The Resistance of Micrococcus Freudenreichii in Laboratory High-Temperature-Short-Time Pasteurization of Milk and Ice Cream Mix

Abstract

M. freudenreichii was found to possess about the same degree of heat resistance in milk as ''was formerly possessed by the heat-resistant Esckerichia coli (no. 3U) and was used as the test culture in laboratory pasteurization studies. Its growth on desoxy-cholate lactose agar (BBL) permitted differentiation from other bacteria present. An improved laboratory pasteurization technique was devised for conducting HTST pasteurization tests which should give results similar to those obtained in commercial HTST pasteurization units. Five ml. of milk or ice cream mix in a 120 X 20 mm. glass vial closed with a rubber sleeve stopper was tempered to the temp, under test. Then 0.05 ml. of the suspension of the culture was injd. through the stopper using a 1 ml. tuberculin syringe equipped with a 2-in., 20-gauge hypodermic needle. The pasteurized sample was cooled in ice water. The length of time required for 99.99% destruction of M. freudenreichii at a given temp, was 2-4 times greater for ice cream mix than for milk, this difference decreasing with increased temps.