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

We report the effect of using moderate hydrostatic pressure, 40-140MPa, at moderate temperature (38-58 degrees 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 degrees C, and at a more rapid rate at higher temperatures. Increasing pressure leads to an increase in spore inactivation below 95MPa; 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 (R-2>0.9). The thermal resistance values (Z(T)) of B. subtilis spores are 30 degrees C, 37 degrees C, and 40 degrees C at 60, 80, 100MPa. The increase in Z(T) value at higher pressures indicates lower temperature sensitivity. The pressure resistance values (Z(P)) are 125, 125 and 143MPa at 38 degrees C, 48 degrees C, and 58 degrees 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 140MPa. We show that at temperatures <60 degrees C, B. subtilis spores are inactivated at pressures below 100MPa. This finding could have implications for the design of the sterilization equipment.