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Lesson 14. PHYSICO-CHEMICAL CHANGES TAKING PLACE DURING MANUFACTURE OF CONDENSED MILK-II
PHYSICO-CHEMICAL CHANGES TAKING PLACE DURING MANUFACTURE OF CONDENSED MILK-II
During the processing of evaporated and sweetened condensed milks the physico-chemical changes that take place are further elaborated here.
14.2.1 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.
14.2.2 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 NH2 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.
14.2.3 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.
14.3 Cooked Flavor
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.
14.3.1 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.
14.4 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.
14.4.1 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 PO4 content in the milk and thus provides possibility of precipitation. This can be controlled by addition of casein stabilizer.
14.4.2 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. Table 14.1 below shows this effect.
14.5 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 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.
14.6 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 CO2 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.
14.7 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 as shown in Table 14.2.
Table 14.2 Effect of storage, time & temperature on acidity of evaporated milk
This increase in acidity is mainly due to chemical changes as there was no increase in microorganisms during storage.
14.8 Sweetened Condensed Milk
14.8.1 Age thickening
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.
14.8.2 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.
14.8.3 Oxidative changes
These changes can be prevented by keeping head space oxygen to a minimum.
14.8.4 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.
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.