Graphical Procedure for Comparing Thermal Death of Bacillus stearothermophilus Spores in Saturated and Superheated Steam

Abstract

SHULL, JAMES J. (Wilmot Castle Co., Rochester, N. Y.) AND ROBERT R. ERNST. Graphical procedure for comparing thermal death of Bacillus stearothermophilus spores in saturated and superheated steam. Appl. Microbiol. 10:452-457. 1962-The thermal death curve of dried spores of Bacillus stearothermophilus in saturated steam was characterized by three phases: (i) a sharp initial rise in viable count; (ii) a low rate of death which gradually increased; and (iii) logarithmic death at maximal rate. The first phase was a reflection of inadequate heat activation of the spore population. The second and third phases represented the characteristic thermal death curve of the spores in saturated steam. A jacketed steam sterilizer, equipped with a system for initial evacuation of the chamber, was examined for superheat during normal operation. MNeasurements of spore inactivation and temperature revealed superheat in surface layers of fabrics being processed in steam at 121 C. The high temperature of the fabric surfaces was attributed to absorption of excess heat energy from superheated steam. The superheated steam was produced at the beginning of the normal sterilizing cycle by transfer of heat from the steam-heated jacket to saturated steam entering the vessel.

Kinetics of Heat Activation and of Thermal Death of Bacterial Spores

“…than activated spores, a time delay would result before all remainiing viable spores were activated and equally susceptible to heat, and the logarithmic portion of the death curve would extrapolate to a point in excess of the direct counit. The latter situation was observed by lialvorson (1958) and inferred in the report of Shull and Ernst (1962). Shull and Ernst (1962) demonstrated that heat activation can be responsible for a signiificant portion of the initial lag in the thermal death curves of spores of Bacillus stearothermophilus, but it was still niot clear how much of the lag could be attributed to heat activation, and whether the death of the spores obeyed first-order kinetics.…”

“…The latter situation was observed by lialvorson (1958) and inferred in the report of Shull and Ernst (1962). Shull and Ernst (1962) demonstrated that heat activation can be responsible for a signiificant portion of the initial lag in the thermal death curves of spores of Bacillus stearothermophilus, but it was still niot clear how much of the lag could be attributed to heat activation, and whether the death of the spores obeyed first-order kinetics. We have re-examined this system in an effort to resolve three questions.…”

“…Therefore, any system in which the temperature changes during a significant portion of the experiment would give rise to distortions in the thermal death curve. Consequently, the curve reported by Shull and Ernst (1962) is probably distorted as the result of a relatively long heating time. Because the data at hand are subject to this procedural error, answers to the questions posed must await acquisition of more exacting experimental data or application of the hypothetical equations to data available in other laboratories.…”

“…This concept has led to the use of D values (the time required to reduce the number of viable microorganisms by 90 %7 under specified treatment conditions) and first-order reactioni-rate constants (k) to describe the resistance of microorganisms to heat; these values, in turn, have been employed to predict extinction of a population of microorganisms under the specified treatmenit conditions. However, several investigators have reported nonlogarithmic thermal death curves for certain strains of bacterial spores (Halvorson, 1958;Stern and Proctor, 1954;Shull and Ernst, 1962). In such cases, D values cannot be applied meaningfully, and a more generally applicable description is desired.…”

Comparison of Survivor Curves of Bacillus subtilis Spores Subjected to Wet and Dry Heat

“…These data suggest that there may be differences between wet and dry heat survivor curves. Shull et al (1962) found that the survivor curve of dried B. stearothermophilus spores heated in saturated steam had three phases: (1) an initial rise; (2) a low rate of death which increased slowly;…”