Non-thermal atmospheric pressure plasma: Screening for gentle process conditions and antibacterial efficiency on perishable fresh produce

Fresh fruits and vegetables, destined to be eaten raw or minimally processed only, harbor the risk of conveying pathogenic microorganisms. Factors such as weather conditions, which favor survival or growth of microorganisms, and improper handling during cultivation or in the postharvest chain, can contribute to outbreaks of food-borne illness. Application of chemical sanitizers or physical treatments often shows a limited efficiency or does not meet consumer acceptance. Availability of gentle and effective techniques for disinfection of fresh produce, therefore, is highly desirable. Non-thermal gas plasma (NTP) treatment is a promising novel technique to reduce the microbial load on fresh fruits and vegetables. However, knowledge on practical applicability of NTP for fresh fruits and vegetables is very limited. In this study, chlorophyll fluorescence imaging (CFI) was used to elucidate suitable process parameters for application of an atmospheric pressure plasma-jet (kINPen 09, INP Greifswald, Germany) on corn salad, a perishable leafy green. Keeping a distance of 17 mm to the plasma-jet, corn salad leaves could be treated for up to 60 s at a fixed power (8 W) and 5 L min− 1 of argon mixed with 0.1% oxygen. Surface temperature on leaves did never exceed 35.2 °C. Antibacterial tests were performed on corn salad, cucumber, apple, and tomato and achieved an inactivation of artificially inoculated Escherichia coli DSM 1116 of 4.1 ± 1.2, 4.7 ± 0.4, 4.7 ± 0, and 3.3 ± 0.9 log units, respectively, after 60 s treatment time. Additional tests with a dielectric barrier discharge plasma and indirect plasma treatment within a remote exposure reactor, fed by a microwave induced plasma torch, did not result in equivalent levels of quality retention as observed using the plasma-jet. Development of gentle non-thermal disinfection methods aims to provide the industry with new tools to actively improve the microbial status of fresh produce beyond the preventive benefits of good hygiene practices and the limited efficacy of post-harvest washing. The presented study shows how cold plasma can be applied to heat-sensitive lettuce leaves without detrimental effects to product quality. The additional microbiological tests offer insights into the antibacterial capacity of cold plasma on different produce surfaces. The results contribute to prompt the development of appropriate large-scale plasma sources to establish a new plasma-based sanitation technique for fresh fruits and vegetables, which should also be implementable into running process lines.