Purpose: This study was performed to compare thermal inactivation kinetics of spores from high pressure (HP) resistant nonproteolytic C. botulinum type B and F and thermally resistant Bacillus cereus spores suspended in ACES buffer (0.05M, pH 7.0).
Methods: Spores of nonproteolytic C. botulinum strains, Ham-B, Kap 9-B, and 610-F, were prepared using biphasic media. Bacillus cereus spores (F4430/73 and 9818) were prepared using a Nutrient Agar (NA) method. Spores diluted in ACES buffer (0.05M, pH 7) to 105-106 CFU/mL were heat sealed in sterile NMR tubes and subjected to temperatures of 80-91°C in an oil bath. Clostridium botulinum survivors were determined by a 5-tube MPN method using TPGY broth after incubation for 10 weeks and B. cereus survivors by plate count using NA after 48 hour incubation.
Results: Thermal D-values of HP resistant nonproteolytic C. botulinum strains and thermally resistant B. cereus strains decreased as process temperature increased from 80 to 91°C. Thermal D-values at 80°C for Ham-B, Kap 9-B, and 610-F were 5.15, 9.60, and 3.39 minutes, respectively, while B. cereus F4430/73 and 9818 were 369 and 371 minutes, respectively, at 80°C. D-values of the C. botulinum strains decreased to <1.0 minute at 87°C, while B. cereus F4430/73 and 9818 were 21.9 and 31.6 minutes, respectively, at 91°C.
Significance: Based on D-values, results indicate that thermally resistant B. cereus spores, although inactivated by high pressure in combination with heat, are much more resistant to heat alone than HP resistant nonproteolytic C. botulinum spores. The data may be useful for eventually identifying surrogate strains for process validation.