Purpose: We investigated the practicality and efficiency of a combined treatment with UV-C radiation and chemical sanitizers for food industry to inactivate internalized human pathogens in fresh produce.
Methods: Iceberg lettuce was grown in the greenhouse for 4 weeks and then translocated to a growth chamber for Salmonella inoculation. All the lettuce was contaminated with green fluorescent protein-labeled Salmonella (108 CFU/ml) on the leaf surface and harvested in 2 days. Lettuce was rinsed with ethanol, AgNO3 and DI water to remove the surface bacteria after collection. The internalized Salmonella was confirmed with plate count method and confocal microscope. For inactivating the internalized Salmonella, five groups of lettuce were treated for 10 min using different methods: UV-C (200 μW/cm2; dose=1.2 kJ/m2); chlorine (100 ppm); peracetic acid (PAA, 80 ppm); UV-C+chlorine; and UV-C+PAA. The reduction of internalized Salmonellain each treatment was determined with plate count method.
Results: Salmonella was observed in the interior region of the lettuce leaves. The internalization level was 2~3 log CFU/g of lettuce. When applying one single type of disinfectant, UV-C inactivated 98.9% (1.95 log) of the internalized Salmonella, which was higher than chlorine (89.3%, 0.97 log) and PPA (93.4%, 1.18 log). In contrast, the combined methods performed more effectively: UV-C+chlorine and UV-C+PAA reduced the internalized Salmonellaby 99.8% (2.70 log) and 100% (2.87 log), respectively.
Significance: The internalized Salmonella in the lettuce was effectively inactivated applying the combined treatments with UV-C radiation and chemical sanitizers. This research provides a platform for future studies about inactivation of internalized pathogens with non-thermal processing and practical application of UVC irradiation in the fresh produce industry.