The Milk Salts and Their Interaction with Casein
In all milks that have been examined in sufficient detail, it is apparent that the concentrations of calcium and phosphate are in excess, sometimes considerably in excess, of the solubility of solid calcium phosphate. Indeed, for a limited number of species there is direct evidence of the existence of such a phase in association with casein micelles and for cow’s milk the composition and structure of the colloidal calcium phosphate have been the subject of many experimental and some theoretical investigations. So important is the calcium phosphate for the integrity of the casein micelle, and hence for the behaviour of the casein in milk processing, that there is a need to concentrate on the interaction of the two rather than to follow historical precedent and consider the milk salts in isolation or treat the interaction with casein as an afterthought. In this chapter, the basic physical chemistry (thermodynamics, kinetics) of the milk salt-casein system is first considered but centred on a cluster model of the nature of the colloidal calcium phosphate. The chapter goes further, however, in arguing in favour of a particular point of view regarding one of the most important biological functions of casein, i.e. that the casein micelle is a particular solution to a wider problem in biology of controlling type I (cooperative) phase transitions using rheomorphic proteins. Caseins prevent pathological calcifications of the mammary gland and this function has shaped the nature of these proteins to a large degree.
KeywordsCalcium Phosphate Casein Micelle Amorphous Calcium Phosphate Dicalcium Phosphate Octacalcium Phosphate
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