Thermal Resistance of Bacillus stearothermophilus Heated at High Temperatures in Different Substrates

The effect of mushroom extract, with or without acidification with glucono-δ-lactone, and the overnight incubation of the spores in CaCl₂, on the heat resistance of B. stearothermophilus ATCC 12980 spores was studied. The temperature range considered was 121 to 140°C for mushroom extract and CaCl₂ and 121 to 145°C for double-distilled water as a reference substrate. The results indicated that mushroom extract without added acid significantly reduces the thermal resistance of the spores in comparison to the double-distilled water. Acidification of the mushroom extract reduces the heat resistance of spores of B. stearothermophilus at 121 °C. However, above 130°C lowering of the pH did not significantly reduce the thermal resistance of the spores, and so no generalizations should be made with regard to the effect of the pH when high temperature-short time (HTST) processes are being considered. Overnight incubation in CaCl₂ and subsequent heat treatment lead to increased heat resistance at 121 °C compared to that observed in double-distilled water. However, at 130°C and above CaCl₂ did not increase the apparent heat resistance of the spores.