17A-18 |
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L. H. CHOI, Department of Food Science, Purdue University, 745 Agricultural Mall Dr., West Lafayette, IN 47907, K. D. Hayes, Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907-2009, and S. S. Nielsen, Dept. of Food Science, Purdue University, 745 Agricultural Mall Dr., West Lafayette, IN 47907. Plasminogen activators (PA) play an important role in the plasmin enzyme system in milk and blood. There are two inherent forms of PAs found in milk, tissue-type PA (tPA) and urokinase-type PA (uPA). In milk and blood, PAs activate plasminogen to plasmin. Plasmin contributes beneficial or detrimental characteristics to dairy products. The few studies conducted on PAs have found conflicting results about the composition of the PAs in bovine milk. After evaluating the studies, it was hypothesized that both PAs are present in bovine milk, but different results can be attributed to the sample preparation and measuring methods. The objective of this project was to determine and compare the type and composition of PAs in fresh milk using different sample preparation and measurement methods. Samples of fresh milk were defatted and separated from the somatic cells immediately after collection. Milk samples were treated using two different sample preparation techniques (White et al., 1995; DeHarveng and Nielsen, 1991). The supernatants generated were tested using the same coupled activity assay, but with two different chromogenic substrates (SpectrozymeŽ PL and S-2251). Using S-2251 and both sample preparation methods, the results were consistent with results reported by White et al. (1995) and showed tPA to be the major PA associated with the casein fraction of milk with very little uPA. SpectrozymeŽ PL measured both uPA and tPA in all of the samples with a majority being tPA. However, unlike S-2251, SpectrozymeŽ PL measured uPA to be half as much as tPA. These results suggest that the measured PAs in milk are greatly influenced by sample preparation and measurement methods. In addition, researchers may be able to influence the activation of plasminogen by both uPA and tPA in milk.
Session 17A, Dairy Foods: Cheese and microbiology
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