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L. WANG, S. E. Sombers, K. D. Hayes, and L. J. Mauer. Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907-2009 Plasmin, and its inactive form plasminogen (PG), is the most significant protease contributing to proteolytic activity in dairy products. Existing knowledge of the plasmin system is mainly based on measurements of plasmin activities. Better understanding of the plasmin system is hindered by lack of a method to accurately quantify and characterize distribution of plasmin system components in bovine milk. Objectives of this research were to investigate the effect of BLG bound to micelles, as a result of heat treatment, on the location and concentration of PG.PG was labeled with Alexa Fluor 594 (Molecular Probe). Micelles isolated from fresh milk were reconstituted in Jenness-Koops buffer. beta-lactoglobulin (BLG) and labeled-PG were added, then the mixture was heated at 85ºC for 16 s and ultracentrifuged to separate micelle from whey fractions. Labeled-PG levels in each fraction were determined by fluorescent measurement. Fluorescent confocal microscopy showed distribution of labeled-PG. Milk protein compositions and concentrations were measured by SDS-PAGE and BCA. Prior to heating, 60% of the total PG was associated with micelles and 40% with whey. After heating, micelle-associated PG increased to 80%. Addition of BLG in heated samples caused an additional 5-fold decrease of PG in whey fractions. These results were confirmed by fluorescent confocal microscopy on un-fractionated samples. SDS-PAGE and BCA analysis revealed that 70% of BLG was adhered to micelles in a heat-induced manner. The results indicated that PG might have associated with BLG through disulfide bond in milk serum and then co-adhered to micelles upon heating. For the first time, PG location in milk systems affected by cheese-making conditions was illustrated. Heat and BLG affect PG location in milk and result in higher levels of PG in cheese curd. This information provides the basis for future studies to control PG activation during cheese ripening.
Session 32, Food Chemistry: Component interactions and processing effects
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |