Antimicrobial Efficacy of Essential Oils and Their Primary Constituents Against Escherichia Coli O157:H7 on Organic Leafy Greens

Abstract

The purpose of this study was to determine the effectiveness of plant-based essential oils and their primary constituents against Escherichia coli O157:H7 during the washing and short-term storage of organic leafy greens. Organic baby and mature spinach, and romaine and iceberg lettuce were inoculated with E. coli O157:H7 at 5-log10 CFU g-1. Essential oils of cinnamon, oregano and lemongrass and their primary constituents cinnamaldehyde, carvacrol, and citral at 0.1, 0.3 and 0.5% (v/v) concentrations, along with controls of hydrogen peroxide and water, were used to wash the inoculated leafy greens for one or two minutes. The leafy greens were then stored at 4 or 8 oC and bacterial populations determined on day 0, 1, and 3 of storage. All essential oils and their primary constituent treatments showed significant (P<0.05) reduction of E. coli O157:H7 populations on all leafy greens. Oregano essential oil was the most effective essential oil with concentrations of 0.5% showing the greatest reduction, providing non-detectable growth after initial application (day 0). Similarly, carvacrol was the most effective compound providing non-detectable growth on all leafy greens on day 3 for all concentrations. There was no significant difference (P<0.05) between 1 and 2-minute treatment exposures on all leafy greens. Storage temperatures of 4 and 8 oC showed significant difference only in controls, with higher growth at 8 oC storage. Higher concentration (0.3 and 0.5%) of both essential oils and compounds exhibited no-detectable growth after 3 days in both 4 oC and 8 oC storage. This study provides evidence that plant-based essential oils, as well as their isolated compounds, can act as effective natural antimicrobials for washing organic leafy greens during short-term storage.