Determination of Lethality Rate Constants and D-Values for Bacillus atrophaeus (ATCC 9372) Spores Exposed to Dry Heat from 115°C to 170°C

摘要

Dry heat microbial reduction is the NASA-approved sterilization method to reduce the microbial bioburdennon spaceflight hardware for missions with planetary protection requirements. The method involves heating thenspaceflight hardware to temperatures between 104°C and 125°C for up to 50 hours, while controlling the humiditynto very low values. Collection of lethality data at temperatures above 125°C and with ambient (uncontrolled)nhumidity conditions would establish whether any microbial reduction credit can be offered to the flightnproject for processes that occur at temperatures greater than 125°C. The goal of this research is to determinenthe survival rates of Bacillus atrophaeus (ATCC 9372) spores subjected to temperatures higher than 125°C undernboth dry (controlled) and room ambient humidity (36–66% relative humidity) conditions. Spores were depositedninside thin, stainless steel thermal spore exposure vessels (TSEVs) and heated under ambient or controlled humiditynconditions from 115°C to 170°C. After the exposures, the TSEVs were cooled rapidly, and the sporesnwere recovered and plated. Survivor ratios, lethality rate constants, and D-values were calculated at each temperature.nAt 115°C and 125°C, the controlled humidity lethality rate constant was faster than the ambient humiditynlethality rate constant. At 135°C, the ambient and controlled humidity lethality rate constants were statisticallynidentical. At 150°C and 170°C, the ambient humidity lethality rate constant was slightly faster thannthe controlled humidity lethality rate constant. These results provide evidence for possibly modifying the NASAndry heat microbial reduction specification.