Essential oil microemulsions inactivate antibiotic-resistant Salmonella Newport and spoilage bacterium Lactobacillus casei on Iceberg lettuce during 28-day storage at 4°C

Authors: ARELLANO, STEPHANIE - University Of Arizona; LAW, BIBIANA - University Of Arizona; Friedman, Mendel; RAVISHANKAR, SADHANA - University Of Arizona

Submitted to: Food Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2021
Publication Date: 6/5/2021
Citation: Arellano, S., Law, B., Friedman, M., Ravishankar, S. 2021. Essential oil microemulsions inactivate antibiotic-resistant Salmonella Newport and spoilage bacterium Lactobacillus casei on Iceberg lettuce during 28-day storage at 4°C. Food Control. 130. Article 108209. https://doi.org/10.1016/j.foodcont.2021.108209.
DOI: https://doi.org/10.1016/j.foodcont.2021.108209

Interpretive Summary: Cinnamon, lemongrass, and oregano plant essential oils contain organic compounds that have shown antimicrobial activity. Certified as Generally Recognized as Safe (GRAS) by the FDA, these phyto-antimicrobials are appealing to consumers because they provide a natural alternative to chemical preservatives such as chlorine and hydrogen peroxide. In the present study, we discovered that the applications of microemulsions of the three essential oils have the potential to increase the safety, quality, and shelf-life of minimally processed vegetables such as Iceberg lettuce while providing eco-friendly and healthful options to the consumers. We found that the microemulsions caused strong inhibition of the antibiotic-resistant Salmonella Newport bacteria on contaminated Iceberg lettuce during storage for 28 days and 4o C compared to our previous studies with non-emulsified essential oils. The antimicrobial emulsions also protected the lettuce against Lactobacilli casei that cause spoilage of the lettuce leaves. In addition, recordings of the visual appearance of treated Iceberg lettuce leaves over a 28-day storage period also demonstrated that the microemulsions show reduced adverse effects on the visual quality such as browning discoloration compared to chlorine and hydrogen peroxide treatments. It seems that the antimicrobial microemulsions can serve as effective natural sanitizer alternatives to inactivate antibiotic-resistant Salmonella Newport and reduce the population of Lactobacilli casei spoilage bacteria on fresh-cut Iceberg lettuce during long-term storage. This study provides an innovative way to apply plant antimicrobials to leafy greens.

Technical Abstract: The produce industry commonly uses chlorine to wash vegetables post-harvest. However, chlorine has disadvantages because it is not sustainable to the environment, is not user friendly, is corrosive to equipment, and loses efficacy in the presence of organic matter. Because consumers prefer natural over chemical sanitizers, alternatives for wash sanitizers must be employed to better meet the needs for improving food safety. Plant-based antimicrobial microemulsion in the wash water were evaluated for their efficacy against antibiotic-resistant pathogenic Salmonella enterica serovar Newport and Lactobacillus casei bacteria that cause spoilage on Iceberg lettuce. Oregano oil, lemongrass oil, and cinnamon oil emulsified with a plant-derived saponin were evaluated for improved solubility of the oil in water. Iceberg lettuce (10 g) was washed thoroughly and inoculated with Lactobacilli casei (6.0 log colony-forming units (CFU)/g) or Salmonella Newport (6.0 log CFU/g). The Iceberg leaves were separately treated with 0.1%, 0.3%, or 0.5% of the emulsions, 50 ppm chlorine, and 3% hydrogen peroxide, stored at 4o C, visually observed, and analyzed for surviving populations of both bacteria on days 0, 3, 7, 10, 14, 21, and 28. The efficacies of the antimicrobials were concentration- and storage-time dependent. The microemulsions exhibited a 2.3-4.37 log CFU)/g reduction in the Salmonella population throughout during storage at days 0-28. The microemulsions were also effective against the Lactobacilli resulting in 0.11-4.25 log CFU/g reduction during storage. Visual observation of treated leaves indicated that the 0.1% oregano microemulsion showed the best visual appeal in Iceberg leaves inoculated with Salmonella and that the 0.5% lemongrass microemulsion most effective in reducing browning of the Iceberg leaves inoculated with Lactobacilli. This study demonstrates the potential of essential oil microemulsions to inactivate foodborne pathogenic and spoilage bacteria on Iceberg lettuce, thus providing natural and effective alternatives to chemicals for produce decontamination and shelf-life extension.