ACID-INDUCED DISSOCIATION OF CASEIN MICELLES IN MILK - EFFECTS OF HEAT-TREATMENT

The effects have been studied of heat treatment of skim milk at temper atures from 70 to 90 degrees C on the dissociation of caseins and mine rals from the micelles during acidification at 5 and 22 degrees C. The amounts and types of caseins that were dissociated depended on the pH and temperatures of acidification and heating. When acidification was carried out at 5 degrees C, more caseins were dissociated in the heat ed milks than in the unheated milks; the reverse occurred after acidif ication at 22 degrees C. Heating casein micelles dispersions that were free of serum proteins had no effect on casein dissociation during ac idification at 22 degrees C, but an effect similar to that observed fo r heated milk was apparent at 5 degrees C. Solubilization of Ca, inorg anic phosphate, and Mg from the micelles during acidification remained largely unaffected by heat treatment of milk. Heat treatment of milk appeared to weaken the interaction forces between casein components wi thin the micelles, and, consequently, micelles dissociated more readil y when pH was lowered at low temperatures. At higher temperatures, thi s dissociation was compensated, to a large extent, by the introduction of new crosslinks within the micelles through hydrophobic interaction s between whey proteins and casein components.