Significance of Variations in Observed Slopes of Thermal Death Time Curves for Putrefactive Anaerobes

Abstract

Information on the thermal resistance characteristics of specific spoilage organisms is needed t o estimate the lethalities of heat treatments applied in the pasteurization or sterilization of foods. Data now in the literature provide substantial basis for the assumption that, f o r a given organism under specified conditions, thermal death time is an exponential function of temperature. Thus, the resistance characteristics of a spoilage organism are designated by a thermal death time curve which, for practical purposes, is a straight line on semilogarithmic paper defined by a slope or z value and a point or F value (1). These data are also sufficient to demonstrate that the F value for a given organism depends upon the nature of the medium in which its determination i s made (5, 6).Slope or z value of the thermal death time curve also appears to be influenced b y the medium in which it is determined but relevant data are less conclusive. Thus, reported heat resistance data for spores of Cameron's Putrefactive Anaerobe 3679, a typical low-acid canned food spoilage organism, can be interpreted as evidence that z values of its thermal death time curves vary when determined in different food substrates (6). However, it is also notable that replicated determinations in a single food substrate have yielded z values exhibiting a range of variation comparable to that observed from determinations in several different substrates. For example, Townsend, Esty, and Baselt (14) using asparagus, peas, spinach, and milk as substrates, observed composite z values for P. A. 3679 spores varying from 16.9 to 21.1. At the same time individual determinations in asparagus yielded z values ranging from 16.6 to 30.0.In a previous paper, Reynolds, Kaplan, Spencer, and Lichtenstein (6) reported data on the characteristics of thermal death time curves for P. A. 3679 spores heated in 13 different vegetable substrates. The purpose of this report is to present additional thermal death time data on P. A. 3679 and Clostridiunz botulinum spores and to determine whether the variations in z values that were observed when spores were heated in a number of different substrates, represent real interactions between substrate and relative spore resistance or whether they can be attributed to uncontrolled variables in the experimental methodology 1ist.d. METHODSThe methods, media, techniques, and nomenclature used in this study mere tbose previously aescribea (6). Spore suspensions of Clostridim botulinum 313-B were prepared 173

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