Aims: This study aimed to compare the efficacy of plasma-activated water (PAW) generated by two novel plasma reactors against pathogenic foodborne illness organisms. Methods and results:The antimicrobial efficacy of PAW produced by a bubble spark discharge (BSD) reactor and a dielectric barrier discharge-diffuser (DBDD) reactor operating at atmospheric conditions with air, multiple discharge frequencies and Milli-Q and tap water, was investigated with model organisms Listeria innocua and Escherichia coli in situ. Optimal conditions were subsequently employed for pathogenic bacteria Listeria monocytogenes, E. coli and Salmonella enterica. DBDD-PAW reduced more than 6-log of bacteria within 1 min. The BSD-PAW, while attaining high log reduction, was less effective. Analysis of physicochemical properties revealed that BSD-PAW had a greater variety of reactive species than DBDD-PAW.Scavenger assays designed to specifically sequester reactive species demonstrated a critical role of superoxide, particularly in DBDD-PAW.Conclusions: DBDD-PAW demonstrated rapid antimicrobial activity against pathogenic bacteria, with superoxide the critical reactive species. Significance and impact of study: This study demonstrates the potential of DBDD-PAW produced using tap water and air as a feasible and cost-effective option for antimicrobial applications, including food safety.

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The effect of plasma activated water (PAW) generated with a dielectric barrier discharge diffusor (DBDD) system on microbial load and organoleptic quality of cucamelons was investigated and compared to the established sanitizer, sodium hypochlorite (NaOCl). Pathogenic serotypes of Escherichia coli, Salmonella enterica, and Listeria monocytogenes were inoculated onto the surface of cucamelons (6.5 log CFU g-1) and into the wash water (6 log CFU mL-1). PAW treatment involved 2 minutes in situ with water activated at 1500 Hz and 120 V, and air as the feed gas; NaOCl treatment was a wash with 100 ppm total chlorine; and the control treatment was a wash with tap water. PAW treatment produced a 3 log CFU g-1reduction of pathogens on the cucamelon surface without negatively impacting quality or shelf life. NaOCl treatment reduced the pathogenic bacteria on the cucamelon surface by 3-4 log CFU g-1, however, this treatment also reduced fruit shelf life and quality. Both systems reduced 6 log CFU ml-1pathogens in the wash water to below detectable limits. The critical role of superoxide anion radical (·O2-) in the antimicrobial power of DBDD-PAW was demonstrated through a scavenger assay, and chemistry modelling confirmed that ·O2-generation readily occurs in DBDD-PAW generated with the employed settings. Modelling of the physical forces produced during plasma treatment showed that bacteria likely experience strong local electric fields and polarization. We hypothesize that these physical effects synergise with reactive chemical species to produce the acute antimicrobial activity seen with the in situ PAW system.

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Plasma-activated water (PAW) is an emerging sanitizer in the fresh food industry, where food safety must be achieved without a thermal kill step. Here, we demonstrate PAW generated in situ to be a competitive sanitizer technology, providing a significant reduction of pathogenic and spoilage microorganisms while maintaining the quality and shelf life of the produce item.