Analysis of survival rates and cellular fatty acid profiles of Listeria monocytogenes treated with supercritical carbon dioxide under the influence of cosolvents

In the present study, we identified several process variables that significantly affect the efficiency of supercritical carbon dioxide inactivation of the food-borne pathogen Listeria monocytogenes. Treatment with SC-CO₂ completely disabled the colony-forming activity of the cells (8-log reduction) within specific treatment time (10-50 min), pressure (80-150 bar), and temperature ranges (35-45 °C). Microorganism inactivation rates increased proportionally with pressure and temperature, but the inactivation rate decreased significantly when cells were suspended in phosphate-buffered saline rather than in physiological saline. Additionally, when the microbial cell suspension was 80-100% (w/w) of water, the SC-CO₂-mediated reduction in CFU ml^{- 1} was 4-8 log higher at the same treatment conditions than in typical cell suspensions (a water content of 800-4000% [w/w]) or dry preparations that had only 2-10% (w/w) of water. The addition of a fatty acid, oleic acid, decreased the effectiveness of the microbial inactivation by SC-CO₂, but the addition of a surfactant, sucrose monolaurate, increased the effectiveness. Therefore, cosolvents for SC-CO₂, including water, a fatty acid, and a surfactant in this study, were found to greatly influence on the inactivation effectiveness. The extraction of cellular substances, such as nucleic acid- and protein-like materials and fatty acids, was monitored by spectrophotometry and GC/MS and increased with SC-CO₂ treatment time. Additionally, using scanning and transmission electron microscopies, we investigated morphological changes in the SC-CO₂-treated cells. The effects of the variables we have described herein represent a significant contribution to our current knowledge of this method of inactivating food-borne pathogens.