Inactivation of Bacillus Spores in Dry Systems at Low and High Temperatures

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Abstract
A plot of the thermal resistance of B. subtilis var. niger spores (log D value) against temperature was linear between 37 and 190.degree. C (z = 23.degree. C), provided that the relative humidity of the spore environment was kept below a certain critical level. The corresponding plot for B. stearothermophilus spores was linear in the range 150-180.degree. C (z = 29.degree. C) but departed from linearity at lower temperatures (decreasing z value). The z value of 29.degree. C was decreased to 23.degree. C if spores were dried before heat treatment. The straight line corresponding to this new z value was consistent with the inactivation rate at a lower temperature (60.degree. C). Bacterial spores which are treated in dry heat at an environmental relative humidity near zero are apparently inactivated mainly by a drying process. By extrapolation of the thermal resistance plot obtained under these conditions for B. subtilis var. niger spores, the D value at 0.degree. C would be about 4 yr.