Modifications in milk proteins induced by heat treatment and homogenization and their influence on susceptibility to proteolysis

The influence of the sequence of processing steps on protein integrity and susceptibility to proteolysis of skimmed and whole milks was examined. Laboratory-scale devices were used to homogenize and heat treat the milk. The results were confirmed in skimmed and whole milks processed in a direct ultrahigh temperature (UHT) pilot plant, with and without a homogenization step following heat treatment. The effects of homogenization varied depending on whether it preceded or followed heating. In general terms, homogenization slightly enhanced whey protein denaturation. But, although homogenization increased the concentration of micellar -lactoglobulin (beta-Lg) in whole milks, it led to high levels of soluble denatured beta-Lg in skimmed milks. Homogenization after heating had a promoting effect towards proteolysis in skimmed milk, with kappa-casein suffering the highest degradation. This was probably because these samples had high levels of soluble beta-Lg and kappa-casein that could have enhanced the susceptibility to proteolysis. Whole milk samples presented the lowest proteolysis levels, probably because of an increased complex formation between kappa-casein and beta-Lg. (C) 2002 Elsevier Science Ltd. All rights reserved.