Effect of pH on the speciation and solubility of divalent metals in human and bovine milks.

Computer models estimated the ligand speciation and solubility of calcium, magnesium, zinc, and copper over a pH range for low molecular weight fractions characteristic of either human or bovine milks. Above pH 4 calcium is the only metal predicted to precipitate. Most of the remaining soluble calcium, magnesium, and zinc should be complexed with citrate. The solubility of calcium, magnesium, and zinc in human and bovine milks was measured experimentally from pH 2 to 7. The solubility of all three metals decreased as the pH increased. Calcium and zinc were soluble over a narrower pH range in bovine milk than in human milk. Increasing the levels of either calcium or inorganic phosphate alone in decaseinated human milk did not affect the solubility of zinc, but when both calcium and inorganic phosphate were added at levels comparable to bovine milk the solubility of zinc decreased at the higher pH's. The decreased solubility of zinc in skimmed milks in pH's characteristic of the small intestine is likely due to coprecipitation of zinc with calcium phosphate--a reaction not predicted for milk systems from known chemical solubility product data.