The Heat Resistance of Bacterial Spores at Various Water Activities

SUMMARY The heat resistance of the spores of six species of bacteria varied with water activity (aw) at which the spores were heated, although the magnitude of the changes differed greatly between species. At all aw values there was an approximately linear relation between the logarithm of the number of viable spores and the time of heating. The slopes of these straight lines were used to describe the observed death-rates as the time (D value) required to decrease the population by one log. unit. For all six species the greatest heat resistance was manifest at aw values of about 0.2–0.4, the maximum D values at 110° now varying from about 2 to 24 hr. At aw values less than 0.2 the heat resistance decreased; for spores rigorously dried over P2O5 (0.00 aw) the D values at 110° now varied between about 30 sec. and 30 min. When the spores were heated at aw values above 0.4 the resistance of 4 species decreased considerably, being lowest at 1.00 aw; with spores of Bacillus coagulans and B. stearothermo-philus the heat resistance decreased less at the high aw values; at 1.00 aw their D values were slightly greater than at 0.00 aw. At the high aw values the D values at 110° varied from less than 0.1 sec. for Clostridiuum botulinum type E to about 40 min. for B. coagulans and B. stearothermophilus. The Q10 for thermal death was about 10 at high aw values, decreasing to about 2 at aw values below 0.3. Under very moist conditions spores of B. stearothermophilus were about 50,000 times more heat resistant than were spores of C. botulinum type E; but at aw values less than about 0.5 this ratio fell to about 10. The convergence of this ratio resulted from more than a 100,000-fold increase in the resistance of the type E spores, and only a 20-fold increase in the resistance of the spores of B. stearothermophilus.