Effect of a Novel Supercritical Carbon Dioxide (CO2) Drying Process on Foodborne Pathogens Inoculated onto Coriander and Strawberries

Introduction:  Dried foods are considered microbiologically stable foods, having a long shelf-life at ambient temperature. However, even though dried foods show adverse conditions toward microbial growth, they may still host pathogenic microorganisms, which may proliferate upon rehydration. Although it is often assumed that drying processes have a lethal effect on vegetative organisms, little is known about bacterial pathogens survivability throughout drying processes.

Purpose: In this study, we investigated the inactivation of three pathogens inoculated onto coriander and strawberries, after drying with supercritical CO₂.

Methods: The fresh products, inoculated with three strains of Escherichia coli O157:H7, Salmonella Typhimurium and Salmonella Thompson or Listeria monocytogenes, were treated with pure, supercritical CO₂ in the following conditions: 1) pressurization to 80 bar at 35°C, immediately followed by depressurization; 2) pressurization to 100 bar at 40°C, immediately followed by depressurization; and 3) pressurization to 80 bar at 35°C, followed by depressurization after 150 min. The depressurization rate was kept at 5 bar/min. Enumeration of E. coli O157:H7, Salmonella spp. and L. monocytogenes was performed by standard plate count.

Results:   After pressurization/depressurization, coriander lost less than 10% of its initial weight, whereas, after 150 min at 80 bar/35°C, the average mass loss was 88.7%. Independent of the type of treatments, four to six log inactivation of E. coli O157:H7 and Salmonella spp. strains were noted for coriander. Listeria monocytogenes strains were found to be more resistant, showing a four log reduction. On strawberry, E. coli O157:H7 and Salmonella spp. strains were only reduced by two to three log units, whereas in this case, L. monocytogenes strains were more susceptible, and again a four log reduction was noted.

Significance:   This study indicates that supercritical CO₂ can be used for drying, while offering a good inactivation of bacterial pathogens. Moreover, microorganisms resistance to the process were strongly influenced by the food matrix.