CONDENSED AND DRIED MILKS

Module 6. Heat stability of milk and condensed milk

Lesson 16
HEAT STABILITY OF CONDENSED MILK

16.1 Introduction

Heat stability of milk, concentrated and dried milks is of commercial importance. The term heat stability of evaporated milk refers to the relative resistance of the milk to coagulations in the sterilizer. Webb Bell and Deysher defined heat stability as “the time necessary to initiate coagulation at 115°C (239ºF)”.

16.2 Importance

1. Heat stability is important in the processing operation of concentrated product manufacture.

2. In the manufacture of recombined evaporated milk from anhydrous milk fat and skimmed milk powder, heat stability adjustment is important before sterilization. This is because of the convenience with which milk may now be stored and transported in the form of anhydrous milk fat and skimmed milk powder.

3. In the manufacture of recombined UHT milks

4. In the manufacture of beverages e.g., coffee whiteners – must not coagulate or cause “feathering” when added to coffee usually under adverse conditions of temperatures and pH.

5. In the manufacture of low heat/high heat milk powders and used in admixture with fresh fluid milks.

6. In the manufacture of skim milk powders, stability of ‘instant’ dried skim milk when added to hot coffee showed that the formation of insoluble material was inversely related to the maximum coagulation time of the reconstituted powder measured at 140°C and directly related to the casein number of milk protein (i.e. % of milk protein precipitated at pH 4.6).

16.3 Heat Stability & Concentrated Milks

• Although evaporated milk is produced in quantity, the process is very arbitrary.

• Sommer & Hart (1926) found that apart from the destabilizing affect of albumin, the heat stability of the product is controlled by the ratio of calcium and magnesium to phosphate and citrate. Adjustment of the ratio by the addition of these salts, mainly phosphates, permits successful sterilization of the product in most cases. This ‘salt balance’ theory still remains as the major basis of the commercial sterilization of evaporated milk.

• Powders made from milk which had not been preheated and which had a high level of undenatured whey protein nitrogen generally have a lower heat stability on reconstitution than had preheated powders. This reduction is even more marked when the powder is recombined as 18 % SNF, 8 % fat evaporated milk. In contrast, when the powders are reconstituted to the total solids level of the skim milk from which the powder was derived, the reverse effect is obtained and the non-preheated milk gives higher heat stability times. This effect is significant in the selection of powders for the preparation of sterilized milks.

• The heat stability of the recombined product is, therefore determined by the skimmed milk powder, homogenization, and the final pH adjustment using phosphate stabilizers before sterilization. This creates a more complex situation as the combined heat stability characteristics of both the imported skimmed milk powder and the local milk supply will determine the suitability of the final product for sterilization.

• In general, heat stabilization of 15 min at 120°C can be taken as guide indicating adequate stability for commercial sterilization.

16.4 Optimum Heat Stability

The problem is not of achieving maximum heat stability at the cost of other defects but is to obtain optimum heat stability as to keep desired viscosity, body and texture. The maximum heat stability will eliminate the danger of curdling during sterilization but it would yield evaporated milk so lacking in body, i.e. viscosity that it would cause the milk to be “Rough” after sterilization. The stability may range from a few minute beyond the time held at sterilizing temperature to slightly over double the time held at sterilizing temperature.

16.5 Factors Influencing Heat Stability

The factors that influence heat stability of evaporated milk are related to the inherent properties of the milk such as chemical composition and freshness and the process of manufacture. The more important factors are:

16.5.1 Acidity

In case of average normal milk, any increase of developed acidity has adverse effect on the heat coagulation of milk or heat stability of milk. This is true of the titratable acidity and of pH. It is further proved that the optimum acid reaction for maximum heat stability is dependent upon its relation to several variables of milk, such as salt balance and amount of milk proteins. Hence the optimum acid reaction for one lot of milk may or may not apply to any other lot of milk. The normal range of titratable acidity of freshly drawn milk is 0.15 to 0.16%. The sensitiveness of heat stability of milk and evaporated milk emphasizes the importance of getting fresh milk at condenser.

16.6 Effect of Concentration and Sterilizing Process on Acidity

The acidity of evaporated milk is higher than that of fresh milk from which it is made. The titratable acidity of evaporated milk before sterilization is approximately equal to that of fresh milk times the ratio of concentration. The acidity of milk after sterilization is more than that of before sterilization. This is attributed to the formation of acid resulting from the action of processing heat on some of the milk constituents. The pH of evaporated milk likewise is lower than of fresh milk. Fresh milk averaged approximately pH 6.6 while evaporated milk falls in range of pH 6.15 to 6.3

16.7 Milk Proteins

The nitrogenous constituents of milk are the pivot on which heat stability centres. They are the substances which are coagulated. The relation of all other ingredients or factors to the heat coagulation of milk is important only in so far as they influence the sensitiveness of the major proteins of milk to heat. Proteins of greatest importance here are casein and lactalbumin. Both are coagulated by heat. It is reported that 68.6% of total albumin found in fresh milk is precipitated in evaporated milk after sterilization. In normal evaporated milk of correct viscosity, the casein coagulated does not exceed 10% of total present.

• The casein is of primary importance. It represents 80% of the total proteins in milk. It curdles at ordinary temperature by acid. In fresh sweet milk, it will coagulate at 136°C or higher but even at flash pasteurizing temperature it loses its power to react normally to rennet. The condensing of milk lowers heat coagulation point of casein. The heat stability of casein is greatly influenced by the balance of mineral salts of milk. Webb stated “the problem of heat coagulation of milk is in reality a problem of heat stability of calcium caseinate system”.

• The lactalbumin is present to the extent of 15% of the milk proteins. It is partially coagulated by heat in normal milk and completely in the presence of an acid medium. It is neither affected by rennet, nor by acid at ordinary temperature nor, by the mineral constituents of milk.

• When forewarming is kept above 90°C, the initial viscosity and the increased viscosity are in direct relation to the amount of albumin added. But when forewarming temperature is held below 60°C, the initial viscosity remains the same and the changes which are small have no relation to the albumin content of milk.

• Sommer demonstrated that even a small increase in lactalbumin content greatly reduces the time of sterilization to coagulate the evaporated milk.

• Both the casein and albumin content of milk vary with season of year due to change in lactation period of animals. Variation is greater in case of albumin content as compared to casein. The percent of casein and albumin is higher at the beginning and towards the end of lactation period. Colostrum milk tends to decrease the heat stability of the milk.

• Thus, the effects of protein composition specifically concern the ratio between κ-casein and β-lactoglobulin. The larger the amount of β-lactoglobulin, the higher the maximum HCT and the lower the minimum. This is explained by β-lactoglobulin enhancing the dissociation of κ-casein at pH >6.7, which results in formation of more strongly depleted micelles. The higher maximum HCT at pH 6.6 may result from an increased association of β-lactoglobulin with the micelles, which may enhance colloidal repulsion.

16.8 Products of Bacterial origin other than Acidity

Bacterial contamination of milk may lower the heat coagulation point of evaporated milk through activities other than acid formation. Certain species of bacteria produce rennin and rennet like enzymes. Heavy contamination with this type of organisms is capable of lowering the heat coagulation temperature. Thus, bacteria causing sweet curdling when present in large number will lower heat stability of milk.

16.9 Mineral Salts Balance

The temperature at which evaporated milk curdles in the sterilizer is affected, and to a larger extent controlled by the balance of the milk salts.

16.10 Sommer and Hart’s Theory of the Salt Balance

They were first to demonstrate that aside from albumin, salt balance has a greater effect on heat stability and salt balance is readily changed by other changes like acidity, thus affecting, coagulation temperature.

• It was observed that casein has maximum heat stability when in combination with a definite optimum amount of calcium. When the Ca++ content available for the calcium casein complex is above or below this optimum combination, the casein is less stable to heat. The calcium contained in the milk distributes itself between casein, phosphates and citrates. In addition, the Mg present reacts by replacing the Ca in the PO4 and citrates. The effect of calcium and Mg being basic radicals is opposed to the effect of PO4 and citrates which are acid radicals. The calcium casein combination is at its optimum of heat stability when above two groups of mineral salts are in balance, hence the term salt balance. An excess or deficiency of either group accelerates heat coagulation.

• If coagulation in heat test is due to deficiency of Ca and Mg, it can be prevented by the addition of proper amount of soluble Ca or Mg salts such as Ca and Mg acetates or chlorides. Such milk may also be stabilized by a slight increase in acidity because the increased acidity changes secondary PO4 to primary PO4 and the primary PO4 have little or no effect on the salt balance. This change diminishes the amount of PO4 that ties up the Ca and more Ca++ is available to satisfy the Ca++ - casein equilibrium. In such cases slight increase in acid thus improves the Ca-casein balance and actually raises the heat coagulation point.

• If troublesome heat coagulation is due to high Ca++ and Mg++ it can be prevented by addition of the proper amount of PO4 or citrate such as di-sodium phosphate or sodium citrate.

• It is found in most cases that low heat stability is due to excess of Ca and Mg. Hence the heat stability is mainly controlled by exclusive addition of salts of PO4 and citrates. And since PO4 is cheaper of the two, it is preferred over citrate.

• The variation in concentration of the ions such as buffer salts as PO4, citrate especially those of citrate have a greater effect on the coagulation temperature than slight variation in the pH of normal milk within range of 6.58 to 6.60. But both factors must be considered.

• It is concluded that each lot of milk represents a separate colloidal system and that for each system there is an optimum combination of salt balance, due to such factor as pH with which optimum heat stability is attained.

• Thus, the main effect of salt composition i s through the calcium and phosphate contents. The addition of a certain salt to milk can strongly disturb all salt equilibriums involved. Addition of calcium and phosphate to milk, up to the concentrations that are found in concentrated milk, causes its heat stability at pH > 6.8 to be equal to that of concentrated milk, i.e., zero.

16.11 Factors related to Process of Manufacture

In view of the problems associated with the concentrated and dried milks, the more important processing operations affecting the heat stability of these products are described below:

16.11.1 Forewarming

The temperature at which the fresh milk is forewarmed is an important factor in the control of heat coagulation of evaporated milk. For unconcentrated milk, preheating mainly causes a shift of the heat coagulation curve to lower pH values, and the stability at the pH optimum is hardly affected. But a very intensive preheat treatment (e.g. a few minutes at 150 ° C) causes heat stability to increase over the whole pH range and the minimum in the curve disappears. Perhaps, the second heat denaturation of b -Lg, which occurs near 140 ° C, is involved.

Preheating of milk at 80°C, 90°C and 120°C in the presence of aldehyde and sugar significantly increased heat stability of buffalo milk. Maximum heat stability of concentrated buffalo milk can be achieved by preheating milk at 80°C.

Concentrated (evaporated) milk hardly can be sterilized without preheating the milk before concentrating. The heat stability maximum is shifted to lower pH values and becomes higher. The pH of the milk before preheating also affects the heat stability of the evaporated milk, its optimum being, for instance, 6.45. The beneficial effect of preheating must be caused at least partly by reactions of the b -Lg, addition of this protein to the milk decreases the heat stability of evaporated milk, but this decrease can be eliminated largely by proper preheating.

Preheating at ultra-high temperatures for short times produces a ‘concentrate’ which is very stable to subsequent heat processing but the product has a low viscosity and is therefore unsuitable for the manufacture of satisfactory evaporated milk. The stability of recombined, evaporated milk is considerably improved if the original milk is preheated at 120°C for 2 min. The maximum stability occurs at the acid side of the natural pH and pH manipulation might best be done using a mixture of NaH₂PO4 and Na₂HPO₄ rather than HCI.

In practice, the pre-heating temperature for the manufacture of heat – stable skimmed milk powder, temperatures of 85-90°C with holding periods of 10-20 min have been commonly used. However, there is evidence now of the use of higher temperatures in excess of 100°C with shorter holding periods.

16.11.2 Homogenization

• Homogenization of skim milk has little effect on its heat stability, but the stability of fat – containing products is decreased by homogenization, the magnitude of the effect being dependent on the temperature at which homogenization is performed, the extent of destabilization is greatest at 60°C and least at 80°C.

• Homogenization has no significant influence on the heat stability of buffalo milk. However, after concentration, the heat coagulation time of homogenized milk decrease significantly. Homogenization causes a significant increase in the molar ratio of (Calcium + Magnesium) / (Phosphate + Citrate) which in turn destabilize homogenized concentrated milk. A reduced stability with increasing homogenization pressure from 3.5 to 34.5 MPa is observed. The destabilizing effect of homogenized milk could be partly offset by two stage homogenization (20.7 MPa followed by 3.5 MPa) or by the addition of phosphate stabilizer (0.08% w/v) or by homogenizing at high temperature (65°C).

16.11.3 Concentrated milk

• Homogenization has little effect on milk, but it renders concentrated milk far less heat stable. The detrimental effect can be offset for a considerable part by preheating milk before concentration.

• Heat stability of fat containing concentrated milk products is found to decrease by homogenization.

• Concentration (31 % TS) before homogenization result into more stable product.

• The heat stability of concentrated milk could be enhanced to a greater extent by high temperature fore warming (145°C, 5 sec), two stage homogenization and addition of sodium phosphate.

• Homogenization tends to slightly lower the heat stability of evaporated milk. This tendency increases with increasing homogenization pressure, and as the level of heat stability determined by the forewarming treatment drops.

a. With milk of high heat stability such as results from HTST forewarming at 120°C for 4 minutes, the pressure of homogenization has no effect on heat stability.

b. With the milk of normal heat stability such as result from forewarming at 90°C for 10 minutes, the homogenizing pressure lowers the heat stability only slightly.

c. With milk of low heat stability, such as results from forewarming at 65.5°C, the heat stability drops considerably with increasing homogenizing pressure.

16.11.4 Homogenizing pressure

Milk forewarmed at 90°C for 10 minutes having total solids 26.44% have heat stability as shown in Table 16.1 below

Table 16.1 Heat stability and the corresponding temperature and pressure of homogenization

Excessive homogenizing pressure that is more than 210 kg/cm² definitely lowers the heat coagulation temperature of evaporated milk.

16.11.5 Storage

Normal milks can be stored at 4°C for at least one week without significant change in heat stability, suggests that the activity of indigenous milk proteinase is of little consequence to heat stability, at least under normal circumstances. The growth of proteolytic psychrotrophs in milk on prolonged storage at refrigeration temperature does not have a significant effect on the heat stability of milk until they reach populations of ~10 CFU/ml.

16.11.6 Concentration process

· The concentration of evaporated milk has a marked effect on heat stability. As the concentration increases, the heat coagulation temperature drops. Particularly with 25.9% TS, a forewarming temperature slightly below the boiling point is suitable. At higher concentration, however, the evaporated milk becomes increasingly curdy.

· The titratable acidity of evaporated milk after sterilization calculated as lactic acid, definitely exceed that contained in the fresh milk times the ratio of concentration.

· The increased concentration of acid, albumin and casein respectively tends to lower the heat coagulation. Sommer and Heart suggested that with the higher acid reaction in the concentrated milk, there is an appreciably smaller fraction of secondary phosphates, which in turn has the effect of accentuating the unfavourable influence of the usual excess of calcium in the milk.

· For every percent of difference in concentration within the limits of 16-26% SNF, there was a change of 1.25 to 1.50°C in temperature of heat coagulation.

· A concentration below 14% SNF and high forewarming temperature (95°C) causes to lower the heat coagulation point while with concentration above 14% SNF, the same forewarming temperature increases the heat stability.

· “Low-heat stability can no longer be considered a factor which might limit evaporated milk to 26% solids content” because an improvement in the heat stability can be obtained by keeping optimum forewarming temperature at 120°C with a holding time of 3-4 minutes.

· Concentrated buffalo milk is less stable than concentrated cow milk.

· Concentration of buffalo milk result in decrease in pH and in the proportion of calcium, magnesium, phosphate and citric acid in dissolved phase and increase in the ratio of calcium / phosphate and (calcium + magnesium) / (phosphate + citric acid).

· The stability of the calcium – caseinate – phosphate complex to various coagulating agents declines on concentration.

· Concentration induces major changes in milk system such as:

(i) closer packing of casein micelles

(ii) A higher concentration of denaturable whey proteins (native whey protein > 0.9% decreased HCT)

(iii) Precipitation of calcium phosphate which also causes a significant decrease in pH. If the pH of a 9% SNF milk is 6.6, that of a 26% solids concentrate will be ~ 6.2 and of a 40% concentrate ~ 6.0.

(iv) Due to water binding by proteins, lactose and salts there is very little free water in 3:1 concentrate.

(v) Milk concentration by ultrafiltration (rather than by evaporation) is much more heat stable than evaporated milk.

(vi) Concentration increases the rate of dephosphorylation, preheating has no effect on rate of dephosphorylation of unconcentrated milk but reduce the rate for concentrated milk.

16.11.7 Additives

Urea and lactose

• Urea does not significantly increase the stability of concentrated milk (25% TS) although it does act synergistically with aldehydes.

• Addition of simple aldehydes brings about large increase in heat stability of milk over a comparatively wide pH range. Some sugars which react readily as an aldehyde on heating are also shown to stabilize concentrates at high temperature (120°C).

Alkali and salts

• Sodium hydroxide @ 0.05 to 0.10% and trisodium phosphate @ 0.075% (additions before fore warming) increase the heat stability of 18.5% to 22.5% total solids skim milk concentrates but calcium chloride has the reverse effect.

• In order to ensure concentrated milk to be heat stable, it is essential to employ stabilizing techniques before homogenization. The primary effect of stabilizer is a consequence of influence on the pH of the milk.

Salts in buffalo milk concentrates

Maximum increase in heat stability is noticed in preheated (120°C, no holding) buffalo milk concentrate (35% TS) when disodium hydrogen phosphate is added @0.3% level. Addition of trisodium citrate at the same level is also found to increase stability but to a lesser extent. Added casein has some stabilizing effect on HCT of buffalo milk concentrate (1:2).

16.11.8 Effect of heating the milk after concentration

The stabilizing effect of HTST is not confined to forewarming the fresh milk before condensing. The special heat treatment may be applied either before or after concentration of the milk but the temperature of heat treatment that reduces maximum stability is found to vary. Thus when employed as a part of forewarming process, a temperature of 120°C for 3-4 minutes yield maximum heat stability. When HTST heating is applied to the milk after concentration, a temperature of 150°C with no holding period produces maximum heat stability.

16.12 Heat Stability of Recombined Concentrated Milks

• Heat stability for recombined EM is of utmost importance. It may be defined as the time taken for milk / milk concentrate to thicken or coagulate when heated under standardized conditions. It is a particular problem in evaporated product because of the concentrated nature of the product and need for sterilization.

• The main factors affecting heat stability of recombined concentrated milks are similar to those for concentrated milks. However, concentration of milk and addition of butter oil during recombination has complicated relation with heat stability which is not yet fully understood.

• Correction for improvement of heat stability is achieved by the similar methods as listed for the concentrated milks. However, heat treatment or addition of stabilizing salts such as phosphates together with dried buttermilk is in practice and regarded as most successful.

• As heat stability of concentrated milks manufactured from whole milk or powders varies with season and stage of lactation, attempts have been made to extend the period over which heat stable powders can be manufactured.

The addition of sweet cream buttermilk powder before heat treatment increases overall heat stability, perhaps due to its high phospholipids content. Similarly, incorporation of soya lecithin has marked positive influence on heat stability of recombined evaporated milk.