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

Abstract

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. • This study provides an innovative way to apply plant antimicrobials to leafy greens. • Essential oil microemulsions inactivated resistant Salmonella on Iceberg lettuce. • Microemulsions inhibited spoilage Lactobacillus growth allowing probiotic benefits. • Treated lettuce leaves showed reduced browning discoloration than untreated controls. • Microemulsions can serve as natural sanitizers on stored fresh-cut Iceberg lettuce.