Remember the aphorism, “Perfect is the enemy of good”? It applies to water sanitizers for keeping leafy greens safe.
The Center for Produce Safety (CPS) hosted a Sept. 30 webinar presenting the findings of a study on food safety in leafy greens. The study, partially funded by CPS, looked at how effective common irrigation water sanitizers are at reducing or eliminating E. coli already present in the soil or on leafy greens. It also looked at which E. coli infection scenarios posed the biggest risk.
Generally, the researchers found both common water sanitizers used by industry reduced microbial loads on plants and in soils. They also found that the “animal intrusion” scenario — for example, deer entering fields and leaving droppings — posed the highest and most persistent contamination risk.
“Hopefully, the results of this study are helpful to industry, not only growers but also regulators, because it really helps us understand … additional positive impacts of water treatment to ultimately reduce risks for consumers,” said the study’s principal investigator, Channah Rock of the University of Arizona.
Background on the Study
The research was spawned in part because food safety attention surrounding illness-causing E. coli in leafy greens has shifted to agricultural water. Researchers found a problem, however.
“While significant progress has been made in understanding pathogen reduction within water, a critical knowledge gap remains regarding the impact of treated water on pathogens already present on plant surfaces or in soil,” the report’s abstract reads.
In an attempt to fill that gap, researchers inoculated romaine and the soil the greens grew in with different strains of E. coli. These strains included the 2006 spinach outbreak strain, the 2018 romaine outbreak strain and a generic strain. Plants were inoculated at differing concentrations of E. coli — one intended to mimic atmospheric deposition, one mimicking animal intrusion and the last replicating a treatment failure.
The plants were then treated with irrigation water containing common sanitizers — either peracetic acid (PAA) or calcium hypochlorite (chlorine) — at low and high concentrations. Researchers then looked for the level and persistence of illness-causing E. coli on the plants and in the soil.
Study’s Findings
Overall, both of the trial sanitizers reduced — but did not eliminate — illness-causing microbial loads both on plants and in the soil, regardless of contamination level.
“No sanitizer was completely effective, nor at differing concentrations,” said Kerry Cooper of the University of Arizona, one of the co-principal investigators (PI) on the study. “We see a lot of variation within the different concentrations as well as between the different strains when it comes to the efficacy of the different sanitizers and the different concentrations.”
For example, PAA was more effective at reducing pathogens on the plant in the short term, but chlorine was more effective in the long term. The different strains also showed different reactions to the sanitizers, both on the plant and in the soil.
Efficacy also differed depending upon the contamination scenario. Of the three contamination scenarios tested, the animal intrusion mimicking fecal contamination posed the most risk. It had the greatest microbial load and persisted longer on both plants and in soil.
“Essentially what we’re seeing is bacteria is able to hold on for extended periods of time when we’re comparing it to the other two contamination type scenarios,” Rock said. She also reported that the animal intrusion contamination scenario had the least response to the two sanitation treatment methods.
Still, water sanitizing treatment did reduce microbial loads. And that reduces risks to consumers; an important takeaway.
“Ultimately, what we see is that applying the chemical sanitizer decreased the relative risk in all cases,” said Hunter Quon of Arizona State University. Kerry Hamilton, also of ASU and one of the study’s co-PIs, added that while there is no way to remove all risk, water sanitizer does help reduce the risks to consumers.
Advice to Growers
From the findings, researchers had some recommendations for growers. The first is to optimize their approach to using sanitizers for leafy greens.
“That ultimately is going to help maximize microbial reduction and limit some of that treatment variability,” Rock said.
Regarding the high-risk contamination potential of animal incursion, the report recommends growers “develop enhanced mitigation strategies” to deal with the potential.
“The last recommendation or take home is monitoring of your systems can really help to support confidence that they are operating as intended,” Rock said. She cited the changing requirements of the Food and Drug Administration’s Produce Safety Rule, specifically in Subpart E, backing the importance of water systems data.
“We believe that FDA and other state inspectors are going to be looking for is confidence in your water treatment system,” she added. “The only way that you can do that is to have monitoring information and data that you can present to show their efficacy and to show that they are operating as intended.”
