Development of Off-Flavors in Ultra-High Temperature and Pasteurized Milk as a Function of Proteolysis

Abstract

The relationship between proteolysis and off-flavor development in ultra-high temperature and pasteurized milk was investigated. Milks were subjected to proteolysis by different concentrations of three psychrotroph enzymes for 20 h at 35°C. Proteolysis, measured as the increase in trichloroacetic acid-soluble free amino groups was subsequently determined colorimetrically with trini trobenzene sulfonic acid. Bitter off-flavors were determined by a 10-membered panel. Background trichloroacetic acid-soluble free amino groups of .819, .822, and .817 μmoles/ml were determined for raw, pasteurized, and ultra-high temperature milk, respectively. Increases in free amino groups were significant when pasteurized milk was incubated at both 4°C (.87) and 35°C(.925)for 20 h. Ultra-high temperature milk was approximately twice as sensitive as pasteurized milk to the action of crude proteolytic enzymes, but unlike pasteurized milk, it did not coagulate when exposed to high concentrations of enzyme. When increasing volumes of crude proteases were added to ultra-high temperature and pasteurized milk, proteolysis could be detected in samples which had not developed off-flavors. Off-flavors were detected in ultra-high temperature milk at proteolysis (change μmoles/ml) of .554, .355, and .287 for the three proteases. Corresponding values for pasteurized milk were .499, .355, and.746. Electrophoretic profiles of caseins isolated from protease-treated ultra-high temperature and pasteurized milk did not differ markedly from those of untreated milk. In all samples receiving enzyme treatment a slow moving band was detected that increased in intensity with increased enzyme concentration. Detection of low levels of proteolysis with trini trobenzenesulfonic acid might have some application in monitoring stored ultrahigh temperature milk. © 1981, American Dairy Science Association. All rights reserved.