Efficacy of Chlorine and Peracetic Acid to Reduce Shiga Toxin-Producing Escherichia Coli and Impact of Simultaneous Nitrogen-Based Fertilizer Use on Microbial Die-Off in Preharvest Agricultural Water

Abstract

Several foodborne disease outbreaks in the United States have been linked to the consumption of various types of leafy greens in which irrigation water was suspected as the potential source of contamination. To reduce potential produce contamination from agricultural water, the U.S. Food and Drug Administration (FDA) has proposed regulations/metrics which would require growers to assess their agricultural water systems. In some cases, this would mean monitoring their water quality and taking corrective action, by way of antimicrobial treatments, when agricultural waters are deemed as a “reasonably likely foreseeable hazard”. Additionally, the Arizona and California Leafy Greens Marketing Agreements (AZ/CA LGMA) require growers utilizing surface water for overhead irrigation to treat their water within 21 days of harvest to meet acceptable risk indicators; generic Escherichia coli (E. coli) (non-detect per 100mL) and Total Coliform bacteria (<99 MPN/100mL). For many growers, this will be the first time that water quality data may necessitate them to use an antimicrobial treatment before irrigation can be applied safely. Additionally, growers are faced with a myriad of options related to antimicrobial water treatment with very little guidance on the most appropriate treatment option for their ranch, or the requirements needed for successful implementation. With limited guidance, water treatment decisions are likely to be unsuccessful and expend both excess time and money while seeing little to no reduction in potential pathogen loading in an agricultural water source and thus little to no reduction in microbiological risk. To provide guidance on antimicrobial agricultural water treatment options available to industry, the efficacy of two antimicrobial treatments Peroxyacetic Acid (PAA) and Calcium Hypochlorite (Cl) were tested, in triplicate. Tests were executed for various rates of each antimicrobial product (sanitizer), 6 & 8 PPM for PAA and 2 & 4 PPM for Cl. For each sanitizer at each PPM, tests were conducted at temperatures 12°C and 32°C. To evaluate sanitization efficacy, the team measured the reduction of a 109 CFU/mL cocktail of Shiga toxin-producing E. coli (STEC) strains (ATCC MP-9 and 43895) in four water sources from across the southwestern United States (Yuma and Maricopa, AZ, Uvalde and Edinburgh, TX). Four different water sources were used to gauge if water quality impacted sanitization efficacy. The experimental design was based on an EPA/FDA protocol to assess the efficacy of an antimicrobial product to reduce foodborne bacteria in pre-harvest agricultural water (https://www.fda.gov/media/140640/download) . This protocol dictates that STEC cocktail be added to agricultural water then equilibrated at either temperature (12°C or 32°C); post equilibration, each sanitizer, for each concentration, is injected into the mixed solution. The appointed contact time (1 or 5 minutes) is given and then the solution is neutralized and evaluated. To further growers’ comprehension of best management practices for successful antimicrobial treatment application, the impact of two nitrogen-based fertilizers (UAN32 and CAN17) on the efficacy of Sodium Hypochlorite 6% (chlorine) and PAA against naturally occurring coliforms was also evaluated. The first study provides evidence that chlorine meets EPA’s required 3-log reduction of pathogens in order to receive label approval. At a one-minute contact time, the chlorine treatment resulted in log reduction values (LRVs) ranging from 3.24 to 6.15 regardless of temperature, dose/PPM, or water source. PAA however did not perform as well with LRVs ranged from 0.0 to 1.10 with higher reduction occurring at the higher temperature and dose of PAA. When the contact time of PAA treatment was increased to five minutes, LRVs increased and ranges from 1.5 to 5.4 were observed; the efficacy of the sanitizer increased with increased solution temperature. Furthermore, the addition of nitrogen-based fertilizer to the water source in tandem with treatment application significantly affected the antimicrobial capabilities of chlorine. For chlorine, when applied unaccompanied an average log reduction of 3 logs was seen. However, LRVs decreased on average by 1.34 logs when fertilizer was introduced: with the greatest reduction in efficacy resulting in a nearly 2-log decrease. Contrarily, combined application of PAA and either fertilizer showed little to no interaction with a 0.4 log increase in disinfection efficacy when UAN32 was used. Results indicate that a prolonged contact time may be needed to meet regulations when PAA is used as an antimicrobial treatment. As well, growers must be cautious when applying fertilizer conjointly with antimicrobial treatment to their agricultural waters to ensure compliance with new proposed food safety metrics.