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

The produce industry commonly uses chlorine to wash vegetables during post-harvest practices. 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. Hence, alternatives to wash sanitizers are required to better meet the needs of the industry. Essential oil-based antimicrobial microemulsions in the wash water were evaluated for their efficacy against antibiotic-resistant Salmonella enterica serovar Newport and spoilage bacterium Lactobacillus casei on Iceberg lettuce. The microemulsions that were assessed included oregano oil, lemongrass oil, and cinnamon oil along with a plant-based emulsifier 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 microemulsions, 50 ppm chlorine, and 3% hydrogen peroxide, stored at 4 °C, 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 dependent on concentration and storage-time. The use of microemulsions resulted in a 2.3–4.37 log CFU/g reduction in the Salmonella population at various time points during days 0–28. They were also effective against Lactobacillus, resulting in 0.11–4.25 log CFU/g reduction during storage at days 0–28. The data on visual observation of treated leaves summarized in the tables and shown as figures indicated that the 0.1% oregano oil microemulsion had the best visual appeal in Iceberg leaves inoculated with S. Newport and the 0.5% lemongrass oil microemulsion showed improvement in reduced browning of the Iceberg leaves inoculated with Lactobacillus casei. This study demonstrates the potential of essential oil microemulsions to inactivate foodborne pathogenic and spoilage bacteria on Iceberg lettuce, thus providing effective produce decontamination strategies.