1887

Abstract

SUMMARY

The heat resistance of the spores of six species of bacteria varied with water activity (a) at which the spores were heated, although the magnitude of the changes differed greatly between species. At all a 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 ( value) required to decrease the population by one log. unit. For all six species the greatest heat resistance was manifest at a values of about 0.2–0.4, the maximum values at 110° now varying from about 2 to 24 hr. At a values less than 0.2 the heat resistance decreased; for spores rigorously dried over PO (0.00 a) the values at 110° now varied between about 30 sec. and 30 min. When the spores were heated at a values above 0.4 the resistance of 4 species decreased considerably, being lowest at 1.00 a; with spores of and the heat resistance decreased less at the high a values; at 1.00 a their values were slightly greater than at 0.00 a. At the high a values the values at 110° varied from less than 0.1 sec. for type E to about 40 min. for and The Q for thermal death was about 10 at high a values, decreasing to about 2 at a values below 0.3. Under very moist conditions spores of were about 50,000 times more heat resistant than were spores of type E; but at a 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

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1966-06-01
2021-10-24
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