Inactivation of dried spores of Bacillus subtilis 168 by a treatment combining high temperature and pressure

Specific treatments combining high temperatures of up to 150 degrees C and moderate pressure of up to 0.6 MPa have been applied to Bridling subtilis 168 spores conditioned at different a(w) levels (between 0.10 and 0.70) corresponding to different residual water contents within the spore core. The spores were treated as a dry powder in a pressurized nitrogen environment or in water/glycerol solutions. These thermodynamic conditions were intended to prevent any water evaporation from the spore core during time/temperature treatments. Our results clearly show that retaining liquid water in the core by applying pressure during the treatment resulted in greater spore destruction (between 2.4 and 4.9 log at 150 degrees C, 120 s and a(w) 0.5 in powder) than the destruction observed after the treatment at atmospheric pressure (0.7 log), during which the water rapidly evaporated because its boiling point was reached. Moreover, we found that the water activity level of the spore had a significant impact on spore destruction: the higher the a(w) level, the greater the spore inactivation. We obtained similar results from spores heat-treated in powder and in water/glycerol solution at the same a, confirming the strong influence of this parameter. We hypothesized that the increased spore inactivation was related to the well-known thermal sensitivity of vital organic molecules such as proteins, enzymes, and ribosomes in the presence of water.