Fast-responding thermal-death-time tubes for the determination of thermal bacteria inactivation

Comparison of heating block and water bath methods to determine heat resistance in Shiga-toxin producing Escherichia coli with and without the locus of heat resistance

This study found variability in the time required for tubes of media in heating block wells to reach 60 °C, resulting in significant effects on heat resistance measurements. To determine the extent that methodology changed heat resistance measurements, we compared the heat resistance of Shiga-toxin producing Escherichia coli (STEC) strains with and without the locus of heat resistance (LHR) using both heating block and water bath methods. A total of 34 strains of STEC were used along with a generic E. coli which has been identified as heat-resistant and used as a positive control. The E. coli strains were incubated in a water bath and a heating block set at 60 °C to determine come up time to 60 °C (T0) and for 6 additional minutes (T6) to calculate the D₆₀ value. After incubation, the colony forming units (CFU) were enumerated and mean log CFU/mL from biological replicates was calculated. To compare reductions from T0 to T6, standard deviations among replicates within heating method and correlation of the D₆₀ values generated across methods were determined using Mixed model and Correlation analyses. Our findings indicate that the method chosen to evaluate heat resistance of E. coli can dramatically influence results as there was not a significant correlation between D₆₀ values for the same isolate determined by water bath and heating block methods. The water bath method generates more reliable and consistent heat resistance data and should be used in future evaluations of heat resistance in E. coli. Moreover, PCR screening for the LHR would only be moderately useful for predicting phenotypic heat-resistance of E. coli. Considering water bath data only, LHR-positive STEC isolates were either moderately heat-resistant (1 to 5 log reduction) or heat-sensitive (> 5 log reduction). As LHR-negative STEC were also moderately heat-resistant, prediction of phenotypic heat resistance from genotype requires further refinement.

Performance of a Heating Block System Designed for Studying the Heat Resistance of Bacteria in Foods

Knowledge of bacteria's heat resistance is essential for developing effective thermal treatments. Choosing an appropriate test method is important to accurately determine bacteria's heat resistances. Although being a major factor to influence the thermo-tolerance of bacteria, the heating rate in samples cannot be controlled in water or oil bath methods due to main dependence on sample's thermal properties. A heating block system (HBS) was designed to regulate the heating rates in liquid, semi-solid and solid foods using a temperature controller. Distilled water, apple juice, mashed potato, almond powder and beef were selected to evaluate the HBS's performance by experiment and computer simulation. The results showed that the heating rates of 1, 5 and 10 °C/min with final set-point temperatures and holding times could be easily and precisely achieved in five selected food materials. A good agreement in sample central temperature profiles was obtained under various heating rates between experiment and simulation. The experimental and simulated results showed that the HBS could provide a sufficiently uniform heating environment in food samples. The effect of heating rate on bacterial thermal resistance was evaluated with the HBS. The system may hold potential applications for rapid and accurate assessments of bacteria's thermo-tolerances.