Effect of psychrotrophic bacteria on the plasmin system of fresh milk and dairy products
Abstract
Milk handling conditions in the dairy industry allow psychrotrophic bacteria to dominate the microflora and produce heat-stable proteases that can disrupt the casein micelle. From previous studies it was noted that when Pseudomonas strains grew in nonfat dry milk to levels of 10 8 cfu/m1, a shift in plasmin activity from the casein to the whey fraction resulted. The goals of this research were to study the effect of growth of Pseudomonas strains during cold storage of fresh milk on plasmin activity, the mechanism by which changes in plasmin location occurred, and how this change affected Mozzarella cheese ripening. Two bacterial strains, Pseudomonas fluorescens M3/6, and Pseudomonas spp. SRM28A, were inoculated at 103 cfu/m1 into fresh milk and incubated at 7°C for 3 days. Bacterial counts were 108 cfu/m1 by day 3. Samples collected every 24 hr were analyzed for plasmin activity. With psychrotrophic bacterial growth, the plasmin level in casein fractions decreased significantly. It was hypothesized that the change in plasmin location from the casein to the whey fraction was caused by proteases from the Pseudomonas acting on the casein micelle. An isolated extracellular protease from Pseudomonas fluorescens M3/6 was incubated with fresh milk at 5°C for 24 hr. Casein and whey fractions from samples collected during incubation had decreased plasmin and plasminogen activities in the casein fraction within 10 hr of incubation and increased activities in the whey fraction starting at 7 hr of incubation. The migration of plasmin from casein to whey could reduce plasmin levels in the curd during cheesemaking operations. This could affect the ripening process in cheeses dependent on plasmin for proteolysis. The relationship between plasmin and psychrotrophic bacterial growth as it affects ripening of Mozzarella cheese was studied. Milk used for Mozzarella cheesemaking was inoculated with Pseudomonas fluorescens M3/6 and kept at 5°C for 2 days. Cheese was ripened for 6 weeks, and samples taken during this time were analyzed for plasmin activity and proteolysis of caseins. High psychrotrophic bacterial loads affected plasmin content in cheeses made with this milk. Changes in plasmin content apparently did not affect the ripening process of the cheese.
Degree
Ph.D.
Advisors
Nielsen, Purdue University.
Subject Area
Food science|Biochemistry
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