Properties of casein micelles in high pressure-treated bovine milk

High pressure (HP) treatment of milk alters many of its constituents and properties; in this study, HP-induced changes in casein micelles were examined. HP treatment of milk at 250 MPa increased casein micelle size by ∼30%, whereas treatment at 400 or 600 MPa reduced it by ∼50%. Pre-denaturing whey proteins in milk by heat treatment or preventing interactions between β-lactoglobulin and κ-casein, by addition of sulphydryl-blocking or reducing agents to milk prior to HP treatment, had little influence on HP-induced changes in casein micelle size. Addition of ethanol to milk, prior to HP treatment, resulted in the formation of large casein aggregates. On treatment at 250 MPa, larger increases in casein micelle size were observed in milk reconstituted from larger micelles, suggesting that increases in micelle size at this pressure are probably due to the formation of casein aggregates. HP treatment reduced the L^*-value of milk; these changes were rapidly reversed on storage of milk at 37°C, but maintained on storage at 5°C. Similar to untreated milk, micelles in HP-treated milk were susceptible to dissociation by urea and tri-sodium citrate. Altered properties of casein micelles in HP-treated milk may influence the properties of products made from such milk.