Inactivation of Bacillus subtilis and Geobacillus stearothermophilus inoculated over metal surfaces using supercritical CO2 process and nisin

•SC-CO2 alone was not able to inactivate bacterial spores on metal surfaces.•Pressure cycles improved inactivation efficiency.•A synergic sterilizing effect was observed between nisin and SC-CO2.•No morphological changes in bacterial spores were observed after treatment.•CO2 specificities exert an important role in the inactivation mechanism.

The use of supercritical CO2 (SC-CO2) as sterilizing agent of liquids and solid surfaces is arising as one of the most promising alternative sterilization techniques. This technology shows potential to substitute current chemical and thermal processes that, despite efficient, can damage the material. In this study, the effect of supercritical CO2 treatment (with and without ethanol as co-solvent) on the sterilization of Bacillus subtilis and Geobacillus stearothermophilus spores inoculated over metal surfaces were evaluated under pressure of 30 MPa and different temperatures (30 and 60 °C) and treatment times (60, 120 and 180 min). The effects of pressure cycles (ΔP = 30 MPa) and the addition of a natural antimicrobial agent (nisin) were also tested. Considering the tested conditions, SC-CO2 treatment alone was not able to efficiently reduce spore population of microorganisms. However, an interesting synergic sterilizing effect (more than 7 log cycles) was observed when SC-CO2 treatment was used in combination with low concentrations of nisin.

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