The effects of different media, additives, pH and incubation temperatures were studied on the recovery of heat‐treated spores of Bacillus stearothermophilus. Raising the pH from 6.5 to 7.3 increased the shoulder and decreased the subsequent exponential rate of inactivation with some of the media. Addition of starch or charcoal caused a significant increase in the level of recovery. No significant differences were observed by using two incubation temperatures (50° or 60°C). Yeast‐peptone‐tryptone‐dextrose medium plus starch was found to be the best of the media used.

The relation between the amount of cortex, measured as total hexosamine, as diaminopimelic acid and as muramic lactam, and the heat resistance of spores of five different strains of Bacillus stearothermophilus was studied. Electron micrographs of thin sections of the spores were made to relate the structure of the spores to chemical and thermal characteristics. It was found that the amount of the cortex was significantly related to heat resistance of the spores. Strains with more electron-dense and better organized cortices were found to express higher heat resistance.

Summary. A digital computer program is described which fits log‐linear regression lines to survival curves and thermal destruction time lines and performs appropriate statistical tests. The use of the program is illustrated with heat resistance data from 4 strains of B. stearothermophilus spores at 8 time–temperature relationships. D, F, zD and Q10 values were calculated by the computer program. The heat resistance of the strains was in the order of B

The relation between heat resistance characteristics and the composition of the bacterial spores was studied in five strains of Bacillus stearothermophilus. A relationship was found to exist between the dipicolinic acid (Dpa) and calcium content of the spore and the heat resistance characteristics. The stability of the Dpa chelates was also significantly related to heat resistance.

A simple and accurate technique for the determination of the heat resistance of spores is described. The technique combines a modified capillary tube method with a solid heating block. The come-up time of spore suspensions was found to be short and simple and accurate technique is suggested for the correction of the come-up times. Experimental results are presented for the destruction of spores of Bacillus stearothermophilus at 120 degrees which indicates the accuracy and reproducibility of the new method.