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Y. D. LIVNEY and D. G. Dalgleish. Dept. of Food Science, Univ. of Guelph, Ontario Agricultural College, Guelph, ON N1G 2W1, Canada One of the most important heat-induced interactions in milk is that between b-lactoglobulin (b-Lg) and k-casein (k-CN). It is central to the improved acid gelation of heated milk, which is crucial in yoghurt production. Conversely, it dramatically decreases the rennetability of milk by creating an alternative steric barrier to micelle aggregation, an undesirable phenomenon for cheese making. Heat-induced interaction between b-Lg and k-CN occurs via sulfhydryl-disulfide interchange; however, it has never been established which of the 5 cysteines of b-Lg and the 2 of k-CN participate in the reaction, and whether there is any steric specificity to the interaction. To gain a better understanding of this important interaction we have studied the specificity of disulfide bond formation between these two proteins in a model system. A mixture of bovine b-Lg and k-CN at a 1:1 molar ratio (similar to the ratio in milk) was heated at pH=6.7 to 90°C for 10 min then quickly cooled to ambient temperature. Iodoacetamide was then added to block unreacted thiol groups and prevent post-digestion shuffling of disulfide bonds. The mixture was digested by proteolytic enzymes and the digests were analyzed by Reverse-Phase-HPLC and MALDI-TOF Mass Spectrometry, and compared with theoretical digestion patterns. One such specific intermolecular disulfide bond identified was bC66-kC88. The position of bC66 on the surface of the native structure of b-Lg makes it a favorable reactive site. Additionally, from other identified bonds, it appears that both kC11 and kC88 may serve as anchoring points for the adhering whey protein complex. Apparently, this is the first report of identification of specific heat induced disulfide bonds between bovine k-CN and b-Lg. Such basic molecular level understanding of the mechanisms of interactions between milk proteins may facilitate manipulation of their functional behavior. We are continuing to study the details of these reactions.
Session 64, Dairy Foods: Milk proteins
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