Inorganic constituents of milk. III. The colloidal calcium phosphate of cow's milk.

Experiments involving the depletion of milk calcium phosphate (MCP) with EDTA have shown that it has a Ca to phosphate ratio of 1.61 +/- 0.04, close to that of dental enamel. The calcium phosphate also contains small amounts of citrate (citrate/Pi = 0.097 +/- 0.011 and Mg (Mg/Pi = 0.044 +/- 0.011). These experiments and model calculations indicate that the Ca binding capacity of casein is not reduced by mineralization with calcium phosphate and hence that ion binding sites and nucleation sites are distinct, or nearly so. Calculations of the ionic equilibria in the aqueous phase of milk show that there is an invariant ion activity product for a molecular formula close to that of dicalcium phosphate. It is suggested that MCP is a mixture of salts, with dicalcium phosphate a precursor of more basic salts such as octacalcium phosphate and hydroxyapatite.