CLASSIFICATION, STANDARDS AND COMPOSITION OF CREAM
Introduction Cream is the fatty portion of milk which rises to the top of milk on quiescent storage and is rich in fat. It is produced by separation of un-homogenized whole milk. The concentration of non-fat components (SNF) in cream is in the proportion to the amount of non-fat components transferred from the original milk from which cream is obtained. The indigenous product similar to cream is malai.
Legal Definition According to Food Safety and Standards Regulations (FSSR) 2011, Cream including sterilized cream means the product of cow or buffalo milk or a combination thereof. It shall be free from starch and other ingredients foreign to milk. It may be of following three categories, namely:-
1. Low fat cream-containing milk fat not less than 25.0 percent by weight. 2. Medium fat cream-containing milk fat not less than 40.0 percent by weight. 3. High fat cream-containing milk fat not less than 60.0 percent by weight. Note:- Cream sold without any indication about milk fat content shall be treated as high fat cream.
Classification The fat in cream varies from 18-85%. As the fat percentage in cream increase the components of milk in cream gradually decreases. The SNF content constitutes lower proportions than present in milk. Broadly, cream may be classified into two groups: 1.
Market Cream : The cream used for direct consumption
2. Manufacturing or Industrial Cream: It is used in production of various milk products. Different types of cream following under these groups are:
Description of Cream as per Codex Alimentarius Commission 1. Cream is the fluid milk product comparatively rich in fat, in the form of an emulsion of fat-inskimmed milk, obtained by physical separation from milk. 2. Reconstituted cream is cream obtained by reconstituting milk products with or without the addition of potable water and with the same end product characteristics as those of cream. 3. Recombined cream is cream obtained by recombining milk products with or without the addition of potable water and with the same end product characteristics as those of cream. 4. Prepared creams are the milk products obtained by subjecting cream, reconstituted cream and/or recombined cream to suitable treatments and processes to obtain the characteristic properties as specified below. 5. Prepackaged liquid cream is the fluid milk product obtained by preparing and packaging cream, reconstituted cream and/or recombined cream for direct consumption and/or for direct use as such. 6. Whipping cream is the fluid cream, reconstituted cream and/or recombined cream that is intended for whipping. When cream is intended for use by the final consumer the cream should have been prepared in a way that facilitates the whipping process. 7. Cream packed under pressure is the fluid cream, reconstituted cream and/or recombined cream that is packed with a propellant gas in a pressure-propulsion container and which becomes Whipped Cream when removed from that container. 8. Whipped cream is the fluid cream, reconstituted cream and/or recombined cream into which air or inert gas has been incorporated without reversing the fat-in-skimmed milk emulsion. 9. Fermented cream is the milk product obtained by fermentation of cream, reconstituted cream or recombined cream, by the action of suitable micro-organisms, that results in reduction of pH with or without coagulation. Where the content of (a) specific micro-organism(s) is(are) indicated, directly or indirectly, in the labelling or otherwise indicated by content claims in connection with sale, these shall be present, viable, active and abundant in the product to the date of minimum durability. If the product is heat-treated after fermentation the requirement for viable micro-organisms does not apply. 10. Acidified cream is the milk product obtained by acidifying cream, reconstituted cream and/or recombined cream by the action of acids and/or acidity regulators to achieve a reduction of pH with 2|Page
or without coagulation In general, the following types of cream are relevant to market and industrial purposes: Table Cream: This type of cream contained 18 to 22% fat and used for eating directly. Various food preparations are made from this cream as fruit cream etc. Light Cream or Thin Cream: Fat percentage in this type of cream is about 20 to 25 and used for table purpose and for manufacturing of cream cottage cheese etc. Coffee Cream: This is light or thin type fresh cream containing 18 to 25 percent fat. It is used mostly in coffee making. If this cream has any lactic acid, it becomes coagulated after adding to coffee. These cream coagulates are known as “Cream feathers”. This feathering may be prevented by addition of sodium citrate to this slight acidic cream. If the water which used in coffee making has more calcium, accelerate the cream feathering. The addition of 0.05% sodium citrate also reduces the hardness and thickness of cream plug in cream bottles. Heavy Cream: This type of cream contains 30 to 40% fat and used in production of butter and ice cream. Sometimes it can be used in paneer production. Plastic Cream: The cream of 30-40% fat re-separated to obtain the cream of 80 per cent and this rich cream is known as plastic cream. Mostly this type of cream is used for ghee production. This cream can be produced directly from milk which is separated in an especially designed plastic cream separator. Whipped Cream: When the normal cream (30-40% fat) is incorporated with air, the air bubbles are stabilized by protein adsorption. The fat globules are assemble around these air bubbles. As whipping continues, these air cells subdivided into smaller one each with adsorbed layer and in numerable fat globules. At the maximum stiffness, these smaller air cells (nuclei) are surrounded by films of adsorbed protein which is so thin that drainage is at a minimum and the structure of dry foam is set up. The favorable temperature for whipping is 10Type equation here. C. Over whipping may produces a buttery product. The cream pasteurization and sugar addition declines the whipping rate in cream. Some of the dairy plants used Nitrous oxide gas for cream whipping. Whipped cream is used in cakes and ice cream etc. for decorative purpose. Sour Cream: This type of cream are produced by inoculating sweet, pasteurized and homogenized cream with a starter culture containing lactic acid producing and aroma producing microorganism and incubated to proceed fermentation. This cream has 0.6% lactic acid, clean flavour and smooth texture. Sometimes rennet can be added @ 0.03 ml/gal cream to get more firm product. Sour cream should be stored at 40 oF or less temperature to check the further increase in acidity.
Clotted cream: To obtain the clotted cream, milk is scalded for fifteen minutes at about 85-90 oC in shallow pan and allowed to form a cream layer. Considerable evaporation occurs from the cream layer and on cooling over a period of twenty four hours, the clotted cream may be removed. The main difference between ordinary cream and clotted cream are the decreased ratio of SNF/water and relatively higher proportion of protein in the clotted cream. Frozen Cream: Cream is frozen to improve its keeping quality. Pasteurize the ordinary cream (40-50% fat) at 77 oC for 15 minutes and packing in paper, plastic or tin containers after cooling at 4 oC or below. Freeze quickly this packaged cream and store at 12 oC or below. This stored cream is used during shortage in preparation of ice-cream etc. During storage the fat globule membrane may ruptured by ice crystals. Therefore, this cream tends to Oil-off on thawing. This Oiling-off impairs the whipping characteristics of the ice cream. Sterilized or Canned Cream: Homogenize the 20% cream at 80 oC using pressure 175 kg/cm2 in the first stage and 35 kg/cm2 in the second stage and cooled to 16 oC immediately. Tri-sodium phosphate is added @ 0.2% as a stabilizer to the cream. This cream is packaged in tin cans or bottles and sterilized in following manner: Coming up time 15 Minutes to 114± 1oC
Holding time 15 Minutes to 114± 1 oC Cooling time 15 Minutes to room temperature Reconstituted Cream: By vigorous emulsification of unsalted plain butter in milk or separated milk or reconstituted milk or condensed milk, the fat globules are dispersed and coated with a layer of adsorbed protein, the resulted product is closely resembles with cream and known are reconstituted cream. Synthetic Cream: In the artificial cream, butter fat is substituted with margarine fat and resulted product is known as synthetic cream. Margarine is an imitation and substitute of butter. In production of synthetic cream, the refined oils are carefully blended to have some physical cream, the refined oils are carefully blended to have some physical properties resemble to butter fat. Synthetic cream may have some emulsifying agents such as egg yolk, vegetable lecithin and glycerol- Mono or distearated etc.
Whey Cream: Separation is sometimes used to remove fat from cheese whey and resulted cream is known as whey cream. It is slightly differs in composition from regular fresh cream obtained from milk.
Composition of Cream Cream generally contains all the constituents of milk but non-fat constituents are inversely proportional to the fat content. Composition of cream Cream with 30% fat
Cream with 50% fat
SNF of cream can be calculated by taking the ratio of water to SNF of milk If ratio of water to SNF of milk is 10:1, then the approximate SNF in cream = (100 - F/11) where F is fat content of cream. SNF in cream can be estimated as:
Physico-Chemical Properties Some of the important physico chemical properties relevant to cream processing and consumption are discussed below:
It is the resistance offered by the liquid to flow. Consumer judges the richness of cream by its viscosity. The factors affecting the viscosity of cream are given below: Fat percentage: Higher the fat % greater is the viscosity. Temperature: Higher the temperature lower is the viscosity. Separation conditions: Higher temperature of separation lowers the viscosity.
Homogenization: Homogenization increases the viscosity of cream. Single stage homogenization increases viscosity more than the double stage homogenization.
Cooling: Cooling of cream increases the viscosity. Ageing: The viscosity of cream increases with storage period. Clumping: It refers to the tendency of fat globules to adhere loosely to one another and form clumps. Clumping depends on fat globule size, temperature (maximum at 7 oC), agitation and method of separation. The greater is the clumping, greater will be the viscosity.
Whipping quality Whipping means emulsion of gas or foam production by beating of cream. Whipped cream has remarkable stability. This is used in cakes, ice creams and for decorative purposes. The most satisfactory fat content for production of whipping cream is 30-35%. The optimum aging period is 24 hours at 4 oC . Homogenization, acidity and stabilizers in cream reduces the whip-ability of cream.
Titratable acidity (TA) There is an inverse relationship between the percent fat and percent titratable acidity. Fresh cream has lower acidity percentage than milk. Formula for Acidity:
Specific gravity Specific gravity of cream is inversely proportional to the fat percentage as shown below: Fat percentage in milk
CHEMISTRY OF CREAMING AND FACTORS AFFECTING THE SAME-I
Introduction When milk is allowed to stand un-disturbed for some time, an upward motion of the fat globules takes place, leading to the formation of a surface layer on milk in which the percentage of fat is considerably increased. This upward motion of fat based on the fact that milk fat is lighter than the skim milk portion. At 16°C, the average density of milk fat is 0.93 and of skim milk is 1.0404. Therefore when milk (a mixture of fat and skim milk) is subjected to either gravity or centrifugal force, the two components, cream (fat-rich portion of milk) and skim milk (reducedfat portion of milk), by virtue of their differing densities, separate from each other.
Purpose of cream separation 1. To obtain a fat-reduced or fat-free milk 2. To concentrate milk fat for the production of high-fat products 3. To standardize the fat content of milk 4. To recover fat from milk The cream separation process has significant economic importance, as it controls the efficiency of the fat separation. The key objective is to manufacture skim milk with the lowest possible fat content, which corresponds to good separation efficiency. Knowledge of the basics of the fat separation is important for an optimal de-creaming process. Cream separation is based on the facts that fat exists in poly-disperse system in an emulsified state and that the specific density difference between milk fat (p = 0.93 g/cm3) and skim milk (p = 1.035 g/cm3) is fairly large. Basically two processes for fat separation are possible, natural de-creaming and separation with machines. Natural creaming has no industrial significance.
Separation Processes There are two methods of cream separation viz., · Gravity Method · Centrifugal Method
Cream separation by gravity method When milk is allowed to stand undisturbed for some time, there is a tendency of fat to rise. The velocity or rate at which the fat globules rise is given by the following equation, which is known as Stoke’s Law: 8|Page
V = (2/9) * Gr2 * (ds - df) / N Where, V = rate of rise of fat globule in centimeter per seconds r = radius of fat globule G = Force of gravity (981 dynes) ƞ= Viscosity of skim milk ds = density of skim milk df = density of fat globule From, Stoke’s Law it is observed that theoretically velocity increases with: a. Increasing radius of fat globule, b. Increasing difference in densities of skim milk and fat c. Decreasing viscosity of skim milk However, in practice the factors affecting the rate of rise of fat in gravity method of separation are:
Size of fat globules: As the size of fat globules increases, the rate at which fat rises also increases. Larger fat globules rise faster than smaller ones. Thus, in buffalo milk gravity creaming occurs faster due to the larger fat globules than those in cow milk.
Temperature: As temperature increases, viscosity decreases.
Clumping: A clump or cluster acts like a single globule in so far as movement through skim milk is concerned. Thereby the effective ‘r’ is increased, which in turn increases velocity, as shown below
Effect of size of fat globules on its rate of rise Diameter of fat globule or cluster (µm) 3.2
Rate of rise (mm/h)
There are five various methods for separating the cream using gravity method: 9|Page
i. Shallow Pan Method: Milk is allowed to stand in a pan of 10 cm depth and 45-60 cm diameter at 7°C for 24 h. During this time, cream rises to the surface. ii. Deep Pan Method: Milk is allowed to stand in pan of 20” depth and 8 to 12” diameter at 10°C for 24 h. These tall cans have glass on one side of can and a faucet placed near the bottom. Skim milk is drawn through the faucet. iii. Water Dilution Method: Milk is diluted with water and allows standing for 12 h at 37.7°C temperature. Water would make the milk less viscous, thus facilitating the rising of the fat globules. iv. Scalding Method: Heating and cooling of milk slowly causes the formation of cream layer at surface of milk v. Jersey Creamery Method: Milk is heated to 40°C using hot water in the jacketed vat and then cool to 10°C using chilled water in place of hot water in the jacket of Vat. The cream will be separated rapidly on cooling, immediately after heating the milk, by increasing the difference in densities of milk fat and serum. Gravity method being very slow, it is no longer used commercially for cream separation.
Cream separation by centrifugal method Milk is fed to machine through flow regulator. Milk comes to regulating chamber from milk basin by milk faucet. When milk enters the revolving bowl through milk regulator of machine, it is subjected to a gravity and centrifugal force. Centrifugal force is about 3000 to 6000 times more than gravitational force. Fat (0.9) and skim milk (1.037) are varying in their specific gravity. When fat and skim milk are subjected to centrifugal force, the difference in density affect the fat and skim milk i.e. (heavier Portion) affected more intensely than the fat (lighter portion). So skim milk is forced to the periphery and fat portion (cream) moves towards the centre. Cream and skim milk forms separated vertical walls within the bowl and goes out through separate outlets near the axis of rotation. The cream outlet is at higher level than skim milk outlet. The rate or movement of a fat globule in machine is estimated by following Stoke’s equation. V = r2 *((as - df) / n) * N2 * R * K Where, V = rate of movement of a single fat globule r = radius of fat globule ds = density of skim milk df = density of fat N = Revolution per minute of bowl 10 | P a g e
R = Distance of fat globule from axis of rotation. K = Constant N = Viscosity of skim milk It will be seen from the above that the speed (rate) of cream separation is increased by: · greater radius of the fat globule · greater difference in density between skim milk and fat · greater speed of the bowl · greater size of the bowl · lower viscosity of skim milk
Characteristics of gravity and centrifugal methods Gravity and centrifugal cream separation compare as shown below:
Particulars Nature of force causing Separation Speed of separation Direction of movement of fat and skim milk particles Bacteriological quality of cream or skim milk Fat % of cream Skim milk Scale of operation Fat % recovered in cream
Gravity Method Gravitational force Extremely slow
Centrifugal Method Centrifugal force Practically instantaneous
10-25% only 0.2 % above Small not more than 90
18-85 % 0.1 or below Large 99-99.5
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CHEMISTRY OF CREAMING AND FACTORS AFFECTING THE SAME-II
Introduction Normally the objective of separation is to attempt to recover all the fat in the whole milk within the cream fraction, with the minimum amount of fat being retained in the skim milk. Skimming efficiency is assessed as the fat content of the skim milk. If the skim milk is to be converted into skim powder or casein then it is important that the fat content be low, to meet various specifications and functional requirements of these products.
Factors Influencing the Fat Percentage of Cream The important factors influencing the fat percentage of cream by centrifugal separation are discussed below: 1. Position of the cream screw The cream screw/outlet consists of a small, threaded, hollow screw pierced by a circular orifice through which the cream emerges. This screw can be driven IN or OUT, thus bringing it nearer to, or away from, the centre of rotation. Similarly, the skim milk screw/outlet is for the removal of skim milk. Once the cream screw or skim milk screw has been adjusted, the cream separator delivers, under normal conditions, a definite ratio of skim milk and cream, which is usually 90:10 (or 85:15) by volume. Basically, any change in the separation procedures which alters the relative quantities of skim milk and cream will influence the fat test of the cream. By altering the position of the cream screw (or skim milk screw), the ratio of skim milk to cream changes. Thus, when the cream screw is moved IN towards the axis of rotation, a higher fat percentage in cream is obtained, and vice versa; this is because the force tending to discharge cream through the orifice is decreased ('R' in the formula F = KWRN2 is decreased),. A smaller proportion of cream is therefore discharged, which, contains the same quantity of fat, resulting in a higher fat percentage. Screwing OUT the cream screw produces thinner cream. Similarly, the skim milk screw OUT results in richer cream, and vice versa. 2. Fat percentage in milk The higher the fat percentage in milk, the higher the per cent fat in cream, and vice versa. Since practically all the fat in milk is contained in the cream, the cream from the separation of high-fat milk has a higher fat content than that from low-fat milk; a greater fat content in cream, the amount of which remains unaltered in the two cases, will obviously show a higher fat percentage in it, and vice versa. 3. Speed of the bowl 12 | P a g e
The velocity of a fat globule is proportional to the square of the rotational speed so an increase in bowl speed will have a very major effect on separation efficiency. An increase in bowl speed however requires an increase in energy input and a more robust design to withstand the large forces at the bowl periphery. The separator also generates more noise. For this reason bowl speeds have not increased significantly as skimming efficiency is quite adequate at moderate speeds of 4000-6000 rpm. The higher the speed of the bowl, the higher will be the fat % in cream. The higher the speed of the bowl, the greater will be the centrifugal force and more rapidly the skim milk leaves the bowl with higher fat % in cream. 4.Rate of the milk flow The higher the rate of milk inflow, the lower the fat percentage in cream, and vice versa. When the rate of inflow increases, the discharge from the cream outlet increases. As the skim milk discharge remains constant (with constant centrifugal force); more cream containing the same amount of fat results in a lower fat test, and vice versa. 5.Temperature of milk An increase in temperature of milk leads to both an increase in density difference between milk fat and skim milk. So increase in temperature will lead to lower separation efficiency. Higher temperature will lead to disruption of fat globule which will result in heavy fat losses in skim milk. The fat losses are higher at 70 O
C than at 54.5 OC. The optimum separation temperature is 40 OC. Higher temperature leads to protein
denaturation and phospholipids. Cold milk separators that will operate at temperatures less than 10 OC are available. These allow separation of milk as it is received at the factory, and although fat losses to skim milk are somewhat higher they do sometimes allow substantial savings in energy and capital costs. In some cheese making operations heat treatment of the milk is undesirable and cold milk separators offer some advantages. In addition cold milk separators produce cream with greater phospholipids content which gives better whipping properties. The major modification in a cold milk separator is wider disc spacing than in a conventional model to allow adequate flow of the more viscous cold cream. 6. Amount of water or skim milk added to flush the bowl The greater the quantity of water or skim milk added to flush the bowl, the lower the fat percentage in cream, and vice versa. The addition of more water or skim milk will cause an increase in the amount of cream produced, which, with the same fat content, will show a lower fat test.
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Factors affecting fat losses in skim milk The important factors influencing the fat losses in skim milk obtained through centrifugal separation are discussed below: 1. Temperature of milk The lower the temperature of milk, the higher the fat losses in skim milk and vice versa. For efficient separation, the temperature of milk should be above the melting point of fat, so that the milk fat in the fat globules is entirely in liquid form. A satisfactory temperature for separation is around 40 OC. The higher the temperature, the more efficient is the separation. There is no marked increase in efficiency after 43-49 OC. On the other hand, separation at low temperatures (in warm-milk separators) may lead to partial clogging of the bowl due to high viscosity of cream at these temperatures, resulting in a greater fat loss in skim milk. 2. Speed of the separator bowl The lower the speed of the bowl, the higher the fat loss in skim milk, and vice-versa. At below-rated speed there will be more fat loss in skim milk because insufficient centrifugal force is generated for efficient cream separation. However, at above rated speeds, the skimming efficiency will not increase greatly. 3. Rate of milk in-flow The higher the rate of inflow of milk, the higher will be the fat losses in skim milk, and vice versa. If the rate of inflow is increased above the designed capacity of the separator, the milk passes through the bowl too rapidly and do not to allow for complete separation, thereby resulting in a higher fat loss in skim milk. On the other hand, underfeeding the separator does not greatly increase the efficiency of the separation. 4. Position of cream screw A good separator is designed to give efficient skimming within a fairly wide range of positions of the cream screw, so that the fat test of the cream can be varied without influencing the efficiency of skimming. With most separators, the position of the cream screw has little effect on the fat test of skim milk until the cream test is above 45 to 50 per cent. From this point up to a 60 per cent fat test in cream, the fat content of the skim milk increases. Separation of very thick cream at low temperatures may lead to higher losses due to clogging of the bowl with viscous cream. 5. Mechanical condition of the machine 14 | P a g e
Unsatisfactory mechanical condition of cream separator causes greater loss in skim milk. Vibration of separator: This reduces the efficiency of separation by disturbing the currents of cream and skim milk. Vibration is caused by installation on an insufficiently firm foundation, the bowl being out of balance, bearings being worn out, the axis of rotation not exactly vertical, etc. Condition of the discs: Discs in an unsatisfactory condition suffer a loss of skimming efficiency due to the uneven flow of the counter-current streams of cream and skim milk between them. An unsatisfactory disc is one which is out of shape, dirty, scratched or rough. 6.Amount of separator slime in the bowl If too much slime accumulates, the fat loss in skim milk increases; this is caused not only by a disturbance in the even flow of the counter-currents of cream and skim milk, but by reduction in the centrifugal force (because of decrease in the 'effective' diameter of the bowl). Separator slime (which is usually considered identical with clarifier slime) consists of the slimy mass which accumulates inside the bowl shell of the cream separator. It is made up of foreign matter, milk proteins, leucocytes, fragments of the secreting cells from the udder, fat calcium-phosphate and other minerals, bacteria and, occasionally, red blood corpuscles. 7. Size of the fat globules The greater the number of fat globules of less than 2 µm size, the higher the fat loss in skim milk and vice versa. Fat globules of less than 2 µm size usually enter the skim milk, as they are not subject to sufficient centrifugal force to be recovered in the cream. 8. Degree and temperature of separation The higher the degree and temperature of agitation, the greater will be the loss of fat in skim milk and vice versa. Agitation of hot milk causes the disintegration of normal fat globules in to smaller ones which escape the effect of centrifugal force there by leading to more fat loss in skim milk. 9 .Presence of air in milk The greater the amount of air in milk the higher the fat losses in skim milk. If the milk delivered to the separator contains entrapped air bubbles, centrifugal force will disturb the counter-current streams of cream
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and skim milk between the discs, and lower the efficiency of separation. The effect of air in the milk is greater with hermetic than with non-hermetic cream separators. 10. Acidity of Milk The higher the acidity of milk, the lower the efficiency of separation, the lower the stability of casein particles which in turn get precipitated and clog the bowl there by lowering the efficiency of separation.
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COMPOSITION OF BUTTER Butter is principally composed of milk fat, moisture, salt and curd. It also contains small amount of fat, lactose, acids, phospholipids, air, microorganisms, enzymes and vitamins. The proportion of principal constituents in butter is largely controlled by the method of manufacture and this is turn is chiefly regulated to conform to the standards of butter prescribed by regulatory authorities such as codex and FSSAI. Composition of butter
Quantity (% w/w)
RIPENING AND NEUTRALIZATION OF CREAM
Introduction Butter, a fat rich dairy product obtained by churning cream and working the granules thus obtained into a compact mass, has been a staple item of diet in many countries of the world. Up to the middle of the nineteenth century, manufacture of this product was mainly confined to the farm on cottage scales. It was only after the development of centrifugal cream separator in 1879, fat testing methods by Babcock (1890) and Gerber (1892) together with introduction of artificial refrigeration and pasteurization around 1980, the industrial production of butter developed rapidly. Prior to 1970 most of the world’s butter was manufactured by batch-process. However, since World War-II, continuous processes have been introduced to achieve increased manufacturing efficiencies. Regardless of manufacturing method employed, the essential feature of churning evolves destabilization of cream emulsion by means of mechanical agitation. Butter and other fat spreads can be characterized by the type of emulsion. In milk or cream, fat is dispersed in the continuous phase of serum while in butter, there is a reversal of phase i.e. fat becomes the continuous phase with serum dispersed in it. This phase reversal is carried out by churning cream in butter churns. Steps involved in the conventional process of butter making which comprises preparation of cream and churning and working are given in Figs. 18.1 and 18.2 and described below.
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Preparation of Cream Commercial butter can be produced from both sweets as well as cultured cream. Very little cultured butter is produced in India and U.S.A., although in Europe and Canada, cultured butter is an important product. However, most creamery prefer to produce butter from sweet cream as it result in sweet butter milk which has better economic value than sour butter-milk that results when sour/cultured cream is churned.
Flow diagram of cream preparation for butter manufacturing
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Flow Diagram of Butter Manufacturing 1. Neutralization of cream Sour cream must be neutralized to make butter of good keeping quality. It is under stood that by neutralization of cream acidity of cream is reduced. Churning of High acid cream may cause high fat loss
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which can be prevented by neutralization. In pasteurization of sour cream, the casein curdles, by entrapping fat globules, as the bulk of curd goes in butter milk, causing high fat loss.
Objectives of neutralization 1. The objectives of neutralization are to reduce the acidity in cream to a point (0.14 -0.16%) which permits pasteurization without risk of curdling, to produce butter which keeps well in cold storage 2. To avoid excess loss of fat which result from the churning cream i.e excessively sour. 3. To guard against undesirable flavors which may result when a cream of high acid which is subjected for pasteurization at higher temperatures. 4. To improve the keeping quality of butter from high acid cream. Salted-acid-butter develops a fish flavor during commercial storage at -23 to -29 OC. 2. Theoretical basis of cream neutralization Soda neutralizer NaHCO3 + CH3 CHOHCOOH----- CH3 CHOHCOONa +H2O + CO2 84
NF = Neutralization factor The quantitative relationship between amount of lactic acid present in solution and amount of pure neutralizer required to give exact neutralization is fixed and definite amount of CO2 is driven off by air. Ex: 90g of lactic acid requires 84g NaHCO3 (or) 106g Na2CO3 (or) 40g of NaOH for neutralization. 3. Expression of acidity in cream Acidity in cream is present chiefly in the serum portion and not in fat. The acidity in cream as a whole known as cream acidity (C.A.) while the acidity per cent found in serum is known as cream serum acidity (C.S.A). Cream serum acidity is more reliable than cream acidity. There is a relationship between CA & C.SA as below: CA %
Serum % in cream
4. Factors affecting neutralization Accurate neutralization of sour cream is important to get a desired quality product. Neutralization is influenced by several factors such as: i. Accuracy in sampling. ii. Accuracy in testing. iii. Accuracy in estimation of amounts of cream and neutralizer. iv. Careful weighing the quantity of neutralizer. v. Thorough mixing of neutralizer in cream prior to pasteurization. 5. Method of neutralization of cream There are five essential steps to follow for cream neutralization. These are: 1. Adoption of definite standard of churning acidity 2. Correct estimation of acidity 3. Calculating the amount of neutralizer to be added 4. Adding neutralizer in the correct manner 5. Checking results by re-testing acidity
Adoption of a definite standard of churning acidity Acidity of cream at churning time controls the flavour and keeping quality of the butter. Therefore, it is important to decide that at what acidity the cream shall be churned. 21 | P a g e
Churning acidity should be kept upto that maximum acidity where freedom from chemical deterioration of butter (fishy flavour) with age can be ensured. For cream of average richness (about 30%), fishy flavor can be prevented by keeping the churning acidity to 0.3% maximum. The safe maximum limit of churning acidity varies with the richness of the cream. Since the acidity of cream is chiefly contained in the cream serum, cream serum acidity adjustment would give better results.
e.g. 30% fat cream, cream acidity 0.25%
= 0.357% Usually serum acidity is kept at 0.35% to achieve best keeping quality of butter. Ex.1 cream test 20% fat, serum acidity 0.35% is desired what should be the acid test of the cream? % cream serum: 100-20 = 80 % serum acidity = 0.35 % acid in cream = % cream serum ×% serum acidity = 0.80 × 0.35 = 0.28% Correct Estimation of Acidity Representative sample should be taken. Weight of cream should be accurately measured as the final amount of neutralizer will be dependent on the weight of cream. Effect of CO2 on acidity should be taken care especially in high acid cream (acidly > 0.65%) Calculating the amount of neutralizer to be added
N. F. for Sodium bicarbonate
2.43 22 | P a g e
Magnesium hydroxide Sodium hydroxide
Adding neutralizer in the correct manner Neutralizer should be dissolved or emulsified in clean water, diluted to approx. 20 times its weight with water; the solution must be distributed quickly & uniformly throughout the entire batch of cream and mixed thoroughly with cream. For efficient mixing, neutralizer is usually sprayed onto the surface of well agitated cream. While the neutralizer is added, the cream should be agitated vigorously and continuously. Agitation of cream is preferable for 5-10 min after neutralization. Temperature at the time of neutralization should be 30oC. (High enough for smooth consistency & low enough to prevent abnormal heat curdling of the sour cream) The above precautions are essential, if efficiency of neutralization, protection of butter against neutralizer flavour, oily metallic flavour and mealy body, avoidance of pasteurizing difficulties and prevention of excessive fat losses in the buttermilk are to be assured. Checking Results of Neutralization by Re-testing for Acidity Acidity should not be checked immediately after neutralization because of the following reasons:
In case of lime and Magnesia neutralizers, the neutralizing action is slow. It completes after pasteurization & cooling.
In case of soda neutralizers, CO2 is liberated and this reacts acid toward the phenolphthalein indicator. After pasteurization, expulsion of CO2 is largely accomplished. Therefore, testing acidity after pasteurization would give correct results.
6. Role of carbon dioxide in neutralization of cream with sodium bicarbonate Fresh cream always contains some dissolved carbon dioxide (as carbonic acid) which reacts with sodium hydroxide during titration and shows a higher acidity test. But the carbon dioxide does not react with sodium bicarbonate neutralizer and consequently over neutralization results. 7. Double neutralization with lime and soda The objectives of double neutralization are: 1. To avoid the intense effect on flavor of a large amount of any one neutralizer with high-acid cream. 2. To avoid production of excessive carbon dioxide by the use of sodium bicarbonate with high-acid cream For this purpose the cream is first neutralized with lime neutralizer and brings the cream acidity down to 0.3-0.4 percent. Next use soda neutralizer to bring the cream acidity down to the desired level. 8 .Neutralizing Precautions 23 | P a g e
In order to secure the desired results, i.e., accurate acid reduction, absence of objectionable neutralizer flavor and of excessive fat losses, make sure of the correctness of weight of cream and acid test. Do not heat the sour cream above 85 to 90OF, before neutralization, use the correct amount of neutralizer in properly diluted form, distribute it evenly over the cream, and continue agitation of the neutralized cream for 5 to 10 minutes before starting to pasteurize. 9. Type of Neutralizers Neutralizers in order to accomplish the purpose, for which they are used in the creamery, must have alkaline properties. They must be alkalis, alkaline earths or their substances. An alkali is a substance that has the property of neutralizing acids, forming salts with them. The neutralizers used for reducing acidity in cream belong to either one or the other of two groups namely. Lime Neutralizers Soda Neutralizers
Lime neutralizers The principal constituent of the majority of lime neutralizers is calcium. Many of the lime neutralizer’s available for cream neutralization also contain some magnesium. The various commercial lime neutralizers differ from one another chiefly with respect to the proportion of calcium and magnesium they contain. They are conveniently placed in three groups, as follows: a. Low magnesium limes: Containing 5% or less of magnesium. A well Known brand of creamery lime belonging to this group is peerless lime. b. Medium magnesium limes: Containing about 30-35% magnesium. To this group belong such brands ad Kelly Island lime, Neutra-Lac and Neutra-Lime. c. High magnesium limes: Containing about 45 to 55% magnesium. All wood lime is an outstanding representative of this group. All magnesium limes in the form of magnesium oxide and magnesium carbonate are also available. They are artificially prepared limes and demand a higher price than the natural limes. Their effect on the flavor of cream and butter however is outstandingly favorable. Calcium carbonate - low solubility, low alkalinity unsuitable for cream neutralization and action is very slow. In general the medium and high magnesium limes react somewhat more satisfactorily in the cream than the low magnesium limes. The higher the magnesium oxide content of lime the greater is its alkalinity and its neutralizing strength.
Soda neutralizers 24 | P a g e
Soda neutralizers commonly used in the creamery are: 1. Bicarbonate of soda or baking soda 2. Sodium carbonate or soda ash 3. Mixtures off baking soda and soda ash, such as Sodium sesquicarbonate, Neutralene and Wyandotte. 10. Comparison between lime and soda neutralizers 1. Purity: Soda Neutralizers will have less than 0.1 impurities whereas lime neutralizers will have sand and clay. 2. Solubility: Sodium Neutralizers are completely soluble whereas lime neutralizers are slightly soluble in waste. 3. Action: Sodium Neutralizers act quickly. Lime Neutralizers react slowly. 4.
Action on casein: Soda neutralizers will have solvent and softening action and assist in minimizing clogging during processing. Lime neutralizers tend to granulate (or) precipitate casein which results in bitter lime flavor on pasteurization.
5. Acid reaction: Soda Neutralizers act on serum acidity first. In lime neutralizers the calcium has natural affinity towards casein, lime particles attach themselves mechanically to casein and it is not completely available for neutralizing the lactic acid. The neutralizing capacity of lime neutralizer is 80-85%. 6.
Foaming effect: Soda neutralizers will produce violent foam when there is high acidity NaHCO3 has more effect than Na2CO3. Lime neutralizers do not create foam.
7. Neutralizing strength: Soda neutralizers are weaker alkalis than lime neutralizers about twice as many kilograms of soda neutralizers are required to neutralize given amount of acid as compared to lime neutralizers. 8. Material cost: Lime neutralizers are cheaper than soda neutralizers. 9. Effect on texture of butter: In case of high acid cream especially neutralizes to a low point there is a tendency to butter from lime neutralizer cream will give less smooth texture than soda neutralizers. 10. Effect on flavor of butter: Soda neutralizers produce soapy type of flavor. Lime neutralizers produces course lime flavor. In over all the butter will give neutralizer flavors.
Standardization of cream 25 | P a g e
It refers to adjustment of fat to desired level. It is done by adding calculated quantity of skim milk or butter milk. Desired level of fat in cream for butter making is 33 to 40 per cent. Standardization to both higher and lower level leads to higher fat loss in butter milk. Reduction of fat by adding water should be avoided as it interferes ripening of cream and also results in butter with “flat” or “washed off” flavour.
Pasteurization of cream It refers to adjustment every particle of cream to a temperature not less that 71OC and holding it at that temperature for at least 20 min or any suitable temperature-time combination using properly operated equipments. The main objectives of pasteurization are: (i) it destroys pathogenic microorganisms in cream so as to make it, and the resultant butter, safe for human consumption. (ii) It also destroys bacteria, yeast, mould, enzymes and other biochemical agents that may lower keeping quality. (iii) It also eliminates some of the gaseous and training substances. A number of equipment viz. LTLT (law temperature long time, 74 OC for 30 min); HTST (high temperature short time, 85 oC for 15s.) and Vacreator, a direct steam injection method, can be employed for this purpose. More severe heat treatment of cream should be avoided as higher, the temperature the greater the migration of copper from the milk serum into milk fat globules. This increases the level of copper associated with the milk fat making it more prone to the development of oxidative rancidity and reduce the shelf-life of butter. Pasteurization of cream for making ripened cream butter is commonly carried out at higher temperature than for sweet cream butter e.g. 90-95oC for 15 or 105-110oC with no holding. Severe heat treatment denatures whey proteins, particularly lactoglobulins, exposing-SH groups which act as antioxidants and can enhance starter growth.
Ripening of cream Ripening refers to the process of fermentation of cream with the help of suitable starter culture. This step can be eliminated if sweet-cream butter is desired. The main object of cream ripening is to produce butter with higher diacetyl content. Ripening improves the keeping quality of salted butter but it reduces the keeping quality of a salted butter. Starter culture consisting of a mixture of both acid producing (Streptococcus lactis, S.cremories) and flavour producing (S.diacetylactis, Leuconostoc citrovorum and/or Leuc. dextranicum) organisms is added. Amount of starter added depends on several factors and usually ranges between 0.5-2.0 percent of the weight of the cream. After being thoroughly mixed, the cream is incubated at about 21oC till desired an acidity is reached. Cream is subsequently cooled to 5-10� C to arrest further acid development.
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Biosynthesis of diacetyl is not sufficient above pH 5.2. Stopping fermentation of cream by cooling at pH 5.1-5.3, results in a milder flavour; whereas continuing fermentation upto pH 4.5-4.7 results in higher levels of both diacetyl and lactic acid, giving more pronounced flavour. 1. Purpose The fundamental objects of cream ripening are to produce butter with a pleasing, pronounced flavor and aroma, and to produce this flavor and aroma uniformly from day to day. Ripening also influences somewhat the exhaustiveness of churning and it affects the keeping quality of the butter variously, according to quality of original cream, churning acidity, and whether made into salted or unsalted butter. 2. Starter culture Mixture of both acid producing organisms (Lactococcus lactis, L. cremoris) and flavour producing organisms (S. lactis subsp. diacetylactis, Leuconostoc citrovorum and/or Leuconostoc dextranicum). Starter culture is added at the rate of 0.5 to 2.0% of the weight of cream and incubated at about 21 oC till desired acidity is reached. Usually it takes 15-16 hrs.
Effect of Cream ripening on butter Effect on Flavor and Aroma The mildly acid and pronounced “nutty” flavor that is characteristic of the pleasing flavor of good butter is usually accompanied by a high, attractive aroma. The typical butter flavor is due to the presence of diacetyl in combination with lactic acid, carbon dioxide, acetoin and intermediary products such as acetaldehyde, and probably other aromatic products as yet not definitely determined. These substances are the products of fermentation, brought about by the associative action of lactic acid-producing bacteria and citric acidfermenting bacteria. These bacteria are propagated and their flavor and aroma substances produced in the starter. During cream ripening the starter that is added to the cream, therefore, functions in two ways. It seeds the cream with species of bacteria that are capable of producing the desired aroma and flavor substances in the cream, and it adds to the cream the aroma and flavor substances produced and already contained in the starter. Factors which Influence the Diacetyl + Acetoin Content of Butter Cream ripened with a normal starter shows a varying ratio of diacetyl to acetoin. In the case of 20% cream, a ripening temperature of 17˚C. (62.6˚F) yields the largest amount of diacetyl. With increasing fat content the amount of diacetyl + acetoin increases. Cream testing 40% fat yields higher diacetyl content in the butter than 20% cream, or whole milk. The diacetyl content of butter to affect its flavor as follows:
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Amount of Diacetyl
Absence of diacetyl
0.2 to 0.6 ppm diacetyl
0.7 to 1.5 ppm diacetyl
Distribution of Diacetyl + Acetoin by the Churning Process Butter contains a relatively small proportion of the diacetyl and acetoin content of the cream from which it is made. Fresh buttermilk contains larger amounts of diacetyl and acetoin than the corresponding butter and than the original cream, at churning time. The serum of butter contains larger amounts of diacetyl + acetoin than the fat of the same butter. The wash water contains appreciable amounts of diacetyl + acetoin. Churning 2 liters of cream yields the following amounts of diacetyl in the buttermilk, wash water and butter: Butter milk
6.14 mg Diacetyl
First wash water
3.16 mg Diacetyl
Second wash water
0.00 mg Diacetyl
Third wash water
0.00 mg Diacetyl
0.30 mg Diacetyl
The following amounts of diacetyl + acetoin are present in washed and unwashed butter: Diacetyl (mg/kg)
Diacetyl + Acetoin (mg/kg)
These findings indicate that in commercial manufacture of butter, particularly in the case of unsalted butter, excessive washing gives the finished butter, even when made from properly ripened cream, a disappointing “washed-out” flavor. Effect of Cream Ripening on Keeping Quality of Butter The development of flavor and aroma in butter by cream ripening, or by any other process of manufacture, can be of value only, provided that it does not impair or destroy the keeping quality of the resulting butter. The ripening of cream affects the keeping quality of butter in two fundamental ways, namely, by its control of age deterioration due to bacterial causes, and by its influence on age deterioration due to chemical causes.
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Bacteriological effect The ripening of cream improves the keeping quality of butter as far as keeping quality is dependent on freedom from age deterioration due to biological causes. Cream ripening assists in controlling bacterial deterioration in butter. In butter made from ripened cream there is a great prevalence of lactic acid bacteria and a relatively high acidity and probably an abundance of lactate salts. These agencies are antagonistic to the great majority of flavor-damaging organisms that may be present in the butter, thus retarding their action, preserving the fresh or desired flavor, and prolonging the keeping quality of the butter. Chemical Effect Cream ripening does not improve the chemical stability of butter. On the contrary, under average commercial conditions of manufacture, the ripening of cream to a full aroma and flavor shortens the life of salted butter. The usual flavor defects that develop with age in butter made from fully ripened cream are oily-metallic, fishy and sometimes tallowy flavor. There is a tendency also to intensify the well-known cold storage flavor. This is especially true of salted butter made from cream that arrives at the factory in sour fermented condition, is neutralized, pasteurized and re-ripened to a high acidity. It applies also, through to a somewhat lesser extent, to salted butter made from ripened sweet cream. It does not apply to unsalted butter. Salted butter made from sweet, unripened cream, or from sour cream, neutralized and pasteurized, keeps better from the standpoint of absence of flavor deterioration due to chemical causes, than salted butter made from the same cream ripened to a full flavor and aroma. 3.Percent Acid to which the Cream should be Ripened For fresh consumption salted butter, cream of moderate richness (30% fat) may safely be ripened to about 0.25 to 0.30% acid. For salted butter of commercial cold storage, it has been found preferable not to ripen the cream and to churn it at an acidity of about 0.21% acid or lower. In case of unsalted butter, the cream may be ripened to any acidity without jeopardizing keeping quality. 4.Cooling and ageing Cooling and ageing are processes which prepare the cream for subsequent operation of churning. When cream leaves the pasteurizer, the fat in the globule is in liquid form. When cream is cooled, fat crystallization starts, cream will not churn unless the butter fat is at least partially crystallized. If solidification of fat is not sufficient, the fat losses in butter are high. Rate of cooling has an important influence on the body and texture of butter. The temperature to which cream is cooled is chosen is such a way that the butter produced is of optimum consistency and cream churns to butter in a responsible time of about 35-45 minutes. Churning at too high temperature may give butter with “greasy” body which may work
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up too quickly and become sticky. Generally cooling temperature in summer should be 7-9OC and that if in winter (10 OC-13 OC). Crystallizing of the milk fat during aging Before churning, cream is subjected to a program of cooling designed to control the crystallization of the fat so that the resultant butter has the right consistency. Butter fat contains varying amounts of soft and hard fats. The relative amounts of fatty acids with high melting point determine whether the fat will be hard or soft. Soft fat has a high content of low-melting fatty acids and at room temperature this fat has a large continuous fat phase with a low solid phase, i.e. crystallized, high-melting fat. On the other hand, in a hard fat, the solid phase of high-melting fat is much larger than the continuous fat phase of low-melting fatty acids. In butter making, if the cream is always subjected to the same treatment it will be the chemical composition of the milk fat that determines the butter's consistency. A soft milk fat will make a soft and greasy butter, whereas butter from hard milk fat will be hard and stiff. Pasteurization causes the fat in the fat globules to liquefy. And when the cream is subsequently cooled a proportion of the fat will crystallize. If cooling is rapid, the crystals will be many and small; if gradual the yield will be fewer but larger crystals. The more violent the cooling process, the more will be the fat that will crystallize to form the solid phase, and the less the liquid fat that can be squeezed out of the fat globules during churning and working. The crystals bind the liquid fat to their surface by adsorption. Since the total surface area is much greater if the crystals are many and small, more liquid fat will be adsorbed than if the crystals were larger and fewer. In the former case, churning and working will press only a small proportion of the liquid fat from the fat globules. The continuous fat phase will consequently be small and the butter firm. In the latter case, the opposite applies. A larger amount of liquid fat will be pressed out; the continuous phase will be large and the butter soft. So by modifying the cooling program for the cream, it is possible to regulate the size of the crystals in the fat globules and in this way influence both the magnitude and the nature of the important continuous fat phase. Treatment of hard fat For optimum consistency where the iodine value is low, i.e. the butterfat is hard, as much as possible of the hardest fat must be converted to as few crystals as possible, so that little of the liquid fat is bound to the crystals. The liquid fat phase in the fat globules will thereby be maximized and much of it can be pressed out during churning and working, resulting in butter with a relatively large continuous phase of liquid fat and with the hard fat concentrated to the solid phase. The program of treatment necessary to achieve this result comprises the following stages: 30 | P a g e
rapid cooling to about 8oC and storage for about 2 hours at this temperature; heating gently to 20 21oC and storage at this temperature for at least 2 hours (water at 27 - 29oC is used for heating)
cooling to about 16oC
Cooling to about 8oC causes the formation of a large number of small crystals that bind fat from the liquid continuous phase to their surface.
Treatment of medium-hard fat With an increase in the iodine value, the heating temperature is accordingly reduced from 20-21oC. Consequently a larger number of fat crystals will form and more liquid fat will be adsorbed than is the case with the hard fat program. For iodine values up to 39, the heating temperature can be as low as 15oC. Treatment of very soft fat Where the iodine value is greater than 39-40 the "summer method" of treatment is used. After pasteurization the cream is cooled to 20oC. If the iodine value is around 39 - 40 the cream is cooled to about 8oC, and if 41 or greater to 6oC. It is generally held that aging temperatures below the 20 o level will give a soft butter.
Churning of Cream It is during the churning process that cream is converted into butter. Here the fat gloubles are disrupted under controlled conditions to destabilize o/w emulsion and bring about agglomeration of milk fat. What happens during churning has been explained by various theories of churning as discussed in lesson 8.3. The sequence of events that occur during churning is as follows: i) Churning is initiated by agitation of cream causing incorporation of numerous air bubbles into the cream. ii) With incorporation of air there is increase in the volume of cream and air plasma interface. iii) Surface active (such as frictional, impact, concussion etc.) causes partial disruption of fat globule membrane iv) The fat film, thus formed, serve as a foam depressant causing the air bubble to burst. v) The liquid fat also serves as cementing material causing fat globules to clump together and eventually butter grains are formed which floats in plasma i.e. butter milk.
Initial working Working of butter is essentially a kneading process in which butter granules are formed into a compact mass. During this operation, any excess moisture or buttermilk is removed. However, the emulsion (w/o) at this stage is not fully stable.
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Salting of butter In conventional process, butter may be salted by adding salt to butter churn after initial working of butter. Salt to be added must be high quality e.g. IS 1845:1961, with low level of lead, iron and copper. The grain should be fine, all passing through IS: sieve-85 (aperture 8424). It should be 99.5 to 99.8% sodium chloride and microbial count should be less than 10/g. Salt sets up osmotic gradient which draws water from the butter grains. This can lead butter to be leaky. Salted butter should therefore, must be thoroughly worked. Salt may be added either in dry form or as saturated brine solution.
Adjustment of moisture After the addition of salt, the moisture content in butter is adjusted by adding calculated amount of additional water. In most countries, maximum limits of 16% is placed on the level of moisture. Amount of water is to be added in a batch of butter is calculated as follows:
Starter distillates, if required, may also be added at this stage to enhance the flavour of resultant butter, if cream has not been cultured.
Final working of butter The objective of working butter is to incorporate moisture and uniformly distribute added moisture and salt in butter. During this process remaining fat globules also break up and form a continuous phase, and moisture is finally distributed to retard bacterial growth in butter. It is safer to slightly over-work butter than to under-work. Under-worked butter may be leaky in body with large visible water droplets and may develop “mottles” on standing. Moisture droplet size normally ranges from 1 to 15 micron and there are approximately 10 billion droplets per gram of butter. Working affects the colour of butter (making is slightly light). Working also increase air content (this favors growth of microorganisms, oxidative effects and therefore poor keeping quality). Vacuum working of butter may be carried out with advantage to reduce the air content of butter. Vacuum range from 15-40 cm of Hg may be used. Air content of conventional butter range from 3-7% by volume with an average of 4 ml/100 while that of vacuum worked butter it is about 1 ml/100g.
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CHURNING OF CREAM
Introduction Churning is the process of converting cream into butter through appropriate mechanical manipulations leading to the conversion of oil-in-water (O/W) emulsion of cream into water-in-oil (W/O) emulsion desired in butter. The emulsion change accompanied by removal of buttermilk and working of butter yields the desired structure and texture in the product.
Theories of Churning The conversion of oil-in-water (O/W) emulsion of cream into water-in-oil (W/O) emulsion to form butter has been explained by various theories of churning. These are discussed below: (1)The phase reversal theory This theory was proposed by Fischer and Hooker in 1927, the theory is therefore also referred as Fischer and Hooker’s theory. According to this theory churning is a process of phase reversal i.e. changing of oil in water emulsion (O/W) to water in oil emulsion (W/O). The stability of emulsion is related to the relative volumes of the two constituents present. When oil and water are mixed together, the resulting suspension may be a suspension of (o/w) or suspension of w/o. The type of emulsion obtained depends on the proportion of the two main constituents present, the order in which they are added and the type of emulsifier used. In churning cream, initially the ratio of surface area to volume (S/V) of the fat globules is large. When the churning proceeds, surface area decreases and with progressive churning, surface area keeps on decreasing. The reduced surface area can no longer hold all the butter milk so it breaks i.e. separates out. Agitation of cream during the churning process causes coalescence and clumping of fat globules until eventually the ratio of surface area to volume of the fat units becomes so small that the reduced surface area can no longer contain the butter milk in stable form. The O/W emulsion then suddenly breaks; giving butter grains consisting of an emulsion of W/O and free butter milk. The supportive evidence of this theory is established by the fact that in normal butter, water is not in continuous phase. It has been demonstrated that plastic cream, containing 80-82% fat, conducts electricity and it responds to the pH determination showing water is in continuous phase but butter is a very poor conductor of electricity and pH determination cannot be done on butter but only on serum separated from it.
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Microscopic structural studies conducted by Rahn (1928) revealed that butter is not a true W/O emulsion. A proportion of globular fat are still intact in worked butter. He explained that since butter fat is cooled and largely crystallized before the start of churning, true W/O type emulsion is rarely possible. (2) The Foam Theory This theory was put forward by Rahn (1928). According to Rahn, cream (and also milk) contains a foam producing substance which gets solidified gradually when cream (or milk) is agitated. During churning first foam is produced. The fat globules then, due to surface tension, tend to concentrate on the foam bubble and thus are bought into such close contact that clumping of fat globules take place. Subsequently the foam producing substance assumes a solid character and the foam collapse. The fat globules then coalesce and butter is formed. According to Rahn’s theory, fat in cream at churning time is completely crystallized and the pass in to butter with their membrane intact and thus butter is a compact mass of fat globules in which butter milk, water and air are distributed as small globules. Rahn’s theory was based on his findings that air was necessary for normal churning of butter. Application of normal amount of mechanical agitation, in the absence of air did not result in churning of cream. The effect of overloading of churn resulting in increased churning time supported this theory (in case of overloading the churn, there was no sufficient space in the churn for the formation of required amount of foam hence more time). This theory was, however, subsequently criticized because of the fact that foam formation i.e. presence of air, is not required in some of the continuous butter making processes developed subsequently. (3) King’s Theory King’s theory was proposed in 1930 and 1953 and it is regarded as the modern theory. According to this theory, what happens during churning is mid-way between the ‘Phase Reversal theory’ and Foam theory. The modern concept has been summarized by Mc Dowall as follows: i) The fat in the cooled cream, at churning temperature, is present as clusters of fat globules. And within each globule it is present partly in solid and partly in liquid form. ii) Agitation (churning) breaks up the clusters and causes foam formation. The globules become concentrated to some extent in the film around the air babble in the foam and thus are brought into close contact of each other. iii) The movement of the globules over one another in the foam film and the direct concussion between them causes a gradual wearing away of the emulsion protecting surface layer (of phospholipid protein complex). The globules then adhere together to form larger and larger 34 | P a g e
particles. Eventually these particles become visible as butter grains. The grains enclose some of the air from the foam. The fat still mainly remains in globular form. iv) The working of the butter grains causes the globules to move over one another. Some of them, under the effect of friction and pressure cause some yields out a portion of the liquid fat, others are broken during working. Finally there is enough free liquid fat present to enclose the water droplets, air bubbles and intact fat globules. Factors Influencing Churn ability of Cream The factors which influence the churnability of cream can be classified into two groups as i. The factors related to the initial character of the cream ii. The conditions in the process of manufacturing Factors related to the initial character of the cream includes chemical composition of the butter fat, size of fat globules, richness of cream and viscosity of cream while factors related to processing conditions are churning temperature, fullness of churn, speed of churn, design of churn etc. All these factors are discusses in the following sections. (1) Chemical composition of butter fat Churnability of cream is greatly influenced by the proportion of soft fats (low melting point fat) and hard fats (high melting points). This proportion determines the degree of fat solidification in the cooled cream. If the proportion of soft fats is more than the churning period will be shortened, butter made will have less firmness and there will be more fat losses in butter milk. If the proportion of soft fats is low, it will prolong the churning period. (2) Richness of cream The amount of fat in cream affects its churnability considerably. The richer the cream the sooner will be the completion of the churning provided the cream is not rich enough to be so thick as to cause the cream to adhere to the inside of the churn and thus escape agitation. If rich cream is churned at a high temperature the butter will form in a remarkable short time, providing all other conditions are favourable. Thin cream churns much more slowly, and can be churned at high temperature than thick cream, without injuring the quality of butter when rich cream is churned at a high temperature and the butter forms in a short time (about 10 min), the butter will usually be greasy in body and will not contain a much of butter milk, which will be more or less difficult to remove on washing. When thick cream is churned, the butter does not break in the form of small round granules, as it does when thin cream is churned. When thick cream is (36 to 38% fat) is churned at as high a temperature as is consistent with getting a good texture, the best result are obtained. This type of cream produces less butter milk and consequently less part loss in the butter milk and this will give increase over run and the breaking of the butter at the end of the churning will be such as to cause the granules to appear large and flaky, rather than small round granules. The more flaky granules of butter will retain more moisture than the small, harder granules under the same treatment. 35 | P a g e
When thick cream is churned and the temperature is moderately high, it is almost impossible to churn the butter into granules. This condition causes butter from thick cream to contain more moisture than butter from thin cream. (3) Viscosity of cream The more viscous the cream, more time is required to complete the churning process. More viscosity diminishes the freedom of movement of the fat globules, lessens their opportunity of being brought together and retard coalescence, thereby increases churning time. (4) Size of fat globule Cream containing large fat-globules (avg. diameter 4.6µ) churn more quickly than cream containing small globules. Cream containing small fat globules (avg. diameter 3.4 µ) churn with difficulty and require twice as much time to break as the large globule cream. The butter made from such cream has short grains and crumbly character. (5) Churning temperature The temperature is one of the most influential factors in determining the churnability of cream. The higher the temperature of cream, the sooner the churning process will be completed. Too high a churning temperature is however not desirable. It causes the butter to contain soft lumps instead of in a flaky granular form. This is deleterious to the quality of the butter. It causes first a greasy texture of butter, and secondly, it causes the incorporation of too much butter milk in the butter. This butter milk contains lactose, curd, and water, which when present together in butter, are likely to sour and in other ways deteriorate the butter. Curd and lactose should be excluded from butter as much as possible, in order to eliminate food for bacteria which may be present. Too low temperature is also undesirable although it is better to have the temperature a little low rather than too high. Cream at low temperature becomes more viscous. On agitation in the churn such cream if it is very thick will adhere to the sides of the churn and rotate with it without agitating; consequently no churning will take place. Too low a temperature brings the butter in such a firm condition that it takes up salt with difficulty, and when this hard butter is being worked, a large portion of the water in the butter is expressed, and the overrun will be lessened to a great extent without increasing the commercial value of the butter. The degree of hardness of the fat in cream is the governing factor in deciding the temperature during churning. The hardness of the fat depends upon: (1) The season of the year. (2) The individuality of the cow. (3) The stage of lactation period. (4) The kind of food fed for the cows. 36 | P a g e
All these factors influence the melting point of butter fat- The higher the melting point of butter fat, higher is the churning temperature and the lower the melting point of the fat; the lower is the churning temperature. During spring, the cows yield milk containing a longer proportion of soft fats; consequently the churning temperature is always lower in the spring than in the winter. During the winter, when the cows are fed on dry food chiefly the harder fat increases in quantity, therefore, a higher churning temperature is necessary during that time. The nature of food fed affects the melting point of butter to a considerable extent. Cotton seed will cause butter to become hard. When larger amount of cotton seed is fed, the butter assumes a crumby, hard condition. It can be concluded that the churning temperature may vary between wide limits, but the average desirable churning temperature according to the season is Winter --- 10-13°C; Summer--- 7-9°C (6) Amount of cream in churn For maximum agitation to take place during churning, the cream must dash from side to side or from top to bottom. Optimum load for maximum agitation should be one third to one half full. Overloading of the churn diminishes free space in the churn, diminishes concussions and leads to increase in churning time. The overloaded cream may be churned at higher temperature so that churning time is not prolonged but this is not recommended as higher temperature increases fat loss in butter milk and produces soft and leaky butter. Under loading the churn is not economical for the manufacturer and at the same time the cream will adhere to the inner side of the churn and delay churning. (7) Nature of agitation Proper agitation is necessary for churning cream into butter. The speed of the churn provides agitation to cream. So, the maximum speed of the churn is the speed that yields the maximum amount of agitation. It is dependent on the ratio of centrifugal force and gravity force. Centrifugal force should be less than gravitational force.
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(8)Churning Difficulties The causes of churning difficulties are usually associated with the peculiar character of the cream and particularly where the source of cream is confined to a single herd. Usual causes of prolonged churning time and difficulty in formation of butter are excessive hardness of fat, small fat globules, use of thin cream for butter making and high protein content in cream. i) Excessive hardness of fat: Winter cream usually contains more hard fats. Use of such cream for butter making prolongs churning time because it diminishes the ability of fat globules to coalesce during churning. ii) Small fat globules: cream that contains small fat globules takes more time for churning as the ratio of membrane material to fat increases and thus provides increased protection to fat globules.
iii) High protein content: such cream delays butter formation because of increased viscosity that minimizes the force of concussion between the globules. iv) Use of thin cream: Such cream also have increased protection due to higher membrane protein to fat ratio and also due to intervening serum that keeps globules apart during churning.
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Introduction Butter colours are required to manufacture uniform quality butter all the year round as the natural components (like carotene) responsible for colour in butter, varies according to many factors like season, breed, stage of lactation, feed among others. Therefore, a calculated amount of colour is added in butter so that it does not have influence of varying natural colouring components present in milk or cream.
Desirable Properties of Butter Colours – It should be free from ingredients injurious to health – It should be free from undesirable odors and flavors – Strength should be such that only a small quantity is required – It should have such permanency of emulsion as to prevent settling out upon standing – It must be oil soluble
Types of butter colours Butter colours are classified on the basis of their source as vegetable butter colours and mineral butter colours.
Vegetable butter bolours This class of butter colours is derived from plants. The most common is the colour obtained and extracted from the seed of the annatto plant (Bixa orellana). The extract of vegetable butter color is made by boiling the annatto seed in oil for several hours. During the latter period of the process the heat is raised to a high temperature, about 115°C for extraction of annatto principle in permanent emulsion with oil. The mixture is then filtered through heavy canvas, either by gravity or under pressure.
Mineral butter colours The colouring principle of this class of butter colours is derived from harmless oil soluble coal tar dyes. This coal tar dyes are mixed with the neutral oil, boiled and filtered. The coal tar dyes certified by USDA are • Yellow A B (Benzeneazo- β- naphthlyamine) • Yellow O B (Ortho- Tolueneazo- β- naphthylamine) 39 | P a g e
Mineral colors have greater concentration of coloring principle and therefore less of the butter color is needed to produce desirable color in butter than is the case with vegetable colors. The emulsion of mineral butter color in oil is more permanent as compared to vegetable butter colors. FSSR 2011 has permitted some food colors to be incorporated in butter. FSSR 2011 permitted colors in butter
Quantity of butter color to be added The amount of color that must be added varies greatly under diverse conditions and may vary from none to about 250 g for every 100 Kg of butter. Manner of adding butter color The butter color should be preferably added to the cream while loading the churn. Alternatively, it may be added to and mixed with salt just before final working of the butter. It is then worked into the butter and distributed uniformly.
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CHEMISTRY OF GHEE
Introduction Competition in the organized ghee production sector leads to create brand value to the product. This brand value comes with quality and special features associated with the product. Today, consumers are much aware of the brand quality through media. Manufacturers or producers are looking forward continuously for innovations that helps to create market for their product, may be through mode of packing, through product aesthetic quality.
Ghee Composition and Changes During Manufacture Ghee majorly consists of milk lipids and richest source of milk fat of all Indian Dairy products. The constituents of ghee tend to vary with the method of its manufacture. Chemically ghee is a complete lipid of glycerides, 97-98% triglycerides. Small amount of diand mono-glycerides are also present in traces. Also cow milk ghee is different from buffalo milk ghee in terms of its composition. Fatty acid composition of buffalo milk ghee also varies from cow milk ghee. The amount of butyric acid is significantly higher in buffalo than in cow ghee. The levels of short chain fatty acids caproic to myristic are significantly higher in cow than buffalo ghee where as levels of palmitic and steoric are higher in buffalo than in cow ghee. Major fatty acid in buffalo and cow milk fat is given in table 36.3. Ghee made from buffalo milk is white (lack of carotenoids) with greenish tinge and that made from cow milk is golden yellow. The characteristic colour of buffalo fat has been attributed to “tetrapyrozole” pigments- “biliverdin and bilivubin” this pigment is conjugated to a protein in milk, but is released during the manufacturing process of ghee making. Thus, imparting yellowish-green colour to buffalo ghee. During manufacturing, water gets evaporated and fat present in the cream or butter getting concentrated (curd particles (MSNF present in cream or butter) starts settling at the bottom during clarification process. Flavour formation in ghee happens during fermentation of cream and during clarification process. Colour development and granulation also happens during clarification for the subsequent packaging of ghee.
Flavour Formation in Ghee Free fatty acids, carbonyls and lactones are the major groups of compounds contributing to ghee flavour. The flavour profile is affected by method of preparation temperature of clarification and storage period. 41 | P a g e
1. Carbonyls The quantity of carbonyls is directly proportional to the temperature of clarification. “Head space” and “volatile” carbonyl content of fresh desi cow ghee is higher that of buffalo ghee, where as total carbonyl content of fresh desi buffalo; whereas total carbonyl content of fresh desi buffalo ghee is higher than that of cow ghee. Carbonyl content found to increase during storage. 2.Lactones The lactone level in buffalo ghee has found to be higher than that in cow ghee. It was the highest in direct cream (DC) ghee, followed by creamery butter (CB) and lowest in desi ghee. The lactose level in butter (12 ppm) increased 1.9, 2.4, 2.8 and 3.0 fold on clarifying at 110 OC, 120 OC, 140 OC and 180 OC respectively. Clarification butter at 100-120 OC doubles the lactones level from butter. The lactone level in ghee showed a significant rise on storage.
Flavour Components of Ghee Major flavor components in ghee Components
Formation of flavour components are due to (i) Heat interaction between the native carbohydrates and protein system of cream; (ii) Due to heat effect on the unfermented residue as well on fermented metabolic products formed by ripening process. Cream constituents like lactose, citrate and glucose were responsible for the increase in ghee flavour components. Flavour in ghee is the resultant of four different mechanisms, they are (i)
Hydrolysis - Free fatty acid formation 42 | P a g e
Oxidation - Saturated and unsaturated aldehyde, ketones, alcohols and hydrocarbons.
(iii) Decarboxylation - Alkan-2-Ones (iv) Dehydration and Lactonization - Lactones 1. Texture of ghee When ghee is stored at room temperature, it crystallizes into three distinct fractions or layers, (i) Oily (ii) granular semi-solid at the bottom and (iii) hard flakes portion floating on the surface and sticking to the sides of the container. According to Singhal et al. Layer formation in ghee could be prevented by storing it at 20 OC or below immediately after preparation. Ghee thus solidified could subsequently be stored at higher temperature without formation of layer. The liquid portion of ghee varies with storage temperature, shape and size of container, repeated heating and agitation, ripening of cream/butter, storage and handling, external seeding etc. 2. Market quality of ghee Consumer judge the quality of ghee base on its inherence flavour, colour and appearance. Ghee should have characteristic pleasant, nulty, slightly cooked rich aroma. Ghee flavour is best described as lack of blandness, sweetly rather than acid. Golden yellow to light yellow colour of ghee is appreciated largely. Granular appearance of the product rather more score as it is important quality as well as purity preventer of ghee. Apart from above sensory characteristics, its chemical and other physical preventers are evaluated to judge the quality of ghee and also to prevent adulteration of ghee. (i)
Refractive Index: It is the ratio of the velocity of light in vacuum to the velocity of light in the sample medium. More generally, it is expressed as the ratio between the sine of the angle of incidence to the sine of the angle of refraction when a ray of light of a definite wave length wave length (usually 589.3 µm the mean of the D-lines of sodium) passes from air into the fat. In case of milk fat reading is normally made at 40 OC using Abbe refractometer and its values range from 1.4157 to 1.4566. This value is low in comparison to the other fats and oils. The RI if ghee is influenced by both the molecular weight and the degree of saturation of the component fatty acids. RI could be used as indicator of adultration.
Iodine Number: It is defined as number of grams of iodine absorbed by 100 g of fat under specified conditions. Thus constant is a measure of the unsaturated linkages present in a fat. The iodine number for milk fat falls within the range of 26 to 35 which is low in comparison to other fat and oils. This is estimated using Wig’s method. One molecule of halogen compound is absorbed by each unsaturated linkage and the 43 | P a g e
absorption is expressed as the equivalent number of grams of iodine absorbed by 100 g of fat. (iii)
Reichert-Meissl Number (RM Number): This is defined as number of ml of n/10 Sodium hydroxide required to neutralize the steam volatile water soluble fatty acids distilled from 5 g of ghee under precise conditions specified in the method. It is primarily measure of butyric acid and caproic acid. The value for milk fat ranges between 17 to 35 and it is above that of all other fats and oils. Therefore, milk fat contains more of these acids than any of the fats.
Polenske Number: It is defined as number of ml of N/10 Sodium hydroxide required to neutralize the steam volatile water insoluble fatty acids distilled from 5 g of fat under precise conditions specified in the method. Caprylic acid, capric acids which are somewhat steam volatile but longely insoluble in water are indicated mainly in Polenske number and it ranges from 12 to 24 for milk fat.
Saponification Number: It is defined as the number of milligrams of potassium required to saponify one gram of fat. The value ranges from 210 to 233 and more often falls in the range of 225 to 230. This constant is an indication of the average molecular weight of the fatty acid present. Saponification value is more useful in detecting the presence of minerals oils such as liquid paraf fims in ghee as they are not acted upon by alkali and such a sample doesn’t form a homogeneous solution on saponification.
Melting Point: Melting point for milk fat ranges from 30 OC to 41 OC as reported in literature.
Factors Affecting The Market Quality of Ghee (i)
Raw-materials (Milk, Dahi, Cream, Butter) used for ghee making Milk used should be clean, fresh and strained. Use of ripened cream, butter improves the flavour score of ghee.
Method of preparation and temperature of clarification Flavour compounds of ghee vary according to its method of preparation. For example desi methods have more volatile Carbonyl compounds than cream method. Temperature of clarification also has influence on the quantity of Carbonyl compounds and lactones formation during ghee production.
Type of feed It is the main factor affecting variation in fatty acid composition of milk fat. Roughages in the feed mainly consist of cellulose contribute to the formation of fatty acid of 4 to 16 44 | P a g e
carbon chain length and lipid content of the feed contributes to the formation if long chain fatty acid if C16 and above. Animals fed with cotton seed meal will have high amount of C1010 and C1210 fatty acids. (iv)
Season In winter and monsoon, the granulation is more due to changes in the fatty acid profile. Winter ghee showed higher acidity, melting point and grain size where as in summer the saponification value was found to be higher.
Grading of Ghee The quality of ghee can be judged by physical and chemical analysis. Customer can only perceive appearance, taste and aroma of ghee. Therefore grading i.e. classification according to its quality and purity is necessary to assure the customer. The Agricultural Produce (Grading & Marking) Act, 1936 empowers the Central Government to fix quality standards, known as “AGMARK” standards and to prescribe terms and conditions for using the seal of “AGMARK”. The word 'AGMARK' is a derivative of "Agricultural Marketing".
Objectives of AGMARK i). To assure the consumer a producer of pre-tested quality and purity ii). To enable manufacturers of high grade product to obtain better returns iii). To develop an orderly marketing of the commodities by eliminating malpractices when transferring from producer to consumer. AGMARK is a certification mark of Government of India to ensure the purity and quality of Agricultural and allied products in India. The Act empowers the Directorate of Marketing and Inspection to
Fix grade designation indicating the quality of the produce Define the quality indicated by each grade designation Specify the grade designation mark to represent particular grade designation Specify the manner in which the article could be packed, sealed and marked and Authorize a person or a body of persons to use the grade designation marks under prescribed condition.
AGMARK Ghee Specifications Grade designation marks for ghee- The grade designation mark shall consist of a label specifying the name of the commodity, grade designation and bearing a design consisting of an outline map of India with the word “AGMARK” and the figure of rising sun with the words produce of India and resembling the design as set out as follows.
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AGMARK grades of ghee Grade Special General Standard
Letter and Circular border colour Red Green Chocolate
The labels shall be printed on the watermark paper of the Government of India and shall have a micro tint back ground bearing the words “Government of India” in olive green color Each label shall have printed thereon a serial number along with a letter or letters denoting the series, e.g. A054987. Each label shall have printed thereon the approximate weight content of the package on which it is affixed.
Design of label
1. Special Grade
2. General Grade
The word “Regional” shall be printed on each label used on a package of the ghee not conforming to the normal physical and chemical constants specified as follows.
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AGMARK standards of ghee Parameters Special Grade Baudouin Test Negative Butyro-refractomer 40.0- 43.0 reading at 40 OC Reichert Meissl Not less than 28.0 value Polenske value 1.0 - 2.0 Moisture content Percentage of Free Fatty Acid (as oleic acid)
General Grade Negative
Standard Grade Negative
Not less than 28.0
Not less than 28.0
1.0 - 2.0 Not more than Not more than 0.3% 0.3% Not more than 1.4 Not more than 2.5
1.0 - 2.0 Not more than 0.3% Not more than 3.0
For cotton tracts areas such as part of Saurshtra and Madya Pradesh following standards are applicable. AGMARK standards for ghee produced in cotton tract areas Parameters Baudouin Test Butyro-refractomer reading at 40 OC Reichert Meissl value Polenske value Moisture content Percentage of Free Fatty Acid (as oleic acid)
Special Grade Summer Negative
41.5 - 43.0 Not less than 23.0
42.5 - 45.0 Not less than 21.0#
0.5- 1.2 Not more than 0.3% Not more than 1.4
0.5 - 1.0 Not more than 0.3% Not more than 2.5
General and Standard grade have Percentage of Free Fatty Acids (as Oleic acid) shall not exceed 2.5 and 3.0 respectively. According to the law it is not compulsory for every trader and manufacturer, to get his produce certified under AGMARK symbol. Presently it is only a voluntary scheme of the Government.
Requirements of High Grade Ghee Consumer judges the quality and accepts it on the basis of three main attributes, taste and aroma(flavour), granularity and colour.
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1. Flavour: High grade ghee should have natural sweet and pleasant odour, an agreeable taste and it should be free from rancidity and any other objectionable falvour. A pleasant, nutty, slightly cooked aroma is appreciable in the product. 2. Texture: Large uniform grains with very little liquid fat is desirable, greasy texture is objectionable. Upon melting ghee should be clear, transparent, free from sediment and foreign colouring matter. 3. Colour: The colour should be uniform throughout, it should be bright yellow for cow milk ghee and white with or without a yellow or greenish tint for buffalo milk ghee.
Adulterants in Ghee Adulteration of ghee in India is more prevalent especially in unorganized sector. Being the most expensive fat people started to adulterate the product to make profits. Major adulterants of ghee are as follows: i).
Vanaspati (Hydrogenated vegetable oil). Because of close resemblance in its texture most commonly used this as adultrant to ghee. ii). Refined (de-odourized) vegetable oil. iii). Animal body fat. Government has made it compulsory that all Vanaspati must contain a maximum of 5% of Sesame oil which can be identified in ghee by a simple colour test (known as Baudouin test). By means of this Adultration of ghee with Vanaspati an extent of 3% can be detected.
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PHYSICO-CHEMICAL CHANGES TAKES PLACE DURING KHOA MAKING AND STORAGE OF KHOA Factors Affecting Quality of Khoa
(1)Quality of milk In India both cow and buffalo milks are used for manufacture of traditional dairy products depending on the availability and suitability of milk for a particular product. However, the quality of the products greatly depends on the type of milk used for their preparation. Yield of Danedar is and Dhap type of Khoa from buffalo milk is 24% and 25%. a) Species of Animal: Buffalo milk is preferred over cow milk for khoa making because it yields a product with soft, loose body and smooth granular texture which is highly suitable for the preparation of sweets. The buffalo milk gives a greater out burn than that from cow milk. The average yield from buffalo milk is 21.6-23% and from cow milk it is 18.3-18.5% due to the higher T.S. Content in buffalo milk moisture content in khoa. Khoa from cow milk is of inferior quality due to its dry surface, sticky and sandy texture and salty taste (due to higher chloride and citrate content (Citrate buffalo milk 0.18%, cow milk 0.18% and chloride: buffalo milk 0.07% and cow milk 0.10%)) which is not considered suitable for manufacturing of best quality khoa based sweets. b) Fat content in milk: A minimum fat level of 4% in cow milk and 5.5% in buffalo milk is essential for production of khoa with desirable body and texture and to meet the FFA requirements also. A lower fat content results in undesirable hard body and coarse texture in the finished product which is not suitable for good quality sweets preparation. The fat level higher than the minimum improves the quality of the final product. (The higher emulsifying capacity of buffalo milk fat due to the presence of higher proportion of butyric acid containing triglycerides (50%) than only 35% in cow milk fat and higher fat content in buffalo milk is responsible for smooth and mellowy texture of buffalo milk khoa) c) Acidity of milk: Fresh sweet milk yields the best quality khoa, while developed acidity in milk produces an undesirable, coarse texture sour smell and bitter taste in to the khoa which is unsuitable for sweet preparation. d) Presence of additives and adulterants: Neutralization of high acidic milk improves the texture, but does not improve the flavour of khoa. Further it gives saltish taste in the final product.
Presence of colostrum in milk has a marked effect on the colour of khoa, which is deep yellow with cow colostrums. It also gives pasty texture in the final product making it not suitable for sweet making. Adulteration of milk with water produces a brown khoa and a proportionate reduction in the yield. Texture and flavour of khoa are not affected much when adulterated with starch
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the physical quality of khoa is considerably affected. It develops a harder body and a pasty sticky texture not suitable for the preparation of confections.
Speed of Stirring: The speed of stirring should be optimum. It depends on the type of machine/ method. In case of traditional method the optimum speed is about 100 rpm where as in continuous systems it depends upon the type of machine used (it is slightly more). The optimum speed of stirring prevents burning of milk solids and helps in developing desirable body and texture in khoa. Low speed results in to the burning of khoa. Higher speed makes the product pasty and sticky.
Temperature of desiccation: To obtain good quality khoa, milk should be maintained at the boiling temperature till it reaches a paste consistency and then temperature is lowered to 88°C till the pat formation stage. The dehydration should be stopped when the pan contents starts leaving the pan surface and shows a tendency to stick together. Continued heating at higher temperature at advanced stage of khoa making results into undesirable flavour (cooked) and texture (hands dry). The colour of such product is also brown. Slow heating is not only more time consuming but also produces sandy texture and brown colour.
(2) Homogenization of milk Homogenization of cow/ buffalo milk produces a softer body in khoa, as against an homogenized milk. The khoa from homogenized milk also shows lower fat leakage, less browning, and a reduced patting tendency as compared to that obtained from unhomogenized milk. Physico-Chemical Changes in milk during Khoa making The changes in physico-chemical characteristics of milk during khoa making take place due to three actions. 1) concentration 2) heating and 3) Stirring and scraping 1) Change of state The removal of moisture from milk results into concentration of milk solids. This eventually changes the state of milk from liquid to solid/ semisolid. All the constituents including lactic acidity increase in proportion to the degree of concentration. pH decreases. 2) Development of cooled flavour Heating of milk causes changes in proteins resulting in the production of sulphydryl compounds by denaturation of whey protein particularly β-Lactoglobulin. 3) Coagulation of casein Due to the combined action of heat and concentration coagulation of casein tends to 50 | P a g e
increase logarithmically with milk solids concentration and forms a complex with denatured whey protein. Convention of soluble calcium and phosphate to colloidal form and interaction between protein compounds. Super-saturated solution of lactose: From a dilute solution in milk, lactose is present in khoa as a super-saturated solution. Most of the lactose is present as α-hydrate in khoa. Free fat formation: (Free fat in khoa is 60% of the total fat in khoa, Cow khoa = 50% of the total fat). By vigorous stirring and scraping, the fat globule membrane ruptures, thereby releasing considerable amount of free fat in khoa. The water dispenses as fine droplets in mass of the khoa. Change in colour intensity: The colour of khoa becomes intense with brownish tinge due to formation of melanoiodins pigment. The browning reaction is maillard type reaction due to interaction between aldose group of lactose and free amino group of casein. Increase in iron content: From 2 to 4 ppm iron content in milk, the iron content of khoa increases more than 100 ppm due to the incorporation of additional quantities of iron from the karahi and the khunti by vigorous scraping. Chemical Changes in Khoa During Storage
Moisture: Loss of moisture takes place resulting into dry and hard surface of khoa. The extent of evaporation however depends on the temperature of storage and packaging material. The higher the storage temperature the higher is the rate of evaporation. More the water vapour transmission rate (WVTR) of package faster the loss of water from khoa. Minimum loss of moisture takes place in case of tins and maximum loss in parchment paper.
Acidity: Lactic acidity increases with the storage period. Higher percentage increase take place at higher temperature and vice-versa. The pH normally decreases during storage corresponding to the increase in acidity.
Deterioration in fat: The peroxide value increases due to oxidative spoilage during storage of khoa, particularly at ambient temperature. The hydrolytic rancidity in khoa take place thereby increasing the free fatty acids measured as % oleic acid.
Breakdown in milk protein: There is break down of milk protein in khoa as recorded by the increase in tyrosine value.
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Changes in organoleptic quality: The changes in organoleptic quality of khoa during storage include development of off flavour (stale, sour, oxidized, etc) hardening of body and texture and increase in intensity of colour.
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Chemistry of Milk Coagulation During Paneer Production The phenomenon of coagulation involves formation of large structural aggregates and network of protein in which milk fat globules gets embedded. Acid and heat treatment causes the physical and chemical changes in casein. Heating causes interaction of ßlactoglobulin with ĸ-casein and the complex formed between ß-lactoglobulin and αlactalbumin. Acidification initiates the progressive removal of tri-calcium phosphate from the surface of the casein and it gets converted into mono-calcium phosphate. Further calcium is progressively removed from calcium hydrogen caseinate to form soluble calcium salt and casein. Colloidal dispersion of discrete casein micelles changes into large structural aggregates of casein. Under such a circumstance dispersion is no longer stable, casein gets precipitated and forms coagulum. Fat is embedded in the casein network.
Factors Affecting Quality and Yield of Paneer (1) Type of milk Paneer prepared from buffalo milk possess desirable frying properties, body and texture as compared to cow milk. The cow milk paneer is soft, weak and fragile and during cooking it tends to disintegrate. However, cow milk and buffalo milk mixed in equal quantity yields better product than cow milk. Paneer made from skim milk has chewy and rubbery texture and hard body. (2) Quality of milk Milk must be fresh and free from off falvour. Growth of psycrotrophic organisms should be minimized to restrict the off-flavour development. Acidic milk having a titratable acidity of more than 0.20% lactic acid yields a product of inferior quality. Milk with COB positive and low acidity (sweet curdling) is not suitable for paneer making. Paneer made from such milk has weak body and texture, more moisture, acidic smell and not safe for human consumption. (3) Type, strength and temperature of coagulant Product yield and moisture retention are directly influenced by the type and concentration of the acid and the mode of delivery and blending into the hot milk. Citric acid is generally used as a coagulant. Lemon or lime juice or vinegar imparts a typical flavour to the product. 1% solution of citric acid yields good quality of paneer. Sufficient acid is added gently but quickly blended with the milk (within one min) to reach optimum pH of coagulation. Normally 1.8 to 2.0 kg citric acid is required for coagulating 1000lit of milk. High acid concentration imparts acidic flavour, hardness and causes greater solids loss. Whey cultured with Lactobacillus acidophilus at a level of 2% and incubated overnight at 37 OC can be used as a substitute for citric acid. However acidic whey must be heat treated to destroy these lactic organisms before use to prevent loss of shelf life of paneer. Coagulation temperature influences the moisture content of paneer. It is reported that an increase in temperature from 53 | P a g e
60 OC to 86 OC decreases the moisture content from 59 to 49%. However, optimum coagulation temperature for best organoleptic and frying quality product is 76 OC. (4) Heat treatment of milk This is one of the technological requirements of the process which affects the sensory and microbiological quality of paneer. The objective of heating milk is to prepare it for rapid isoelectric precipitation, control the moisture content, develop typical body and texture, create conditions conducive to the destruction of pathogenic and other microflora present in milk and ensure safety as well as keeping quality of the final product. The milk is heated to 90 OC without holding or 82 OC for 5min in order to maximize the total solids recovery. Whey proteins especially β-lactoglobulin and α-lactalbumin form a complex with Қ-casein and retained with the curd thus increasing the yield of the product. The high heat treatment imparts desirable cooked flavour by controlled liberation of sulphydryl compounds. (5)
It influences the moisture content of paneer. An increase in temperature from 60 OC to 86 OC decreases the moisture in paneer from 59 to 49%. At 70 OC, paneer made from buffalo milk has the best organoleptic and frying quality in terms of shape retention, softness and integrity. (6)
pH of coagulation
The optimum pH of coagulation of milk at 70 OC is 5.30-5.35 for better product quality and maximum recovery of solids when made from buffalo milk. The moisture retention in paneer decreases with the reduction in pH and consequently the yield also decreases. At pH more than 5.35 the paneer is very soft with fragile and crumbly body. Optimum pH for paneer preparation from cow milk is 5.2.
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CHHANA Factors influencing quality of chhana (1)Type of milk Cow milk produces chhana with moist surface, light yellow colour, soft body, smooth texture and mildly acidic flavour which is more suitable for sweet preparation than buffalo milk chhana. Buffalo milk produces chhana with hard body and coarse texture, with white colour and greasy surface. Sweets prepared from buffalo milk chhana are hard, coarse and less spongy. Colostrum milk produces pasty Chhana with deeper yellow colour and unsuitable for rasogolla production. (2)Quality of milk 1.Fat level Minimum 3.5 to 4% fat in cow milk and 5% fat in buffalo milk gives a satisfactory body and texture in chhana. Lower than 3.5% fat leads to hard body and coarse texture while higher fat level results in greasy surface. 2. Acidity Acidic milk produces chhana with sour smell and bitter taste hence unfit for sweet making. Addition of neutralizer to slightly acidic milk helps in getting chhana suitable for sandesh and not for rasogolla. Milk with 0.25-0.28% LA can be used by adding 0.2% Sodium Citrate followed by thorough washing of the coagulum. (3)Type, strength and quantity of coagulant 1.Type Organic acids, like citric acid, lactic acid or sour whey are normally used. Lactic acid produces granular chhana suitable for rasogolla making. Citric acid gives pasty texture suitable for sandesh making. However, dilute solution of citric acid can also be used for making chhana suitable for rasogolla. Sour whey with (0.9% LA) can also be used for producing good quality chhana. Calcium lactate produces chhana with bright white colour, soft body and smooth texture and pleasant flavour and most suitable for sandesh making. 2. Strength Low acid strength (0.5%) results in very soft body and smooth texture suitable for rasogolla but unsuitable for sandesh making. The optimum strength of coagulant should be between 0.5 to 0.8% citric or lactic acid to produce good quality chhana suitable for making both rasogolla and sandesh. However, calcium lactate of 4% solution produces most satisfactory quality chhana. 3.Quantity 55 | P a g e
The quantity of coagulant required is dependent on the type of milk. Generally, 2 to 2.5 g of citric acid per kg of fresh milk and 2.5 to 3.9 gm of lactic acid and 6 to 12 gm of calcium lactate per kg are required individually for complete coagulation. 4.Temperature and pH of coagulation As the coagulation temperature decreases, the moisture content of chhana increases resulting in softer body and smooth texture. Higher coagulation temperature imparts graininess and hardness to chhana. Optimum coagulation temperature of cow milk is 80 to 85 OC and pH is 5.4 and that of buffalo milk is 70 to 75 OC and pH is 5.7. 5. Speed of stirring during coagulation Higher speed of stirring during coagulation reduces the moisture content of chhana and increases its hardness, optimum speed is 40-50 rpm. 6. Method of Straining Delayed straining produces a soft and smooth texture chhana than immediate straining. Delayed straining gives a higher proportion of moisture, yield, recovery of milk solids and lower hardness. Delayed straining is recommended for buffalo milk. 7. Effect of heat treatment given to milk The recovery of milk solids and yield of chhana is influenced by the heat treatment given to milk prior to acidification. The heat treatment prior to acidification involves temperature to which milk is heated, the rate of heating, temperature to which milk is cooled and the rate of cooling. Several heat induced changes occur during heating; denaturation of whey proteins and their subsequent association with casein micelles, precipitation of calcium phosphate onto the casein micelles and dissociation of κ-casein from the micelle. The degree of denaturation of whey proteins depends on time temperature combination during heating and is mainly determined by maximum temperature to which milk is heated. Whey protein denaturation at above 70 OC is two step process, first an infolding of whey proteins takes pace followed aggregation. α-lactalbumin shown the higher resistance to denudation and it reported that 95 OC temperature denaturates all the whey proteins.
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CHEMISTRY OF CHEESE Milk-clotting Enzymes from Plants Enzymes from many plant sources may be used as clotting enzymes in cheesemaking but most of the plant proteases are strongly proteolytic and cause extensive digestion of the curd, resulting in reduced yields, bitter flavors and pasty-bodied cheese. 1.Papain The latex of the plant Carica papaya yields papain and several other proteases. It has powerful milk clotting activity but it is also highly proteolytic. It requires a free sulfhydryl group for its catalytic activity. 2.Ficin Ficin is present in the latex of several species of the genus Ficus (fig) such as Ficus glomarata, Ficus religiosa and Ficus carica but the best source is Ficus carica. Cheese made with ficin develops bitter flavor which decreases in intensity during curing. 3.Others Bromelain from pineapple has also been considered as a possible substitute for calf rennet. An enzyme extracted from Withania coagulans was also used in the manufacture of Surati and Cheddar cheese. Extracts from the flower petals of Cynara cardunculus were used for the manufacture of Serra cheese from sheep’s milk by Portuguese farmers. 4.Microbial Rennet A large number of microorganisms are known to produce milk clotting enzymes. The possibility of a few of the microorganisms proving successful as rennet substitutes may appear from the fact that they play an important part during the manufacture of cheese. Microbial enzymes are known to exhibit considerable variations in the range of activity, substrate specificity and mode of action. Even more important is the fact that they can be produced economically on any desired scale. Many hundreds of bacterial and fungal cultures have been investigated for milk clotting enzymes and their proteolytic abilities. Milk clotting enzymes from bacteria like Streptococcus liquefaciens, Micrococcus caseolyticus, Bacillus cereus, B. polymyxa, B. mesentericus, B. coagulans and B. subtilis have been used as coagulating enzymes. Milk clotting enzymes from fungi are also used during cheese making. Some of the fungi producing such enzymes are Aspergillus nidulans, A. galucus, Syncephalastrum racemosum, and Cladosporium herbarum.
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Various strains of these organisms behave differently; therefore, the cheese maker should use only well tried and tested rennet for the types of cheese contemplated. The activity of various microbial rennets varies according to pH and enzyme system. The protease of the Mucor miehei coagulants degrade casein fairly rapidly in the pH range 5.5-7.0; but inspite of this, the incidence of bitter cheese using this enzyme is low. The enzyme is very sensitive to temperatures in the region 37-45°C and is destroyed at 70°C. This enzyme is used successfully for many types of cheese. Mucor pusillus extract is also used as microbial rennet. It is highly proteolytic than calf rennet or the Mucor miehei extract. An increase in calcium ion concentration in milk decreases the clotting time but activity of this enzyme is not so pH-dependent as for other coagulants. It tends to give hard curds because of its high proteolytic activity and the curd tends to lose fat into the whey, thereby giving lower yield as compared to others. Thus, this enzyme is usually used in combination with other enzymes. The enzyme extracted from Endothia parasitica is more caseolytic than those from Mucor sp. or calf rennet and tends to produce more bitter flavors in high moisture cheese. Continuous research is being carried out in exploring the milk coagulating activity of enzymes from different cultures. Recently, commercial starter ‘natto’ (Bacillus subtilis) has been studied for its milk clotting activity and it was found to be a potential rennet substitute. 5.Recombinant Chymosin many rennet substitutes are used for cheesemaking but many of these proteolytic enzymes from microbial or plant origin cause flavor, texture and yield changes in certain types of cheese that are different than those produced by calf rennet. Further, they are not suitable for long ripening cheeses as they have a different range of non-specific activities than chymosin and do not produce the correct flavors on prolonged ripening. Recombinant rennet/chymosin can be prepared by gene transfer technology in which the bovine chymosin is cloned in a suitable production strain and the enzyme is produced by fermentation. This enzyme can be isolated and used in cheese making as a coagulant having properties similar to that of calf rennet. Escherichia coli, Kluyveromyces lactis and A. niger var. awamori have been successfully used for production of calf rennet. In addition to the beneﬁt that such chymosin can be produced in large-scale fermentors at low cost, recombinant and highly pure chymosin also has some other advantages such as speciﬁc, low proteolytic activity, predictable coagulation behavior and vegetarian approval. With the advent of various sources of rennet substitutes, a number of firms are manufacturing these substitutes at commercial level. Among these microbial rennet is most widely used.
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Enzymatic Coagulation of Milk The enzymatic coagulation of milk is essentially a two stage process (Fig. 16.3). As discussed earlier, the casein micelle is stabilized by κ-casein layer on the surface of the micelle. The enzymes present in rennet (proteinases) hydrolyse κ-casein layer to form paracasein micelles which aggregate in presence of calcium and thus milk is coagulated. The hydrolysis of the κ-casein layer is called as the primary phase of rennet coagulation while the aggregation of paracasein micelles in presence of calcium is called the secondary phase of rennet coagulation of milk. CASEIN
PARA-K- CASEIN + GLYCOMACROPEPTIDES Enzymatic hydrolysis of casein
The amino acid chain forming the κ-casein molecule consists of 169 amino acids. Rennet enzymes act specifically at 105 (phenyl alanine)-106 (methionine) bond of this amino acid, thereby splitting it into two parts. One part consists of amino acids from 1-105, called as para-κ-casein. This part is insoluble and remains in the curd together with αs and β-casein. The other part of amino acids from 106-169 is soluble part. These amino acids are dominated by polar amino acids and the carbohydrate, which gives this part its hydrophilic properties. This part of the κ-casein molecule is called the glycomacro-peptide and is released into the whey in cheesemaking. The formation of the curd is due to the sudden removal of the hydrophilic macropeptides and the imbalance in intermolecular forces caused thereby. Bonds between hydrophobic sites start to develop and are enforced by calcium bonds which develop as the water molecules in the micelles start to leave the structure. This process is usually referred to as the phase of coagulation and syneresis. Hydrolysis of κ-casein during the primary phase of rennet action releases the highly charged, hydrophilic C-terminal segment of κ-casein (macropeptide), as a result of which the zeta potential of the casein micelles is reduced from -10/-20 to -5/-7 mV and the protruding peptides are removed from their surfaces, thus destroying the principal micelle-stabilizing factors (electrostatic and steric) and their colloidal stability. When roughly 85% of the total κ-casein has been hydrolyzed, the stability of the micelles is reduced to such an extent that when they collide, they remain in contact and eventually build into a three-dimensional network, referred to as a coagulum or gel. Factors Affecting Rennet Coagulation The primary and secondary phase of rennet coagulation are affected by some of the compositional and environmental factors like milk composition, temperature, pH, calcium content, pre-heating of milk, rennet concentration etc. The effects of all these factors are summarized here.
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1.Composition of milk Variation in the composition of milk mainly affects the rate of coagulation and the curd firmness. Fast coagulation results in firmer curd. The rate of clotting is largely dependent on the nature of the casein micelles and the equilibrium with the calcium phosphate and calcium ions. The firmness of the curd are affected by pH value, calcium concentration, temperature, fat content and the ratio of rennin to casein. The rennet coagulation time (RCT) is markedly affected by the protein content in milk. RCT decreases with protein content in the range of 23%. Further increase in milk protein level i.e. more than 3% result in a slight increase in gelation time. This is due to decrease in rennet:casein ratio, which necessitates an increase in the time required to generate sufficient hydrolysis of κ-casein to induce aggregation of paracasein micelles. A minimum protein content of 2.5-3.0 is necessary to obtain gel in about 30-40 minutes during cheesemaking. Increase in fat content also results in decreased RCT but the effect is lower than that of the protein content. 2.Heat treatment of milk Heating milk to pasteurization temperature has beneficial effect on rennet coagulation due to heat induced precipitation of calcium phosphate and a concomitant decrease in pH. But heating further to higher temperatures causes other effects which in combination dominate the positive effects of heating to pasteurization. Some such effects are: • Whey protein denaturation and the interaction of denatured β-lactoglobulin with micellar κcasein • The deposition of heat-induced insoluble calcium phosphate leading to reduction in the concentration of native micellar calcium phosphate. This micellar calcium phosphate is important for cross linking para-κ-casein micelles and their aggregation during gel formation. 3.Set temperature The principal effect of set temperature is on the secondary phase of enzymatic coagulation, which does not occur at temperatures below around 18°C. Above this temperature, the coagulation time decreases to a broad minimum at 40-45°C and then increases again, as the enzyme becomes denatured. In cheesemaking, rennet coagulation normally occurs at around 31°C. This is necessary to optimize the growth of starter bacteria which will not survive the temperature more than 40°C. In addition, the structure of the coagulum is improved at the lower temperature, which is therefore used even for cheeses made using thermophilic cultures. 4.Rennet concentration The rate of enzymatic coagulation is directly related to the concentration of enzyme. Increase in concentration of rennet decrease RCT. During cheesemaking, rennet is added in such a concentration so as to coagulate the milk 30-40 minutes. More rennet concentrations can be used to shorten the coagulation time but it leads to retention of more rennet in the curd which 60 | P a g e
has pronounced effect in ripening of the cheese, particularly proteolysis. Some studies also suggest that using increased concentration of rennet may jeopardize the curd firming rate and curd firmness. 5.Concentration of calcium ions The concentration of calcium ions mainly affect the secondary phase of enzymatic coagulation. Increased calcium concentration is beneficial for coagulation of milk. For this reason, sometimes CaCl2 is added to milk prior to cheese making. This promotes cheese making via three beneficial changes, viz. an increase in calcium ion concentration, an increase in the concentration of colloidal calcium phosphate and a concomitant decrease in pH (the addition of CaCl2 to 0.02%, i.e. 1.8 mM Ca, reduces the pH by ~ 0.05-0.1 units, depending on protein level).
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CHEMICAL, PHYSICAL, MICROBIOLOGICAL AND SENSORY CHANGES Cheese Ripening Unique characteristics of each cheese variety are determined by the curd manufacturing operations but these characteristics are largely developed during ripening process. For example, the type of microflora established in the curd is determined by the manufacturing process but its effect on cheese characteristics develop largely during the ripening process. Ripening involves microbial and chemical changes which are responsible for development of typical characteristics of varieties of cheeses. Microbial changes involve death and lysis of the starter cells, development of non-starter microflora and growth of secondary microflora. Ripening usually causes softening of the cheese texture due to hydrolysis of the casein matrix, change in pH and change in water binding ability of the curd. Flavor production is largely described by a series of biochemical events taking place during ripening. The primary events occuring during cheese ripening are metabolism of residual lactose, lactate metabolism, proteolysis and lipolysis. These reactions are mainly responsible for textural changes and development of flavor in cheese. However, many secondary changes occur simultaneously and modify cheese texture and flavor. Since the biochemistry of cheese ripening is complex, the objective of this chapter is to present an overview of the principal biochemical pathways contributing towards cheese ripening. Metabolism of Residual Lactose Lactic acid bacteria (LAB) is added in the form of starter culture to cheese metabolize lactose to lactate. The rate and extent of acidification influence texture of the curd by controlling the rate of demineralization. The pH of the cheese curd is largely determined by the extent of acidification during manufacturing process. This influences the solubility of the casein, which in turn affects the texture of curd. pH also affects the activity of enzymes involved in ripening, thereby having an indirect effect on cheese texture and flavor development. Most of the lactose is lost in whey during cheese manufacturing. However, low levels of lactose remains in the curd. This residual lactose is converted to L-lactate during early stages of ripening by the action of starter bacteria. The rate of conversion is dependent on temperature and salt-in-moisture levels of the curd. Starter activity is stopped very quickly at the end of manufacturing operations due to low pH, salt addition and lesser amount of fermentable lactose. Lactose that remains unfermented by the starter is probably metabolized by non-starter lactic acid bacteria (NSLAB) flora present in curd and they convert the residual lactose to D-lactate. D-lactate can also be formed by the 62 | P a g e
racemisation of L-lactate. Lactate Metabolism Lactate produced by fermentation of residual lactose serves as an important substrate for a range of reactions occuring during cheese ripening. L-lactate can be racemised to D-lactate by NSLAB flora. D-lactate is less soluble than L-lactate which results in the formation of Ca-D-lactate crystals. These crystals are not harmful but they appear as white specks on the surface of the mature cheese. Lactose can be metabolized to acetate, ethanol, formate and CO2 depending on the population of NSLAB and availability of O2. In cheese wrapped with film, oxidation of lactate occurs to a lesser extent due to low level of O2 available. Late gas blowing is a defect which is caused by anaerobic metabolism of lactate by Clostridium tyrobutyricum to butyrate and H2. The release of H2 causes cracks in cheese during ripening. The above mentioned metabolisms contribute negatively towards cheese ripening. There are some positive contributions also of lactate metabolism. This is essential for cheese varieties characterized by the development of large eyes during ripening such as Emmental cheese. Propionibacterium freudenrichii metabolise lactate to propionate, acetate, CO2 and H2O. Propionate and acetate contribute to the flavor of cheese while CO2 is mainly responsible for eye formation. Lipolysis Milk fat is essential for the development of the correct flavor in cheese during ripening. Cheddar and other cheeses normally made from whole milk do not develop correct flavor when made from skim milk or milks in which milk fat has been replaced with other lipids. Lipids may undergo oxidative or hydrolytic degradation in foods but the redox potential of cheese is very low, so mainly hydrolytic degradation of lipids takes place in cheese. The triglycerides present in cheese are hydrolyzed by lipases which result in the formation of fatty acids. Sources of lipases in cheese are:• Milk (particularly unpasteurized) • Rennet • Starter culture • Starter adjuncts 63 | P a g e
• Non starter bacteria (may come through ingredients or contamination) • Exogenous lipase (if added deliberately)
Low level of lipolysis is required for the development of flavor of cheese but excessive lipolysis causes rancidity. Lipolysis of milk fat results in production of free fatty acids which contribute to the flavor of cheese and also act as precursors for development of other flavor compounds in cheese like esters, lactones, ketones and aldehydes. These secondary fat-derived compounds can be very potent flavor compounds. Fatty acid esters are produced by reaction of fatty acids with an alcohol; ethyl esters are most common in cheese. Thioesters are formed by reaction of a fatty acid with a thiol compound formed via the catabolism of sulphur-containing amino acids. Fatty acid lactones are formed by the intramolecular esterification of hydroxyacids; γ- and δlactones contribute to the flavor of a number of cheese varieties. n-methyl ketones are formed by the partial β-oxidation of fatty acids. Liberation of short and medium chain fatty acids from milk fat by lipolysis contribute directly to cheese flavor. This degree of flavor development depends on the variety of cheese. For example, it is very extensive in some hard Italian varieties, smear cheeses and blue mold cheeses. Excessive lipolysis causes rancidity in cheese varieties like Cheddar and Gouda.
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Production of flavor compounds from fatty acids during cheese ripening: Proteolysis Proteolysis is the most important and complex of all the events during ripening of cheese. The extent and pattern of proteolysis is also used as an index of cheese ripening and quality of cheese. Proteolysis contributes significantly towards development of texture and flavor in cheese. Textural changes (softening of cheese curd) occur due to breakdown of protein network and release of carboxyl and amino groups resulting in the binding of more water and thus decrease water activity (aw). Proteolysis leads to the formation of peptides and free amino acids which contribute to cheese flavor. These amino acids also act as precursors for many reactions like transamination, deamination, decarboxylation, desulphuration, catabolism of aromatic compounds such as tyrosine, phenylalanine, tryptophan, etc. and generate many important flavor compounds. Proteolysis in cheese is catalysed by proteinases and peptidases and they originate from the following sources: • Coagulant • Milk • Starter LAB • Non starter LAB • Secondary starters (e.g. P. camemberti in Camembert cheese and P.
roqueforti in Blue cheese) • Exogenous proteinases or peptidases, if added for accelerated ripening of cheese
In majority of cheese varieties, casein is initially hydrolyzed by the residual coagulant, often chymosin, which results in formation of large and intermediate-sized peptides. These peptides are then hydrolyzed by enzymes derived from starter and non-starter microflora of the cheese. The production of small peptides and amino acids is caused by the action of microbial proteinases and peptidases, respectively. The final products of proteolysis are amino acids, the concentration of which depends on the cheese variety. The concentration of amino acids in cheese at a given stage of ripening is the net result of the liberation of amino acids from the caseins by proteolysis and their catabolism or transformation into other amino acids by the cheese microflora. Medium and small peptides contribute to a brothy background flavor in many cheese varieties; short, hydrophobic peptides are bitter. Amino acids contribute directly to cheese flavor as some amino acids taste sweet (e.g. Gly, Ser, Thr, Ala, Pro), sour (e.g. His, Glu, Asp) or bitter (e.g. Arg, Met, Val, Leu, Phe, Tyr, Ile, Trp). 65 | P a g e
Proteolysis in cheese during ripening Microbiology of Cheese Ripening Microorganisms including bacteria, yeasts and molds are present in cheese and contribute to the ripening process through their metabolic activity. The enzymes released by these microorganisms also add to the various metabolic activities like proteolysis, glycolysis and lipolysis. Microorganisms in cheese may gain entry through their intended addition in the form of starter culture and they may be associated with the ingredients used in cheese making. The microflora associated with cheese ripening may be divided into two groups - the starter bacteria and non-starter bacteria. Starter bacteria are primarily responsible for acid production during manufacture to reduce the pH of milk to the desired level. The secondary microflora do not play any active role during cheese manufacture but contribute to the ripening process. The factors controlling the growth of microorganisms in cheese include water activity, 66 | P a g e
concentration of salt, oxidation-reduction potential, pH, ripening temperature, and the presence or absence of bacteriocins (produced by some starters). Water activity Water activity (aw) is defined as the ratio of the vapor pressure of water in a material (p) to the vapor pressure of pure water (po) at the same temperature. Its value ranges from 0 to 1.0. It expresses the water availability rather than total water present in the system. Water activity of cheese reduces during ripening process. This may be due to several reasons like: • Evaporation of moisture if the cheese is not vacuum packed or paraffin coated • Hydration of proteins bound water rendering it unavailable for bacterial growth • Hydrolysis of proteins to peptides and amino acids and of lipids to glycerol and fatty
acids • The salt and organic acids (lactate, acetate, and propionate) dissolved in the
moisture of the cheese reduce the vapor pressure Growth of microorganisms at low aw is characterized by a long lag phase, a slow rate of growth, and a reduction in the maximum number of cells produced. Each of these factors helps to limit the number of cells produced. LAB generally have higher minimum aw values than other bacteria. The amount of salt in moisture in cheese also affects the growth of microorganisms in cheese. The salt normally used in cheese making is about 2% of the weight of the curd. Salt is added to cheese mainly to suppress growth of unwanted microorganism and to assist the physico-chemical changes in the curd. The growth of unwanted microorganisms is essentially curbed by reduction in aw as salt act as a humectant. Oxidation-Reduction potential The oxidation-reduction potential (Eh) is a measure of the tendency of the solution to either gain or lose electrons when it is subject to change by introduction of a new species. The Eh of milk is about +150 mV whereas that of cheese is about -250 mV. As cheese ages, the products of proteolysis and lipolysis may reduce the Eh of cheese. This reduction of Eh makes cheese an anaerobic system, in which only facultatively or obligately anaerobic microorganisms can grow. Anaerobic sporeforming organisms present in the cheese may germinate and grow, causing defects like bitter and putrid flavor. Obligate aerobes, like Pseudomonas spp., Brevibacterium spp., and Micrococcus spp., will not grow within the cheese, even when other conditions for growth are favorable. Eh is therefore 67 | P a g e
important in determining the types of microorganisms that grow in cheese. pH Most bacteria require a neutral pH value for optimum growth and grow poorly at pH values below 5.0. The pH of cheese curd after manufacture generally lies within the range 4.5-5.3, so pH is also a significant factor in controlling bacterial growth in cheese. LAB, especially lactobacilli, generally has pH optima below 7, and Lactobacillus spp. can grow at pH 4.0. Most yeasts and molds can grow at pH values below 3.0, although their optimal range is from 5 to 7. B. linens, an important organism in smear-ripened cheese, cannot grow below pH 6.0. Micrococcus spp., which are commonly found on the surface of soft cheeses, cannot grow at pH 5 and only slowly at pH 5.5. Temperature The ripening temperature of cheese mainly depends on two considerations: the temperature should be such that the growth of undesirable spoilage causing and pathogenic bacteria can be checked and at the same time, this temperature should be conducive for various ripening reactions essential for development of typical flavor, body and texture of cheese. Ripening temperature for Cheddar cheese is 6-8°C while Camembert and other mold-ripened cheeses are ripened at 10-15°C. Emmental cheese is ripened initially for 2-3 weeks at a low temperature (~ 12°C), after which the temperature is increased to 20-24°C for 2-4 weeks to promote the growth of propionic acid bacteria and the fermentation of lactate to propionate, acetate, and CO2. The temperature is then reduced again to around 4°C. Use of higher ripening temperature is one of the techniques used for accelerated ripening of cheese but it also stimulates the growth of other microorganisms present in cheese.
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PHYSICO-CHEMICAL CHANGES TAKES PLACE DURING MANUFACTURING OF CONCENTRATED MILKS Introduction
In course of manufacture of concentrated and dried milks, raw milk is subjected to various pretreatments, which in turn determine to a great extent, the physical, chemical, microbiological and organoleptic characteristics as well as the shelf life of the finished product. The technological aspects of various pretreatments and their significance in the manufacture of dried milks are discussed here. The following pretreatments are given to milk during the processing schedule: 1. Filtration and/or centrifugation 2. Standardization 3. Forewarming /Preheating 4. Concentration / Concentrate heating 5. Homogenization Filtration and / or Centrifugation (Separation, Clarification and Bactofugation)
Filtration The basis for high quality and fine flavor of the product is "clean milk" in preference to "cleaned milk". However, for sanitary, ethical as well as for technological reasons the milk that is received at the dairy is filtered and/or centrifuged. Weigh can strainers, strainers and filters in the milk line or a duplicate filter unit with by-pass connection are general filtration techniques. Bactocatch method developed by Alfa-Laval Co., Sweden, involves filtration of the milk through ceramic filters prior to pasteurization. This microfiltration technique removes 99.6% of the bacteria in the milk so that the following normal low pasteurization results in 50% life extension when milk is kept at 8°C. The advantages of this process are:
Bacillus cereus level is reduced appreciably
Absence of disagreeable flavour that sometimes is present in overheated milk
Excellent quality low heat powder for cheesemaking, least adverse effect on rennetability, curd characteristics etc.
Low heat powder having extended shelf life. 69 | P a g e
Centrifugation There are three types of centrifugal processes to which milk may be subjected during manufacture of dried milks. These are
Separation proper i.e. skimming
Centrifugal clarification is done in hot or cold condition in a centrifugal triprocess separator, which serves the purpose of clarification. A clarifier removes leucocytes, cellular debris and particles from earth or fodder gaining entry into milk which might act as a catalyst for chemical reaction during storage of powders. The Bactofugation process, originated by Professor Simonart is designed to remove bacteria from milk by means of centrifugation. Basically the bactofuge is a clarifier with one inlet and one outlet for the treated liquid. Additionally, there are two nozzles of about 0.4m diameter fitted into the bowl wall for the discharge of skim milk or what is called 'bactofugate' rich in protein and bacteria. The machine is characterized by a much greater efficiency of removal of bacteria than conventional clarifiers. Bactofugation is a selective method of removing bacteria, the size and density of the bacteria being the criteria of their removal. Bactofugation of milk is always applied in combination with heat treatment, a temperature of 55-65°C offering the optimum bactofugation effect. A combination of heat treatment and bactofugation results in a greater reduction of bacteria than is possible by heat treatment alone. Likewise, bactofugation of milk by single pass process is less effective (90% removal of bacteria) than two pass process where two bactofuges are arranged in series (bacterial reduction may be up to 99.9%). To improve the quality of raw milk for dried milk, bactofugation has been suggested by some workers. Though clarification has also some beneficial effects, cream separation has no effect on ultimate quality of powder. However, some losses of constituents, though negligible, do occur while bactofuging the milk. Heating (130 - 140°C the bactofugate and remixing it with treated milk can eliminate the losses due to bactofugation. Influences of centrifugation (i.e. clarification and bactofugation) of milk on quality of milk powders can be summarized as follows: 1. Centrifugation prior to concentration improves casein dispersion and heat stability. 2. Improved shelf life of powder due to reduced tendency of oxidation and lipolysis due to removal of slime, foreign materials (which may act as chemical reaction catalyst), leucocytes, lipolytic bacteria and enzymes etc. 3. Improved solubility to powder (1.8-2.0%) when clarification is applied at 35- 40°C. 4. Decrease in sediment formation 5. Better organoleptic quality of resultant powder. 70 | P a g e
6. Improved bacteriological quality of finished product. Standardization
Milk intended for dry milk manufacture is standardized to obtain a product of expected quality, to meet the legal standards and to optimize yield and profit.
In manufacture of dried milks, the milk fat and SNF are so standardized that the finished product will meet the standards of not less than 26% fat (BIS, IDF and ADPI standards for whole milk powder).
The fat to SNF ratio adjustment is done by addition of calculated amount of cream or skim milk to the milk. This purpose is accomplished in the present day industry by use of tri-purpose separator which has the standardizing device too. Other computerized systems are also used commercially.
Preheating / Forewarming of Milk
In manufacture of milk-powders milk is subjected to heat treatment in two stages(A) Prior to concentration (B) After concentration, before atomization. Heating prior to concentration has the following objectives Preheating of milk to at least the boiling temperature of the first effect evaporation is essential, so that evaporation can start immediately the milk enters the heating tubes. The most common boiling temperatures of the first effect in the food industry are between 40-100°C. Heating of milk to a temperature exceeding the boiling temperature of the first effect are required for the purpose of: (a) Enzyme inactivation, e.g. lipoprotein lipase. (b) Pasteurization effect - improved microbiological quality of resultant product due to microbial inactivation. Heat treatment employed in manufacture of low and high heat powders had profound effect on bacterial profile. Both pasteurization and high heat treatment (90°C, 10 min) almost completely eliminate psychrotrophic bacteria. Pasteurization had little effect on either thermoduric or spore counts whereas high heat treatment brought about significant reduction in bacterial numbers. (c) To lengthen the running time of evaporators (as most of the whey proteins are denatured when milk is preheated before evaporation).
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(d) To generate specific properties in the resultant milk powders, e.g. heat stability, viscosity, etc. by altering the characteristics of some of the components of milk such as casein, whey protein, mineral balance etc. (e) To improve oxidative stability of whole milk powder during storage. This is due to production of -SH groups and other reducing substances in milk on account of heating. The actual treatment required depends on many factors, including the composition of milk as influenced by the breed, season, climate etc. and the equipment including type and construction of heater. The influence of spray drying on WPNI is negligible. Typical time temperature combination for different types of powders TYPE HIGH HEAT MEDIUM HEAT LOW HEAT
TEMPRATUE 82 o c 79 o c 74 o c
TIME 30 Min 3 Min 15 Sec
Concentration / Heating after concentration/ concentrate heating The viscosity of the concentrate has bearing on drying efficiency as well as certain properties of the dried milks. In general, high concentrate viscosity can be the source of many practical problems by increasing the likelihood of blockage of feed lines and in the calandria of evaporators. Therefore, it is often desirable to reduce the viscosity of concentrate to as low a viscosity as possible. The viscosity of concentrated milk is significantly affected by temperature. To achieve desired low viscosity of the feed, the concentrate has to be heated to 70-75°C prior to atomization. This in turn has the following advantages: (a) Saving of energy as it is more efficient to heat the concentrate by means of steam or hot water than to use heat from the air supplied to the dryer. The evaporation capacity of the dryer is increased by ~ 5% when the concentrate is supplied to the atomizer at 70°C instead of 50°C. (b) The average size of the droplets into which the concentrate is atomized is reduced due to the lower viscosity. This facilitates the drying making it possible to reduce the air outlet temperature and still maintain the specified moisture content of the powder. The lower outlet temperature results in increased capacity and gentler drying which results in improved powder quality especially solubility. (c) As the temperature conditions in the last stages of the evaporator and in the feed tank for the dryer can be favorable for growth of micro organisms, heat treatment of the concentrate immediately before drying will contribute to safeguard the bacteriological quality of the powder. Heating of the concentrate must be done in such a way that no holding takes place, i.e. the concentrate heater must be placed immediately before the atomizer. This is necessary in order to avoid the so called "age-thickening" which occurs very rapidly at temperatures above 60°C. The time dependent thickening of hot concentrate is irreversible and results in very high viscosity. Table 4.3 shows changes in viscosity of concentrated skim milk (39%TS) held for varying 72 | P a g e
period at 75°C. Table 4.4 shows the effect of storing concentrated skim milk (39%TS) at 50°C and 60°C on powder solubility. Since concentrate is a complex system its heating has to be done carefully to minimize adverse effects of heating on chemical and physical characteristics of the concentrate. Otherwise, such change will reflect on the quality of resultant milk powders. Tables 4.5 and 4.6 show influence of concentrate heating on the various characteristics of whole milk powder. Heating of milk after concentration helps to retain higher percentage of antioxidants. Homogenization
Homogenization is used as one of the pretreatments in manufacture of whole milk powder as well as some specific products.
The purpose of homogenizing milk/concentrate prior to spray drying is to reduce the size of the fat globules and consequently to change the physical structure in such a way that the dried product obtained has a low free fat content.
The type of homogenizer, the stage at which product is homogenized, the temperatures and pressures of homogenization employed have profound effect on the various properties of the concentrate and this in turn have influence on the quality of resultant milk powders.
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PHYSICO-CHEMICAL CHANGES TAKES PLACE DURING MANUFACTURING OF CONDENSED MILK Introduction
Concentrated milks are liquid milk preserves with considerably reduced water content. Water is removed by evaporation. Preservation is achieved either by sterilization, leading to a product called evaporated milk , or by creating conditions that do not allow growth of microorganisms. The latter is generally realized by addition of a large quantity of sucrose and exclusion of oxygen. The resulting product is called sweetened condensed milk . These products were initially meant for use in regions where milk was hardly or not available. The milks were packaged in small cans. The contents were often diluted with water before consumption to resemble plain milk. Currently, alternative products are used more often, such as whole milk powder or recombined milk. For the concentrated milks, some alternative forms of use are developed, and processing and packaging have been modified. The consumption of sweetened condensed milk has greatly declined. Product Properties
1.Flavour & colour ● Maillard reactions are of paramount importance for the flavor and color of evaporated milk. The temperature and duration of the heat treatment during manufacture determine the initial concentration of the reaction products, but ongoing Maillard reactions occur during storage, especially at a high temperature. The milk eventually develops a stale flavor also due to Maillard reactions. The flavor after a long storage time differs considerably from that directly after intense heating. This is because the complicated set of reactions involved leads to different reaction products at different temperatures. A sterilized milk flavor may be appreciated by some people when the milk is used in coffee. Off-flavors due to autoxidation need not occur. ● When the milk is used in coffee, the brown color is often desirable to prevent the coffee from acquiring a grayish hue. The brown color depends greatly on the Maillard reactions, although the color of the fat plays a part. 2.Viscosity The viscosity of evaporated milk is often considered an important quality mark. Many consumers prefer the milk to be viscous. However, it should pour as like thin cream. This can be achieved by sterilization in such a way that visible heat coagulation is barely prevented. UHT evaporated milk is always less viscous and, therefore, κ -carrageenan is often added. 3.Age thinning If the original milk contains bacterial lipases and proteinases due to growth of psychrotrophs, these enzymes may remain active in the evaporated milk and lead to strong deterioration, i.e., soapy-rancid and bitter flavors, and to age thinning. Evaporated skim milk may even become more or less transparent due to proteinase activity. 74 | P a g e
4.Nutritive value The nutritional value of evaporated milk can be significantly decreased as compared to that of plain milk. In-container sterilization can destroy up to 10% of the available lysine, about half of the vitamins B1, B12, and C, and smaller proportions of vitamin B6 and folic acid. All of these changes are far smaller when UHT heating is applied. The nutritive value of the sweetened and unsweetened condensed milk is very high. Both of them are rich in fat and fat soluble vitamins viz. A, D, E & K and body building proteins, bone forming minerals and energy giving lactose. While sweetened condensed milk is especially high in energy giving sucrose because of the added sugar, evaporated milk is suitable for infant feeding since it makes a soft curd and is easily digested. 5.Heat Stability Concentrated milk is very less stable during sterilization than non-evaporated milk, and the fairly intensive homogenization applied decreases the heat stability further. Moreover, evaporated milk should increase in viscosity during sterilization. Essentially, the viscosity increases by incipient coagulation. A subtle process optimalization is needed to meet these requirements. Therefore, the milk must be preheated before evaporation in such a way that most serum proteins are denatured. Otherwise, the evaporated milk forms a gel during sterilization due to its high concentration of serum proteins. Preheating is, for example, for 3 min at 120°C. 6.Effect of stabilization The pH should always be adjusted. Preheating and evaporation have lowered the pH to about 6.2 or 6.1, and that is far below the optimum pH. In practice, Na 2HPO4 . 12H2O is usually added, but NaOH can also be used. 7.Effect of homogenization Homogenization of evaporated milk does not lead to formation of homogenization clusters. It is often observed that a slight homogenization increases HCT, which cannot be easily explained. UHT heating of evaporated milk after homogenization is not possible. Even traditional sterilization is difficult if the milk is highly concentrated or if the evaporated milk is intensely homogenized. There are some other factors affecting heat stability. It can be improved by lowering the calcium content of the milk before evaporation by means of ion exchange. Addition of 0.05% H2O2 or about 15 µ mol Cu2+ (from 0.5 to 1 mg . kg − 1 ) after preheating but before evaporation tends to increase the heat stability. 8.Creaming Creaming of evaporated milk eventually leads to formation of a solid cream plug that cannot be redispersed. This may be due to bridging of adjacent fat globules because of ‘fusion’ of the fragments of casein micelles in their surface layers. Accordingly, intensive homogenization is 75 | P a g e
necessary. A higher viscosity of the evaporated milk often involves a slower creaming, but the relations are not straightforward. Generally, a high viscosity is due to approaching heat coagulation. The homogenized fat globules tend to participate in this coagulation and hence to form clusters, which will cream rapidly. κ -carrageenan is often added to decrease creaming rate. The homogenization has an adverse effect on the heat stability and, consequently, the homogenization pressure should not be high. The largest fat globules exhibit creaming, and therefore the aim should be to have the relative diameter of the globule size distribution as small as possible. The width is greatly affected by the type of homogenizer used.
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AGE THICKENING AND GELATION OF CONDENSED MILK
When evaporated milk is kept, its viscosity may initially decrease slightly. This may be explained in terms of the casein micelle aggregates formed during sterilization which changes from an irregular to a spherical shape, and as a result the effective volume fraction decreases. Subsequently, the viscosity increases, and it becomes strongly dependent on the shear rate, the milk displays a yield stress, and a gel is formed that firms rapidly.
The mechanism involved in the age thickening or gelation is not quite clear. However, in most cases, gelation is not caused by proteolytic enzymes or Maillard reactions, although the latter parallel the gelation. Also, gelation is not related to heat coagulation. It does not depend significantly on the pH, and its rate increases rather than decreases after lowering of the calcium content. Electron microscopy reveals that thread-like protrusions appear on the casein micelles, which eventually form a network. It is likely that a slow change in the micellar calcium phosphate is at least partly responsible for the changes observed, but a definitive explanation is still lacking. Age thickening and gelation occur faster in UHT-evaporated milk. It may then be due to proteolysis caused by enzymes released by psychrotrophs, but also if such enzymes are absent, fast age gelation occurs. A more intense sterilization after evaporation delays gelation whereas it is faster in more concentrated milk and at a higher storage temperature. It is observed that addition of sodium polyphosphate delays gelation considerably. Addition of citrate or orthophosphate often accelerates gelation, presumably because of binding of calcium. Polyphosphates may be hydrolyzed to yield orthophosphate, especially during heating. Consequently, addition of polyphosphate does not counteract gelation of in-bottle-sterilized evaporated milk. Conventional evaporated milk only gels if kept for a long time at a high temperature. Extensive Maillard reactions also occur. Rapid gelation can occur if the evaporated milk before its sterilization is kept refrigerated at 4°C for a few days. Sweetened Condensed Milk
The sweetened condensed milk is highly concentrated. Its mass concentration ratio is very high and because of this and the high sugar content, the product is highly viscous, i.e., η a is approximately 2 Pa . s, about 1000 times the viscosity of milk. The product is somewhat glassy in appearance because the fat globules show little light scattering as the refractive index of the continuous phase is almost equal to that of fat. The turbidity of the product is largely due to lactose crystals. Most of the lactose crystallizes because of its supersaturation. Recombined Concentrated Milks
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1.Viscosity Viscosity of recombined EM is one of the important aspects in consumer acceptance which is decided by original powder and conditions of processing and stabilization. Age thinning is a problem when high storage temperatures are employed. High total solids and HTST method of sterilization give age thickening or gelation defect. Viscosity, the most important property of recombined SCM, should be ~ 35 poise in fresh product which can increase up to 3 times, yet giving good acceptance. Too low viscosity poses air incorporation at filling and fat separation problems. Too high initial viscosity accelerates rate of age thickening. Means to control viscosity of recombined SCM, in order of importance are: 1. Manufacture, 2. Selection of milk powder, 3. Homogenization, 4. Pasteurization and 5. Storage temperature. 2.Colour The color of recombined EM varies with use of anhydrous milk fat, powder and level of heat treatment. Colour and flavour of recombined SCM are affected by raw materials used and severe pasteurization conditions employed. Homogenization is found to lighten the colour. 3.Flavour Flavour of the product can be improved by use of buttermilk powder due to higher phospholipids content. Holding the product for some period, at least for 7 days after production and before release, allows post manufacturing hydration of dry ingredients and normalization of flavour characteristics. The most preferred flavour in recombined WMP results when fresh whole milk, commercial additives and fresh buttermilk powder contributing number of flavour compounds lost during the preparation of anhydrous milk fat are included in the mix. Incorporation of distilled monoglycerides or buttermilk powder provides best flavour stability on extended storage of recombined WMP. Powders with moisture level below 3% provide best flavour initially and during storage. Moisture content >5% leads to crystallization of lactose resulting in lumpy powder, increased free fat and increase in colour development within 2-3 months. 4.Fat separation The efficient homogenization, absorption of casein on fat globule, product viscosity and minimum fat globule clustering controls the fat separation in recombined EM. High ambient 78 | P a g e
temperature increases the defect. Addition of carrageenan gives best control of fat separation but have greater tendency towards gelation. Formation of a stable fat globule from anhydrous milk fat and SNF is very important for recombined evaporated milk. Homogenization reduces fat globule size as well as increases viscosity thereby reduce fat separation tendency. Commonly employed pressures are up to 10 MPa, about 3.5 MPa being most common. Homogenization of reconstituted skim milk with milk fat (at 50 kg/cm 2 or 150 kg/cm2) increases viscosity, while recombined SCM made using recombined cream (0.7-0.9 % protein, 20.2-20.9 % fat) age thickens slowly. Age thickening is also accelerated by higher storage period and temperature as well as by higher proteins and lower whey protein nitrogen with changed casein fraction. The pasteurization conditions can be used to control viscosity but is less convenient to use. 5.Lactose crystallization The second important quality determining factor for recombined SCM is lactose crystallization which can be controlled by storage temperature and using dry seeding procedure. 6.Other factors important for quality aspects • Fortification of recombined SCM with 1000-3000 IU of Vit-A and 30 mg of ascorbic acid is recommended. • Holding period of up to 2 weeks before release of recombined SCM in the market is desirable to allow post manufacturing hydration of dry ingredients and normalization of flavour characteristics. High temperatures preheat treatment of milk, as part of manufacture of full-cream milk powder; can reduce the susceptibility of product to oxidation during storage as a result of development of sulfhydryl compounds. However, such heat treatment is not always compatible with the development of required specific functional properties.
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PHYSICO-CHEMICAL CHANGES DURING MANUFACTURE OF CONDENSED MILK-I Introduction
There are numerous changes occurring in the condensed milks during their manufacture. They are because of the inherent properties of the milks, any additives like sugars, stabilizers etc. added and the processing variables. Some of the product specific changes of commercial importance are discussed here. Changes Caused by Concentration
Apart from the increase of most of the solute concentrations, removal of water from milk causes numerous changes in properties, which often are approximately proportional to concentration factor. The changes also depend on other conditions, such as heat treatment and homogenization. Some important changes in properties are:
The water activity decreases.
The Calcium ions activity increases only slightly because calcium phosphate, which is saturated in milk, turns into an undissolved state. As the water content decreases, association of ionic species increases and also ionic groups of proteins are neutralized.
The conformation of proteins changes because ionic strength, pH, and other salt equilibria change. When milk is highly concentrated, the solvent quality decreases. Thus, the tendency of the protein molecules to associate and to attain a compact conformation is increased. Coalescence of casein micelles causes them to increase in size. This increase is smaller if the milk has been intensely preheated, presumably because β-lactoglobulin and other serum proteins have become associated with casein.
Osmotic pressure, freezing point depression, boiling point elevation, electrical conductivity, density and refractive index increase and heat conductivity decreases.
The viscosity increases and the liquid become non-Newtonian and finally solid-like.
The diffusion coefficient of water decreases from approximately 10−9m2s−1 in milk to 10−16 m2s−1 in skim milk powder with a small percentage of water.
1.Viscosity Viscosity is defined as resistance to the motion of the molecules of a fluid body among themselves caused by internal frictions as opposed to mobility. This is measured by viscometer. 2.Effect of sterilization on viscosity of product
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Viscosity of fresh milk changes by the preheating of milk. The condensing operation causes a slight but definite increase in viscosity. This is due to increase in concentration of milk solids. The increase in viscosity that gives the finished product its full body, however occur during process of sterilization. In general HTST yields a thin body and low viscosity, while LTLT process yield a full body and little more viscosity. 3.Increase
In sterilization process, the rate of thickening is greatest, shortly before the occurrence of a visible coagulation. It is observed that the thickening does not proceed rapidly until above 10 min before coagulation. This is because the thickening or increase in viscosity during sterilization appears to be a part of the function of coagulation. Thickening in sterilization is in fact the beginning of coagulation. It is very slow and gradual and in normal commercial process of sterilization, covering a period of approximately 20 minutes at the full sterilizing temperature. The proportion of milk proteins coagulated while sufficient to increase viscosity, is too limited for curd to become visible. The increase in viscosity which milk undergoes toward the end of sterilizing process has to do with progressive coagulation of the milk proteins. Table-13.1. below shows the viscosity changes during successive steps in manufacture of evaporated milk: It is observed further that for a heavy creamy body, the heat stability of milk should not exceed 30–40 minutes. Milks with heat stability in excess of 50 min will be exceedingly thin at the end of sterilization at 115°C for 30 min unless an increase in solids content is depended upon to build up body. The viscosity produced during sterilization is controlled mainly by the heat stability of milk. Therefore, factors which affect the heat stability of milk will certainly affect the viscosity of milk. Following factors gives low heat stability, high viscosity and very thick body and vice-versa: (1) High acidity of fresh milk (2) Low forewarming temperature (3) High concentration (4) High homogenizing pressure (5) Excessive holding of evaporated milk at ordinary temperature before sterilization Consumer desires a product with a good body that suggests richness. The heat stability sufficient to avoid visible coagulation or curdling during sterilization process is required but maximum heat stability is not desirable because such high stability results into objectionably thin milk. 4.Effect of storage on viscosity Evaporated milk becomes thinner with age. This loss of viscosity increases with the temperature of storage. The decrease in viscosity begins immediately. The rate at which viscosity is lost is accelerated at high storage temperature Thus it is observed that at or above 30°C, evaporated milk looses as much as 40% of viscosity (original) during first 10 days of storage. While at 15.6°C or below, the age thinning is very slight. It is reduced after which the rate of thinning is 81 | P a g e
much more gradual. Age thinning of evaporated milk may therefore be definitely retarded and the attainment of final viscosity delayed by use of relatively low storage temperature. A study shows viscosity loss upon storage for 110 days of 58.75% at 26°C, 40% at 15.6°C & 11.25% at 7.2°C. But this depends upon quality of raw material (milk) used. Better grade milk lost less viscosity than the low grade initial milk with the same temperature and time of storage. 5.Gel formation on storage In case of some evaporated milks, the viscosity increases later in the storage period even to the gel formation. This tendency is greatest with milks that had received relatively light heat treatment and those of high solids concentration. These results of gel formation in prolonged storage are supported by commercial experiences especially with samples with high TS evaporated milks incubated at tropical temperatures. Further it is observed that evaporated milks that have been sterilized by the HTST process (135°C for 0.5 min) thicken quickly during storage. When the gel is broken before it becomes firm, the product wheyed off. When sterilized in the conventional manner (115.6°C for 20 min), gel formation is slow and holds the water better than from the HTST process. Since long fluidity in storage is a marketable property of primary magnitude, it appears that the more severe heat treatment of commercial process (115.6°C for 20 min) that retards thickening in storage is commercially preferable to the less drastic treatment of momentary exposure to high sterilizing heat. Neither the mechanism underlying age thinning nor that responsible for age thickening appears to be well understood. It may be due to insufficient time for the attainment of new solubility equilibrium of the Calcium and Magnesium salts when using HTST method of sterilization. When solutions of these salts are heated, it takes considerable time to attain that. Their peculiarity of being less soluble at higher temperature may be contributing to the reaction of these salts. In the HTST method of sterilization, conditions are thus less favorable than those of the LTLT procedure (115.6°C for 20 min). This suggests the possibility that, less of Ca and Mg content is rendered insoluble. This condition might encourage the protein particles to swell and to form a gel or it may be as low continuation of the coagulation process accompanied by an operation of caseinate molecule which finally produces an irreversible gel structure. 6.Protein separation during storage The proteins of some evaporated milks have a tendency to settle during storage. This is found more common in evaporated skim milk than evaporated whole milk. Homogenization retards protein separation in evaporated whole milk. It is observed that, fat in evaporated milk is a factor to maintain a normal dispersion of milk constituents during long period of storage. It is observed in this respect that fat acts to counter balance the protein settling to bottom. 7.Fat Separation in Storage Our aim is to get Evaporated Milk of good body and to avoid fat separation. During age thinning and before age thickening sets in, there is definite tendency to objectionable fat separation. This 82 | P a g e
tendency emphasizes importance of efficient homogenization that ensure reduction of at least 90% of fat globules to 2 µ or smaller. Such efficiency can be obtained by taking following care:
Daily check up of homogenization Valve
Keeping the clearance surface of valve and valve seat smooth and intact by regrinding or resurfacing
Making sure of the continued accuracy of the pressure gauge
Examining fat globule size
It is observed that HTST sterilization provides little opportunity to control body hence through a heavier body, control of the extent of fat separation can be achieved. Again here the LTLT procedure (115.6°C for 20 min) proves helpful. Fat separation in this more viscous body is slower and less intense. In case of LTLT, fat layer get re-emulsified easily than in case of HTST sterilization process.
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PHYSICO-CHEMICAL CHANGES TAKING PLACE DURING MANUFACTURE OF CONDENSED MILK-II Introduction
During the processing of evaporated and sweetened condensed milks the physico-chemical changes that take place are further elaborated here. Colour
1.Relation of caramelization to discoloration Browning of milk is attributed to reaction of sugar in milk. But when solution of sucrose, lactose, dextrose are heated separately, it do not show any sign of browning or caramel flavour. The colour changes in evaporated milk are not in direct relation of time, but it is the reaction of catalytic nature. Similarly, solution of sugar and proteins when heated, a marked brown colour develops and colour could not be removed by washing and also whey is clear showing no sign of any colour. 2.Relation of amino acid sugar complex to browning It appears that in the lactose - protein reaction, responsible for the darkening of milk proteins, certain amino acids are involved. The sugar combines with NH 2 group which is alkaline, neutralizing the alkalinity and leaving the acid group free, thereby causing a reduction of pH accompanied by the formation of highly coloured product. It is well known that sugar heated with an alkali turns brown; hence the amino acid - sugar complex which is alkaline would tend to assume a brown colour. This would be the case especially with a reducing sugar such as lactose, because of the availability of the aldehyde groups, which are naturally present in reducing sugar. This is further proved as if we add formaldehyde, (being having a great affinity for amino acid) to evaporated milk at the time of sterilization, milk remains quite white. 3.Effect of steps of processing on color It is found that time and temperature of heating is an important factor contributing towards colour of the milk. In addition it is noted that 1. Within the time of forewarming applied generally, the product suffers no decisive change in colour. If it is for long period (30’), effect is intense. 2. Homogenization tends to diminish the colour of the product because of a finer sub division of the fat globules which prevent such penetration of the rays of light as it would reveal the butter fat more nearly in its natural colour which is yellow. 3. Storage causes progressive darkening with increase in time and temperature. At (5°C) there is no change in colour. It is observed that for each 10°C decrease in forewarming temperature, the holding period is increased ~ 2.5 fold without causing an increase in colour. 84 | P a g e
Though cooked flavour is of little significance in our country, but still its mechanism is to be studied. Heat treatment in processing tends to produce a cooked flavour in evaporated milk. This is true also of fresh milk and milk products in general. It is generally observed that cooked flavour accompanies the darkening of colour. HTST process produces less cooked flavour than LTLT process. HTST sterilization treatment (135°C for 0.5 min) also reduces the extent of sterilization, decreases the viscosity and also fat separation etc. occur. So because of these limitation factors, LTLT process is important. 1.Reaction involved in production of cooked flavour The reactions involved in production of cooked flavour are not sufficiently clear. It is observed that when heating skim milk, cream, milk, EM, etc., -SH compounds are produced (provided heated to high temperature or at low temperature for longer time) from one or more proteins present. However, it is found that -SH compounds are wholly responsible for cooked flavour. Five hours heating at 70°C produces full cooked flavour. It is shown that in the reaction causing cooked flavour of milk by heat, O2 is taken up and CO2 is produced. Oxidation condition inhibits the formation of -SH group. Sediments of Mineral Salts
There is a tendency in some evaporated milk upon ageing for granular deposit to form in the can. This deposit has a whitish colour, it is gritty and insoluble and seemingly of non-crystalline character. On analysis these granular structure are found to contain Tri Calcium Citrate Ca3(H5O7)2 and Tri Calcium Phoshate - Ca3 (PO4)2. These salts have peculiarity of being less soluble in hot rather than in cold solution. 1.Effect of processing on mineral salts The tendency for increased concentration of milk solids to cause an increase in the amount of sediment produced is well known to evaporated milk manufacturers. An increase in concentration necessarily increases Ca, Mg, citric acid and PO 4 content in the milk and thus provides possibility of precipitation. This can be controlled by addition of casein stabilizer. 2.Effect of temperature of storage on sediment formation The precipitation of mineral salts in the form of white sand like deposits is an age defect of Evaporated Milk. The temperature of storage appears to be a controlling factor in this respect. Effect of HTST Sterilization
By this process, natural property of milk is retained better than that with the process of low temperature long duration. The major advantages are to avoid cooked flavour and colour changes. By HTST sterilization, heat stability will be better in concentrated product than with conventional method of sterilization, but the problems that are encountered in this process are insufficient viscosity and thin body which ultimately increase the tendency of fat separation on 85 | P a g e
storage. Self stability of product is relatively small and gelation may occur during storage period. Some of the different types of processes developed are: (1) Tin Sterilization: Sterilization at 127°C-130°C for about 2 minutes to 40 seconds. (2) Continuous Flow Sterilization: Here temperature is 145°C or higher with only few seconds holding and with subsequent aseptic canning. For avoiding the problems due to this sterilization process, increase the viscosity and reduce the age thickening. Several methods are supplementing temperature treatment which may be given before or after sterilization. They may be given to the product during coming up period or cooking period and get the advantage of avoiding cooked flavour. Effect of Heat Treatment on the Acidity
When heat treatment is given to milk in manufacture of concentrated milk, the acidity measured in terms of H+ concentration and titratable acidity is increased. The rate of formation of acid in concentrated product is in proportion of time and temperature treatment. It is also related to concentration of lactose. If the milk is heated in open pan, then the acidity will slightly drop in the beginning followed by an increase in acidity which will continue till the coagulation of casein. Initial decrease in acidity is due to loss of CO 2 dissolved in milk which usually contributes to 0.01-0.02% of the total acidity expressed as lactic acid. The increase in acidity on further treatment is due to breaking down of casein causing cleavage of phosphorus containing acid, probably nucleic acid and to a lesser extent to oxidation of lactose. The actual increase in acidity due to conventional temperature time ratio of forewarning in SCM is very slight. This slight increase in acidity is compensated by the initial loss of acidity due to expulsion of CO2. Whereas in case of evaporated milk, the reaction of sterilization heat treatment in conventional type procedure is more and in addition, the milk is also heated at higher temperature of forewarming. Due to this treatment, usually there is increase in acidity approximately 0.05 - 0.1% during the sterilization process than the normal acidity due to concentration. Effect of Storage, Time & Temperature on Acidity
The increase in acidity is in direct relation with time and temperature of storage. The acid producing reaction due to heat treatment is continued even when the concentrated product is held in storage room. The rate of acid formation will be very slow depending upon the temperature of storage. SCM of 64.5% sugar ratio having initial acidity of 0.43% and the original bacterial count 4381/g when stored at different temperatures resulted in to increase in acidity. This increase in acidity is mainly due to chemical changes as there was no increase in microorganisms during storage. Sweetened Condensed Milk
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The main change in sweetened condensed milk during storage is presumably age thickening and, finally, gelation. Sweetened condensed milk is far more concentrated than evaporated milk. However, it does not thicken markedly faster with age. It is usually assumed that added sucrose inhibits age thickening. Sucrose increases the Ca2+ activity. A difference with evaporated milk is that an initial decrease in viscosity before age thickening is not observed. The viscosity in sweetened condensed milk increases almost linearly with time. The following main factors affect age thickening in sweetened condensed milk:
The variation in the type of milk and season occurs among batches of milk.
Higher preheating treatment yields higher initial viscosity, and gel can form earlier. Hence, UHT heating is now generally applied.
The addition of sugar at later stage in the evaporating process results in lesser age thickening.
Higher concentration factor gives more age thickening.
The influence of added salts varies widely and depends on the stage at which it is added. Salts are added up to 0.2%. Adding a small amount of sodium tetrapolyphosphate (e.g. 0.03%) mostly delays age thickening considerably, whereas adding more may have the opposite effect.
Age thickening considerably increases with storage temperature.
2.Maillard browning Ongoing Maillard reactions are inevitable. Brown discoloration is stronger as the storage temperature is higher, the milk is evaporated to a higher concentration, and more intense heating is applied. Additional Maillard reactions occur if the added sucrose contains invert sugar. 3.Oxidative changes These changes can be prevented by keeping head space oxygen to a minimum. 4.Lactose crystallization Sweetened condensed milk contains around 38 to 45 g lactose per 100 g water. The solubility of lactose at room temperature is about 20 g per 100 g water, but in sweetened condensed milk the solubility is about half as much due to the presence of sucrose. It implies that 75% of the lactose tends to crystallize, meaning about 8 g per 100 g sweetened condensed milk. Due to the high viscosity, nucleation will be slow and only a few nuclei would be formed per unit volume of milk, leading to large crystals. Without special measures, the product will obtain a relatively high quantity of large crystals. These crystals settle and are responsible for a sandy mouth feel. Although the crystals may not be so large as to be felt singly in the mouth, they can be large enough to cause a non-smooth impression. To avoid this, they should be smaller than about 8 µm in length. 87 | P a g e
Preventing crystallization is not possible and, accordingly, a large number of crystals should be obtained. Satisfactory results can be reached by using seed lactose. Adding 0.03% seed lactose represents 0.004 times the amount of lactose to be crystallized. The final size of the crystals in the product should not exceed 8 µ m. Consequently, the seed lactose would contain enough seed crystals (one per crystal to be formed) if its crystal size does not exceed about (0.004 x 83)⅓, i.e., 1.25 µm. Such tiny crystals can be made by intensive grinding of α -lactose hydrate.
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PHYSICO-CHEMICAL CHANGES TAKES PLACE DURING MANUFACTURING OF DRIED MILKS WPN INDEX AND HEAT DENATURATION Introduction
Heat treatment of the original product or the concentrate can cause denaturation of serum proteins; the conditions during spray drying are rarely such as to cause extensive heat denaturation. The extent of denaturation is an important quality mark in relation to the use of milk powder. For instance, if the powder is to be used in cheese making, practically no serum protein should have been denatured in view of the rennetability; in infant formulas, on the other hand, the rennetability should be poor. WPNI
The extent of the denaturation of serum protein can be used as a measure of the heating intensity applied. This is true also where denaturation by itself may be of no importance, but other changes associated with intense heat treatment are. An example is the flavour of a powder to be used in beverage milk, which requires a mild heat treatment. An intensive heat treatment is needed for some other uses, for instance, to acquire good stability against heat coagulation in the manufacture of recombined evaporated milk, or a high viscosity of the final product when making yogurt from reconstituted milk. It is also desired if milk powder is used in milk chocolate; presumably, Maillard products contribute to its flavor. The whey protein nitrogen index (WPN index) is generally used to classify milk powders according to the intensity of the heat treatment(s) applied during manufacture. To that end, the amount of denaturable serum protein left in the reconstituted product is estimated, usually by making acid whey and determining the quantity of protein that precipitates on heating the whey. This can be done by Kjeldahl analysis of protein nitrogen or by means of a much easier turbidity test that is calibrated on the Kjeldahl method. The result is expressed as the quantity of undenatured serum protein per gram of skim milk powder. Whey protein nitrogen indices for heat classification of milk powders
Damage Caused by Heating
High drying temperatures can result in undesirable changes in the dried product. Generally, it is only after the powder has been dissolved again that the changes involved are noticed. Three quite different categories of undesirable changes can be distinguished: 89 | P a g e
1.Heat denaturation and killing of microbes a. T he reaction rate is highly dependent on temperature, but the reaction is much slower and less dependent on temperature at low water content. b. Some results for the inactivation of phosphatase are showing that, a higher average drying temperature and the higher dry-matter content of the liquid gives more inactivation because this goes along with a higher viscosity and hence, on average, larger drops. Therefore, a longer heating time is needed at a dry-matter content in which the inactivation rate still is appreciable. However, the largest drops contain the greatest amount of material. c. Killing of bacteria is higher as the average drying temperature is higher. The initial increase of survival of bacteria with increasing dry-matter content is presumably due to a substantial decrease in heat sensitivity of the organism. d. Heat denaturation of globular proteins and consequently, inactivation of enzymes and killing of microorganisms greatly depend on water content. It may also occur that removal of water increases the concentration of a reactant or catalyst for heat inactivation; this is presumably the case for chymosin in whey, because at, say, 40% dry matter, a w and diffusivity are not greatly lowered. e. When spray-drying a starter culture, survival of bacteria is of paramount importance. Often, a relatively large proportion of an inert material, generally maltodextrin, is added to the liquid before drying, which lowers the temperature sensitivity of the bacteria. In this way, survival rates over 80% can be achieved. 2.Insolubility a. Part of the protein may be rendered insoluble during the drying process due to heat coagulation. b. The powder contains particles that do not dissolve in water, but the amount is a tiny fraction of the powder. shows that insolubility increases with increasing outlet temperature and increasing dry-matter content of the milk. It also greatly depends on drier design. Presumably, heat coagulation mainly occurs in some (large) drops or powder particles that recirculate in the drier and become wetted again. A cumulation of high temperatures and high dry matter content for a relatively long time then causes the problem. Modern driers tend to give very small insolubility figures. 3.Formation of hair cracks a. These can form at high drying temperatures because the outer rind of a drying droplet soon reaches a glassy state; the pressure gradients developing in the particle then cause these very thin cracks to form.
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b. In case of whole milk powder, part of the fat can now be extracted from the powder by solvents like chloroform or light petroleum. The extractable fat is often called free fat, but that is a misleading term.
Autoxidation of lipids Autoxidation of lipids follows quite a different pattern. The reaction rate is high for low a w . Possible causes are that water lowers the lifetime of free radicals, slows down the decomposition of hydroperoxides, and lowers the catalytic activity of metal ions, such as Cu 2+. The rate of many reactions greatly depends on the water content; shows Rate of Maillard reactions (—) and of protein becoming insoluble (---) in concentrated skim milk at high temperature (~80°C) as a function of water content. Because of an increase in the concentration of reactants, the rate of bimolecular reactions at first increases due to removal of water. On further increase of Q* , the reaction rate often decreases again; this decrease would be caused by reduced diffusivity. A good example is the Maillard reaction. The irreversible loss of solubility of milk protein in milk powders, and the rate of gelation of concentrated milks show a trend similar to curve 5. On removal of water from milk, it thus is advisable to pass the level of approximately 10% water in the product as quickly as possible. Relative reaction rate ( Kr) of various reactions plotted against the water activity (aw) of (concentrated) skim milk (powder). The upper abscissa scale gives the water content (% w/w). (1)
aureus ascorbic (e.g.,
; acid; lipase);
(5) Maillard reaction (non-ezymatic browning). Thus, the influence of some process variables on product properties is inevitably complex as the relationship between two parameters may depend on other variables and also it is mostly not possible to vary only one process variable. However, it is observed that a two-stage drying provides increased possibilities to make powder with various desirable characteristics.
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THE MILK POWDER SYSTEM Introduction
The milk powder manufactured undergo several changes while conversion from liquid milk to dry product. They are:
The manufacturing process employed on milk (~87% water, aw=0.99) eliminates majority of water till it reaches the final stage of drying, the powder (~4% moisture, aw<0.6). The process not only eliminates the water from milk but also changes the physical structure i.e. the arrangement of the components of milk in space. In the process, the sol changes to a dust.
The dried milk exhibits a well pronounced dual physical structure – the primary structure, which is the internal build-up of the powder particles from the milk solids and small amounts of mois¬ture and air and – the secondary structure which represents a typical powder, a system of closely packed solid particles in a gas.
The structural elements of dried milk particles are – lactose, either amorphous or partly crystalline, casein micelles and whey protein particles, fat in globular or non-globular (free fat) forms and air as spherical cells. The fat, protein and air are presumably dispersed in continuous phase of amorphous lactose.
The whole milk and infant milk powders are rich in fat (20-37%) and hence need protection against oxygen and moisture, therefore, generally tinplate cans are used and the packaging is done under nitrogen cover to replace air with nitrogen in the tins. Skim milk powders have very low fat (1.5%) and as such a good moisture barrier material like multi wall paper sacks with one polythene liner is used. Recently, some skim milk powder has also been sold on polythene bags and even high density polythene bottles are being tried for retail packs.
The milk powders are concentrated mass of milk components in the form of particles with small amount of moisture and air. If sufficient care is not taken to protect them from high humidity, high temperature of storage and the entry of air, the deterioration processes are accelerated.
These changes while manufacture occurs in a sequence. They are discussed here:
Changes of State of the Drying Droplets
Atomizing pure water in a drying chamber in the usual way causes the water droplets to reach the wet-bulb temperature and to vaporize within 0.1s at this temperature.
The presence of dry matter in the droplets, however, makes an enormous difference. T he diffusion coefficient of water decrease substantially with increasing dry-matter content. Accordingly, the vaporization is significantly slowed down.
In the drying droplet, temperature equalization occurs in less than about 10 ms. i.e. the temperature is virtually constant throughout a droplet during drying. 92 | P a g e
Drying Stages • Initially, the droplet has a very high velocity relative to the drying air. • Therefore, there is a first stage during which circulation of liquid in the droplet occurs; this circulation greatly enhances transfer of heat and mass. • For a droplet of 50 µ m diameter, this stage lasts for ~2 ms. In this time, the droplet covers a distance of about 10 cm and loses a small percentage of its water. • Its velocity compared to the air decreases to the extent that the formed surface tension gradient of the drop surface arrests internal circulation of liquid. • But in the second drying stage, the difference in velocity between drop and air is still great enough to accelerate water transport. • The transport in the droplet occurs by diffusion but in the air by convection. • After about 25 ms the relative velocity of the droplet has decreased so far that the water transport has become essentially equal to that from a stationary droplet. • Relative to the air, the droplet then has covered a distance of a few decimeters and has lost about 30% of its original water.
• In the third stage, lasting at least a few seconds, the droplet loses the rest of the water by diffusion. Drying Process
During drying the air temperature decreases and the humidity of the drying air increases. Moreover, the droplets vary in size and the smallest ones will dry fastest.
In practice, the mixing is always between the two extremes. For driers with a spinning disk, the situation tends to be fairly close to perfect mixing. In most driers with nozzles, it tends to be closer to concurrent flows. In all situations, the drying time may very by a factor of, say, 100 between the smallest and the largest drops. This is of great importance for fouling of the drying chamber; the largest drops have the greatest chance of hitting the wall and of being insufficiently dry to prevent sticking to the wall.
Another factor that affects drying rate is the presence of vacuoles in the drying drops. It leads to significantly faster drying.
1.Concentration gradients A rapid decrease in drying rate of the droplets occurs after the water content is reduced to, say, 15%. A strong concentration gradient forms rapidly during drying. The higher the drying temperature, the stronger the effect. Therefore, stronger gradients occur for cocurrent drying. A dry outer layer, i.e., a kind of rind, is formed and because of this, the water transport is slowed down considerably. The temperature can rise significantly in the dry outer layer because a dried 93 | P a g e
material assumes the air temperature, not the wet-bulb temperature. In other words, the decrease in temperature near the surface of the droplet caused by consumption of the heat of vaporization becomes far smaller because the vaporization of water is slower. Because temperature equalization happens very quickly, the whole drying droplet increases in temperature. The concentration gradients are not of a lasting nature. The relatively dry outer layer of the droplet soon becomes firm and eventually glassy. This causes the droplet to resist further shrinkage. The droplet can react by forming vacuoles or by becoming dimpled. Especially at a low water content of the particles, so-called hair cracks may be formed. 2.Aroma retention Besides water, the drying droplets lose other volatile components, including flavor compounds (aroma). In many cases, however, the loss of flavor compounds is far less than expected, in spite of their volatility because of the following conditions • The effective diffusion coefficient of most flavor components in the relatively dry outer layer
of the droplet decreases far stronger with decreasing water content than the diffusion coefficient of water does due to the greater molar mass. • Thus, the aroma retention - retaining flavor compounds during drying - increases with * Droplet size because in larger droplets, the outer layer from which the flavor components get lost has a relatively smaller volume and * Drying temperature because, at a higher temperature a solid rind forms more rapidly. • Formation of vacuoles diminishes aroma retention, especially if hair cracks develop in the particles and the vacuoles come into contact with the surrounding air. • Dissolved milk powder often has a cooked flavor, which results from the flavor compounds formed during preheating and possibly during evaporation. • During drying, conditions are mostly not such that off-flavors are induced. On the contrary, a considerable part of the volatile sulfhydryl compounds is removed. A cooked flavor mainly results from methyl ketones and lactones formed by heating of the fat and from Maillard products which are almost absent in skim milk powder. 3.Water Activity If the water content of a product decreases, its water activity ( a w ) also decreases. Water activity is expressed as a fraction. In pure water a w = 1; in a system without water, a w = 0. Due to the very low water activity in milk powders, it exhibits following changes
The hygroscopicity increases: Usually, a (dry) product is called hygroscopic if a small increase of a w causes a considerable increase in water content. Obviously, this mainly concerns milk powder with very low water content. 94 | P a g e
Diffusion coefficients decrease: At low water content the effect is very strong. The diffusion coefficient of water decreases from approximately 10 -9m2 s-1 in milk to 10-16m2s-1 in skim milk powder with a small percentage of water.
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The Physico-Chemical Changes During Storage of Milk Powders
It has been observed that the life of roller dried whole milk powder from cow's milk is about 10 months; the spray dried product however cannot be stored more than 6 months at 17°C and half of this time at 27°C. Although the milk powders are not sterile products, the low water activity does not permit microbial growth and microbial activity. The same is true of enzymic activity. The changes occurring in milk powders on storage are of physical, chemical and physicochemical in nature. Both the lipids and the non-lipid constituents of milk undergo such changes and normally the rates of such reactions are accelerated with the elevation of storage temperature and levels of moisture and oxygen in the product. The quality of milk and the processing parameters adopted while manufacture of milk powders are also the major factors determining their storage stability. 1.Water activity (aw) The most important variable determining the rate of undesirable changes in milk powder is the water content . When comparing different types of powder, it is probably easiest to consider water activity ( a w ). The relationship depends on the composition of the product.
The higher aw of whole milk powder as compared to skim milk powder of the same water content is caused by the fat not affecting aw .
Whey powder has aw slightly different from that of skim milk powder because in a dry product the soluble constituents (especially sugar and salts) decrease a w somewhat less than casein. This is only true, however, as long as all lactose is amorphous, which often does not apply to whey powder.
The aw is considerably reduced if lactose crystallizes without absorption of water by the powder, at least if aw is less than about 0.5.
Crystalline lactose binds water very strongly, and that is also why the usual oven-drying methods to estimate the water content do not include the bulk of the water of crystallization. If the water content excluding the water of crystallization is taken as a basis, then a w is even higher for the powder with crystallized lactose.
It is thus advisable to make milk powder sufficiently dry and to keep it in that condition. If it is not hermetically sealed from the outside air, it will attract water in most climates. The higher the temperature, the higher the water activity. Because several reactions are faster at a higher aw , this implies that a temperature increase may well cause an extra acceleration of deterioration.
The deterioration effect may be especially strong if the powder loses its glassy state. A lactose–water mixture will be at most ambient temperatures in the glassy state if its water 96 | P a g e
activity is below 0.3. Because lactose is the dominant component of the amorphous material in a powder particle, about the same relation is supposed to hold for the powder. This means that most dairy powders are in the glassy state (i.e., the nonfat part of the material), except if the water content is high and the temperature is also high. A change in conditions leading to a glass–liquid transition will strongly accelerate most reactions and physical changes occurring in a powder. 2.Colour, flavor and off flavor characteristics The user of dried milk is concerned mainly with the flavour defects which arise during storage. Following aspects are important which affect the colour and flavour of dry milks:
The use of poor quality milk is not desirable for manufacture of good quality dry milk.
Some feed taints may be removed during the course of vacuum evaporation or drying, but not always.
Acidity already developed in the milk will be retained in the powder, which acquires an unpleasant flavour.
Neutralization of acid milk has been practiced, but the flavour of the resulting powder remains unsatisfactory and the colour may darken. Very acid milk cannot be dried by the roller process. First quality powder should not exceed an acidity of 0.15 (as lactic acid), and no powder should exceed 0.17%.
Assuming that good quality milk has been used, the most likely defect of freshly made powder is some degree of burnt flavour. A distinct burnt flavour is only likely to occur in spray powder following considerable overheating in the drying chamber, but may arise much more readily in roller powder owing to overheating on the rollers or faulty adjustment of the scraper knives. In any case, roller powders always possess a distinct cooked flavour. The flavour of spray powder is influenced considerably by the milk preheating temperature used. Thus "low-heat" powder or powder from "ultra-high temperature" processed milk gives reconstituted milk having a flavour close to that of pasteurized milk. "High-heat" powder when reconstituted possesses a more cooked flavour, which however, is not unpleasant and indeed is preferred by some consumers. In practice, storage changes produce various flavour defects which mask other factors and after 2-3 months the "low-heat" powder is commonly inferior if special precautions have not been taken.
The colour and flavor changes will progress during prolonged storage if conditions like elevated temperature in combination with increased moisture content prevail. Although much research remains to be done, two major types of deterioration are well recognized, affecting fat in one case and protein plus lactose in the other. Ultimately these affect the flavour, solubility, colour and nutritional value of the powder. Two major categories of reactions are: (a) The oxidation of lipids and 97 | P a g e
(b) The non enzymatic browning commonly known as the Millard reaction. The former is associated with the tallowiness whereas presumably both these reactions contribute to stale and allied flavours. With the advancement of storage, the discolouration will also progress and fluorescence substances are also produced. 3. Fat decomposition Oxidation of the fat leading to the production of flat, and finally marked tallowy off-flavours is a major storage defect of full cream powder and some times occurs to a lesser extent in the residual fat content of separated milk powder. Some of the observations made by different workers are
The chemical changes result from the addition of oxygen to the double bonds of unsaturated glycerides, giving peroxides which can be estimated by chemical means.
In the oxidation of linoleic acid, the C-11 methylene group is preferential point of attack. Many other volatile aliphatic compounds such as alcohols, lactones, esters, acids and hydrocarbons arise from oxidation of unsaturated fatty acids.
At first, the peroxides accumulate but later they decompose to various aldehydes, ketones and acids which impart the unpleasant flavours.
Aldehydes are detectable with colour reagents but the sense of taste is much more sensitive than chemical tests.
The reaction commences with a slow linear induction phase, followed by a period of rapid exponential change and combination with oxygen, large quantity of O2 can be absorbed. The break point at which the oxidation changes from first to second phase is generally found to be at 37 weeks in whole milk powder.
Peroxides are formed when the container contains an excess of free O2, but later decomposition of peroxides exceeds the rate of their formation.
The keeping quality of individual "low heat" powders varies considerably - from 4-12 months in temperate climates and perhaps half this time in hot climates.
Though the development of poor flavour is parallel to O2 absorption, the ratio is not constant and the most stable powders require the most O2 in order to deteriorate to a given flavour value.
The odour and flavour profiles of dried milk with addition of antioxidants (BHA/BHT & Maillard reaction mixtures from histidine and glucose) and stored under air and nitrogen are as follows
The Maillard reaction products are as effective as BHA/BHT in retarding the development of oxidative off flavours and off odours.
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Samples to which antioxidants are added or samples stored under nitrogen retain a higher acceptance for storage times as long as 84 weeks.
The 2, 3 butadione has a sweet and buttery odour and flavour at low concentrations and the branched chain aldehydes, the Strecker aldehydes are generally described as having fruity odours and flavours at low concentrations.
The sulphur compounds are known to impart cooked odours and flavours.
Some compounds, 1-octen-3-one and 1-octene-3-ol that are known to give rise to metallic and mushroom odour and flavour respectively in oxidatively deteriorated milk products.
Factors affecting oxidative stability of lipids in dried milks include oxygen, moisture, temperature, light and metals like copper and iron. These influence the system depending upon their fat content, presence or absence of antioxidant and packaging under the nitrogen atmosphere.
The use of antioxidants instead of nitrogen gas packaging, derived from various materials showing promise as practical antioxidants are: wheat germ oil, gum guaiac, rice bran concentrate mixed tocopherols, oat flour and special preparations made from ethyl gallate, ascorbic acid, nordihydroguaiaretic acid (NDGA), thiourea, hydroquinone monobenzyl ether, flavones, querectin, ascorbyl, palmitate, histide, tryptophan, phosphoric acid either alone or in combination of a few.
For addition of any antioxidant legal acceptance is must and for the present in India, no antioxidant other than lecithin, ascorbic acid and tocopherol is permitted accepting BHA not more than 0.01% by weight in milk powder. In infant milk no antioxidant is permitted.
Inspite of some fairly promising indications that antioxidants retard tallowiness in dry milks, packaging in inert gas is the most efficient method at present available for preventing fat deterioration.
4.Fat hydrolysis True rancidity results from the hydrolysis of fat to free fatty acids such as butyric acid. The lipolytic enzymes involved can be derived from the milk itself or they may be of microbiological origin. This defect has become unusual since strict attention is now given to both milk production and plant hygiene and higher preheating temperatures which inactivate lipases. 5. Other Forms of fat deterioration and their prevention The other flavours are described as "stale" or most characteristically as like "Coconut". A ‘toffee” flavour may also be a form of the same defect. They are produced only from milk fat and do not occur in dried separated milk. The defect is accelerated by high-temperature storage and by high moisture content, but is independent of O2 and occurs in gas packed powder. This 99 | P a g e
form of deterioration seems to occur at a fairly early stage during storage, it develops before the fat oxidation defect from which it also differs in flavour. There is strong evidence that the change is caused by a rearrangement of monoethenoid fatty acids in milk fat to form lactones and that δ-decalactone in minute quantities is the substance responsible for the coconut flavour. The origin of this lactone is thought to be 9-deconoic acid. These rearrangements involve no O2 and the empirical formulae remain the same. No definite method of prevention has been suggested except low-temperature storage. In the absence of special precautions, oxidation occurs rather more rapidly in spray than in roller powders, but in recent years much progress has been made in extending the life of spray powders. With full-cream powder the main cause of deterioration is overcome if fat oxidation is prevented, other possible defects being within the control of manufacturer. Hence, to keep the autoxidation within reasonable limits for a long time, following measures should be taken: 1. High Temperature Preheating: The milk should be intensely heated to form antioxidants. The problem is, of course, that the heat treatment also causes a distinct cooked flavor. Compared with pre-heating the milk at 73.8°C, preheating at 82.2°C improves the keeping quality of the powder, whilst at 87.8°C, the increase is 3-5 times. The mechanism is not fully understood, but in general it is associated with the formation of trace of protein decomposition products containing a sulfhydryl group which function as antioxidants and prolong the induction period. Natural milk proteins contain no free -SH groups, but in the denatured protein formed in heated milk, such groups become unmasked and reactive. The first action may be the formation of cysteine followed by further reaction at the exposed sulfur ends of the chains to split off H 2S or methyl mercaptan. Traces of H2S are detectable in the heated milk. The action of -SH group is uncertain, but may be due in part to prevention of the catalytic action of heavy metals by removing them as sulfides. 2. High temperature preheating may also prevent changes produced by enzymes derived from the raw milk or from bacteria, thus improves the bacteriological quality of the powder and preserves the vitamin content during storage. 3. The rate of autoxidation strongly increases with decreasing a w ; however, to prevent other types of deterioration (especially Maillard reactions) a w should be as low as possible. The effective Q 10 of the autoxidation reaction in milk powder is relatively low (about 1.5) because a higher temperature also causes higher a w . The most suitable water content is generally 2.5 to 3%. 4. Packing in Inert Gas: This depends upon prevention of the chemical reaction by removing the O2 from the container and replacing it with N2 or CO2 usually the former. It is the only method which completely prevents oxidation for practical purposes and can extend storage life up to 7-10 years even at high temperature. Oxygen should be removed as effectively as possible by gas flushing. A problem is that the vacuoles in the powder particles contain some air, hence, O2. Either the powder should contain hardly any vacuoles or the gas flushing should be repeated after a few days. Equilibrating the gas inside and outside the vacuoles by diffusion takes several days in whole milk powder (several weeks in skim milk powder). Equilibration is faster if the powder particles have a greater number of cracks.
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5. Addition of Antioxidants: Of the more successful substances “Ascorbic acid” added @0.03% in the liquid milk prolongs powder keeping quality by several months but disappears during storage. The most effective antioxidants appear to be esters of gallic acid particularly Ethyl or Propyl gallate. The incorporation of as little as 0.07% of ethyl gallate in the powder prolongs the keeping quality to about two years when only low temperature preheating has been used. Ethyl gallate is non-toxic and has no effect on flavour; it does not disappear during storage and appears to remain unchanged. Keeping quality may be extended still further by combining high-temperature preheating with the addition of ethyl gallate. In this way, it is possible to produce powders having a keeping quality of from 3-4 years in temperate climates. Nordihydroguaiaretic acid has also been found effective when added @ 0.04% of the weight of the fat. 6. The rate of deterioration increases rapidly with rise of storage temperature. The keeping quality of an unstable powder may fall as low as 6 weeks in a tropical climate. 7. Oxidation is more rapid in Powder of high acidity. 8. The powder should be packaged in such a way that air and light are kept out. Generally, this implies packaging in cans. 9. Rigorous measures should be employed against contamination of the milk with heavy metals such as copper and iron which act as catalysts and shorten the induction phase. Stainless steel equipment should be used. 10. Other less certain factors are the possible presence of oxidizing enzymes of microbiological origin in the milk and seasonal variations in the glycerides-composition of the fat. 11. Intensive homogenization of the concentrate should be carried out. 6.Protein - Lactose / Maillard Reactions Millard reactions between protein and lactose produce a series of storage defects which often occur together in a progressive sequence of stale flavours, increasing loss of solubility, unpleasant "glue" flavour and darkening or brown discoloration. These changes may occur in all types of powder but are associated predominantly with separated dried milk and tend to be most pronounced in roller dried powders. They are greatly accelerated by high-storage temperatures and in addition, the moisture content of the powder is a major factor. Critical moisture levels appear to be about 5% for spray powder and 4% for roller powder. Above these figures, the defects are accentuated. This reaction in addition to accounting for the major part of browning, the protein-carbohydrate complex or its decomposition products also result in production of reducing substances, fluorescent compounds and disagreeable flavour materials. About 80 different compounds have been isolated and identified which include furanics, lactones, pyrazines, pyridines, acetyl pyrrole, amides, pyrolidinone, succinate, glutarimides, carboxylic acids, acetone, 2-heptanone, maltol etc. Some of the findings in this regard are 101 | P a g e
Maillard reactions increase considerably with water content and with temperature. They lead to browning and to an off flavor. The ‘gluey’ flavor that always develops during storage of dry milk products with too-high water content is usually ascribed to Maillard reactions; the main component appears to be o -aminoacetophenone.
If extensive Maillard reactions occur, they are always accompanied by insolubilization of the protein. Accordingly, the insolubility index increases when milk powder is stored for long at a high water content and temperature.
One of the first reaction products detected in Maillard reaction is hydroxymethyl furfural (HMF), the concentration of which increases with storage time, temperature and moisture content. Many of the products of Maillard reaction are aroma substances (aldehydes, reductones, other furfurals etc.) which have an appetizing odour, but high levels of these compounds may cause considerable odours and off flavours. The ε-amino groups of lysine are mainly involved and because the compounds formed during the reaction such as fructoselysine, furosine etc. are resistant to enzymes, the content of available lysine is reduced, but usually only to a small extent. Another result of the reaction is the production of substances which lead to colour changes, but only in a more advanced stage of the reaction.
Browning reactions of the Maillard type occur if the storage of powdered dairy products at ambient temperature and having the moisture content ≥ 5%. The HMF content is further increased in milk powder with added iron or added Vitamin A.
With the progress of Maillard reaction, the decrease of amino-N is closely related to increase in combined sugar, increase in oxygen absorption and carbon dioxide production, increase in colour and reducing power and decrease in solubility .
A number of compounds have been shown to inhibit the browning reaction. In milk products, active -SH groups serve as natural inhibitors in retarding the heat induced browning, but the mechanism is not understood. Sodium bisulphite, sulphur dioxide and fo rmaldehyde also inhibit browning in milk system as well as in simpler amino acidsugar solutions.
7.Deterioration of a high moisture powder proceeds in a series of stages
First the lactose absorbs moisture and changes to crystalline L -lactose monohydrate containing 5% water of crystallization.
In powders of low moisture content, the lactose is present in the amorphous α and βforms (1: 1.5) with only 0.5 to 5.0% of lactose hydrate. If the moisture content and the temperature remain low, the lactose preserves its amorphous form and no deterioration occurs. If the moisture content is high, as much as 40-60% of the lactose may be hydrated in a fresh powder and at levels of 6.5 to 7.0% for full-cream powder and 7.5 to 8.0% for separated milk powder crystallization becomes rapid and complete (dried milk can also absorb moisture very readily from the air). Such powders may show an apparent decrease in their moisture content. There is often a loss of free-flowing properties and possibly 102 | P a g e
some definite "Caking" of the powder. The impervious lactose envelope around spray powder particles probably changes to a crystalline lattice.
The free lactose content of the powder then falls and an insoluble protein-lactose compound is formed which contains lactose equivalent to the quantity of the free soluble lactose which has disappeared. The reaction is presumed to occur between the aldehyde group of the lactose and a protein-amino group. The amino group mainly involved is that of lysine, and about 40% of the original lysine disappears, which considerably reduces the nutritional value of the powder. As the protein - lactose reaction proceeds, the remaining protein also becomes progressively insoluble and eventually the loss of solubility may be pronounced, particularly of roller -dried milk.
The final stage involves a decomposition of the protein - lactose compound to form products which include substances having an unpleasant glue-like flavour and a brown colour. The reaction involves absorption of O2, and the evolution of CO2, but the exact nature of the breakdown and the identification of the decomposition products are uncertain. In general, the O2 reacts preferentially with the fat in full-cream powders, and tallowy flavours predominates, whilst the stale and gluey flavours are more characteristic of separated milk powder, but both forms of deterioration can proceed simultaneously.
If the powder is gas packed, the protein lactose reaction and loss of solubility may still occur, but the final changes are much less serious. There may be some development of brown colour and a slight caramalize flavour, but unpleasant flavours are unusual. It would appear that development of the gluey flavour requires a supply of O 2. Gas packing is therefore partially effective but is not a real solution.
The manufacturer can avoid these defects of browning more simply and with greater certainty by producing powders by limiting heat treatments and time and of low moisture content and using moisture-proof packaging combined with low storage temperatures.
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PHYSICO-CHEMICAL CHANGES TAKES PLACE DURING MANUFACTURING OF ICE-CREAM PREPARATION OF ICE CREAM
1.Homogenizing Mix Homogenization of ice cream mix is a most essential step to make a permanent and uniform suspension of the fat by reducing the size of the fat droplets to a very small diameter, preferably not more than 2 µm. Advantages of homogenization Proper homogenization of the mix will never allow the fat to form the cream layer
More uniform ice cream
Improved whipping ability
Shorter ageing period
Less opportunity for churning to occur in freezer
Less stabilizer is required 104 | P a g e
Homogenization of mix is usually done at temperature ranging from 63 to77°C. A pressure of 2000 to 2500 psi (135 to 170 kg/cm2) with one valve or 2500 to 3000 psi (170 to 200 kg/cm2) on the first and 500 psi(35 kg/cm2) on the second stage will usually give good results for an average mix. 2.Pasteurization of Mix Pasteurization is done to destroy all the pathogenic bacteria in the mix so as to render the final product safe for human consumption Advantages of pasteurization are:
It renders the mix completely free of pathogenic bacteria.
It dissolves and helps to blend the ingredients of the mix.
It improves flavour.
It improves keeping quality.
It produces a more uniform product
Rapid heating and holding of the mix at definite temperature and rapid cooling below 5°C ensures proper pasteurization. The temperature time combination for pasteurization of the mix as per BIS is as follows
For Batch method – 68.5°C for not less than 30 min
HTST method - 80°C for not less than 25seconds
Vacreation - 90°C for not less than 1-3 seconds
UHT pasteurization – 98.8 to 128.3°C for not less than 0-40 seconds
High temperature pasteurization is preferred as there is a greater bacterial kill resulting in low bacterial count in ice cream
Better body and texture
Protection against oxidation
Saving of stabilizer
Saving of time, labour and space
Increased capacity 105 | P a g e
3. Cooling of Mix After pasteurization, the mix should be rapidly cooled to a temperature below 4°C using a plate heat exchanger. Unless the mix is cooled to a temperature of 4°C or lower, it will become very viscous and the ice cream will not melt down smoothly. Also, temperatures below 5°C retard the growth of bacteria. 4. Ageing of Mix The cooled mix is left to age preferably for a period of 24 h at 4°C. The changes that occurs during ageing are
Hydration of milk proteins
Crystallization of fats
Absorption of water by any added hydrocolloids
Viscosity is increased largely due to the previously mentioned changes.
Ageing is substantially completed within 24 h and longer period should be avoided to control spoilage by psychrotrophs.
4. Freezing of Mix
Changes during freezing process The function of the freezing process is to freeze a portion of the water in the mix and to incorporate air into the mix. This involves:a) Lowering the temperature of the mix from ageing temperature to the freezing point b) Freezing a portion of water in the mix c) Incorporating air into the mix d) Cooling ice cream from the temperature at which it is drawn from the freezer to hardening room temperature. The first phase of freezing process accounts for the freezing of 33 to 67 per cent of the water and the second phase (hardening process) accounts for freezing another 23-57 per cent depending on the drawing temperature. The temperature of the mix which is put into the freezer drops very rapidly while the sensible heat is being removed and before any ice crystals are formed. This process takes less than a minute or two. Meanwhile, the rapid agitation reduces the viscosity by partially destroying the gel structure and by breaking up the fat-globule clusters and also hastens incorporation of air into the mix. 106 | P a g e
When the freezing point is reached, the liquid water changes to ice crystals which appear in the mix. These ice crystals are practically pure water in a solid form, and thus the sugar as well as the other solutes becomes more concentrated in the remaining liquid water. Increasing the concentration of the solutes slightly depresses the freezing point and when the freezing point is continuously lowered, more ice crystals are formed increasing the concentration of sugar and other solutes in the remaining liquid water until the concentration is so great that further freezing will not occur. Thus all the water does not freeze even after long periods in the hardening room. 5. Ice Cream Hardening
In hardening process, the aim is to reduce the temperature of the product to at least 0 oF in the center of the package as quickly as possible. After the ice cream reaches this point, it is only necessary to store it at a uniformly low temperature to prevent ice melting and recrystallization.
Objectives of hardening ice cream
The physical nature of ice cream when drawn from the freezer is such that it is seldom practicable to market it in this form.
To freeze more water in the ice cream that has been drawn from the freezer and filled in the container to obtain better consistency.
To make ice cream stiff enough to hold its shape.
Hardening time: The time necessary for the temperature at the center of the package to drop to -18°C is known as ‘hardening time’. A hardening time of 6-8 h for 19 liters (5 gal). package is considered as ‘excellent’ operation when performed in hardening rooms. When hardening tunnels are used, the rate of hardening is several times faster.
Hardening process After ice cream is drawn from the freezer, it is put into containers to be placed in hardening room. Here the freezing process is continued without agitation until the temperature of ice cream reaches -18°C or lower, preferably -26.1°C (-15°F ). Quick hardening is desirable, since slow hardening favours formation of large ice crystals and a corresponding coarseness of texture.
Factors that affect the rate of hardening
Temperature of ice cream when drawn from the freezer
Composition of the mix
Percent overrun taken in ice cream
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Whether several containers are bundled together
Nature of the wrapping material (paper or plastic)
The manner of stacking of the containers
The temperature and velocity of the circulating air
Obstructed versus unobstructed exposure of the containers to the cooling medium.
Very small containers harden quickly, but they warm up quickly when removed from the freezing temperatures. Hence, they are more prone to body and texture damage as a result of heat shock. This also applies to novelties (stick bars, small cups,etc.). Large containers (i.e. 11.34 liters) harden much slower in the interior(cooling is largely by conduction) and must be given ample time to reach -18°Cin the interior. If containers are stacked before they are adequately hardened, deformation may occur and some overrun may be squeezed out causing surface discoloration.
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STABILIZERS AND EMULSIFIERS – SELECTION, MECHANISM OF ACTION, INFLUENCE ON MIX AND ICE CREAM, PROPRIETARY STABILIZER BLENDS Introduction
The stabilizers are a group of compounds, usually polysaccharides that are responsible for adding viscosity to the unfrozen portion of water and thus holding this water so that it cannot migrate within the product. This results ice cream that is firmer to the chew. Without stabilizers, the ice cream would become coarse and icy very quickly due to the migration of the free water and growth of existing ice crystals. Although excellent ice cream products can be made with only the natural stabilizing and emulsifying materials present in milk such as milk proteins and phosphates, additional stabilizers and emulsifiers have potential benefits. Emulsifiers aid in the production of drier ice cream upon extrusion with smoother body and texture and good stand up or melt resistance. The emulsifying agents commonly used in ice cream are mono and diglycerides and polysorbate-80, polyoxyethylene sorbitan monooleate. Milk also contains some naturally emulsifying compounds that aid in the manufacture of ice cream. These include milk proteins, phosphates and citrates. Egg yolks may also be used as an emulsifier as they are high in lecithin. Stabilizers
Major functions performed by stabilizer in ice cream include contributing to body or ‘substance’ and creaminess, imparting a smooth texture and providing melting resistance to ice cream. Excessive use of stabilizers, however, may result in gumminess, poor meltdown and interfere with flavour release. The most important functions of stabilizers is to ‘stabilize’ the product texture i.e. to prevent or minimize detrimental effect of heat shock during storage and distribution.
Selection The following factors are important in choosing a stabilizer:
Ease of incorporation in mix Effect on viscosity and whipping properties in mix Ease of dispersibility in cold and hot mix Type of body produced in the ice cream Effect on meltdown characteristics Ability to retard ice crystal growth Amount required to produce the stabilization Cost Perception as natural Effect on flavour perception
Mechanism of action Capable of imbibing large quantities of water while still remaining dispersed in water and forming colloidal solutions, stabilizers are functionally also termed as hydrocolloids. These 109 | P a g e
viscogenic compounds are primarily polysaccharides although, gelatin a well known stabilizer is a protein in nature. The most apparent effect of stabilizers is the increased viscosity of the continuous liquid phase. The effect of stabilizers on viscosity exhibits considerable interaction with milk constituents. For instance, the basic viscosity of stabilizer solution is generally not affected by heat treatment in the absence of milk solids, but in their presence, within creasing total solids, the effect of stabilizer on heat induced increase in viscosity of the system becomes more pronounced. Upon hardening, the water content of ice cream falls considerably with a concomitant increase in the concentration of stabilizer in the liquid phase. The increased stabilizer concentration together with decreased temperature greatly increases the viscosity, there by substantially reducing diffusion and mobility of the liquid in the frozen product. Also, gel formation induced by stabilizer in the mix is believed to effect, considerably, immobilization of the liquid. The movement of water is hindered partly by the ability of hydrocolloids to form hydrogen bonds, too. Thus, refreezing of water that originated from melting of ice due to temperature fluctuations would not permit the formation of large ice crystals. It should, however, be noted that merely by increasing the viscosity the desired stabilization of ice structure may not be achieved. Increasing the viscosity of a 36% TS mix by addition of 5% polyethylene glycol had no effect on ice crystal size in the frozen dessert. Certain stabilizers such as guar gum are effective thickening agents with no gelling ability. Further, the way stabilizers influence the body and texture perception in frozen ice cream could be related to their molecular structure and orientation besides their gel forming or viscosity building ability. However, extrapolation of results from model systems to ice cream is difficult as the later is a complex system with high concentrations of salt, sugar and stabilizers, which interact with other molecules, e.g. protein.
Influence on mix and ice cream 1. Effect on texture The most profound involvement of stabilizers in promoting smoothness through control of ice crystal size begins after initial freezing and hardening. Once the ice crystals are formed, the influence of stabilizers is to do exactly what it implies… stabilize the size of the ice crystals against growth as a result of temperature fluctuation (heats hock). There is an increase in mean ice crystal size each time the temperature fluctuates. Stabilizers help create a situation where a higher proportion of water which changes state during temperature rise recrystallizes as small crystals. This is thought to occur through the formation of a system (with high viscosity and /or a gel structure) which immobilizes the melted water, thereby reducing the degree to which it can migrate to existing ice crystals and deposit on them when it refreezes. The net effect is to slow the rate of ice crystal growth whenever heat shock occurs. 2. Effect on body characteristics This is a major function at the time the product is drawn, during hardening and at the time of consumption. It involves the participation of the stabilizer ingredients in determining the cohesiveness of the product, particularly those aspects described by such adjectives as chewy, sticky, weak, gummy, etc. 110 | P a g e
3. Effect on whipping and overrun retention The structure which is established as water freezes and stabilizer and its complexes are concentrated also play a role in providing strength to the air cell wall. Stabilizer are therefore involved in the amount of air which is incorporated and the degree to which the air cells are stable. 4. Effect on melting rate and properties of melted product The structure which results from the interaction of gums with water and other ingredients has a direct bearing on the rate of melting and appearance of the melted product. The meltdown appearance function is outcome to the role of stabilizer with respect to protecting against serum separation and fat destabilization in that by reducing the degree of destabilization of fat and protein, stabilizers canavoid the development of curdy appearance in melted product. 5. Effect on sandiness Sandiness is caused by lactose,the defect was more prevalent 50-60 years ago and is rarely seen today. This is because of the type of stabilizers used these days (CMC, natural gums,carrageenan, etc.), which functions in a manner similar to control of ice crystal size through decreasing the mobility of unfrozen water during heat shock. The supersaturated stages in which sugars responsible for sandiness are prone to crystallize in frozen desserts are reached at temperature associated with a high stabilizer concentration. This encourages the distribution of the crystallizing lactose over a large number of small crystals rather than a lesser number of larger crystals which ultimately grow to a size where they can be perceived. Microcrystalline cellulose is particularly effective in this function, probably by providing an additional seeding function which encourages the development of small crystals. 6. Effect of stabilizer during ageing Ageing of ice cream mix is an important processing step with regard to stabilizer action. Hydration of stabilizer is the most obvious effect of ageing, although not all stabilizers require ageing for complete hydration. The effects of ageing are more pronounced with gelatin stabilizers, and improvements in ice cream and freezing performance becomes more evident as the ageing time increases from 4 to 12 h or more . Even mixes containing stabilizers which require no ageing for complete activation benefit from a certain minimum ageing due to milk protein hydration and fat crystallization. Emulsifires
Emulsifier are surface active agents known to improve the sensory quality of ice cream by aiding the whipping process, improving air cell distribution and enhancing the products heat shock resistance. They also impart a dry appearance and stiffness or ‘stand up’ property to the product being extruded from the freezer. These effects are brought about by the inter facial properties of emulsifiers. Their ability to reduce surface tension seems to promote development of smaller but numerous air cells.
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The primary effect of emulsifiers in ice cream is related to their ability to de-emulsify the fat globule membrane formed during homogenization. This de-emulsifications arising from the disruption of the fat globule membranes during freezing, facilitates the agglomeration and coalescence of fat globules,leading to partial churning out of the fat phase. The agglomerated fat globules stabilize air cells. Thus, emulsifiers are used to improve the whipping qualities of ice cream by producing smallerice crystals and smaller air cells, resulting in a smoother ice cream texture and drier, stiffer ice cream. Generally, a mixture of high and low hydrophile-lipophile balance (HLB) emulsifiers, such as mono- and diglycerides and polysorbate 80 are used. HLB concept was put forth by Griffin in the year 1949. HLB number ranges from 0 to 20. The HLB number is derived by calculating the proportion of the molecular weight of the emulsifier molecule represented by the hydrophilic portion and dividing that value by 5. The HLB system can be useful in describing an emulsifiers general characteristics; however, it is not precise enough to be applied as a tool in identifying an exact HLB number which will exactly match a specific emulsion need.
Selection The following criteria are used in choosing an emulsifier
Fat percentage of mix Type of frozen dessert Effect on flavour of product Cost Composition of fat in mix Compatibility with stabilizer used Type of freezer used Method of processing – homogenization Formulation – effect of other ingredients Legal standards
Mechanism of action The two phase emulsion is stabilized by casein micelles adsorbed at the fat globule serum interface in the homogenized ice cream mix. But when an emulsifier such as a monoglycerideis used, the fat globules are covered with an emulsifier layer and the milk proteins form an outer layer. Monoglycerides effectively compete with protein if they form crystals at the interface. Hence, the outer protein layer tends to be repelled from the fat globules. As a result, the emulsion is prone to destabilization by mechanical action during freezing. This destabilizing effect, in conjunction with other processing factors, is considered to be primarily responsible for development of the desired product structure. The dry appearance of ice cream coming out of the freezer is believed to be caused by several phenomena, one of which is an emulsifier induced clustering of fat globules at the liquid air interface. Also, more air cells of smaller size foaming on account of presence of emulsifiers appear to provide more surface with the available liquid, which implies that the liquid is spread over a larger area. 112 | P a g e
The de-emulsification effect of emulsifiers is related not only to the quantity of emulsifiers used but also to the fat content of the product. With increasing emulsifier concentration, fat deemulsification is enhanced and beyond a certain limit, greasy texture and short body result due to butter formation in the freezer. Higher fat levels magnify this de-emulsification phenomenon. Thus, an emulsifier level just enough to provide the correct amount of ‘partial churning’ is necessary, and less emulsifier is required in a high-fat ice cream than in a low fat one. Excessive emulsifier may promote slow melting and curdy meltdown.
Influence on mix and ice cream 1) Effect of type of an emulsifier Different emulsifiers differ structurally, and so their action in ice cream differs, too. Obviously, therefore, the quantity of emulsifier required varies with the type of emulsifier used. Mono- and diglycerides are often used in combination, although the former is more effective. When unsaturated fatty acids such as oleic acid are present in the molecule, these emulsifiers promote better dryness at drawing from the freezer.The fat destabilization responsible for stiffness of the frozen mix is in the decreasing order for mono laurate,monooleate, and mono stearate in that sequence used at 0.1-0.2%, mono- and diglycerides are less likely to cause churning. However, their drying and whipping ability is somewhat limited as compared to polysorbates which are water soluble components of sorbitol. Tween 80 or polysorbate 80 (polyoxyethylene sorbitan monooleate) has a high drying power and aids in heat shock resistance, but the unsaturated fatty acid may undergo auto oxidation to develop an off flavour,particularly when a certain level is exceeded. Used usually at 0.02 to 0.06%,Tween 80 in excess may cause churning especially in soft serve and high fat ice creams. Another polysorbate, Tween 65 (polyoxyethylene sorbitan tristearate) has a little lower drying power as compared to Tween 80 but has excellent whipping properties. Because of its flavour stability, Tween 65 can be used at higher levels (e.g. 0.1%) without any adverse effect on the products flavour. The fat destabilizing ability of different polysorbates is in the order Tween 80> Tween 40 (polyoxyethylene sorbitan monopalmitate) > Tween 60 (polyoxyethylene monostearate)> Tween. 2) Mix processing in relation to emulsifier action It is necessary that for an emulsifier to be effective in ice cream mix, the latter has to be homogenized. While milk protein, casein in particular helps achieve size reduction of fat globules by homogenization,presence of emulsifiers promote greater size uniformity as, for example, mix containing a monoglyceride displays upon homogenization, a very narrow band of size distribution (about 1µ) as compared to mix without monoglyceride (1-5 µ).While homogenization of ice cream mix helps to stabilize the oil-in-water emulsion, it is this stabilizing effect in conjunction with the destabilizing effect of the emulsifier that results in the desirable texture quality. The greatly decreased fat globule size coupled with controlled deemulsification caused by emulsifiers, in essence, brings about the development of the right kind of structure and corresponding texture in ice cream. During ageing of ice cram mix, milk fat crystallizes, much of crystallization taking place during the first hour. Presence of emulsifiers leads to more extensive fat crystallization. The emulsifier caused protein desorption from fat globules is time dependent and takes place during ageing. Further, the initial desorption from the surface of the fat globules, in the presence of emulsifiers, 113 | P a g e
occurs as the removal of a coherent protein layer rather than individual casein particles. The process of deemulsificaiton continues into the freezer to yield the desired body and texture. Emulsifiers and stabilizers contribute to a great extent, to the desired body and texture characteristics of ice cream and frozen desserts. The mechanisms of their actions have been studied extensively in recent times and their role in conjunction with various processing steps have been delineated by various workers. It is well recognized that individual compounds vary considerably in their emulsifying/ stabilizing effects and often a single compound is not entirely satisfactory. Thus, a mixture of two or more emulsifiers/ stabilizers is generally preferred to overcome the drawbacks of individual compounds.