Optimization and evaluation of heat-shock condition for spore enumeration being used in thermal-process verification: Differential responses of spores and vegetative cells of Clostridium sporogenes to heat shock

To evaluate a heat-shock condition for the enumeration of Clostridium sporogenes spores, a surrogate for C. botulinum spores, we examined the heat tolerance of C. sporogenes spores and vegetative cells exposed to a heat shock at 90℃. From the D values of the spores determined in the temperature range of 113-121℃, z value (±SD) and D∧90℃ value were estimated to be 10.16±0.90℃ and 1,071.52 min, respectively, and the inactivation rates were predicted to be only approximately 2% at 90℃ for up to 10 min. Meanwhile, the viable count of spores was significantly higher when activated under a heat-shock condition of 90℃ for over 9 min than those activated for shorter time periods. The heat tolerance of vegetative cells was extremely low, showing a D∧90℃ value (±SD) of 0.21±0.01 min. Finally, 3 different heat-shock conditions were compared: 70℃ for 30 min, 80℃ for 20 min, and 90℃ for 10 min, and the experimental comparative data showed no significant differences in viable spore counts. Consequently, these results support that the heat-shock treatment at 90℃ for 10 min is suitable to activate spores and to inactivate vegetative cells of C. sporogenes.