The effect of bioindicator preparation and storage on thermal resistance of Bacillus stearothermophilus spores

Paper strips inoculated with spores of Bacillus stearothermophilus ATCC 7953 were conventionally dried (lot 1) and lyophilized (lot 2); stored in defined environments of 32 and 86% relative humidity at 10, 25 and 33°C for 210 d; and submitted to moist heat treatments at 121°C. A significant decrease in thermal resistance from initial starting levels was found for lyophilized bioindicators stored at 86% relative humidity. The respective average D _121°C values were 1.55 ± 0.05 and 1.37 ± 0.10 min for lyophilized bioindicators stored at 32 and 86% relative humidity; and 1.65±0.15min and 1.57 ± 0.11 min for dried bioindicators stored in the same environments.