Distribution of minerals and proteins between the soluble and colloidal phases of pressurized milks from different species

Abstract

The pressure-induced (100–400 MPa) changes in casein micelles from bovine, caprine and ovine milks were investigated by studying the distribution of minerals and proteins after separation of the micellar and serum phases by ultracentrifugation. The results showed that pressurization markedly increased the levels of non-sedimentable casein in the three species, as well as the levels of Ca, P and Mg in the serum, but led to only small increases in the Ca²⁺ concentration. The dissociation rates of individual caseins corresponded to the ester phosphate contents, in the order κ-casein, β-casein>α_s1-casein >α_s2-casein. The concentration of non-sedimentable casein was highest in pressurized ovine milk, which also contained higher levels of pressure-liberated serum minerals than the milks from the other two species. In the case of bovine and caprine milks, maximum dissociation from the micelle was found in milks treated at 300 MPa, while in ewes' milk dissociation increased with pressure up to 400 MPa. Denatured β-lactoglobulin (β-lg) was present in the ultracentrifugation pellets of the pressurized milks of the three species, but pressure-denatured β-lg also remained in the soluble fraction. Despite the increases in the content of soluble proteins, all casein and most β-lg were incorporated in the rennet curds of the pressurized milks from the three species.