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Inactivation of Heat Resistant Proteases in Normal Ultra-high Temperature Sterilized Skim Milk by a Low Temperature Treatment

Department of Food Science, North Carolina State University, Raleigh 27607

ABSTRACT

Heat-resistant proteases produced by psychrotrophic bacteria in raw milk survive ultra-high temperature sterilization and cause development of bitter flavor and gelation in sterile skim milk. However, these proteases undergo a unique inactivation at low processing temperatures. A low temperature inactivation treatment of 55 C for 1 h has been proposed as a feasible means to inactivate these heat-resistant proteases in sterile skim milk. The effectiveness of the low temperature inactivation treatment for proteases in normal skim milk was investigated with pilot plant sterilization equipment. All but one of the milk samples tested met Grade A Standards, and all contained heat-resistant protease, although protease activity did not appear to be correlated with the Standard Plate Count, numbers of psychrotrophic bacteria, or numbers of proteolytic-psychrotrophic bacteria in raw milk. Inactivation of heat-resistant proteases in normal ultra-high temperature sterilized skim milk averaged 87 to 90%. No enzyme reactivation was observed in milk samples stored up to 300 days. Milk that did not receive treatment showed signs of gelation and whey separation after 3 mo at room temperature. There was no evidence of gelation in low temperature inactivated sterile skim milk, and the storage time required to produce bitter flavor increased an estimated ten-fold. Treatment did not affect deleteriously the flavor or quality of the sterilized milk.