Casein heat coagulation

Fig. 10.15. Heating of milk. 1-3 Pasteurization 1 high temperature treatment, 2 short time and 3 long time heat treatment 4 and 5 UHT treatment 4 indirect and 5 direct 6 sterilization, a Killing pathogenic microorganisms Tubercle bacilli as labelling organism), b c inactivation of alkaline/acid phosphatase, d, d2, d denat-uration (5, 40, 100%) of whey proteins, e casein heat coagulation,/ start of milk browning...

Cross-linking of proteins. Covalent cross-linking of caseins is evident (by gel electrophoresis) after even 2 min at 140°C and it is not possible to resolve the heat-coagulated caseins by urea- or SDS-PAGE. 

All the heat-induced changes discussed would be expected to cause major alterations in the casein micelles, but the most significant change with respect to heat coagulation appears to be the decrease in pH - if the pH is readjusted occasionally to pH 6.7, milk can be heated for several hours at 140°C without coagulation. The stabilizing effect of urea is, at least partially,... 

Pyne, G. T. 1958. The heat coagulation of milk II. Variations in sensitivity of casein to calcium ions. J. Dairy Res. 25, 467-474. 

Raw milk is standardized to the proper fat and total milk solids content to produce a final product with a minimum of 50% fat on a solids basis and <39% moisture (CFR 1982 Packard 1975). Cheese is made from pasteurized or raw milk, but raw milk cheese must be aged a minimum of 60 days at >1.7°C (CFR, 1982). Minimum temperature and time combinations are normally used for pasteurization of milk for cheese manufacture in order not to interfere with casein micelle coagulation and curd formation. Milk is sometimes heated only to subpasteurization temperatures to dispel dissolved gases, reduce bacterial populations, and kill certain pathogens, thus resulting in a cheese product with improved flavor (Babel 1976). 

Table 3-7 Heat Coagulation Temperatures of Some Albumins and Globulins and Casein...

This review will concentrate on those varieties produced by coagulation at pH values close to the isoelectric point of casein at 20-40°C (e.g., Quarg, Fromage frais, cream cheese) acid-heat coagulated fresh cheese varieties (e.g., Ricotta) were reviewed by Torres and Chandan (1981a,b). 

However, temperature and pH strongly affect casein association and cause changes in micellular structure (cf. 10.1.2.1.2 and 10.1.2.1.3). An example of such a change is the pH-dependent heat coagulation of skim milk. The coagulation temperature drops with decreasing pH (Fig. 10.16 and 10.9). Salt concentration also has an influence, e. g., the heat stability of milk decreases with a rise in the content of free calcium. 

In Solid Form. Dissolve if possible in water or weak alkalies, and apply tests in above order to the solution. If the protein will not dissolve it is probably casein or heat-coagulated protein. Apply the colour tests directly to the solid material. 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

Casein is very stable to high temperatures milk may be heated at its natural pH (c. 6.7) at 100°C for 24 h without coagulation and it withstands heating at 140°C for up to 20 min. Such severe heat treatments cause many changes in milk, e.g. production of acids from lactose resulting in a decrease in pH and changes in the salt balance, which eventually cause the precipitation of casein. The whey proteins, on the... 

The phosphate of casein is an important contributor to its remarkably high heat stability and to the calcium-induced coagulation of rennet-altered casein (although many other factors are involved in both cases). 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

When heated in the presence of whey proteins, as in normal milk, K-casein and /Mactoglobulin interact to form a disulphide-linked complex which modifies many properties of the micelles, including rennet coagulability and heat stability. 

Ca2+ and Mg2 +, along with H+, play especially important roles in the stability of the caseinate system and its behaviour during milk processing, especially in the coagulation of milk by rennet, heat and ethanol. The... 

Although CCP represents only about 6% of the dry weight of the casein micelle, it plays an essential role in its structure and properties and hence has major effects on the properties of milk it is the integrating factor in the casein micelle without it, milk is not coagulable by rennet and its heat and calcium stability properties are significantly altered. In fact, milk would be a totally different fluid without colloidal calcium phosphate. 

On heating at temperatures above 100°C, lactose is degraded to acids with a concomitant increase in titratable acidity (Figures 9.5, 9.6). Formic acid is the principal acid formed lactic acid represents only about 5% of the acids formed. Acid production is significant in the heat stability of milk, e.g. when assayed at 130°C, the pH falls to about 5.8 at the point of coagulation (after about 20 min) (Figure 9.7). About half of this decrease is due to the formation of organic acids from lactose the remainder is due to the precipitation of calcium phosphate and dephosphorylation of casein, as discussed in section 9.4. 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of / -lg with K-casein, and probably as2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

The current explanation for the maximum-minimum in the HCT-pH profile is that on heating, K-casein dissociates from the micelles at pH values below about 6.7, /Mg reduces the dissociation of K-casein, but at pH values above 6.7, it accentuates dissociation. In effect, coagulation in the pH range of minimum stability involves aggregation of K-casein-depleted micelles, in a manner somewhat analogous to rennet coagulation, although the mechanism by which the altered micelles are produced is very different. 

Decrease in pH. After heating at 140°C for 20 min, the pH of milk has decreased to about 5.8 due to acid production from pyrolysis of lactose, precipitation of soluble calcium phosphate as Ca3(P04)2, with the release of H+, and dephosphorylation of casein with subsequent precipitation of the liberated phosphate as Ca3(P04)2 with the release of H+. The heat-induced precipitation of Ca3(P04)2 is partially reversible on cooling so that the actual pH of milk at 140°C at the point of coagulation is much lower than the measured value and is probably below 5.0. 

Hydrolysis of caseins. During heating at 140°C there is a considerable increase in non-protein N (12% TCA-soluble), apparently following zero-order kinetics. K-Casein appears to be particularly sensitive to heating and about 25% of the JV-acetylneuraminic acid (a constituent of K-casein) is soluble in 12% TCA at the point of coagulation. 

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