Acid-Induced Dissociation of Casein Micelles in Milk: Effects of Heat Treatment

The effects have been studied of heat treatment of skim milk at temperatures from 70 to 90°C on the dissociation of caseins and minerals from the micelles during acidification at 5 and 22°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°C, more caseins were dissociated in the heated milks than in the unheated milks; the reverse occurred after acidification at 22°C. Heating casein micelles dispersions that were free of serum proteins had no effect on casein dissociation during acidification at 22°C, but an effect similar to that observed for heated milk was apparent at 5°C. Solubilization of Ca, inorganic 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 within the micelles, and, consequently, micelles dissociated more readily when pH was lowered at low temperatures. At higher temperatures, this dissociation was compensated, to a large extent, by the introduction of new crosslinks within the micelles through hydrophobic interactions between whey proteins and casein components.