Disulfide reactivity and in vitro protein digestibility of different thermal-treated milk samples and whey proteins

The susceptibility of disulfides to reduction and the in vitro protein digestibility were analyzed in ram, pasteurized, ultrahigh-temperature (UHT)-treated, and sterilized milk samples and whey proteins. Both parameters increased from raw or pasteurized to UHT-treated or sterilized milk samples. Whey proteins had impaired digestibility compared with that of the corresponding whole milk samples, especially when extracted from UHT-treated and sterilized milk samples. The amino acid composition of proteins in whey indicated that the ratio of whey proteins to casein gradually decreased in the extracts from raw or pasteurized to sterilized milk samples. Disulfide reactivity of whey proteins was inversely related to the intensity of thermal treatment of milk. The results were consistent with the progressive heat-induced increase in the amount of highly stable proteins in whey from raw to sterilized milk samples. Stability of whey proteins was Likely either intrinsic in the native conformation or conferred during heating by thiol-disulfide interchange reactions also involving K-casein. The possibility of using disulfide reactivity as a parameter in the characterization of thermal-treated milk is considered.