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-temperatur e (UHT)-treated, and sterilized milk samples and whey proteins. Both p arameters increased from raw or pasteurized to UHT-treated or steriliz ed milk samples. Whey proteins had impaired digestibility compared wit h that of the corresponding whole milk samples, especially when extrac ted from UHT-treated and sterilized milk samples. The amino acid compo sition of proteins in whey indicated that the ratio of whey proteins t o casein gradually decreased in the extracts from raw or pasteurized t o sterilized milk samples. Disulfide reactivity of whey proteins was i nversely related to the intensity of thermal treatment of milk. The re sults were consistent with the progressive heat-induced increase in th e amount of highly stable proteins in whey from raw to sterilized milk samples. Stability of whey proteins was Likely either intrinsic in th e native conformation or conferred during heating by thiol-disulfide i nterchange reactions also involving K-casein. The possibility of using disulfide reactivity as a parameter in the characterization of therma l-treated milk is considered.