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: The consumption of fresh-cut leafy greens such as Iceberg lettuce provides nutrients including vitamin C, B1, B2, and folate, all of which are reported to stimulate the immune system. The shift towards healthier lifestyles has resulted in the incorporation of more fresh fruits and vegetables in the human diet, but this has increased the burden of foodborne illness in the Unites States. Shiga toxin-producing Escherichia coli O157:H7 (STEC) was responsible for 57% of the 23 produce-related multistate outbreaks that were due to pathogenic E. coli occurring in years 2010 to 2017. The spoilage organism Pseudomonas fluorescens has been widely known for its presence in minimally processed vegetables accounting for most of the bacterial spoilage in fresh-cut products. Minimally processed fruits and vegetables are highly susceptible to microbial proliferation because of their processing conditions that involve cutting, slicing, or peeling, allowing microbes to gain access to the nutritive substances found within the vascular tissue of the vegetable cut surfaces. To further investigate antimicrobial essential oil treatments against the foodborne pathogen Escherichia coli O157:H7 and spoilage microorganism Pseudomonas fluorescens on Iceberg lettuce, the present study evaluated the effectiveness of oregano, cinnamon, and lemongrass oil microemulsion treatments during a storage period of 28 days at 4°C. The antimicrobial effects of the plant-based microemulsion treatments were found to be concentration and storage-time dependent against the two antibiotic-resistant organisms on Iceberg lettuce. For both Escherichia coli O157:H7 and Pseudomonas fluorescens, the microemulsion treatments not only provided bactericidal and/or growth-inhibitory effects during the 28-day storage period, but some of the treatments also resulted in better visual appeal of the lettuce leaves throughout the 28 days of storage when compared to the untreated positive control. This innovative study complements our related study on the inactivation of pathogenic Salmonella and spoilage Lactobacilli bacteria on Iceberg lettuce leaves.

Technical Abstract: Current industry practices include the use of chemical-based sanitizers such as chlorine to wash vegetables post-harvest. The occurrence of produce-related outbreaks poses concerns about food safety and suggests that better intervention strategies need to be implemented. The use of safe antimicrobial essential oils has been widely accepted because of their anti-inflammatory, anticancer, antiviral, antimicrobial, and antioxidant activities. The objective of this study was to investigate the antimicrobial activities of essential oil-based microemulsions in the wash water against Escherichia coli O157:H7 and Pseudomonas fluorescens bacteria on Iceberg lettuce. Wash microemulsions that were evaluated included oregano oil, lemongrass oil, and cinnamon oil along with a saponin emulsifier for improved solubility of the oil in water. Iceberg lettuce was inoculated with either E. coli O157:H7 (6.0 log colony-forming-units, CFU/g) or P. fluorescens (6.0 log CFU/g), and for comparison dip treated in a phosphate buffered saline (PBS) control, 50-ppm chlorine, 3% hydrogen peroxide treatment or a 0.1%, 0.3%, or 0.5% microemulsion treatment solution. The treatment time was 2 min, and the treated leaves were stored at 4o C, visually observed, and analyzed for surviving populations of E. coli O157:H7 and P. fluorescens on days 0, 3, 7, 10, 14, 21, and 28. Overall, the efficacies of the antimicrobials were concentration and storage-time dependent. The microemulsions showed a 1.95-4.86 log CFU/g reduction in E. coli O157:H7 population during days 0-28, and were also effective against P. fluorescens, resulting in significant reductions (0.32-2.35 log CFU/g) during storage at days 0-28 (P < 0.05). The visual observation of treated leaves indicated that the 0.1% oregano microemulsion resulted in the best visual appeal in Iceberg leaves inoculated with pathogen E. coli O157:H7 and showed better improvement in the quality of the Iceberg leaves inoculated with spoilage bacteria P. fluorescens than the untreated control leaves. The results suggest that 0.5% cinnamon and 0.3% oregano treatments provide natural, eco-friendly, and effective alternatives to chemicals such as chlorine for produce decontamination against E. coli O157:H7 and P. fluorescens.