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Thermal Inactivation of Heat-Resistant Bacterial Spores in Milk Concentrate at Ultra-High Temperatures¹

Departments of Dairy and Food Industries and Bacteriology, University of Wisconsin, Madison

ABSTRACT

The heat resistance of spores of Bacillus coagulans WH-9, Bacillus stearothermophilus 1518, and Clostridium sporogenes (PA 3679) was determined in 3:1 milk concentrate and 0.067 M phosphate buffer (pH 6.2). Although spore inactivation was predominantly logarithmic, the survivor curves were usually curvilinear during the initial period of heat transfer from the heating medium to the contents of the thermal inactivation tubes. Unlike spores of B. coagulans or PA 3679, those of B. stearothermophilus required an exceedingly high heat activation treatment to obtain maximal spore counts. Also, it was apparent that 3:1 milk concentrate was effective in inhibiting the germination of spores of B. stearothermophilus. Thermal resistance curves based on six D-values, indicated F-values of 1.8, 11.7, and 4.3 mm and z-values of 16, 21.9, and 20.3 F, respectively, for spores of B. coagulans, B. stearothermophilus, and PA 3679 suspended in 3 :1 milk concentrate. Spores of the three organisms suspended in phosphate buffer showed F-values of 1.7, 10.3, and 3.4 mm and z-values of 15.6, 21.2, and 19.7 F.

The thermal process used in 3:1 milk concentrate, namely 295 F for 3.5 sec including the come-up time, appears to be adequate to inactivate spores of B. coagulans and PA 3679, even when they are present in large numbers, and spores of B. stearothermophilus if they are present in low numbers.

¹ Published with the approval of the Director of the Wisconsin Agricultural Experiment Station. This work was supported in part by a grant form the U. S. Steel Corporation.

² Present address: Continental Can Company, Inc. 1350 West 76th Street, Chicago 20, Illinois.