Inactivation of Escherichia coli K12 on raw almonds using supercritical carbon dioxide and thyme oil

Abstract

Raw almonds could be contaminated by pathogens, but the current pasteurization practice using propylene oxide in the U.S. has flammability and carcinogenicity concerns. Supercritical carbon dioxide (scCO2) is a water-free technology and is a solvent of essential oils that are effective antimicrobial preservatives. The objective of this study was to investigate the possibility of combining scCO2 and thyme oil (TO) to reduce Escherichia coli K12 inoculated on raw almonds. Raw almonds inoculated with ∼6 log CFU/g E. coli K12 were batch-treated with scCO2 alone or the combination of presoaking in pure TO followed by scCO2 treatments at different combinations of temperature, pressure, and duration. Compared to scCO2 alone treatments, the combination of TO and scCO2 treatments significantly improved the disinfection effectiveness. Temperature had the most significant effect on the log reduction. At 70 °C, the log reduction by the combination treatment was over 4-log CFU/g and the maximum reduction was 5.16 log CFU/g. The findings suggest that the combination of TO and scCO2 may be a potential water-free technology to meet the requirement of over 4-log reduction of target microorganism for almond and other tree nut products.