Effect of calcium on the hydration of casein. I. Water vapour sorption and fine structure of calcium caseinates compared with sodium caseinates in the pH range 4·6–8·0

Abstract

Summary Hydration of Ca and Na caseinates which were prepared from whole casein at different pH levels (range 4·7–8·0) was determined by means of water vapour sorption measurements in the water activity ( a w ) range 0·58–0·95. Water uptake of Ca caseinates was systematically lower than that of Na caseinates prepared at equal pH, the differences increasing with increasing pH and a w . Plots of the water content ν. the amount of added hydroxide at constant a w revealed a linear relationship between water uptake and cation content, suggesting that the increase in water uptake is mainly determined by the amount and type of cations associated with the caseins. In the a w range tested, Ca 2+ adsorbed about 2–7 and Na + 3–18 mol water/mol of cation. This implies that replacement of one mol of casein-bound Na + by Ca 2+ is accompanied by a displacement of 1–11 mol water at a w 0·58–0·95. The loss of water is a consequence of conformational changes induced by the chelating and cross-linking effects of Ca 2+ , which also lead to micelle formation.