METHODS OF ASSESSING THE SPORICIDAL EFFICIENCY OF AN ULTRA-HIGH-TEMPERATURE MILK STERILIZING PLANT. III. LABORATORY DETERMINATIONS OF THE HEAT RESISTANCE OF SPORES OF BACILLUS SUBTILIS IN WATER AND IN MILK

SUMMARY: Thermal death curves for spores of Bacillus subtilis 786 have been determined in water and in milk. Generally a non-logarithmic order of death was observed. Numbers of survivors were lower in milk than in water, suggesting that there may be inhibitory factors in UHT sterilized milk which affect the germination and/or subsequent growth of heated spores.
The thermal death curves for spores suspended in milk yielded Q₁₀ values of about 30 in the range 110–120°. This is higher than the figures previously reported in the literature for R. subtilis spores. Spores of a number of strains of B. subtilis were compared with strain 786 and all gave high Q₁₀ values.
The results obtained in this work have been used to predict the destruction of spores at higher temperatures in a UHT plant (Burton et al. 1958). The calculated values agree well with the results obtained in the plant by Franklin et al. (1958).