14A-11 |
The effect of Bacillus polymyxa protease on plasmin system components |
N. K. LARSON1, K. D. Hayes1, and S. S. Nielsen2. (1) Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907-2009, (2) Dept. of Food Science, Purdue University, 745 Agricultural Mall Dr., West Lafayette, IN 47907 Refrigerated storage of pasteurized milk prevents growth of mesophilic spoilage bacteria. However, refrigeration temperature allows psychrotrophs to dominate the microflora. Studies have shown that nearly one third of refrigerated milk samples were spoiled by gram positive psychrotrophs; of those, 77% were spoiled by B. polymyxa and B. cereus. As Bacillus bacteria grow in milk, they secrete heat-resistant extracellular proteases that may affect the quality of milk. Proteases produced by Pseudomonas fluorescens M3/6 have been shown to affect the plasmin enzyme system in milk. In this system, inactive plasminogen (PG) is converted to active plasmin (PL) by plasminogen activators (PA), all of which are native to milk. The effects of B. polymyxa protease on plasmin system components are unknown. The objective of this research was to determine the effect of B. polymyxa protease on plasmin system components. The B. polymyxa protease was incubated (Tris buffer pH 7.6, 37ºC, 1 h) with each plasmin system component (PL, PG, PG+PA); the resulting proteolytic activity was measured using a spectrophotometric method and visualized using SDS-PAGE. The activity was further tested over a pH range of 3.8-7.9 in an acetate-phosphate buffer system and a temperature range of 5 to 73ºC. The B. polymyxa protease did not affect the activity of PL or PA. SDS-PAGE visualization of B. polymyxa protease activity toward PG revealed a decrease in the intensity of the PG band with an appearance of three breakdown fragments. B. polymyxa protease was found to have strong PA-like activity toward bovine PG. Maximum PG activation occurred under conditions resembling milk storage (5ºC and pH 6.6). The B. polymyxa protease activity toward bovine PG suggests that this protease could be a potent source of PA in pasteurized milk.
Session 14A, Dairy Foods: General developments in dairy technology I
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