMPLING PLANS Sampling plans are statements describing the collection, methodology, and acceptance criteria of testing. Microbiological methods, no matter how accurate and reproducible, are inadequate to appraise the microbiological quality of food without a satisfactory sampling plan. The criteria one uses to control microbiological hazards are: • • •

Education and training Inspection of facilities and operations Testing

Microbiological standard is a criterion that is part of a law or regulation. It is mandatory to follow this standard. Microbiological specification is a criterion that is applied as a condition of acceptance. It is also mandatory if the produce is to be accepted by the buyer. Microbiological guideline is an advisory criterion that is used to monitor food processes or systems. Selection of criteria used to control microbiological hazards are based on the following: • • • • •

Evidence of hazard to health Microbiology of the raw material Effect of the process Likelihood of contamination Consequence of contamination

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• • • • • • • • •

Category of risk to the consumer Cost to benefit ratio Type of food Contaminants Methods Size of sample Limits Distribution of organisms Practicality

Knowledge about possible contaminants is useful in selecting suitable sampling plans. For example, S. aureus is an environment contaminant that can be passed on by improper or unhygienic handling practices. L. monocytogenes can occur in minimally processed or any ready-to-eat fruit products. Effective criteria and standards are required for control of contamination. There is little relationship between Aerobic Plate Counts (APC) and safety. However, APC and quality attributes are related. High APC means poor hygienic practices in the process and, consequently, poor product quality. APC should be used to determine control of the process. In general, coliform and E. coli counts should be used to indicate process control and food contamination. They indicate bad hygienic practices. There is little relationship between yeast and mold (Y/M) and safety. However, Y/M and quality attributes are related. As in the case of APC, Y/M should be used to determine control of the process. Lactobacilli could also be used as a guide to indicate process control and food contamination. Recognizing the need to minimize the microbial food safety hazards in fresh fruits and vegetables including fresh-cut products, the FDA, USDA, and the Center for Disease Control and Prevention have come up with a guide for handling of such products (FDA, 1998). It covers aspects of the control of potential microbial hazards all the way from the farm to the fork. The safety precautions and procedures needed in dealing with water, manure and municipal biosolids, worker health and hygiene, sanitary facilities, field sanitation, pack house sanitation, transportation, and traceability of contaminated products are discussed extensively (FDA, 1998). Plant sanitation and cleanliness of equipment are of the utmost importance in the success of any quality control program. The plant premises and areas surrounding it should be kept completely free of all decomposing materials and pools of stagnant water that generate foul odors and are ideal for the breeding of flies and other insects. Entrances into the plant and areas surrounding it should be well paved to reduce the amount of mud and dirt brought in by workers and clients (Khattak, 1973). These pavements should be regularly cleaned to prevent the accumulation of mud and dirt. Entrances for the handling of raw materials should be kept particularly clean and, if possible, separated from the general entrance to the plant. All floors in the factory should be cleaned regularly and must be properly constructed with good drainage. Water should not be allowed to remain stagnant on the factory floor because this breeds unwanted insects and flies. Doors and windows of the plants should be screened tightly to prevent the entrance of flies and other insects. All equipment must be rinsed and sanitized on a daily basis and, in some instances, whenever there is a change in shift, if this is appropriate. If possible, equipment should be rinsed and sanitized each time a product change is made that will affect the incoming product with contamination of the previous product run. Lighting in all areas of the plant should be sufficient for adequate performance of all assigned duties. Areas where cleaning, sorting, picking, and product inspection are done should be provided with sufficient nonglaring light to enhance work performance. The plant should be designed in such a way as to ensure sufficient ventilation of all factory floors. Adequate ventilation is extremely important to prevent buildup of odors and recontamination of product. The design of the plant should also ensure that temperatures maintained during

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processing are optimal for the product being manufactured. Too high or low temperatures on the factory floor could seriously affect the quality of the final product and also affect the comfort and productivity of workers. Air quality must be controlled to prevent buildup of condensate and bacteria. Water quality must be checked and controlled. Steam facilities must be evaluated for adequacy for all plant operations, especially where steam is used for sterilization or thermal processing. Waste disposal must be properly controlled to prevent the possibility of recontamination. Sufficient attention should be paid to the waste and garbage disposal system to ensure that it is large enough to carry peak loads and does not contribute to contamination of the finished product. All waste and garbage from the factory floor should be properly disposed of immediately and not left open or stored. Health habits of personnel involved in production should reflect a number of basic rules of hygiene, including washing hands in hot water; using plenty of soap; drying hands on a clean cloth or paper towel; keeping fingernails short and clean; using a hairnet or cap; remaining away from food when a person has an infected cut, boil, or other infection of the exposed skin; using sanitized tools and equipment; covering sneezes and coughs by means of a tissue or handkerchief; reporting all cases of diarrhea and fever, etc., and having periodic checkups (Thorner and Manning, 1983).

14.7.2 SANITATION PROCEDURES Sanitation consists of two parts: (1) cleaning and (2) sanitizing. Cleaning means the removal of residue of food, dirt, dust, foreign material, or other soiling ingredients or materials. Sanitizing means the effective bactericidal treatment of clean surfaces of equipment, utensils, and even surfaces of fruits (Thorner and Manning, 1983; Annous et al., 2001). Some of the sanitizers used in the food industry — chlorine, chlorine dioxide, ozone, and peroxides — have a number of limitations in terms of removing some pathogenic bacteria, especially listeria (Kim et al., 1999; Simons and Sanguansri 1997; Anon., 2000). Besides, some of the above sanitizers have adverse effects on nutritional quality (Ozen et al., 2001). The use of steam (Pao and Davis, 2001; Li et al., 2001; Mazzotta, 2001; Suslow and Cantwell, 2001) and hot water (Fleischman et al., 2001) for surface sanitation of fruits and vegetables intended for minimal processing has received considerable interest recently. Acidified electrolyzed water could be used effectively for washing of fruits to rid them of microbes as well as viruses. There have also been a number of reports in the literature on the effect of acidified electrolyzed water as an antimicrobial and antiviral agent (Izumi, 1999; Kim et al., 2000; Koseki and Itoh, 2001; Koseki et al., 2001). Frequent cleaning of plant equipment should be maintained at all times, and all parts should be continually washed by well-located continuous water sprays. This must be done especially on equipment such as inspection belts and conveyor belts used to convey fruits and other products. All utensils used must be absolutely clean. Buckets, knives, and drain pans should be cleaned and rinsed whenever empty or not in use (Khattak, 1973). A sanitation schedule should be designed that includes every area, machine, and all preparation equipment within the premises. Employees should be assigned specific tasks in sanitizing equipment and factory floors, and this should be inspected and rigorously monitored. An employee should be assigned the task of caring for all cleaning equipment such as rags, sponges, mops, and brushes. When not in use, these should be stored in a utility closet and a sink must be provided. Cleaning equipment should be rinsed and cleaned before storage. Cleaning and sanitizing operations must be carried out by properly trained personnel. The choice of cleaning agents for each piece of equipment should be made based on the type of soil and dirt encountered in processing. Soaping alone might not be enough. Detergents and sanitizers must be applied to equipment regularly after initial rinsing and left on long enough before rinsing again to ensure adequate sanitizing of equipment. During all shutdown periods and at the end of each production line, equipment such as inspection belts, packing tables, belt conveyors, filling machines, mills, choppers, cutters, washers, and similar equipment should be thoroughly washed with hot water, steam hosed, and rinsed off

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with clean cold water (Khattak, 1973). At each washing, all debris should be collected and removed to an appropriate disposal container prior to starting the actual rinsing and soaping of the processing structures. All drain gates must be removed and thoroughly washed with soap or detergent on both sides. Drains must be thoroughly washed with soap or detergent and thoroughly rinsed. The factory floor must be rinsed thoroughly and drain gates replaced. After all equipment and the floors have been washed and rinsed, sanitizers should be applied, which should remain on structures to eliminate residual bacteria in the air. The sanitizers should be applied on all the structures where the product comes in contact with the equipment such as all belts, the inside of tanks, centrifuge baskets, stainless steel hoppers, and all other equipment where the final product will have direct contact. All sanitation personnel should wear cloth gloves, thick rubber gloves, rubber boots, rain gear, safety glasses, hairnets, and bump hats prior to starting their daily cleaning duties. This equipment should be kept clean to ensure against recontamination of sanitized equipment.

14.8 SUMMARY Current trends in both the food manufacturing and food service industries indicate that the use of generic-type programs (such as TQM, ISO 9000, HACCP, industry standards) to address both quality and safety requirements of products has become common practice. These programs attempt to cover the entire scope of food manufacturing, from the acquisition of raw materials to the distribution of the finished product, and contain the critical elements required to address the quality and safety concerns of the fruit sector of the food industry. The concepts and tools advocated in these programs are applicable equally for both ensuring quality and safety of fresh fruits and processed fruits.

REFERENCES Alli, I. 1993. Quality control of MAP products. In Principles and Applications of Modified Atmosphere Packaging of Foods. R.T. Parry (Ed.), Blackie Academic & Professional, New York, pp. 101–113. Anklam, E. and Battaglia, R. 2001. Food analysis and consumer protection. Trends Food Sci. Technol., 12: 197–2002. Annous, B.A., Sapers, G.M., Mattrazzo, A.M., and Riodan, D.C.R. 2001. Efficacy of washing with a commercial flatbed brush washer using conventional and experimental washing agents in reducing populations of Escherichia coli on artificially inoculated apple. J. Food Prot., 64(2): 159–163. Anon. 2000. Food Safety and Hygiene. Food Science Australia. August 2000. Anon. 2001. Information bulletin — Public Advisory on food safety measures for cantaloupe. Canadian Food Inspection Service. June 1, 2001. Battaglia, R. 1993. Quality management in food trade with a view toward 1993 in Europe. Food Res. Int., 26: 69–74. BSI Quality Assurance. 1991. Guidance notes for the application of BS 5750 Part 2/ISO 9002/EN 29002 for the food and drink industry BSI, London, pp. 1–12. Burrows, G. 2001. Quality management system and hazard analysis critical control points. In Fruit Processing: Nutrition, Products, and Quality Management. 2nd ed. D. Arthey and P.R. Ashurst (Eds.), Aspen Publishers, Gaithersburg, MD, pp. 249–280. Day, B.P.F. 1988. Optimization of parameters for modified atmosphere packaging of fresh fruits and vegetables. CAP ’88, Schotland Business Research, Princeton, NJ, pp. 147–170. Day, B.P.F. 1993. Fruits and vegetables In Principles and Applications of Modified Atmosphere Packaging of Foods. R. T. Parry (Ed.), Blackie Academic & Professional, New York, pp. 114–133. ERS-USDA, 2001. Product liability and microbial foodborne illnesses, AER-799. 3-8. www.ers.usda.gov/publications/aer799/aer799c.pdf FAO. 1984. Manuals of food quality control 5. Food Inspection (FAO Food and Nutrition Paper) FAO, Rome, Italy, pp. 9–96.

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FDA Guide to minimize food safety hazards for fresh fruits and vegetables. 1998. Food and Drug Administration U.S. Department of Agriculture, Centers for Disease Control and Prevention. October 26, 1998 Fleischman, G.J., Bator, C., Merker, R. 2001. Hot water immersion to eliminate Escherichia coli 0157:H7 on the surface of whole apples: Thermal effects and efficiency. J. Food Prot. 64(4): 451–455. Golomski, W.A. 1993 Total quality management and the food industry. Why is it important? Food Technol., 47(5): 74–79. Haard, N.R. and Chism, G.W. 1996. Characteristics of edible plant tissues. In Food Chemistry. 3rd ed. O.R. Fennema (Ed.), Marcel Dekker, New York, pp. 943–1011. Harding, P.L., Winstoo, L.R., and Fisher, D.F. 1945. Seasonal changes in Florida Oranges. USDA Tech. Bull., 753. Hartz, R.E. and Lawver, K.E. 1965. The effect of sprays on quality factors of canned red tart cherries. Food Technol., 19(3): 103–105. Holdsworth, S.D. 1983. The preservation of fruit and vegetable food products. In Science in Horticultural Series, L. Broadbent (Ed.), Macmillan, London, pp. 61–98. IFT Expert Report on Emerging Microbiological Food safety Issues: Implications for Control in the 21st Century. 2002. Released at IFT International Food Safety and Quality Conference and Expo, Atlanta, GA, Feb. 20, 2002. Izumi, H. 1999. Electrolyzed water as a disinfectant for fresh-cut vegetables. J. Food Sci., 64: 536–539. Johnson, E. 1945. Effect of hormone weed killers on citrus trees. California Citrograph, 30: 305. Kim, J.M., Huang, T.S., Marshall, M.R., and Wei, C.I. 1999. Chlorine dioxide treatment of seafoods reduce bacterial loads. J. Food Sci., 64(6): 1089–1093. Kim. C., Hung, Y.C., and Brackett, R.E. 2000. Role of oxidation-reduction potential in electrolyzed oxidizing and chemically modified water for the inactivation of food related pathogens. J. Food Prot., 63: 19–24. Khattak, L.H. 1973. A Production and Quality Control Manual for the Frozen Fruit and Vegetable Processing Industry of Prince Edward Island. Department of Industry and Commerce, Charlottetown, Prince Edward Island, pp. 2–89. Koseki, A. and Itoh, K. 2001. Prediction of microbial growth in fresh-cut vegetables treated with acidic electrolyzed water during storage under various temperature conditions. J. Food Prot., 64(12): 1935–1942. Koseki, A., Yshida, K., Isobe, S., and Itoh, K. 2001. Decontamination of lettuce using acidic electrolyzed water. J. Food Prot., 64(5): 652–658. Kramer, A. 1973. Fruits and Vegetables. In Quality Control for the Food Industry. 3rd ed. Vol. 2, A. Kramer and B. A. Twigg (Eds.), AVI Publishing, Westport, CT, pp. 157–228. Li, Y., Brackett, R.E., Chen, J., and Beuchat, L.R. 2001. Survival and growth of Escherichia coli 0157:H7 inoculated onto cut lettuce before and after heating in chlorinated water, followed by storage at 5 or 15∞C. J. Food Prot., 64(3): 305–309. Mahoney, C.H. 1962. Flavor and quality changes in fruit and vegetables in the United States caused by application of pesticide chemicals. Residue Rev., 1: 11–23. Mazzotta, A.S. 2001. Heat resistence of Listeria monocytogenes in vegetables: evaluation of blanching process. J. Food Prot., 64(3): 385–387. Ozen, B.F., Han, Y., Floros, J.D., and Nelson, P.E. 2001. Quality characteristics of green peppers treated with Ozone (O3) and chlorine dioxide (ClO2) gases. Abstract of paper presented at IFT Annual Meeting, June 22–27, 2001, New Orleans, LA, 88E-6. Pao, S. and Davis, C.L. 2001. Microscopic observation and processing validation of fruit sanitizing treatments for the enhanced microbiological safety of fresh orange juice. J. Food Prot., 64(3): 310–314. Perera, A.D. and De Silva, T. 1999. Hazard analysis and critical control point (HACCP). In Handbook of Food Preservation. M.S. Shafiur (Ed.), Marcel Dekker, New York, pp. 735–768. Ropkins, K. and Beck, A.J. 2000. Evaluation of worldwide approaches to the use of HACCP to control food safety. Trends Food Sci. Tech., 11: 10–21. Simons, L.K. and Sanguansri, P. 1997. Advances in the washing of minimally processed vegetables. Food Aust., 49(2): 75–80. Stauffer, L.E. 1988. Hazard analysis critical control points. In Quality Assurance of Food, Ingredients, Processing, and Distribution. Food & Nutrition Press, Westport, CT, pp. 19–40. Surak, L.G. 1992. The ISO 9000 standards. Establishing a foundation for quality. Food Technol. (November): 74–80.

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Suslow, T. and Cantwell, M. 2001. Recent findings on fresh-cut cantaloupe and honeydew melon. Fresh Cut (April): 18–20, 32. Thorner, M.E. and Manning, P.B. 1983. Quality Control in Food Service. AVI Publishing, Westport, CT, p. 232. Wogan, G.N. and Marletta, M.A. 1985 Undesirable or potentially undesirable constituents of foods. In Food Chemistry., 2nd ed. O.R. Fennema (Ed.), Marcel Dekker, New York, pp. 689–724. Zepp, G., Kuchler, F., and Lucier, G. 1998. Food safety and fresh fruits and vegetables: is there a difference between imported and domestically produced products? Vegetables and Specialties, Situation and Outlook Report, ERS/USDA, VGS-274: 23–28, April, 1998. Zomorodi, B. 1990. The technology of processed/prepackaged produce. Preparing the product for modified atmosphere packaging (MAP). CAP ’90, Schotland Business Research, Princeton, NJ, pp. 301–311.

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of Fruits and 15 Packaging Vegetables James P. Smith, Devon Zagory, and Hosahalli S. Ramaswamy CONTENTS 15.1 15.2

Introduction ........................................................................................................................356 Growth of Packaging of Fruits and Vegetables.................................................................357 15.2.1 Developments in New Polymeric Barrier Packaging Materials .........................357 15.2.2 Increased Urbanization ........................................................................................357 15.2.3 Market Needs and Consumer Demands for Convenience..................................357 15.2.4 Increasing Energy Costs ......................................................................................358 15.3 Definition and Functions of Packaging .............................................................................358 15.3.1 Container..............................................................................................................358 15.3.2 Protection .............................................................................................................358 15.3.3 Medium of Communication ................................................................................359 15.3.4 Means of Minimizing Costs ................................................................................359 15.3.5 Means of Selling Product ....................................................................................359 15.4 Types of Packaging Materials............................................................................................360 15.4.1 Wood and Textiles ...............................................................................................360 15.4.2 Paper and Board ..................................................................................................361 15.4.3 Glass.....................................................................................................................362 15.4.3.1 Types of Glass Containers .................................................................363 15.4.4 Metal ....................................................................................................................364 15.4.4.1 Steel Cans ..........................................................................................364 15.4.4.2 Aluminum Cans .................................................................................365 15.4.4.3 Aluminum Foil...................................................................................367 15.4.4.4 Composite Cans .................................................................................367 15.4.5 Plastics .................................................................................................................368 15.4.5.1 Flexible Single Films.........................................................................369 15.4.5.2 Coated Films ......................................................................................371 15.4.5.3 Laminated Films ................................................................................371 15.5 Barrier Properties of Packaging Materials ........................................................................372 15.6 Selection of Proper Packaging...........................................................................................372 15.7 Packaging Requirements of Fruits and Vegetables ...........................................................373 15.8 Bulk Packaging of Fresh Fruits and Vegetables................................................................376 15.9 Packaging of Fresh Fruits and Vegetables at the Store Level...........................................378 15.10 Retail Packages ..................................................................................................................378 15.10.1 Plastic Films ........................................................................................................378 15.10.2 Backings...............................................................................................................379 15.10.3 Boxes....................................................................................................................379 15.10.4 Bags......................................................................................................................379

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15.11 Choice of Package .............................................................................................................379 15.12 Packaging of Frozen Fruits and Vegetables ......................................................................380 15.12.1 The Plastic Pouch ................................................................................................381 15.12.2 Laminated Cartons...............................................................................................382 15.12.3 Composite Can ....................................................................................................382 15.13 Modified Atmosphere Packaging.......................................................................................383 15.13.1 Active and Passive Modification .........................................................................383 15.13.2 Equilibrium MAP ................................................................................................384 15.13.3 Microperforated and Microporous Films ............................................................387 15.13.4 Fresh-Cut Fruit Packaging...................................................................................389 15.14 Other Methods of MAP .....................................................................................................389 15.14.1 Individual Seal Packaging ...................................................................................389 15.14.2 Ethylene Absorbents ............................................................................................390 15.14.3 Edible Films.........................................................................................................390 15.15 Public Health Concerns about MAP Fruits and Vegetables..............................................390 15.16 Thermally Processed Fruits and Vegetables ......................................................................391 15.16.1 Canning of Fruits and Vegetables .......................................................................391 15.16.2 Aseptic/Ultra-High-Temperature (UHT) Packaging ...........................................391 15.17 Packaging of Dry Fruit and Vegetable Products ...............................................................393 15.18 Conclusion..........................................................................................................................393 References ......................................................................................................................................394

15.1 INTRODUCTION Fruits and vegetables are an important part of a healthful, balanced diet. They provide us with essential vitamins, are a rich source of protein and fiber, and are also aesthetically pleasing to the eye and the olfactory sense. However, unlike most other food commodities, fruits and vegetables continue to be living organisms even after harvest. As a result of their biological nature, they are subject to physical, chemical, and microbiological deterioration from the time they are harvested until consumption. Physical deterioration includes bruising, softening, and moisture loss, the latter resulting in shriveling of products. Fruits and vegetables also continue to respire after harvest. These biochemical changes result in breakdown of carbohydrates and a buildup of CO2 and C2H4, changes in composition, and changes in the color and odor of many fruits and vegetables. In addition, enzymatic activity, e.g., pectinases, can result in softening of fruit. Microbiological deterioration due to the growth of molds, yeasts, and bacteria results in changes in the color, odor, and texture of products. All of these changes can occur alone or in conjunction with one another. For example, physical bruising will enhance the activity of polyphenoloxidase (PPO) enzymes and cause browning of tissue. Bruising will also facilitate the entry of spoilage microorganisms into the underlying tissues, thereby facilitating spoilage and the survival of human pathogens, should they be present (Wells and Butterfield, 1977, 1999). While good manufacturing practices and proper temperature or humidity control can reduce physical, chemical, and microbiological deterioration, proper packaging can also play an important role in maintaining the quality and shelf life of fresh produce throughout the distribution chain. Many fruits and vegetables undergo further processing, (e.g., thermal processing, freezing, or drying) to inhibit or delay enzymatic and microbial deterioration and to extend product shelf life. Here again, the success or failure of any processing operation is dependent, in part, on the correct choice of packaging container. For example, if dried fruits are packaged in a material that has too low a moisture barrier, the product will pick up moisture from the external atmosphere to a level conducive to mold growth. In other instances, the packaging container is an integral part of the

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food process operation and ensures the quality and safety of the processed products, e.g., canned fruits and vegetables. There is a multitude of packaging materials in today’s marketplace, each incorporating specific properties. The correct choice of packaging is not only dependent on a knowledge of the physical, chemical, and microbiological characteristics of fruits and vegetables, but also on the functional properties of the packaging materials available for a particular product or preservation technology. This chapter will give a brief overview of the properties of the materials most commonly used for packaging of fresh and processed fruits and vegetables and the packaging technologies that can be applied for shelf-life extension of products.

15.2 GROWTH OF PACKAGING OF FRUITS AND VEGETABLES In the past three decades, there has been a tremendous growth in prepackaged fruits and vegetables on supermarket shelves and new food processing or packaging technologies, such as aseptic processing and controlled or modified atmosphere packaging, for shelf-life extension of these products. The growth of packaging, for both short- and long-term preservation of fruits and vegetables, is due to a number of interrelated factors.

15.2.1 DEVELOPMENTS

IN

NEW POLYMERIC BARRIER PACKAGING MATERIALS

The success of any packaging technology as a means of extending the shelf life of food is dependent on the permeability characteristics of the packaging materials surrounding a product. Developments in polymer chemistry have resulted in the production of packaging films such as low-density polyethylene (LDPE), polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) commonly known as Saran, and ethylene vinyl alcohol (EVOH). These films have a range of water vapor, gas barrier, and heat-sealing characteristics that enable them to be used alone or together in laminated and coextruded structures with the desired permeability characteristics for shelf-life extension of products. In addition, developments in high-speed continuous and thermoforming packaging equipment, compatible with the machinability characteristics of these films, have also promoted the growth of packaging of fruit and vegetable products.

15.2.2 INCREASED URBANIZATION In the past, production of food was a major task for the majority of the population. With the industrial revolution, rural populations decreased as people moved into cities to be closer to their workplace. Most urbanized nations are dependent on a food supply chain that extends all the way from the farm gate, which may be thousands of miles away, to the urban meal table. The food processing and packaging industry provides an essential chain in this long link by ensuring that consumers have a constant supply of a variety of fresh and processed fruit and vegetables that are nutritious and safe to eat.

15.2.3 MARKET NEEDS

AND

CONSUMER DEMANDS

FOR

CONVENIENCE

Over the past 20 to 30 years, there have been many changes in consumers’ lifestyles. The fundamental change is in relation to the traditional roles of women, one of which was meal preparation. With over 60% of the women in industrialized countries in the workplace, the time previously available for shopping and food preparation has decreased substantially. The result is that, today, with the need for convenience, many consumers are prepared to pay to have food products that require a minimum of preparation, e.g., prepared salads, frozen vegetables, instant mashed potatoes, and fruit cocktail. In fact, the consumer is indirectly asking the food industry to take over a part of, or in some cases all of, the more time-consuming steps associated with food preparation. In

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many cases, packaging serves as a convenience tool for easy preparation, handling, opening and recycling, portion control, or even as a serving utensil (TV dinners).

15.2.4 INCREASING ENERGY COSTS Increasing energy costs associated with traditional methods of food preservation and storage, such as freezing, have resulted in the growth of less energy-intensive and more economical methods of short- and long-term preservation, e.g., modified atmosphere packaging (MAP). It has been estimated that MAP is 18 to 20% less energy-intensive than freezing for shelf-life extension of bakery products (Aboagye et al., 1986). As a result of these interrelated factors, food packaging technology has gone through a tremendous transformation. Packaging of fruits and vegetables now provides increased consumer information and is used very effectively as a marketing tool. It has clearly evolved from its primary, and previously singular, role of protection to be a more multifaceted tool.

15.3 DEFINITION AND FUNCTIONS OF PACKAGING There are several definitions of the term packaging. Robertson (1992) defined packaging as “the enclosure of products, items, or packages in a wrapped pouch, bag, box, cup, tray, can, tube, bottle, or other container to perform the following functions: containment; protection; or preservation; communication; and utility or performance.” If the device or container performs one or more of these functions, it is considered a package. This definition implies that packaging serves more than one function; i.e., it is polyfunctional. Brown (1992) stated that the functions of a package are “to preserve the quality and freshness of food, to add appeal to consumers, and to facilitate its storage and distribution.” The basic functions required of a package can be grouped under five major categories.

15.3.1 CONTAINER A primary function of any package is to contain and facilitate handling, storage, and distribution all the way from the manufacturer to the ultimate user. However, there are usually various levels of packaging. A primary package is one that comes into direct contact with the contained product, e.g., metal cans, glass jars, and plastic pouches. By law, a primary package must not yield any substance that may be injurious to the health of the consumer. A logical development to facilitate handling even further is to bundle a series of these primary packages together, and this leads us to the concept of secondary packages. Examples of secondary packages are tins of apple juice inside a corrugated box. As methods of handling and transportation have become more sophisticated, these secondary packages are often palletized and secured by strapping with metal or, more commonly, by shrink- or stretch-wrapped film to give yet another level of packaging, tertiary packaging. In turn, these pallet loads may be packed into large metal containers, i.e., quaternary packaging for transportation over long distances by air, land, or sea.

15.3.2 PROTECTION The most important function of any container is to protect the product contained against any form of loss, damage, deterioration, spoilage, or contamination that might be encountered throughout the distribution chain. Packaging can prevent physical damage, e.g., bruising caused by vibrational shocks during transportation or stacking in a warehouse. Proper packaging will also prevent material loss, e.g., potatoes from a weak sack or juice from a leaky can. Packaging can also protect products against moisture loss or gain, dust, and light, especially UV light, which causes deterioration of some light-sensitive products. It can also protect the package contents against temperature

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fluctuations in the transit of chilled and frozen foods. Packaging can also be used to control the availability of oxygen to fruits and vegetables and to protect against loss of flavor or fragrance and help products retain their nutritional value. Proper packaging may also protect against microbial spoilage by bacteria, yeasts, and molds. It can also protect against macrobiological spoilage of stored products due to rodents and insects.

15.3.3 MEDIUM

OF

COMMUNICATION

An important function of any food package is to identify the product and its origin; to inform the consumer how to use the contents; to provide any other information needed or required; and very importantly, to attract the user and encourage purchase of the product. The information a package can convey to the consumer may include the following: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Product manufacturing and best buy dates Proper storage conditions Cooking instructions Size and number of servings or portions per pack Nutritional information per serving Manufacturer’s name and address Cost Suggested recipes Country of origin

15.3.4 MEANS

OF

MINIMIZING COSTS

An important fact often overlooked is that packaging actually reduces costs for the consumer. Packaging reduces food costs by reducing the cost of processing. Foods can be processed where they are grown, waste is treated at the processing plant, and shipping weights are reduced, thereby lowering the cost of transportation. The handling of packages in quantity is important for the economics of bulk storage, warehousing, transport, and distribution. Proper packaging facilitates efficient and mechanized handling, distribution, and marketing of products, thus reducing the high labor costs that would have to be absorbed into the price of the product.

15.3.5 MEANS

OF

SELLING PRODUCT

Another important function of packaging is to help sell the product it contains. Packaging is often referred to as the “silent salesman.” Robertson (1992) concisely summarized the multifunctions of packaging when he stated that “a package must protect what it sells and sell what it protects.” According to Jelen (1985), primary packages should have the following characteristics to facilitate the sale of products: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Sanitary Nontoxic Transparent Lightweight Tamper evident Easy to pick up and handle Easy to fit into cupboards, shelves, refrigerators, etc. Easy to open and dispense from Easy to reclose Returnable, recyclable, or reusable Safe and presents no hazards in the way of broken glass or sharp jagged metal edges

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TABLE 15.1 Functional Requirements of Packaging Materials Functional Property

Specific Factors

Gas permeability Protection against environmental factors Mechanical properties

O2, CO2, N2, H2O vapor Light, odor, microorganisms, moisture Weight, elasticity, heat-sealability, mechanical sealability, strength (tensile, tear, impact, bursting) Grease, acid, water, color Attractiveness, printability, cost Disposability, repeated use, resealability, secondary use

Reactivity with food Marketing-related properties Convenience

Source: Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266.

TABLE 15.2 Commonly Used Packaging Materials Materials

Examples of Use

Wood Cloth Paper Glass Metal Plastics Laminates

Crates, pallets Sacks Bags, boxes, cartons Bottles, jars Cans, aluminum foil Overwraps, bags, cups, bottles Multilayered plastics, cartons

Source: Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266.

The desirable polyfunctional properties of packaging materials are summarized in Table 15.1.

15.4 TYPES OF PACKAGING MATERIALS A variety of packaging materials, each with specific functional properties, is commercially available for packaging fruits and vegetables. These include wood, cloth, paper, glass, metal, and plastic (Table 15.2). Each of these materials will be briefly reviewed.

15.4.1 WOOD

AND

TEXTILES

Wooden containers were traditionally used for the bulk transportation of fruits and vegetables to the marketplace. Wood offers good mechanical protection, good stacking characteristics, and a high weight-to-strength ratio. However, it is not an effective moisture or gas barrier, and it can also be a source of microbial contamination, particularly by molds. With the advent of plastics, wooden containers are gradually being replaced by polystyrene, polypropylene, and polyethylene containers, which are lighter and have lower transportation costs. Textile containers, e.g., jute sacks, are also used sparingly for the bulk transportation of fruits and vegetables to the marketplace. While jute sacks are durable and have a high tear resistance,

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TABLE 15.3 Types of Paper Commonly Used as Packaging Material Product

Characteristics

Kraft paper

Brown, unbleached paper. Good strength and resistant to bursting White paper, may be glossy. Less strength than unbleached paper Translucent paper treated with H2SO4 to gelatinize surface layers High-density paper, very smooth surface

Bleached paper Parchment paper Greaseproof paper Glassine

High-density, greaseproof paper. Transparent, brittle Lightweight paper produced from most pulps

Tissue Paperboard or cardboard Corrugated cardboard

Compacted paper pulp Paperboard sheets interspersed with paper corrugations

Example Heavy duty bags and sacks White bags, wrapping paper Butter and margarine wrap Wrapping paper requiring high resistance to grease Overwraps on candy Lightweight and used to protect soft products from dust and bruising Cartons, boxes, trays, separators Secondary boxes of many kinds

Source: Adapted from Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266; Brown, W. E. 1992. Properties of plastics used in food packaging. In Plastics in Food PackagingProperties, Design and Fabrication, Marcel Dekker, New York, pp. 103–139.

they have a low extensibility, are very poor barriers to moisture and gases, and are subject to mold growth. They are also being replaced by multiwall paper sacks and plastic net sacks.

15.4.2 PAPER

AND

BOARD

Paper and board are still very popular packaging materials in North America. Kelsey (1989) estimated that paper and paperboard packaging comprise about 31% of the approximately 70 million t of paper products produced annually, with a market value of $16 billion. Paper pulp is produced from wood chips by acid or alkaline hydrolysis. The pulp is suspended in water and beaten with rotating impellers and knives to split the cellulose fibers longitudinally. The fibers are then refined and passed through heated rollers to reduce the moisture content and then through finishing rollers to give the final surface properties to the paper. Alkaline hydrolysis produces sulphate pulp and acid hydrolysis produces sulfite pulp. The various types of paper and paperboard containers used as food packaging are shown in Table 15.3. Kraft paper is made from at least 80% sulphate wood pulp. It is a very strong paper, which is used to make grocery bags, multiwall bags, shipping sacks, and specialty bags that require both economy and strength for bulk packaging of powders, flour, sugar, fruits, and vegetables. Bleached papers are more expensive and weaker than unbleached ones, and they have excellent printability. Vegetable parchment is produced from sulphate pulp, which is passed through a bath of sulphuric acid. It has a more intact surface than kraft paper and therefore has greater grease resistance and wet strength properties than kraft paper. Because of its high grease resistance and wet strength, it is used for packaging butter and shortening (Fellows, 1988; Brown, 1992). Sulfite paper is lighter and weaker than sulphate papers. Greaseproof paper is made from sulfite pulp in which the fibers are more thoroughly beaten to produce a closer structure. It is resistant to oils and fats when dry, but these properties are lost when the paper becomes wet. Packaging applications for greaseproof papers include margarine wraps, french-fry bags, inner liners for

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multiwall sacks, and a liner in composite cans for packaging frozen juices. Glassine is a greaseproof sulfite paper that is given a high-gloss finish by the finishing rollers. It is used as wrapping material for candy products and certain bakery products. Tissue paper is a soft, nonresilient paper used to protect fruits against dust and bruising (Brown, 1992). A major disadvantage of paper as a packaging material is its poor barrier properties against moisture, gases, grease, and odors. Furthermore, it cannot be heat-sealed. To improve its barrier and heat-sealing properties, paper is often combined with wax, plastic film, metal foil, or a combination of foil and plastic film. Paperboard is made in a similar way to paper but is thicker in order to protect foods from mechanical damage. The main characteristics of board are thickness, stiffness, the ability to crease without cracking, the degree of whiteness, surface properties, and suitability for printing (Brown, 1992). White board is suitable for contact with food and is often coated with polyethylene, polyvinyl chloride, or wax for heat-sealability. It is commonly used to prevent freezer burn in stored frozen products. Pulp containers are made from paper pulp compressed in molds to remove moisture. Pulp containers are used for egg cartons, low-cost food trays, and cushioning of food products. Corrugated board is the most comnmon form of secondary food packaging and is used by virtually every industry. According to Kelsey (1989), 280 billion ft2 of corrugated board, with a market value of $11.8 billion, was produced in 1986. Corrugated board has an outer and inner lining of kraft paper with a central corrugating (or fluting) material. This is made by softening kraft paperboard with steam and passing it over corrugating rollers. The liners are then applied to each side using a suitable adhesive. The board is formed into cutouts, which are then assembled into cases at the filling line. There are four different flute sizes — A, B, C, and E flutes. These vary in height and the number of flutes per unit length of board. They can be used alone or in combination with one another to give single-face, single-wall, double-wall, and triple-wall corrugated board constructions, as shown in Figure 15.1. Corrugated board has good impact abrasion and compression strength and is mainly used as secondary packaging containers. The most standard type of secondary packaging material is single wall C flute. A high storage humidity may cause delamination of the corrugated material. This is prevented by lining with polyethylene or greaseproof paper or coating with microcrystalline wax and polyethylene (Brown, 1992).

15.4.3 GLASS Glass is one of the most important packaging materials because of its high barrier characteristics. Over 100 billion glass containers, with a market value of approximately $5 billion, are used annually by the food and beverage industry (Kelsey, 1989). Glass is an inorganic substance formed from a mixture of sand (73%), sodium oxide (13%), and calcium oxide (12%), with a proportion of broken glass or culler (15 to 30% of total weight). When heated to a high temperature (2700∞F), the raw materials liquefy. Specific amounts of molten glass or gobs are shaped in a parison mold by the blow-and-blow process or the press-and-blow process. The glass is then annealed at ~1000∞F to remove stresses and cooled under carefully controlled conditions to prevent distortion or fracturing. According to Fellows (1988), glass containers have several characteristics that make them ideal for food and beverage packaging: 1. 2. 3. 4. 5. 6.

They They They They They They

are impervious to moisture, gases, odors, and microorganisms. are inert and do not react with or migrate into food products. have filling speeds comparable to those of cans. are suitable for heat processing when hermetically sealed. are transparent to microwaves. are reusable and recyclable.

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(1)

(2)

(3)

(4)

(5)

FIGURE 15.1 Various types of corrugated board construction: (1) A flute, single wall; (2) B flute, single wall; (3) C flute, single wall; (4) C and C, double wall; (5) A, B, and C, triple wall (courtesy of MacMillan Bathurst, Montreal).

7. 8. 9. 10. 11. 12.

They They They They They They

are resealable. are transparent and display the contents. can be molded into a variety of shapes and colors. are perceived by the customer to add value to the product. are rigid and allow stacking without container damage. can be printed on directly or by using