Nanoemulsified carvacrol as a novel washing treatment reduces Escherichia coli O157:H7 on spinach and lettuce.

Fresh produce continues to be the main source of foodborne illness outbreaks in the United States implicating bacterial pathogens such as Escherichia coli O157:H7 (EHEC). The efficacy of nanoemulsified carvacrol (NCR) as a washing treatment in reducing EHEC on fresh produce was investigated. Fresh baby spinach, Romaine lettuce, and Iceberg lettuce leaves (2.5 cm diameter cores) were spot-inoculated with a five-strain cocktail of nalidixic acid resistant EHEC at ~ 6 log CFU/cm 2 . After air-drying for 1 h, 20 pieces of each inoculated produce leaves were immersed in water-based treatment solutions (200 ml/group), including water alone, 25 or 50 ppm free chlorine, and 0.25% or 0.75% NCR for 2 minutes. Inoculated produce leaves without any treatment served as baseline. Produce leaves were stored at 10°C and surviving EHEC populations were enumerated on days 0, 2, 7 and 14. The viability of EHEC following NCR treatments on the fresh produce was visualized under fluorescence microscope. NCR treatment at 0.75% immediately reduced EHEC populations on Iceberg lettuce by 1.3 log CFU/cm 2 as compared to the produce treated with water alone (P 0.05). After 14-days of storage at 10°C, populations of EHEC on 0.75% NCR treated Romaine lettuce were reduced by 2.3 log CFU/cm 2 as compared to the recovery from 50 ppm chlorine treated samples (P<0.05). Microscopic images revealed that EHEC cells were observed to be clustered on the baseline samples, indicating the development of cell aggregation, as compared to the scattered cells seen on NCR-treated leaf surfaces. Treatments with NCR did not significantly affect the color on the fresh produce leaves during the 14 days of storage at 10°C. Results of this study support the potential use of NCR as a water soluble natural antimicrobial wash treatment for controlling EHEC on fresh produce.