Effect of several environmental conditions on the "thermal death rate" of endospores of aerobic, thermophilic bacteria.

The composition of the recovery medium affected the apparent heat resistance of Bacillus stearothermophilus when the pH of the medium was 7.0 but not when the pH was 6.5. The rate of thermal death at 110 C was exponential. Deviations from exponential rates of thermal death during the initial phases of heating at 96 C were observed with endospores of B. coagulans under different conditions of sporulation. Additionally, the apparent heat resistance was influenced by the composition of the media used for sporulation and recovery and by the composition of the suspending menstruum. The presence of 0.001 m sorbic acid in the suspending menstruum at pH 7.0 and the temperature of incubation of the cultures after heating did not affect the apparent heat resistance of B. coagulans. Several explanations are discussed for the observed deviations from exponential thermal death rates and the effect of the environment on the apparent heat resistance of B. coagulans.