Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins

Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7-5.3) and pepsin concentra-tion (0.110-2.75 U/mL) on the hydrolysis of kappa-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37 degrees C using reverse-phase HPLC, oscillatory rheology, and confocal laser scan-ning microscopy. The hydrolysis of kappa-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate in-creased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ~6.0. Both the increase in pepsin concentration and de-crease in pH resulted in a shorter coagulation time. The extent of kappa-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ~73% to ~33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly af-fected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.

Automated summary

This study investigated the effects of pH and pepsin concentration on the hydrolysis of kappa-casein and coagulation of casein micelles. The results showed that the hydrolysis rate increased with increasing pepsin concentration, was pH dependent, and reached a maximum at pH ~6.0. Increasing pepsin concentration and decreasing pH resulted in a shorter coagulation time, and the extent of kappa-casein hydrolysis required for coagulation was independent of pepsin concentration at a given pH.