Role of indigenous enzymes in proteolysis of casein in caprine milk

Bulk samples of caprine milk were characterized for chemical composition, enzyme activities and rheological properties; plasmin, elastase, and cathepsin D concentrations were measured as 3.22 ± 0.18, 1.14 ± 0.05, and 1.81 ± 0.06 mg L^-1, respectively. Pasteurized caprine milk was incubated with aprotinin, pepstatin, or a mix of these inhibitors at 37 °C for 7 days. Hydrolysis of α- and β-caseins was influenced by the presence of inhibitors: overall serine proteases, i.e., plasmin and possibly elastase mainly contributed to the hydrolysis of caseins whereas the limited proteolysis observed in milk incubated with aprotinin suggested a marginal role for cathepsin D. Pasteurized milk displayed a greater number of peptides than milk incubated with pepstatin, whereas no peptides were detected in samples incubated with aprotinin or a mix of aprotinin and pepstatin. Several unreported peptides were identified by mass spectrometry in caprine milk, many of which showed sequences previously described as bioactive in bovine and caprine milk.