In this study, reversibility of high pressure (HP)-induced solubilisation of alpha(s1)- and beta-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 alpha(s1)- and beta-caseins, with the extent of solubilisation reaching a maximum around 250 MPa. 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 5degreesC, dissociation of caseins was largely irreversible but at 20degreesC, considerable reassociation of caseins was observed. Hydration of the casein micelles was increased by HP treatment at 100600 MPa, 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 250 MPa; Changes in micellar hydration and the level of ultracentrifugally sedimentable solids were largely irreversible at 5degreesC, but partially reversible at 20degreesC. These results indicate that HP treatment increases levels of alpha(s1)- and beta-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. (C) 2004 Elsevier Ltd. All rights reserved.