Produce safety continues to be a challenge because produce undergoes minimal processing prior to consumption and existing sanitizers are not effective in inactivating pathogens. Novel decontamination technologies for produce are required as alternatives to traditional methods. In this project, two non-thermal process techniques were developed based on the interaction of UV light and gallic acid (GA) to enhance the safety of fresh produce.

The first technique is the simultaneous application of UV-A light and GA (UVA+GA). UVA+GA treatment was effective against E. coli O157:H7, and the mechanism behind the synergistic antimicrobial effect was associated with the cellular uptake of GA, generation of reactive oxidative species (ROS), inactivation of enzymes superoxide dismutase, and damage to the bacterial membrane. In the second technique, the antimicrobial activity of GA was enhanced by its prior UV-C exposure (UVC-GA) against E. coli O157:H7 and was persistent for at least 4 weeks. The antimicrobial activity was affected by solution pH and the wavelength of UV-C exposure. The generation of ROS during UV light exposure and photo-oxidized compounds of GA such as quinone contributed to the antimicrobial activity of the UVC-GA solution. Both UVA+GA and UVC-GA treatments can enhance the inactivation of inoculated E. coli O157:H7 on produce such as spinach leaves and tomatoes without affecting the color and firmness. Common environmental stresses could confer complex cross-stress response in E. coli O157:H7 towards UVA+GA and UVC-GA treatments in that both resistance and sensitization can be induced depending on the stress applied and the technology studied. Repeated exposure to moderate UVA+GA or UVC-GA treatment can also select for sub-population that demonstrates higher resistance towards these treatments as well as cross-resistance to other lethal stress such as heat and acid. ROS scavenging enzymes and alternative sigma factor RpoS are highly likely to be associated with the adaptive response process.

In conclusion, both UVA+GA and UVC-GA treatments are promising novel non-thermal techniques that are potential alternative methods for fresh produce disinfection. For future work, a better understanding of the inactivation mechanisms, optimizing of processing parameters, and the development of adaptive response associated with the two treatments need to be explored.