Effect of the high-pressure-release phase on the protein composition of the soluble milk fraction

This paper describes a study on the influence of the high-pressure-release rate on the protein distribution between the soluble and colloidal fractions of milk. Skim milk, without or with sulfhydryl-blocking agents, was pressure treated at 250 and 350 MPa for 5 to 15 min at 25 degrees C, applying different pressure-release rates (pressure-release times between 0.07 to 10 min). The liberation of caseins to the soluble phase and the denaturation of whey proteins were assessed. A significantly higher increase in the content of soluble casein took place during the pressure-release phase as compared with a pressure-holding phase. Denaturation of beta-lactoglobulin mainly took place by -SH-S-S exchange reactions during the holding phase. The present results, thus, show a negligible influence of whey proteins on the increase in the content of nonsedimentable casein in pressure-treated milk and provide evidence for the importance of the pressure-release rate in this process, so that the slower the pressure release rate, the higher the level of soluble casein.