Factors that affect plasmin, plasminogen, and plasminogen activators in milk, cheese, and model systems
Abstract
Plasmin, the major native milk serine proteinase, is an important contributor to proteolytic breakdown during the ripening of certain cheeses. Plasmin system activities include the active enzyme plasmin (PL), the zymogen plasminogen (PG), and plasminogen activators (PA), which convert PG to PL. Factors that influence plasmin system activities in milk are economically relevant for ripened cheese production. The overall objective for this research project was to investigate the effects select factors have on PL, PG, and PA in milk, cheese and milk model systems. The specific objectives were to investigate: (1) The effects cow stage of lactation and number of offspring have on PL, PG, and PA levels in milk casein pellets and pasteurized milk cheese curds. (2) The effect extracellular proteases isolated from select bacteria have on the activity of PAs in milk model systems. (3) The secondary and tertiary structures of bovine PG in the presence and absence of &egr;-amino caproic acid (EACA) in a model system, as determined using Fourier-Transformed Infrared Spectroscopy (FT-IR). Plasmin system activities and kinetic parameters, and bacterial protease action on plasmin system components, were investigated using spectrophotometric and electrophoresis techniques. Plasmin system activities were highest in milk from cows with two or three offspring, and were lower in cheese curd than in casein pellets. The activation of bovine PG was increased in the presence of proteases produced by Pseudomonas fluorescens and Bacillus polymyxa . Bovine PG underwent a large conformational structure change in the presence of EACA. These findings suggest that plasmin system activities in cheese curds may be increased by: (1) selecting milk from cows with two or three offspring to be used for cheese production, (2) adding to milk/curd the protease from Bacillus polymyxa, and (3) limiting heat treatment of casein micelles to enhance bovine PG binding to casein micelles, which should facilitate the extended conformation of PG so it may be activated to PL at a faster rate.
Degree
Ph.D.
Advisors
Nielsen, Purdue University.
Subject Area
Food science
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