Inhibition of the proteolytic activity of indigenous plasmin or exogenous chymosin and pepsin in bovine milk by blood serum

Adding bovine or ovine blood serum (0.5-2.0%, v/v) to bovine milk did not affect the hydrolysis of caseins by plasmin, but equine, and particularly porcine, serum strongly inhibited casein hydrolysis; in milk containing 1.0-2.0% porcine serum, caseins were not hydrolysed by plasmin on incubation at 37°C for 7 d. Porcine, and particularly equine, serum also impaired the coagulation of milk by chymosin or pepsin; equine serum (2.0%) increased the chymosin-induced coagulation time of milk ∼4-fold and a firm chymosin-induced milk gel did not form. Adding heated (70°C for 5 min) serum from any of the species to milk did not influence plasmin-induced hydrolysis of caseins or chymosin- or pepsin-induced coagulation of milk, suggesting that the inhibitor(s) is heat labile. Plasmin- or chymosin-induced hydrolysis of small synthetic substrates was not influenced by blood serum. It is suggested that α2-macroglobulin may be the inhibitory substance, although further study is required to verify this.