Evaluation of Calcium Hypochlorite and Peroxyacetic Acid to Inactivate E. coli and Salmonella in Irrigation Water in Maryland

Addition of antimicrobials to water used for irrigation of fresh produce is recommended to reduce microbial food safety risks. However, there remains a need to validate the efficacy of antimicrobials in irrigation water using the microbial strains and methods recommended by the Environmental Protection Agency (EPA). We evaluated the antimicrobial performance of calcium hypochlorite (Ca(ClO)₂) (2–4 and 10–12 ppm) and peroxyacetic acid (PAA) (6 and 10 ppm) individually in ground and surface agricultural production water in Maryland using the EPA recommended method (No. 94151PA7). Both water samples were slightly alkaline (pH 8.13 and 8.01, respectively) and low in turbidity (0.65 and 4.82 NTU, respectively). A total of 1 mL of EPA-recommended Escherichia coli or Salmonella enterica cocktail (approximately 9.0–9.7 log CFU/mL) was inoculated in 98 mL of irrigation water in three flasks and equilibrated at 12°C or 32°C, resulting in 7.0–7.7 log CFU/mL bacterial levels. One milliliter of sanitizer solution (Ca(ClO)₂ or PAA) was added into the flask followed by mixing for 15 s. At 5 and 10 min, samples were transferred into phosphate buffer saline containing 0.28 g/mL sodium metabisulfite, serially diluted, and plated on quadruplicated TSA-Rifampicin plates for enumeration. Both low and high levels of Ca(ClO)₂ and PAA solutions inactivated over 4.5 log CFU/mL of E. coli and Salmonella cocktails within 5 min at either water temperature, which exceeded the 3-log threshold required by the EPA. Total bacterial inactivation at 10 min exceeded 5 log CFU/mL. The results demonstrated that, in slightly alkaline irrigation water, adequate sanitizing efficacy was achieved with 2–4 ppm of Ca(ClO)₂ and 6 ppm of PAA.