Partial equilibration during dissolution of calcium hydrogen phosphate in aqueous sodium hydrogen citrate: mechanism behind spontaneous supersaturation increasing calcium bioaccessibility

The higher bioaccessibility of calcium from citrates compared to other salts, often explained by the capacity of hydroxycarboxylates like citrate spontaneously to form supersaturated calcium salt solutions, was analyzed using two models for dissolution of calcium hydrogen phosphate in aqueous sodium hydrogen citrate followed by slow precipitation of calcium citrate hexahydrate after a lag phase. During the dissolution calcium ion activity as measured electrochemically increased to a maximum plateau value for a dissolution time almost independent of initial hydrogen citrate concentration but with a supersaturation degree increasing strongly with increasing hydrogen citrate concentration. The difference in time dependence was analyzed by (i) a model assuming total equilibrium among the dissolved species, and (ii) a kinetic model based on coupled differential rate equations for transformations between involved dissolved species and precipitates using numerical integration. In contrast to model (i), model (ii) could quantify differences in time dependence of the concentration of dissolved species. A major difference was that the concentration of calcium hydrogen citrate follows the dynamics of the total calcium concentration in the equilibrium model, while the concentration decays monotonically towards an equilibrium value in the kinetic model. Calcium hydrogen citrate is concluded to be critical for the precipitation of dissolved calcium with the concentration of calcium hydrogen citrate determining the length of the lag phase and the rate of precipitation. Design of robust supersaturation for functional calcium foods and beverages should accordingly aim of minimizing the concentration of calcium hydrogen citrate.