28-3 |
Ionic calcium concentration in milk and milk ethanol stability |
M. J. LIN1, M. J. Lewis, and A. S. Grandison. (1) School of Food Biosciences, University of Reading, PO Box 226, Whiteknights, Reading, RG 6 6AP, United Kingdom Calcium plays a major role in milk stability by participating in casein micelles. There is approximately 30 mM total calcium in milk, including 20 mM colloidal calcium associated with casein micelles, and 10 mM soluble calcium which includes calcium phosphate, calcium citrate and free calcium ions. The concentration of free calcium ions in milk is about 2 mM. There is a dynamic equilibrium for calcium between the different phases, which can be altered by changing pH or temperature. Our objective was to find out the role of calcium in milk stability. The distribution of calcium between phases in milk and its effect on ethanol stability was studied by adjusting pH and by calcium addition. Ultrafiltration (UF) permeates from skim milk was collected at various pH levels for measuring calcium concentration changes in permeate. Ethanol stability and ionic calcium concentration were measured in individual cow’s milks and bulk milk throughout the entire milking period. When milk pH was reduced, the ionic calcium concentration increased and the ethanol stability decreased. Results showed that total calcium level in permeate increased from 8.80 mM at pH 6.46 to 17.40 mM at pH 5.18; and ionic calcium level increased from 1.23 mM at pH 6.46 to 4.62 mM at pH 5.48. Increasing the ionic calcium concentration in milk by adding calcium decreased the milk ethanol stability. Addition of only 1.50 mM extra calcium into milk lowered its ethanol stability to less than 75%, which is considered to be about the threshold for UHT treatment. Results showed that bulk cow’s milk maintained a similar ionic calcium level and ethanol stability throughout the milking period. However, there was considerable variation in both these measurements for individual cow’s milks and it was observed that lower ionic calcium contents corresponded to higher ethanol stabilities. A similar trend was found for individual cow’s milk during storage at 4°C for 7 days.
Session 28, Dairy Foods: Developments in microbiology and chemistry of cheese
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