Dissociation of caseins in high pressure-treated bovine milk

In this study, reversibility of high pressure (HP)-induced solubilisation of α_s1- and β-casein and HP-induced changes in micellar hydration and levels of ultracentrifugally-sedimentable solids in raw skim milk were examined. HP treatment of milk at 100-600MPa resulted in considerable solubilisation of α_s1- and β-caseins, with the extent of solubilisation reaching a maximum around 250MPa. HP-induced solubilisation of caseins was probably a result of solubilisation of colloidal calcium phosphate and disruption of hydrophobic interactions. On storage of HP-treated milk at 5°C, dissociation of caseins was largely irreversible but at 20°C, considerable reassociation of caseins was observed. Hydration of the casein micelles was increased by HP treatment at 100-600MPa, possibly due to HP-induced interactions between caseins and whey proteins; these changes were more extensive at higher pressures. The level of ultracentrifugally sedimentable solids was reduced by HP treatment, with a minimum occurring at 250MPa; Changes in micellar hydration and the level of ultracentrifugally sedimentable solids were largely irreversible at 5°C, but partially reversible at 20°C. These results indicate that HP treatment increases levels of α_s1- and β-caseins in the soluble phase of milk and produces casein micelles with different properties compared to those in untreated milk; therefore, HP treatment may have a considerable influence on the processing characteristics of milk.