Plasmin activity, β-lactoglobulin denaturation and proteolysis in high pressure treated milk

The effects of high pressure (HP) on plasmin activity, β-Lactoglobulin denaturation and proteolysis during subsequent storage of HP treated milk, were studied. Fresh raw milk samples were exposed to a range of pressures from 50 to 800 MPa, for times of 1, 10 or 30 min, at 20°C. Residual plasmin activity and whey protein denaturation were measured immediately post HP-Treatment. Indices of proteolysis were measured during post-HP storage. Treatment at pressures >300 MPa resulted in extensive β-lactoglobulin denaturation. Plasmin activity decreased in milk treated at pressures ≥400 MPa; the loss of activity was not well correlated with β-lactoglobulin denaturation. Compared to raw milk, treatment at 50 MPa had little effect on proteolysis during storage of treated milk measured as increases in pH 4.6-Soluble N and liberation of proteose peptones, but at pressures of 300-400 MPa, proteolysis was increased relative to raw milk. After pressurisation > 500 MPa, proteolysis during storage of milk was less than that observed in raw milk. Overall, HP influenced proteolysis in milk in a way which is different from that produced by heat, in terms of subsequent susceptibility of casein to proteolysis during storage or incubation. In particular, HP treatment at pressures of 300-500 MPa can increase proteolysis in milk, possibly through changes in micelle structure facilitating increased availability of substrate bonds to plasmin, which has implications for products prepared from milk thus treated.