An effective sanitizer for fresh produce production: In situ plasma activated water treatment inactivates pathogenic bacteria and maintains the quality of cucurbit fruit

Abstract

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-1 reduction 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-1 pathogens in the wash water to below detectable limits. The critical role of superoxide anion radical ({middle dot}O2-) in the antimicrobial power of DBDD-PAW was demonstrated through a scavenger assay, and chemistry modelling confirmed that {middle dot}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. ImportancePlasma 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 micro-organisms while maintaining the quality and shelf life of the produce item. Our experimental results are supported by modelling of the plasma chemistry and applied physical forces, which show that the system can generate highly reactive superoxide radicals and strong electric fields that combine to produce potent antimicrobial power. In situ PAW has promise in industrial applications as it only requires low power (12 W), tap water and air. Moreover, it does not produce toxic by-products or hazardous effluent waste, making it a sustainable solution for fresh food safety.