The Antimicrobial Efficacy Of Plasma Activated Water Is Modulated By Reactor Design And Water Composition

Abstract

Plasma activated water (PAW) contains a cocktail of reactive oxidative species and free radicals and has demonstrated efficacy as a sanitizer for fresh produce, however there is a need for further optimization. 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, discharge frequencies of 500, 1000 and 1500 Hz, and MilliQ and tap water, was investigated with model organisms Listeria innocua and Escherichia coli. Optimal conditions were subsequently employed for pathogenic bacteria Listeria monocytogenes, E. coli and Salmonella enterica. PAW generated with the DBDD reactor reduced more than 6-log CFU of bacteria within 1 minute of treatment. The BSD-PAW, while attaining high CFU reduction was less effective, particularly for L. innocua. Analysis of physicochemical properties revealed BSD-PAW had a greater variety of reactive species than DBDD-PAW. Scavenger assays were employed to specifically sequester reactive species, including the short-lived superoxide (·O2-) radical that could not be directly measured in the PAW. This demonstrated a critical role of superoxide for the inactivation of both E. coli and L. innocua by DBDD-PAW, while in BSD-PAW it had a role in L. innocua inactivation only. Overall, this study demonstrates the potential of DBDD-PAW in fresh produce, where there is a need for sterilization while minimizing chemical inputs and residues and maintaining food quality. Highly effective PAW was generated using air as a processing gas and tap water, making this a feasible and cost-effective option. Importance There is a growing demand for fresh food produced with minimal processing, however guaranteeing microbial safety in the absence of a thermal kill step is challenging. Plasma-activated water (PAW) is a promising novel antimicrobial but its use in high-risk applications like the sanitization of fresh produce requires further optimization. This study demonstrated the importance of reactor design in the production of reactive species in PAW with capacity to kill bacteria. Very effective PAW was generated using a dielectric barrier discharge-diffuser (DBDD) system, with antimicrobial activity attributed to the presence of superoxide radicals. The DBBD reactor used air as a processing gas and tap water, highlighting the potential of this approach as a cost-effective and green alternative to chemical treatment methods that are currently used in food decontamination.