Ionic Calcium in Milk and Whey

A calcium-sensitive electrode was used to measure the Ca +* in raw and processed milk. The standard deviation of measurement was --+ 0.24 mmole per liter, and the coefficient of variation was 13%. The Ca ~ of milk was in the range of 1.4 to 2.5 mmole per liter and decreased as the milk was processed. After 24-hr storage at 4 C, the Ca ~ returned to about the same concentration as before processing. Addition of nonfat dry milk solids to skinunilk in sufficient amount to increase the solids-not-fat to 9.8%, increased the Ca ++ to 4.2 mmole per liter. Rennet whey and milk had the same Ca ~. Treatment of milk with anion exchange resin in the chloride form increased Ca ++ , presumably because of the removal of phosphates. The resin-treated milk reacted with rennet more rapidly than the untreated. Dilution of milk with water caused a logarithmic decrease in Ca *+, with a correlation coefficient of --0.9.