HEAT-INDUCED, PH-DEPENDENT DISSOCIATION OF CASEIN MICELLES ON HEATINGRECONSTITUTED SKIM MILK AT TEMPERATURES BELOW 100-DEGREES-C

The effect of pH (6.3-7.1) and temperature (20-90 degrees C) on the di ssociation of casein from the micelles in reconstituted skim milk was investigated. These pH conditions encompass those naturally found, whe reas heat treatments below 100 degrees C are commonly encountered duri ng the processing of milk and milk products. Low levels of casein were rendered soluble at pH below 6.7 regardless of heating temperature, w hereas increasing levels of casein were solubilized as the pH was incr eased from 6.7 to 7.1. This pH-dependent dissociation of the casein mi celles showed an unusual dependence on temperature. Low levels of case in were dissociated at 20 degrees C at all pH values. The quantity of casein solubilized increased with temperature to a maximum dissociatio n at about 70 degrees C and then decreased at higher temperatures. The dissociation behavior of alpha(S)-casein and beta-casein at pH greate r than or equal to 6.7 showed dependence on temperature similar to tha t of the total casein. In contrast, above pH 6.5, the dissociation of kappa-casein increased essentially linearly with increasing temperatur e over the entire temperature range studied. The proportion of beta-ca sein in the soluble casein was essentially constant regardless of the temperature and pH, whereas the proportions of alpha(S)-casein and kap pa-casein varied with both temperature and pH. The results of this stu dy have indicated that, at certain pH, a marked dissociation of protei n 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.