Inactivation behavior of Pseudomonas aeruginosa by supercritical N2O compared to supercritical CO2

Interest is growing for a non-thermal sterilization technique in the food and pharmaceutical industries in order to ensure microbiological safety without the deterioration of product quality. In this study, supercritical nitrous oxide (SC N2O) treatment was carried out in a multi-batch system to examine its bactericidal effect and characteristics, which largely remains unclear. The effect of operating pressure, temperature, mixing intensity, and working volume ratio (defined as the ratio of sample volume to the reactor volume to be filled with SC fluids) on the inactivation efficiency of SC N2O were investigated in comparison with supercritical carbon dioxide (SC CO2) treatment. Pseudomonas aeruginosa was chosen as a model microorganism. A 8-log reduction of P. aeruginosa cell concentration (10% working volume) in neutral phosphate-buffered saline was achieved by the SC N2O treatment accompanying no pH change in the presence of vigorous mixing (600 rpm) within 6 min, in a condition of 37 °C and 10 MPa and its overall efficiency is comparable to the SC CO2 treatment. Among the reaction parameters investigated in this study, mixing intensity appeared to be the most important operating parameter affecting the bactericidal efficiency. lease of intracellular substances in cells as a result of the SC N2O treatment was explained as one of major bactericidal actions by the SC N2O treatment. At the same time, relatively negligible change of proteins or enzyme activities in the cells in case of the SC N2O treatment was observed compared to the SC CO2 treatment. Also, the observation by scanning and transmission electron microscopy shows the milder morphological change of SC N2O-treated cells in comparison with the SC CO2-treated cells. This study suggests that the SC N2O bactericidal application on food products would be a viable option over SC CO2 application, when mild change of enzyme activity and the proteins, or no pH change are desired.