Dry-Heat Destruction of Bacillus subtilis Spores on Surfaces: Effect of Humidity in an Open System

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Bacillus subtilis var. niger spores were tested for dry-heat resistance on stainless-steel strips hung in an oven. Heat resistance was dependent on the relative humidity before and during treatment, which in turn affected the water content of the spores. Higher humidities increased the heat resistance of the spores. D-values ranged from 16.1 min for spores conditioned at <2% relative humidity (RH) and treated at 0.34% RH to 37.6 min for spores conditioned at 89% RH and treated at 1.1% RH. The y-intercept of the regression line ranged from 6.94 × 104 for spores conditioned and treated at the low humidities to 2.00 × 105 for spores conditioned at 89% RH and treated at 0.34% RH. For a constant value of N0, the y-intercept appears to be lowered by low-humidity conditions. The statistic log y0/log N0 is used to measure the downward displacement of the regression line. Values obtained in this experiment range from 0.90 for spores conditioned at <2% RH and treated at 0.34% RH to 1.04 for spores conditioned at <2% RH and treated at 1.1% RH. A combination of linear regression and analysis of variance methods was used for data analysis. The former estimates D-values and y-intercepts, whereas the latter is sensitive to differences between treatments.

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