715. The relation between the chemical composition of milk and the stability of the caseinate complex: IV. Coagulation by heat

1. The variation in the stability of milk protein to heat and the relationship between milk composition and heat stability were examined.

2. The coagulation times of the majority of the milk samples decreased by a factor of about 3 with an increase in temperature of 10°C. over the range 130–150°C. Because of the general proportionality of the coagulation times at 130, 140 and 150°C., the coagulation time at 130°C. only were used as a measure of the stability of the samples to heat.

3. The coagulation times of herd bulk milks ranged from 17·2 to 59·0 min. at 130°C., whereas the range for samples from individual cows was 0·6–86·2 min.

4. Samples of colostrum were very unstable to heat, and milk from cows in late lactation tended to have the longest coagulation times, but otherwise there was little relation between the heat stability of milk and the stage of lactation of the cow.

5. Although colostrum samples were comparatively rich in ionized calcium, their marked instability to heat appeared to be caused solely by their high content of lactalbumin plus lactoglobulin.

6. The stability to heat of the calcium caseinate-calcium phosphate complex in all samples, other than colostrum, could not be closely related either to the concentration and composition of the complex or to the composition and salt-balance of the aqueous phase.

7. When the calcium phosphate content of the caseinate complex was relatively low, the heat stability of the complex tended to be inversely related to the concentration of ionized calcium in the milk, but in general coagulation time was not related to the concentration of ionized calcium.

Interaction between heated κ-casein and β-lactoglobulin: predominance of hydrophobic interactions in the initial stages of complex formation

Mixtures ofκ-casein and β-lactoglobulin (β-lg) were heated at 70 °C in 20 mM-imidazole buffer, pH 6·8 containing 20 mM-EGTA. Aggregation of theκ-casein/β-lg mixture occurred within 90 s and susceptibility to hydrolysis by chymosin decreased significantly within 180 s even though covalent interaction was not detected until after 4000 s. UV-absorbance indicated initial structural destabilization of the heatedκ-casein/β-lg mixture followed by a return ( > 350 s) to a spectrum comparable to the native state, indicating molecular rearrangement. Apparent hydrophobicity ofκ-casein decreased 80% within 250 s compared to a 38% decrease forβ-lg. Under similar conditions, theκ-casein/β-lg mixture (1:1) showed a faster (2·5 times) decrease in apparent hydrophobicity thanκ-casein alone with concomitant exposure of acidic (hydrophilic) groups. The results suggested that the driving force for the rearrangement was mainly hydrophobic, i.e. entropic in origin. The tendency of heated and subsequently cooledκ-casein/β-lg to aggregate reached a maximum after heating at 70 °C for 720 s.