Heat stability of milk

The maximum HCT and the shape of the HCT-pH profile are influenced by several compositional factors, of which the following are the most significant  

and probably a-la, increase the stability of casein micelles at pH 6.4-6.7 but reduce it at pH 6.7-7.0 in fact, the occurrence of a maximum-minimum in the HCT-pH profile depends on the presence of -Ig- 

Addition of K-casein to milk increases stability in the pH range of the HCT minimum. 

Reducing the level of colloidal calcium phosphate increases stability in the region of the HCT maximum. 

Natural variations in HCT are due mainly to variations in the concentration of indigenous urea due to changes in the animals feed. 

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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. 

Acid phosphomonoesterase Hydrolysis of phosphoric acid esters Reduce heat stability of milk cheese ripening... 

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 heat stability of milk is increased by the Maillard reaction, probably via the production of carbonyls (section 9.7). 

Concentration. Concentration by thermal evaporation markedly reduces the heat stability of milk, e.g. concentrated skim milk containing about 18% total solids coagulates in roughly 10 min at 130°C. The stability of the concentrate is strongly affected by pH, with a maximum at around pH 6.6, but stability remains low at all pH values above about 6.8 (Figure 9.20). Concentration by ultrafiltration has a much smaller effect on HCT than thermal evaporation, due to a lower concentration of soluble salts in the retentate. 

Singh, H. and Creamer, L.K. (1992) Heat stability of milk, in Advanced Dairy Chemistry, Vol. 

Fox, P. F. 1981B. Heat stability of milk Significance of heat-induced acid formation in coagulation. Irish J. Food Sci. Technol. 5, 1-11. 

Fox, P. F. and Hearn, C. M. 1978A. Heat stability of milk Influence of dilution and dialysis against water. J. Dairy Res. 45, 149-157. 

Holt, C., Muir, D. D. and Sweetsur, A. W. M. 1978. Seasonal changes in the heat stability of milk from creamery silos in south-west Scotland. J. Dairy Res. 45, 183-190. 

Kudo, S. 1980C. The heat stability of milk Formation of soluble proteins and protein-depleted micelles at elevated temperatures. N.Z. J. Dairy Sci. Technol. 15, 255-263. 

Rose, D. 1962B. Factors affecting the pH-sensitivity of the heat stability of milk from individual cows. J. Dairy Sci. 44, 1405-1413. 

We chose to prepare 14C-methyl-K-casein (M-k-C) as a tracer because of the important role of K-casein in stabilizing casein micelles (8) and because K-casein is known to participate in heat-induced interactions with whey proteins, thereby influencing the heat stability of milk (9). The reductive methylation radiolabeling procedure used low concentrations of reagents (10) and resulted in M-k-C containing approximately 1 fiinol of 14C-methyl groups for every micromole of protein monomer (about 3 /xCi/mg). When tracer M-k-C was added to skim milk, and trichloroacetic acid was added to a concentration of 2%, about 1% of the radioactivity remained soluble. After clotting of the milk with excess... 

Effect of Forewarming and Concentration on 14C-Methyl-/ -Lacto-globulin Distribution in Milk. The stability of milk that is to be concentrated before sterilization is improved by a preliminary heat treatment (forewarming), but concentrated milk may be destabilized by the same heat treatment. An interaction of serum proteins with x-casein may be involved more detail is available in a recent review on the heat stability of milk (17). 

It should be emphasized that the foregoing experiments were not designed to study the mechanisms of heat-induced protein interactions in milk. They merely serve to illustrate the utility of radiolabeled milk proteins in studying this very complex phenomenon. The radiolabeling of other milk proteins along with double-labeling experiments should prove very instructive in future research on the heat stability of milk. 

Effect of Urea on the Distribution of 14C-Methyl-/ -Lactoglobulin or 14C-Methyl-/c-Casein in Heated Milk. Muir and Sweetsur (25) exploited the interesting observations by Robertson and Dixon (26) that the heat stability of milk proteins is related to the urea level of the milk, and that adding relatively small amounts of urea increased the heat stability of milk (27). To examine the urea effect further, we added urea to milk containing either M-/3-L or M-k-C and measured the ultracentrifugal distribution of C-14 after heat treatment. The experiments using... 

These studies demonstrate that treatments that have been reported to modify the heat stability of milk also may change the distribution of tracer milk proteins between the physical phases of milk. Analysis of these interactions may provide useful information about the mechanisms of these effects. In unpublished experiments, we have extended this approach for studying homogenized milk-based systems, also containing a lipid phase, to investigate lipid-protein interactions. Dual-label experiments, for example in milk containing M-/3-L and 3H-methyl-/c-casein, could be applied to the isolation and characterization of protein complexes in milk. Labeled caseins could prove valuable as probes for elucidating micelle structure. 

Figure 3-4 Reactions Involved in Sulfhydryl Polymerization of Proteins. Source From O. Kirchmeier, The Physical-Chemical Causes of the Heat Stability of Milk Proteins, Milchwis-senschaft (German), Vol. 17, pp. 408-412,1962.

Singh, H., and Latham, J.M. (1993). Heat stability of milk aggregation and dissociation of protein at ultra-high temperatures. Int. Dairy J. 3,225-237. 

It is well known that the addition of soluble calcium salts reduces the heat stability of milk, whereas the addition of calcium complexing agents with the appropriate control of pH improves heat stability. Phosphates and citrates have often been used to increase the heat stability of concentrated milks (Augustin and Clarke, 1990 Pouliot and Boulet, 1991 Sweetsur and Muir, 1982a). A reduction in activity by the addition of these salts contributes to the improved heat stability of concentrated milks, but the effects of salts on the equilibrium of caseins between the serum and micellar phases of milk also affect heat stability. 

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