Influence of recovery conditions on apparent heat resistance of Bacillus stearothermophilus spores

Abstract

The effects of post‐treatment environmental factors on the heat resistance of Bacillus stearothermophilus spores (ATCC 12980, 7953, 15951 and 15952) were investigated. Nutrient Agar (NA), Antibiotic Assay Medium (AAM), Dextrose Tryptone Agar (DTA) and Tryptic Soy Agar (TSA) with Ca2+ added to a final concentration of 100 p.p.m. were used as recovery media. No significant differences were seen between D‐values obtained except in the case of strain 12980 when comparing TSA with the other media and for strain 7953 comparing AAM and DTA. The optimum incubation temperature was slightly lower for heated than for unheated spores of each strain, although, in general, 50 °C was adequate. Higher D‐values were obtained at 50–55 °C. The effects of the pH of the medium in the range 5.0–7.0 and the addition of starch and phosphate on heat resistance have also been investigated. Maximum colony counts of heated spores were obtained at pH 7.0 and decreased as pH fell. D‐values were significantly lower at pH ≤ 5.5. Increasing the concentration of phosphate in the recovery medium from 0 to 0.2% resulted in a progressive decrease in spore recovery and D‐values. The addition of starch improved recoverability. The z‐values obtained for the four strains studied under the different recovery conditions were similar with a mean value of 7.58 °C ± 0.28.