Heat-induced, pH-dependent dissociation of casein micelles on heating reconstituted skim milk at temperatures below 100 degrees C

The effect of pH (6.3-7.1) and temperature (20-90 degrees C) on the dissociation of casein from the micelles in reconstituted skim milk was investigated. These pH conditions encompass those naturally found, whereas heat treatments below 100 degrees C are commonly encountered during the processing of milk and milk products. Low levels of casein were rendered soluble at pH below 6.7 regardless of heating temperature, whereas increasing levels of casein were solubilized as the pH was increased from 6.7 to 7.1. This pH-dependent dissociation of the casein micelles showed an unusual dependence on temperature. Low levels of casein were dissociated at 20 degrees C at all pH values. The quantity of casein solubilized increased with temperature to a maximum dissociation at about 70 degrees C and then decreased at higher temperatures. The dissociation behavior of alpha(S)-casein and beta-casein at pH greater than or equal to 6.7 showed dependence on temperature similar to that of the total casein. In contrast, above pH 6.5, the dissociation of kappa-casein increased essentially linearly with increasing temperature over the entire temperature range studied. The proportion of beta-casein in the soluble casein was essentially constant regardless of the temperature and pH, whereas the proportions of alpha(S)-casein and kappa-casein varied with both temperature and pH. The results of this study have indicated that, at certain pH, a marked dissociation of protein from the casein micelles occurs on heating at temperatures below 100 degrees C; this phenomenom has not previously been reported to occur under such mild heating conditions.