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E. A. PARKER and D. G. Dalgleish. Dept. of Food Science, Univ. of Guelph, Ontario Agricultural College, Guelph, ON N1G 2W1, Canada Sodium caseinate is used as an ingredient in many dairy foods and during processing many of these products are heated. It is known that temperatures above 75°C induce the interaction between denatured whey proteins and micellar k-casein via disulfide bonds. However, little information is currently available on the behaviour of the sodium caseinate in milk-based systems during heating. Our objective was to study the behaviour of sodium caseinate in heated systems of skim milk fortified with sodium caseinate. Mixed samples of skim milk and sodium caseinate were prepared and heated for different time/temperature combinations. Ultracentrifugation was employed to separate the solutions into serum and pellet fractions. Protein aggregates from the serum and pellet were separated using size exclusion HPLC, and further characterized using reversed phase HPLC. Results from the unheated serum fractions showed the presence of two protein peaks. One peak, increasing in size with increasing sodium caseinate concentration, contained all four casein proteins and the other contained the major whey proteins. Heating caused a considerable decrease in the whey protein peak. This was accompanied by the appearance of a third peak of a complex of whey proteins and k-casein which increased with temperature and sodium caseinate concentration. This behavior pattern appeared to reach completion after heating at 85°C for 20 min. Conversely, the casein peak changed more significantly at higher temperatures (above 85°C) especially at elevated sodium caseinate concentrations. Only small changes were observed in the pellet samples from the different milks. These results suggest that most of the proteins in the added sodium caseinate do not participate in normal heat-induced interactions in milk. The results show, however, that at elevated temperatures the caseins from the caseinate undergo a separate reaction that causes their precipitation. This research will provide insight into the behaviour of sodium caseinate in prepared milk-based foods.
Session 64, Dairy Foods: Milk proteins
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