Further Studies on the Heat-induced, pH-dependent Dissociation of Casein from the Micelles in Reconstituted Skim Milk

Abstract

When milk at a pH value >6.7 was heated, the quantity of αS- andβ -caseins dissociated increased with increasing temperature to a maximum at about 60 °C, decreased at temperatures between 60 and 100 °C and then increased again at temperatures above 100 °C. This produced a local minimum in the dissociated casein vs. temperature curve at about 100 °C. The dissociation of κ -casein increased essentially linearly with increasing temperature over the entire temperature range. Removal of the whey proteins from the milk had no effect on the dissociation of casein at temperatures up to 60 °C. At higher temperatures, the levels of dissociatedαs - and β -caseins increased so that the local minimum in the dissociated casein vs. temperature profile was eliminated when most of the whey protein was removed from the milk. The level of dissociatedκ -casein decreased with increasing whey protein depletion. The addition of β -lactoglobulin to whey protein-free milk had no effect on dissociation at temperatures up to 60 °C. At higher temperatures, the addition ofβ -lactoglobulin reduced the level of dissociated αs- and β -caseins and increased the level of dissociated κ -casein. Blocking the thiol groups with N-ethyl-maleimide (NEM) caused an increase in the dissociation of αs- and β -caseins at all temperatures. The addition of NEM had no effect on the dissociation of κ -casein at temperatures below 60 °C and reduced the level of dissociation at higher temperatures. Increasing the colloidal calcium phosphate (CCP) level of the milk had little effect on the dissociation of the casein micelles. Reducing the CCP level of the milk increased the level of micellar dissociation but did not alter the pH dependence of the phenomenon.