Moderate hydrostatic pressure–temperature combinations for inactivation of Bacillus subtilis spores

Abstract

We report the effect of using moderate hydrostatic pressure, 40–140 MPa, at moderate temperature (38–58°C) to inactivate Bacillus subtilis spores in McIlvaine's citric phosphate buffer at pH 6. We have investigated several parameters: pressure applied, holding time, pressure cycling, and temperature. The kinetics of spore inactivation is reported. The results show that spore inactivation is exponentially proportional to the time the sample is exposed to pressure. Spore germination and inactivation occur at the hydrostatic pressures/temperature combinations we explored. Cycling the pressure while keeping the total time at high pressure constant does not significantly increase spore inactivation. We show that temperature increases spore inactivation at two different rates; a slow rate below 33°C, and at a more rapid rate at higher temperatures. Increasing pressure leads to an increase in spore inactivation below 95 MPa; however, further increases in pressure give a similar rate kill. The time dependence of the effect of pressure is consistent with the first-order model (R² > 0.9). The thermal resistance values (Z_T) of B. subtilis spores are 30°C, 37°C, and 40°C at 60, 80, 100 MPa. The increase in Z_T value at higher pressures indicates lower temperature sensitivity. The pressure resistance values (Z_P) are 125, 125 and 143 MPa at 38°C, 48°C, and 58°C. These Z_P values are lower than those reported for B. subtilis spores in the literature, which indicates higher sensitivity at pressures less than about 140 MPa. We show that at temperatures <60°C, B. subtilis spores are inactivated at pressures below 100 MPa. This finding could have implications for the design of the sterilization equipment.

Keywords:

• high pressure
• temperature
• B. subtilis
• spore
• inactivation

Acknowledgements

The authors are grateful for the technical assistance of Ms Yannan Dou, Ms Sara Hovakeemian, Mr Lewis W.Y. Lau, Ms Hanyu Wang.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by a grant from the Natural Science and Research Council of Canada (RBM).