Use of an ion-selective electrode to determine free Ca ion concentration in the milk of various mammals

Milk is a heterogeneous fluid in which the colloidal phase is homogeneously dispersed in the aqueous phase. Cal-cium is partitioned between the colloidal and aqueous phases and is in complex electrochemical equilibrium with several major milk components. In human and bov-ine milk, calcium is mainly distributed between the aque-ous and casein micelle in the colloidal phases (Holt & Jenness, 1984; Neville et al. 1994). Caseins form a complex micelle structure that contains approximately 25 000 phos-phorylated monomers that react with calcium phos-phate complexes in the milk to bind 20–40 mole calcium per mole casein (Holt & Jenness, 1984; Neville et al. 1994). Thus, the distribution of calcium between the col-loidal and aqueous phases appears to be governed by the level of casein in the milk (Holt & Jenness, 1984; Nevill