HEAT-INDUCED CHANGES IN THE WHEY PROTEINS AND CASEINS

Changes in the whey proteins and caseins in milk heated at temperature s between 72 and 140-degrees-C for holding times between 15 s and 5 mi n were examined by PAGE and gel permeation and ion-exchange FPLC The e xtent of denaturation of the whey proteins, as measured by loss of sol ubility at pH 4.6 increased with the severity of heating, and most of the immunoglobulins and serum albumin, beta-lactoglobulin and alpha-la ctalbumin were denatured when milks were heated for 5 min at 80, 90 an d 110-degrees-C respectively, and precipitated with the caseins at pH 4.6. The caseins were less sensitive than the whey proteins to heating , and only slight changes could be detected on heating milk for 5 min at temperatures up to 110-degrees-C. On heating for 5 min at 120-degre es-C, or between 1 and 5 min at 140-degrees-C, however, the patterns o f the major caseins on alkaline PAGE and the elution profiles on ion-e xchange FPLC became less distinct. On anion-exchange FPLC there was a marked decrease in the amount of protein in the beta-casein region and an increase in the kappa- and alpha(s2)- regions. On cation-exchange FPLC there was a decrease in the amount of protein in the alpha(s1)- a nd alpha(s2)- regions, and an increase in the amount of protein in the kappa-region and in the amount of a fraction believed to be derived f rom alpha(s1)-casein. Results from SDS-PAGE and nitrogen analysis show ed that, at the temperatures and holding times studied, the degree of proteolysis was slight, and the differences in behaviour of the casein s on alkaline PAGE and ion-exchange chromatography were consistent wit h changes in the charges of the negatively and positively charged amin o acid residues on heating.