Plasmin activity and proteolysis in high pressure-treated bovine milk

In this study, proteolysis resulting from the action of the indigenous milk proteinase, plasmin, in high pressure (HP)-treated raw skim bovine milk during storage was examined. Plasmin activity was reduced by treatment at pressures ≥400 MPa and decreased further throughout 28 d of storage at 5°C. In untreated milk or milk treated at 100 MPa plasmin activity increased during the first 3 d of storage at 37°C, indicating activation of plasminogen; considerably less activation occurred in milk treated at 200-600 MPa. However, considerable decreases in plasmin activity, probably due to autolysis, were apparent in all samples on storage at 37°C for >3 d. Proteolysis, as measured by increases in the level of pH 4.6-soluble N and decreases in the level of β-casein, was very limited on storage of milk at 5°C, with little difference between untreated and HP-treated samples. Proteolysis on storage of milk at 37°C was influenced only slightly by treatment of milk at a pressure ≤250 MPa and was reduced considerably in milk treated at 600 MPa for 30 min, but in milk treated at 300-400 MPa, proteolysis was more extensive than in untreated milk, possibly as a result of HP-induced disruption of casein micelles. Overall, HP can either induce or reduce proteolysis in milk, and may therefore have implications for products made from such milk.