Preservation of Reactive Species in Frozen Plasma-Activated Water and Enhancement of its Bactericidal Activity Through pH Adjustment

Maintaining the characteristics of plasma-activated water (PAW) over a long period is a challenging task as it could compromise its beneficial properties such as antibacterial activity and reactivity. In this study, we aimed to present a practical method to produce PAW for fresh produce treatment. As the use of PAW during the postharvest processing stages could be beneficial, we implemented a strategy to develop frozen PAW (FPAW) containing high concentrations of reactive species and effective in deactivating pathogenic bacteria. However, lowering the freezing temperature from − 20 to − 80 °C and shortening the freezing time failed to maintain the concentrations of hydrogen peroxide (H2O2) and nitrite (NO2−) in FPAW, ultimately reducing its bactericidal activity after a while. Furthermore, we successfully applied pH adjustment to preserve the reactive species and restore the bactericidal activity of FPAW. The reactive species were maintained for up to 10 days by increasing PAW pH from 3.5 to 5 or 7. At the end of the storage period, acidification restored the bactericidal activity of FPAW, significantly reducing the viable populations of Escherichia coli and Staphylococcus aureus. The highest log reduction of E. coli and S. aureus obtained were 4.46 and 3.55 log colony forming units/mL, respectively. These results supported that frozen PAW treated by pH adjustment is a potential strategy for developing effective treatment agents for fresh produce.