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 min in situ with water activated at 1,500 Hz and 120 V and air as the feed gas; NaOCl treatment was a wash with 100 ppm total chlorine; 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 to 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 (·O₂^-) in the antimicrobial power of DBDD-PAW was demonstrated through a Tiron scavenger assay, and chemistry modeling confirmed that ·O₂^- generation readily occurs in DBDD-PAW generated with the employed settings. Modeling 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 synergize with reactive chemical species to produce the acute antimicrobial activity seen with the in situ PAW system. IMPORTANCE 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. Our experimental results are supported by modeling of the plasma chemistry and applied physical forces, which show that the system can generate highly reactive ·O₂^- and strong electric fields that combine to produce potent antimicrobial power. In situ PAW has promise in industrial applications as it requires only 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.

Keywords: Cucurbitaceae; E. coli; Listeria; Salmonella; antimicrobial treatment; cold plasma; food safety; fresh produce; spoilage; superoxide.

FIG 1

Schematic of the experimental design for the treatment of cucamelons by PAW generated by a dielectric barrier discharge diffusor (DBDD) system.

FIG 2

Schematic of the DBDD-PAW reactor containing the cucamelon and bacteria for electric field modeling with COMSOL. (A) DDBD reactor geometry including bacterial cells attached to the cucamelon skin for electric field analysis; (B) built-in mesh geometry for the COMSOL program near the bubble and cucamelon surface; (C) details of the modeled bacterial cells floating in solution close to the bubble (left) and on the cucamelon skin (right).

FIG 3

Survival of pathogenic bacteria in the wash water and adhered to the cucamelon surface following a 2-min wash treatment. (A) Log reduction of bacterial CFU per gram on cucamelon surfaces compared to an unwashed control after 2-min treatment with tap water, NaOCl (total chlorine, 100 ppm, and pH 6.5), PAW, or PAW with the ·O₂⁻ scavenger Tiron. (B) Survival of pathogenic bacteria in the wash solution after tap water, NaOCl, PAW, or PAW and Tiron treatments. P values of <0.05 are denoted by different letters. Error bars represent the standard error of the mean, and the dotted line denotes the limit of detection.

FIG 4

Scanning electron microscopy images of pathogenic bacteria adhered to the surface of cucamelons and treated with a 2-min wash of water, PAW, or NaOCl. Following PAW treatment, many of the E. coli cells exhibited a deflated, dehydrated, and crumpled cellular morphology (arrow on left panel), while some L. monocytogenes cells had a distinct rupturing from their outermost ends (arrow on right panel). The surfaces of the cells of both bacterial species were moderately crumpled following treatment with NaOCl.

FIG 5

Quality parameters of cucamelons during storage following different wash treatments. (A) Counts of total viable mesophilic bacteria; (B) counts of total yeast and molds; (C) texture of the cucamelons, with higher peak force units representing a firmer fruit; (D) total color change of the surface of the cucamelons over time with the light and dark sections combined; (E) quality of cucamelons as scored by a panel marking organoleptic properties. P values of <0.05 are denoted by *, and error bars represent standard error of the mean.

FIG 6

Plasma chemistry modeling of the N₂/O₂ plasma discharge at atmospheric pressure. (A) The mole fraction of important gas species produced by the DBDD plasma under E/N conditions of 30, 40, and 50 Td. NO_x indicates total nitrogen oxide species in the gas phase including NO, NO₂, NO₃, N₂O₃, N₂O₄, and N₂O₅. (B and C) The important production (B) and loss (C) mechanisms and rates of O₂⁻ in the gas phase. The settings used for this modeling were a glass transition temperature of 300 K, a residence time of 0.067 s, and a gas composition of 0.8:0.2 N₂:O₂.

FIG 7

Modeling of the spatial distribution of the normal component of electric field (A), polarization (B), and maximum current density (C) at the peak voltage, 8 kV, in the entire configuration of the DDBD reactor.

FIG 8

Modeling of the electric field distribution and charge accumulation for bacterial cells in the DBDD-PAW system. (A) Electric field analysis, where gray lines show the streamline of the electric field between the bubble and the cucamelon at peak voltage 8 kV. (B and C) Detailed local electric field distribution close to the bacterial cells shown next to the bubble (B) and on the cucamelon skin (C). (D and E) Simulated maximum current density distribution near bacterial cells floating in the water close to the bubble (D) and attached on the cucamelon skin (E).