Low Temperature Inactivation in Milk of Heat-Resistant Proteases from Psychrotrophic Bacteria

Abstract

Development of bitter flavor or gelation in ultra-high temperature sterilized milk may be the result of bacterial proteases which survive the heat treatment. The heat-resistance of these proteases appear to preclude their destruction by heat during processing. These proteases, however, can be inactivated at pasteurization temperatures apparentl by some mechanism other than or in addition to heat. The parameters for this low temperature inactivation of these proteases were investigated. Activity of buffer solutions of bacterial proteases was reduced by more than 90% during 10 min at 55 C. Inactivation of protease in raw or sterile milk was slower and less complete but approached 70% during 60 min at 55 C. Effectiveness of the low temperature inactivation treatment appeared to be independent of protease concentration. The treatment did not alter the flavor or casein composition of raw or ultra-high temperature sterilized milk containing protease much greater than would be expected to occur normally in milk.