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

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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. These proteases undergo a unique inactivation at low processing temperatures. A low temperature inactivation treatment of 55.degree. C for 1 h was 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 1 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 87-90%. 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.