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A B S T R A C TThe anti-listerial effectiveness of thyme essential oil (EO) and EO compounds camphor and verbenone was examined on fresh-cut lettuce, cantaloupe melon and pineapple with modified atmospheres and air in model packages at 4 and 8 C. Listeria monocytogenes was found to be able to survive and grow in all atmospheres on melon and lettuce. However, on pineapple lowest survival was identified, presumably due to product pH. Thyme EO demonstrated the best anti-listerial effect, although direct application of the EO compromised product appearance. While camphor showed no anti-listerial effects, verbenone was found to have anti-listerial properties and maintained high sensory acceptance in fresh-cut fruit. The high growth rates of L. monocytogenes on melon were significantly reduced with the application of verbenone while being completely eliminated on pineapple. The use of thyme EO and verbenone as an antimicrobial dip was successfully applied to reduce growth of Listeria on fresh-cut melon and eliminate growth on pineapple; however growth-reduction was less pronounced in melon when compared to a conventional chlorine dip. Further research will be necessary to optimise conditions in fresh-cut produce treatments with natural products including verbenone and thyme EO to replace current chlorine treatments for improved food safety.

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Listeria monocytogenes is a particular risk for the ready-to-eat food sector because of its ability to grow in various environmental conditions. In the literature, growth and survival of L. monocytogenes on food is tested using inoculation densities ranging from less than 10 to over 10 CFU g. Inoculation densities on food have been rarely tested as a factor for growth. In this study, inoculation densities from 10 to 10 of L. monocytogenes were tested on iceberg lettuce (Lactuca sativa) in modified atmospheres and air in model packages at 4 and 8 °C to identify any potential inoculation density effects. On days 0, 2, 5 and 7, L. monocytogenes was extracted from the lettuce surface and enumerated via selective media. The resulting growth curves identified a significant inoculation density effect at 4 and 8 °C with significantly higher amounts of growth (1-2 logs) when lettuce was inoculated at 10 CFU g as opposed to 10 and 10 CFU g. In contrast, the use of different atmospheres had limited influence on growth of L. monocytogenes. In conclusion, greater emphasis on inoculation density of L. monocytogenes should be taken in inoculation experiments when confirmation of growth or the efficacies of growth inhibiting treatments are tested on ready-to-eat food such as lettuce.