Heat Resistance of Clostridium botulinum Type G in Phosphate Buffer

The heat resistance of two strains of Clostridium botulinum type G in phosphate buffer was studied by the thermal death time (TDT) tube method and the thermal destruction rate (TDR) method. The strains were estimated to have one highly heat-resistant spore among approximately 100 spores or 10,000 relatively heat-labile spores. The heat-labile spores were studied by the TDR method and the heat-resistant spores by the TDT tube method. Decimal reduction times (D) for the heat-labile spores were determined by the slopes of the survivor curves. D values for strain 89 ranged from 0.6 min at 190°F to 6.9 min at 170°F and for strain 2739 from 0.9 min at 200°F to 5.9 min at 180°F. Thermal destruction curves for the heat-labile spores gave z values of 24.0 and 17.5 for two spore stocks of strain 89 and 26.0 for strain 2739. D values for the heat-resistant spores, calculated from the combined data of replicate experiments by the Schmidt probability method, ranged from 0.29 min at 240°F to 1.51 min at 210°F for strain 89 and from 0.25 min at 240°F to 1.48 min at 210°F for strain 2739. Extrapolated to 250°F, the thermal destruction curves of the heat-resistant spores gave D₂₅₀ values of 0.14 to 0.19 min. The thermal destruction curves of the heat-resistant spores were very flat, however, with z values of 37.9 and 49.1 for the two spore stocks of strain 89 and 37.7 for strain 2739. Low-acid canned food processes will provide the same margin of safety for type G as for other proteolytic strains of C. botulinum but ultra high processing temperatures probably will not.