Monitoring of Heat-Induced Proteolysis in Milk and Milk-Resembling Systems

Peptide mapping from the soluble fractions of skimmed milk and a milk-resembling system, obtained either by treatment with trichloroacetic acid (TCA, 40 and 120 g/L) or by adjusting the pH to 4.6, were analyzed by RP-HPLC with UV (205, 214, and 280 nm) and fluorescent detection (excitation at 280 and 270 nm and emission at 340 and 310 nm). Two peptides (p21 and p25), both soluble in 120 g/L TCA, were chosen for following the extent of the heat-induced proteolysis in milk and milk-resembling systems. Both peptides were absent in skimmed raw milk, but were detected in low amounts in Na-caseinate/lactose solution. The rate of formation of p21 and p25 peptides could be fitted to zero-order kinetics. The p25 peptide was formed more rapidly in skimmed milk than in the Na-caseinate/lactose system. As for the p21 peptide, other factors apart from temperature and heating time should be considered for understanding its thermal behavior. Possibly, p21 and p25 peptides could be applied for assessing previous industrial processing.