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. The release 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.