Inactivation of Bacillus subtilis spores by supercritical CO2 treatment

Bacillus subtilis spores were suspended in saline solution (10⁷ cfu/ml) and treated by both conventional heating and CO₂ batch treatment at an operating pressure in the range of 70-150 bar under identical temperature conditions. Temperatures tested were in the range of 36-75 °C. Survival curves indicated significantly higher lethality when spores were treated with supercritical CO₂ (SC-CO₂) rather than with heating alone. These results appear particularly evident at 60 °C, a temperature at which conventional heating gave no spore-inactivation after a treating time as long as 24 h, whereas a 6 h SC-CO₂ treatment led to complete sterilization. At 75 °C spores were partially killed with conventional heating but a treatment of 2 with SC-CO₂ hours assured total inactivation. It is concluded that spore-inactivation during SC-CO₂ treatment was only in part due to thermal effect (at the higher temperature of 75 °C) and there was a significant additional effect caused by CO₂ penetration inside the latent bacteria forms.