The Ion-Exchangeable Calcium in Raw and Heated Skimmilk as Determined by the “Resin Contact Time Method”

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Abstract
The ion-exchange-able Ca in raw and heated skimmilk was determined by an ion exchange resin contact time method. Heating skimmilk at 40[degree], 60[degree], and 80[degree]C for 30 minutes had no discernible effect on the exchangeability of Ca. However, the exchangeable Ca was significantly decreased as the temperature of heating increased from 100[degree] to 120[degree]C for holding times of 30 minutes. When longer periods of heating at 120[degree]C were used (45-60 minutes), a further decrease was noted in the amount of Ca exchanged. The shift in equilibria of Ca due to heat was permanent and did not revert on aging the milk at 5[degree]C for 48 hours. The exchange rate of removal of Ca was decreased by more than 50% for skimmilk heated at 120[degree]C for 30 minutes as compared to raw and pasteurized skimmilk. The specific reaction rate constant for the exchange of Ca from raw skimmilk contacted with 30 ml of IRC-50-K-H buffered at pH 6.8 was 0.0118 second-1, and 0.0039 second-1 for skimmilk heated at 120[degree]C for 30 minutes. The rate phenomenon involved in the decomposition reaction for calcium phospho- caseinate and citrate complexes in skimmilk, including both dissolved and colloidal phases, is such that it behaves as a simple system with a single rate constant.