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 coagulation, determined on the basis of the release of caseinomacropeptides (CMP) soluble in 4% trichloroacetic acid, were inhibited by pressures over 200 MPa. However, the coagulation time decreased as pressure increased up to 200 MPa and then increased again, until at 400 MPa, it 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 κ-casein hydrolysis, indicating that pressure probably favored the aggregation phase. Addition of CaCl 2 enhanced casein aggregation of native or pressurized milk, reducing coagulation time and curd-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 pressure-treated and untreated milk or solutions containing κ-casein, β-lactoglobulin, or both in the presence or absence of denaturing agents showed evidence for the formation of aggregates linked by intermolecular disulfide bonds. Keywords: High pressure; rennet coagulation; protein aggregation