The use of two organic antimicrobials in wash water for leafy greens enables the wash water to be reused without losing antimicrobial activity, according to a new study by researchers at the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS) and Oklahoma State University’s Department of Animal and Food Sciences.

Due to leafy greens’ implication in several significant foodborne illness outbreaks related to Escherichia coli O157:H7, and because approved antimicrobials for organic produce are limited, there is a need for research into solutions for the organic leafy greens sector. Oregano and cinnamon essential oils, and their respective natural compounds, carvacrol and cinnamaldehyde, have been shown to be effective against E. coli O157:H7. Therefore, in their study, the researchers explored the use of natural antimicrobials, as well as hydrogen peroxide and water, in leafy greens wash water, examining their reusability.

The researchers inoculated organic baby and mature spinach, as well as romaine and iceberg lettuce, with E. coli 0157:H7. Each antimicrobial was used five times to wash separate batches of contaminated leafy greens in the same wash water. The leafy greens were stored for three days at 4 °C, and surviving E. coli populations were enumerated after wash and during storage on days 0, 1, and 3. The researchers tested oregano and cinnamon essential oils, carvacrol, cinnamaldehyde, and fulvic acid. Hydrogen peroxide, phosphate buffered saline, and distilled water were used as control washes.

No surviving bacteria was detected in any of the organic sanitizer washes tested, and of the controls, no surviving bacteria was detected in the hydrogen peroxide wash. Oregano essential oil proved to be the most effective, reducing E. coli O157:H7 to undetectable levels after immediate application and throughout the three days of storage, for all five washes in all of the leafy greens. Carvacrol showed similar results, except for mature spinach, where pathogen populations of 0.4–1.3 log₁₀ colony forming units per gram (CFU/g) on washes 1–5, and 0.5–0.6 log₁₀ CFU/g on washes 2–5, were observed on days 0 and 1, respectively. No E. coli O157:H7 colonies were detected in the wash water for all five washes, confirming the effect of antimicrobials during reuse.

Cinnamon essential oil and cinnamaldehyde reduced E. coli populations to undetectable levels on all of the leafy greens by day 3 of storage, although E. coli was detected immediately after application on most of the leafy greens types. Cinnamaldehyde was slightly more effective than cinnamon essential oil. Fulvic acid was the least effective of the antimicrobials tested, but it performed better than hydrogen peroxide and water.

Overall, oregano essential oil and carvacrol exhibited the highest degree of reusability, but cinnamon essential oil and cinnamaldehyde also showed significant antimicrobial effects. The researchers conclude that plant-derived essential oils could serve as effective sanitizers against E. coli O157:H7 and retain their antimicrobial activity with continued use.