Thermal resistance of Bacillus subtilis var. niger in a closed system

Abstract

The heat resistance of B. subtilis var. niger has been measured from 85 to 125°C using moisture levels of percent relative humidity (%RH) ≤ 0.001 to 100 in a closed system. Five curves have been presented to characterize the thermal destruction, using thermal death times defined as F values at a given combination of three moisture and temperature conditions. Reductions of 99.99% (4-log10 cycles) of the initial population were estimated for the 3 moisture conditions. At 110°C, the expected time for a 4-log10 reduction was 1.1 hr at %RH = 100, 3.1 hr at %RH ≤ 0.1, and 54 hr at %RH = 10.7. Goodness of fit tests to examine the adequacy of 3 polynomial models failed to indicate a trend. The linear model (from which estimates of D are obtained) was satisfactory for estimating the thermal death times (%RH ≤ 0.1) in the plate count range. The estimates based on observed thermal death times and D values for the %RH = 100 diverged so that D values generally gave a more conservative estimate over the temperature range 90 to 125°C. Estimates of Z(F) and Z(L) ranged from 32.1 to 58.3°C for the %RH of ≤ 0.1 and 100. A Z(D) value of 30.0 was obtained for data observed at %RH ≤ 0.1. The Z(F) results were obtained from plotting observed log times to achieve a 99.99% reduction in the initial population versus temperature. Estimates of Z(L) and Z(D) were obtained by using linear estimates of L100 ∞ 4D and D values in a similar plot.