Physicochemical characterization of calcium-supplemented skim milk

Abstract

Addition of CaCl2 to skim milk, from 0 to 13.5 mmol·kg-1, leads to significant physicochemical changes in milk salts equilibrium. Indeed, acidification, insolubilizations of calcium, inorganic phosphate and citrate are observed. Calculation of ion equilibria in the resulting milk ultrafiltrates indicates that the observed ion migrations, controlled by solubilities of calcium-phosphate and calcium-citrate, do not cause change in ion activity products of dicalcium-phosphate (calculated as CaHPO4·2H2O and Ca(HPO4)0.7(PO4)0.2). In the same way, the acidification of calcium-supplemented milks in the range of pH 6.7-3.5 shows a solubilization of calcium-phosphate similar to non-enriched milk. So whatever the added calcium concentration, the behaviour of colloidal calcium-phosphate seems to be unchanged. However, at room temperature, addition of calcium to milk originates a decrease in soluble casein concentration and tightly bound water of ultracentrifuged pellets. The consequences are (i) an increase in milk protein hydrophobicity; (ii) a decrease in zeta potential of casein micelles, without modification of their hydrodynamic diameter; and (iii) an increase in milk lightness and turbidity. Furthermore, calcium supplementation of milk seems to lead to a proteinic reorganization of micellar casein, responsible for an increase in micellar density.