The effect of calcium and pH on heat treated micellar casein
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Changes in micelle size observed by particle sizing techniques in micellar casein concentrate as an indication of weakening of the structure of casein micelles were investigated. The changes observed were related to heat-induced dissociation and coagulation of casein micelles depending on milieu conditions characterized by pH, calcium activity, ionic strength, and dissociated casein. It could be shown that, in contrast to dissociation of caseins, the increase in the hydrodynamic radius of casein micelles was closely related to heat-induced coagulation of casein micelles. Dissociation of caseins was found to increase with increasing pH and lower soluble calcium concentration. In addition, low ionic strength in the serum resulted in gel formation of destabilised micelles, whereas at high ionic strength, the formation of distinct large particles could be observed. We therefore concluded that dissociation of caseins from micelles and the coagulation of casein micelles in whey protein free milk systems are opposite reaction paths both destabilising the micellar structure. The coagulation of casein micelles is the subsequent reaction of changes that occur upon heating of casein micelles only under certain milieu conditions. The definition of heat stability as equivalent to heat-induced coagulation therefore needs to be reconsidered.
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