Nanoemulsified carvacrol as a novel washing treatment reduces Escherichia coli O157:H7 on fresh produce

Authors: CHEN, CHI-HUNG - US Department Of Agriculture (USDA); YIN, HSIN-BAI - US Department Of Agriculture (USDA); TENG, ZI - US Department Of Agriculture (USDA); BYUN, SUYEUN - US Department Of Agriculture (USDA); GUAN, YONGGUANG - US Department Of Agriculture (USDA); Luo, Yaguang - Sunny; UPADHYAY, ABHINAV - University Of Connecticut; Patel, Jitu

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2021
Publication Date: 8/19/2021
Citation: Chen, C., Yin, H., Teng, Z., Byun, S., Guan, Y., Luo, Y., Upadhyay, A., Patel, J.R. 2021. Nanoemulsified carvacrol as a novel washing treatment reduces Escherichia coli O157:H7 on fresh produce. Journal of Food Protection. 84(12):2163-2173.

Interpretive Summary: Foodborne illnesses associated with consumption of contaminated fresh produce have increased in recent years. Consumers' preference for least chemicals in food have led research on natural antimicrobials for killing disease-causing bacteria on fresh produce. We evaluated nanoemulsified carvacrol (NCR), a plant-based essential oil as a washing treatment to kill Escherichia coli O157:H7 on fresh produce. Spinach, Romaine lettuce and Icerberg lettuce inoculated with E. coli O157:H7 were washed with 0.25 or 0.75% NCR, or chlorine (25 or 50 ppm) for 2 min and stored at 10°C for 14 days. Fresh produce were analyzed for surviving E. coli O157:H7 populations on days 0, 2, 7, and 14 during storage. The antimicrobial activity of NCR against EHEC was more pronounced on Romaine lettuce, followed by Iceberg lettuce and spinach; populations of EHEC on Romaine lettuce were reduced by 4 log CFU/cm2 (P < 0.05) as compared to the baseline after 14 days. The NCR was treatment did not change the color on the fresh produce leaves during storage. Findings of this study will be helpful to small organic growers in application of natural antimicrobial NCR as a wash treatment for fresh produce.

Technical Abstract: 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), a natural phytochemical, 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/cm2. 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 by plating on CT-Sorbitol MacConkey agar containing nalidixic acid. The viability of EHEC following NCR treatments on the fresh produce was visualized under fluorescence microscope (FM). In addition, the effect of antimicrobial treatments on the color of the fresh produce during storage was determined. The antimicrobial activity of NCR against EHEC was more pronounced on Romaine lettuce, followed by Iceberg lettuce and spinach; populations of EHEC on Romaine lettuce were reduced by 4 log CFU/cm2 (P < 0.05) as compared to the baseline after 14 days. Microscopic images revealed that more viable EHEC cells were observed to be clustered on the baseline samples as compared to the scattered cells seen on the NCR-treated leaf surfaces. Treatments of 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.