Temperature effect on calcium and phosphorus equilibria in relation to gel formation during acidification of skim milk

Levels of micellar, serum and free calcium and micellar and serum phosphorus were studied during gradual acidification of skim milk at 4, 20, 30 and 40 °C. From pH 6.0 to 5.4–5.2, calcium and phosphorus concentrations in milk serum increased for decreasing temperature. For pH < 5.0 the concentrations of serum phosphorus and free calcium were temperature independent, although the concentration of micellar calcium was decreased and the concentration of serum calcium increased with decreasing temperature. The molar ratio of serum calcium/free calcium was 1.71, that of serum calcium/serum phosphorus was 1.35. Gel formation (G′) at pH 4.8, 4.7 and 4.6 showed that for constant pH, G′ was higher for decreasing temperature. Moreover, G′ was increased from pH 4.8 to 4.6 for constant temperature. These last observations could be important for controlling the properties of milk gels formed in low pH ranges and calcium could play a vital role.