Inactivation of internalized Salmonella Typhimurium in lettuce and green onion using ultraviolet C irradiation and chemical sanitizers

Abstract

Aims: The internalized human pathogens in fresh produce are not effectively removed during conventional washing, and therefore, it may cause foodborne illness when the produce is consumed raw. Thus, effective nonthermal processes are needed to prevent this risk. Methods and Results: Green fluorescence protein-tagged Salmonella Typhimurium was either sprayed on the surface of iceberg lettuce or injected into the bottom part (bulb) of green onions to induce bacterial internalization. The contaminated vegetables were collected after 2 days and subjected to surface disinfection. Different fluencies of UV-C radiation (75-900 mJ cm-2) and two fluencies of UV-C (450, 900 mJ cm-2) combined with chlorine and peracetic acid (PAA) were applied to the produce to examine the inactivation efficiency of internalized bacteria. A range of 1·96-2·52 log reduction in the internalized Salmonella was achieved when the lettuce was treated with higher UV-C fluency (150, 450, 900 mJ cm-2) or UV-C combined with chemical disinfectants. Significant reduction (1·00-1·49 log CFU g-1) in internalized Salmonella was observed in green onion treated with UV-C with the fluency of 150 or 900 mJ cm-2 or UV-C-chlorine/PAA. No significant reduction was observed in either lettuce or green onion treatments when chlorine or PAA was used alone. The food quality measured with firmness was not changed during any treatments. However, a slight colour change was observed in lettuce only when UV-C was used at 900 mJ cm-2. Conclusion: High fluency UV-C can significantly inactivate the internalized Salmonella in lettuce and green onion while maintaining the food quality. Significance and Impact of the Study: This research provides applicable research outcomes for developing nonthermal methods to inactivate internalized pathogens in fresh produce. © 2013 The Society for Applied Microbiology.