Investigating the influence of pH and selected heating media on thermal destruction kinetics of Geobacillus stearothermophilus (ATCC10149)

Abstract

Geobacillus stearothermophilus is a thermophilic spore-forming bacterium which is used for intense thermal process validations (10 min–15 min). Several factors including the pH and heating medium influence the heat resistance of such microbial spores. In this study, the influence of using different buffer types including McIlvaine and phosphate was compared to distilled water at pH 7. Also, the effect of pH variations was studied using McIlvaine buffer at pH 3, 4, 5, 6, 7, and 8. All kinetics studies were conducted at 110 to 120 (°C). Results showed higher heat sensitivity (higher Z value) in distilled water (8.27 °C) than in phosphate (9.21 °C) and McIlvaine buffers (9.59 °C); likely being influenced by the high percentage of inorganic salts present in these buffers. The spores had the highest heat resistance at neutral pH, and the associated D values decreased both below and above pH 7.0. The pH effect on enhancing the heat sensitivity was higher at 110 °C, but the overall temperature influence was not significantly different (p > 0.05). Further, both first-order and Weibull models were compared for thermal destruction kinetics of G. stearothermophilus. Results demonstrated some concavity in the log-linear survival curves suggesting a slightly better fit for the Weibull model in some cases, but the model did not show any better prediction than the first-order model for a 3D lethality (Fo) value of 15 min. The combined effect of pH and temperature also showed cubic trends, and the Ea ranged between 240 and 398 (kJ) at pH 4–7.