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

The dry-heat inactivation kinetics of Bacillus subtilis var. niger (ATCC9372) spores has been studied in the temperature range of 120-190 degrees 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 X 10(4), 7.4 X 10(6), 6.3 X 10(7), and 6.6 X 10(8) 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 X 10(5) spores per sample. The D-values obtained at 160 degrees C from these samples were about twice as high as the D-values obtained from samples with a concentration of 6.3 X 10(7) or 6.6 X 10(8) spores per sample. The z-value was found to be independent of spore density. Thus, for the two concentrations 7.4 X 10(5) and 6.3 X 10(7) spores per sample, the z-value was found to be 22 degrees C and constant over the temperature range investigated.