Influence of pH on the effects of high pressure on milk proteins

The effects of pressure treatments (200 and 400 MPa) at different pHs (7.0, 6.7, 6.0 and 5.5), with and without subsequent reneutralization to pH 6.7, on the rennet coagulation properties, Ca2+ concentration, whey protein denaturation, cheese yield and levels of non-sedimentable casein in milk have been investigated. High pressure treatments at 200 MPa significantly reduced the rennet coagulation times over the unpressurized samples at all the pH values tested. Pressurization at 400 MPa led to coagulation times similar to those of the unpressurized samples, except for milk samples treated at pH 7.0, with or without readjustment to pH 6.7, which presented significantly longer coagulation times than their unpressurized counterparts. Pressure-induced beta-Lg denaturation increased considerably at alkaline pH and decreased at acidic pH. In milk samples treated at 400 MPa, protein and moisture retention in the curd were higher as the pH of pressurization increased. Pressurization of acidified milks increased the contents of non-sedimentable caseins as compared with acidification or pressurization alone. In addition, there was a marked increase in the relative amount of all caseins in the serum fraction of milk pressurized at pH 7.0 and, in this case, a bigger proportion of denatured beta-Lg remained in the soluble fraction.