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 similar to 6 log CFU/cm(2). After air drying for 1 h, 20 pieces of each inoculated produce leaf were immersed in water-based treatment solutions (200 mL per group), including water alone, 25 or 50 ppm of free chlorine, and 0.25 or 0.75% NCR for 2 min. Inoculated produce leaves without any treatment served as baseline. Produce leaves were stored at 10 degrees 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 a fluorescence microscope. NCR treatment at 0.75% immediately reduced EHEC populations on iceberg lettuce by 1.3 log CFU/cm(2) as compared with the produce treated with water alone (P<0.05). Antimicrobial activity of NCR against EHEC was comparable to chlorine treatments on day 0 for all produce (P>0.05). After 14 days of storage at 10 degrees C, populations of EHEC on 0.75% NCR-treated romaine lettuce were reduced by 2.3 log CFU/cm(2) compared with the recovery from 50 ppm of 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, compared with the scattered cells seen on NCR-treated leaf surfaces. Treatments with NCR did not significantly affect the color of the fresh produce leaves during 14 days of storage at 10 degrees 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.