Plasmin activity in buffalo milk

Abstract

The proteolytic activity of the indigenous proteinase, plasmin, in buffalo milk and casein was examined under different conditions, e.g. pH, NaCl, lysine or 6-aminohexanoic acid and heat treatments. The plasmin activity, which was primarily associated with the casein fraction, showed large variations between different types of casein preparation; rennet casein had greater plasmin activity than acid or centrifugal casein. The breakdown of buffalo casein by the indigenous proteinase was generally similar to that of bovine casein but minor differences were observed, and buffalo casein hydrolyzate produced by indigenous plasmin corresponded to that of a bovine plasmin-treated sample. Holding the milk at pH in the range 4·8–8·0 for 4 h had no effect on the distribution of plasmin activity between casein and whey, while at pH 4·6 or lower the plasmin activity was completely dissociated from the micelles but remained stable in the supernatant over pH range 4·0–8·0. There was little activity in the casein from milk samples treated with 3–15% NaCl but incubation of casein solutions in the presence of 1–15% NaCl caused proteolysis in all samples but the extent decreased with increasing NaCl concentration. 6-Aminohexanoic acid was more effective in dissociating plasmin from casein micelles than lysine and the micelles were almost devoid of plasmin after treatment with 0·2 m concentrations of either reagent. Heat treatment at 70°C for 10 min increased the plasmin activity in milk while heating at 80°C for 10 min caused losses of only 15 and 40% of the original activity in milk and casein solution, respectively. While heating at higher temperatures caused greater losses of plasmin activity in buffalo milk, some activity still survived in casein dispersions following heating at 90°C for 10 min.