Comparison of plasma-activated water and free chlorine in disinfecting Escherichia coli O157:H7- and Salmonella Typhimurium-inoculated blueberry, cherry tomato, fresh-cut lettuce, and baby spinach

Abstract

The concentration of reactive oxygen and nitrogen species (RONS) in plasma-activated water (PAW) can be increased by increasing the input power, discharge voltage, and discharge time under a constant discharge frequency. However, knowledge regarding RONS concentration prepared under different discharge frequencies is lacking. This study investigated how decreasing the discharge frequency affects the chemical and physical properties of PAW. The highest plasma output efficiency (57%) was observed when the discharge frequency was decreased to 200 Hz, resulting in highest concentrations of hydrogen peroxide (675 μM) and nitrate (32 mg/L), as well as highest oxidation-reduction potential (547 mV) and conductivity (870 μs/cm). Fresh-cut lettuce, baby spinach, blueberry, and cherry tomato inoculated with Escherichia coli O157:H7 and Salmonella Typhimurium were washed with 200 Hz-PAW and 10 ppm free chlorine (FC) for 1–2 min. The pathogens on the produce were inactivated by 1.8–2.5 log CFU/g using PAW, which was higher than that achieved using FC. The residual pathogens were 1.5–3.1 log CFU/mL in PAW washing water, whereas in FC water, they were inactivated below the detection limit (2 CFU/mL). Thus, decreasing the discharge frequency can increase plasma output efficiency to improve PAW properties and confirms that PAW is limited at controlling pathogen cross-contamination.