Inactivation of Salmonella , Listeria monocytogenes and Escherichia coli O157:H7 inoculated on coriander by freeze-drying and supercritical CO 2 drying

Abstract

Abstract Coriander, either fresh or inoculated with three strains of Escherichia coli O157:H7, Salmonella or Listeria monocytogenes, was treated with supercritical CO2 (scCO2, with and without drying) or freeze-dried. After drying in scCO2 for 150 min at 80 bar and 35 °C, the aerobic plate count, yeasts and molds, and the Enterobacteriaceae were reduced by 2.80, 5.03, and 4.61 log CFU/g, respectively. The total count of mesophilic aerobic spores was not significantly reduced by the treatment. Freeze-drying induced lower reductions with 1.23, 0.87, and 0.97 log CFU/g, respectively. After treatment at 100 bar and 40 °C without drying, inoculated strains of E. coli O157:H7, Salmonella, and L. monocytogenes were inactivated by >7.37, >4.73 and 4.99 log CFU/g, respectively. After drying in scCO2 for 150 min at 80 bar and 35 °C, the strains were reduced by >5.18 log CFU/g. Freeze-drying resulted in lower reduction with maximum 1.53, 2.03, and 0.71 log CFU/g, respectively. This study indicated that scCO2 can be used for drying while offering a good inactivation of E. coli O157:H7, Salmonella, and L. monocytogenes as well as most of the bacteria in the vegetative form naturally occurring on coriander. Industrial relevance Although dried foods are considered microbiological stable foods and show adverse conditions to microbial growth, they may still host pathogenic microorganisms, which may proliferate upon sufficient rehydration. Highly contaminated commodities such as herbs and spices can pose a threat to consumer health if not processed carefully. There is therefore a need to develop or improve drying techniques which can provide dried foods while reducing the initial contamination to acceptable levels in a single process. CO2 is a cheap, accessible solvent, with a low critical point (31 °C, 73.8 bar). Moreover, in the supercritical region, CO2 exhibits potent microbicidal properties. Therefore, supercritical CO2 drying could be a valuable alternative non-thermal technique for conventional drying methods, such as air-drying or freeze-drying, when medium to high value-added food products with high initial contamination are involved.