Inhibition of rennets by blood serum

Incubation of calf chymosin with the serum from blood of various mammalian species before addition to milk reduced its milk-clotting activity at 30°C; the inhibitory potency of the sera was in the order horse > goat > cattle > donkey = human. For all species, inhibition of calf chymosin increased on increasing both the incubation time and the level of blood serum. Horse blood serum inhibited other milk coagulants in the order porcine pepsin > Cryphonectria parasitica proteinase = calf chymosin > Rhizomucor miehei proteinase = bovine pepsin. Methylamine-treated blood serum did not inhibit calf chymosin, suggesting that α₂-macroglobulin may be the principal inhibitor of rennet in blood serum.