Dry-heat inactivation of Bacillus subtilis var. niger spores with special reference to spore density

Abstract

The dry-heat inactivation kinetics of Bacillus subtilis var. niger (ATCC 9372) spores has been studied in the temperature range of 120–190 °C. The spores were applied to glass plates of a standardized area (3.24 cm²).Spore preparations of five different spore densities were used (8.3 × 10⁴, 7.4 × 10⁵, 7.8 × 10⁶, 6.3 × 10⁷, and 6.6 × 10⁸ spores per sample, respectively). The heat resistance of the spore was dependent on the number of spores per surface unit. Maximum resistance was observed when the concentration was 7.4 × 10⁵ spores per sample. The D-values obtained at 160 °C from these samples were about twice as high as the D-values obtained from samples with a concentration of 6.3 × 10⁷ or 6.6 × 10⁸ spores per sample. The z value was found to be independent of spore density. Thus, for the two concentrations 7.4 × 10⁵ and 6.3 × 10⁷ spores per sample, the z-value was found to be 22 °C and constant over the temperature range investigated.