Thermal Death Kinetics of Bacillus sporothermodurans Spores Isolated from Ultra-high Temperature Milk

Introduction: Bacillus sporothermodurans is a rod shaped mesophilic bacterium producing highly heat resistant spores (HRS), which can survive ultra-high temperature (UHT) milk processing. The presence of these spores may have implications on the quality and safety of UHT milk and other thermally processed food products. Under UHT conditions, B. sporothermodurans spores have been found to be more resistant than other heat resistant spores, with D₁₄₀ ranging from 3.4 to 7.9 s.

Purpose: The purpose of this study was to establish the heat inactivation kinetics of selected B. sporothermodurans strains with the aim of improving thermal validation during UHT food processing and to evaluate the performance of the linear, Weibull, and biphasic models of inactivation with the aim of establishing the best fit for B. sporothermodurans in UHT milk processing.

Methods: Bacillus sporothermodurans spores were prepared by dispensing one ml of culture onto the sporulation medium [Nutrient broth (25 g/liter), bacteriological agar (15 g/liter), vitamin B12 (1 mg/liter), MnSO₄.H₂O (8.4 mg/liter) and CaCl₂.2H₂O (1 g/liter), pH 6.8)] and then harvesting with physiological saline (8.5 g/liter). Sterile milk was inoculated with the bacterial spores to a concentration of approximately 2x10⁷spores/ml. Thermal inactivation analysis was undertaken and subsequent modeling of the data using the linear, Weibull and biphasic models.

Results: The survival curves indicated a good fit for the nonlinear models for the selected strains. At 130°C, tailing of curves started after approximately 30 s and 35 s treatment time, corresponding to a two-log reduction. The Weibull model consistently proved a better fit than the biphasic and linear models after computation of the mean square error (0.10, 0.14, and 0.77, respectively) and correlation coefficient (0.99, 0.98, and 0.86, respectively).

Significance: The Weibull model should provide the best model for use in thermal inactivation of B. sporothermodurans in UHT milk processing in the food industry.