Modelling ion composition in simulated milk ultrafiltrate (SMUF). I: Influence of calcium phosphate precipitation

Freshly prepared simulated milk ultrafiltrate (SMUF) is a solution that is supersaturated with respect to various calcium phosphate phases that precipitate in time. As a consequence, the ion composition of equilibrated SMUF was found to be significantly different from that of freshly prepared SMUF. This study proposes a thermodynamic ion-speciation model that is able to describe ion equilibria in SMUF. Moreover, it is also able to describe calcium phosphate precipitation in fresh SMUF on its way to equilibrium by using an apparent solubility product for CaHPO4·2H2O as a function of time. The model was validated by experiments in which CaCl2 and Na2HPO4 were added to freshly prepared SMUF. The changes in calcium activity and pH were followed and the precipitates were characterised by X-ray diffraction. The model was able to predict the observed changes.