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

The heat resistance of spores of Bacillus coag'ulans WH-9, Bacillus stearothermophilus 1518, and Clostrid; 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 thennal inhctivation tubes. Unlike spores of B. coagulans or P A 3679, those of B. stearothermophilus required an exceedingly high heat activation treatment to obtain ma..ximal 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, 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 min 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. coagularns and PA 3679, even when they are present in large numbers, and spores of B. stea1'othermophilus if they are present in low numbers.