RENNET COAGULATION OF MILK SUBJECTED TO HIGH-PRESSURES

The rennet coagulation properties of highly pressurized milk have been investigated. The initial rate and extent of the enzymatic phase of c oagulation, determined on the basis of the release of caseinomacropept ides (CMP) soluble in 4% trichloroacetic acid, were inhibited by press ures over 200 MPa. However, the coagulation time decreased as pressure increased up to 200 MPa and then increased again, until at 400 MPa, i t reached values comparable with those of the raw milk. Therefore, the coagulation time of the pressurized milk was not directly related to the degree of K-casein hydrolysis, indicating that pressure probably f avored the aggregation phase. Addition of CaCl2 enhanced casein aggreg ation of native or pressurized milk, reducing coagulation time and cur d-firming time, but did not offset the rate-increasing (up to 200 MPa) or rate-lowering (at 300 and 400 MPa) effects of milk pressurization on subsequent casein coagulation by rennet. The SDS-PAGE studies of pr essure-treated and untreated milk or solutions containing lc-casein, B -lactoglobulin, or both in the presence or absence of denaturing agent s showed evidence for the formation of aggregates Linked by intermolec ular disulfide bonds.