A Method for the Determination of the Thermal Resistance of Bacterial Spores

Abstract

The heat resistance of bacterial spores at temperatures above 212 F was first studied systematically by Bigelow and Esty (1920), who introduced the procedure of heating suspensions in sealed tubes in an oil bath. Townsend, Esty, and Baselt (1938) have described a method uSing a specially designed can or sealed tubes heated in steam in a small retort. More complete details of the construction of the equipment are given by Pilcher (1947). An apparatus for determining spore destruction rates at temperatures higher than 212 F was described by Williams, Merrill, and Cameron (1937). Stumbo (1948) has recently described a technique for studying the resistance of spores in the higher temperature range (250 to 270 F).In the course of investigations of high-temperature short-time processes a method was developed for the determination of the thermal resistance of bacterial spores or "thermal death time" at temperatures of 250 F or higher which possessed the following advantages: (1) more accurate measurement of temperature;(2) reduced lag correction with direct measurement of temperature lag in the product in each experimental run; and (3) the use of cotton-plugged tubes, permitting the addition of subculture media directly into the tube, thus eliminating the possibility of the loss of remaining viable spores during the transfer from sealed tubes.The first two advantages were obtained by using a thermocouple system and recording potentiometer with the thermocouple introduced into a tube of the suspension during the run. The third advantage was obtained by the use of a pressure cooling Vstem whereby the temperature of the tubes could be reduced from process temperature to 212 F under air pressure to prevent spattering of the liquid and blowing of the plugs.The apparatus designed for this purpose has been called a miniature retort and has now been in satisfactory use for approximately two years, during which time some 1,500 retort runs have been made involving more than 15,000 tubes. It is the purpose of this paper to describe the construction of the miniature retort and the principles of its operation for the determination of the thermal resistance of bacterial spores.Construction of miniature retort. The construction of the miniature retort will be described with reference to figure 1. The body of the retort is constructed of a pipe 6 inches in diameter and 12 inches high threaded and capped on the bottom end. To the upper end is attached a 2.5-inch flange with threaded bolts to pass through the 1-inch cover for use with wing nuts to hold the cover in place. In 433 on July 31, 2020 by guest

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