Purpose: To determine the effectiveness of plant-based essential oils against E. coli O157:H7 during the washing and short-term storage of organic leafy greens.
Methods: Organic leafy greens, baby and mature spinach, and romaine and iceberg lettuce were inoculated with a cocktail of E. coli O157:H7 at 5-log CFU/ml. Each essential oil was dissolved in phosphate buffered saline (PBS) at 0.1, 0.3 and 0.5% (v/v) concentrations which were then used to wash the inoculated leafy greens. Leafy greens were kept at refrigerated temperatures and bacterial populations determined on day 0, 1, and 3 for antimicrobial effectiveness.
Results: Statistically significant log reductions for E. coli O157:H7 (P < 0.01) were observed with all essential oils at all concentrations in all types of leafy greens. Oregano oil was the most effective on all leafy greens with a log reduction of 2.4, 3.5 and 4.5 for 0.1, 0.3, and 0.5% concentrations, respectively, on day 0. Lemongrass and cinnamon oils had log reductions between 2.2-2.3, 2.4-2.7 and 4.3 for 0.1, 0.3, and 0.5% concentrations, respectively, on day 0. Log reduction results were also compared to days 1 and 3 of storage. An increasing reduction pattern after each day was seen for all concentrations of essential oils, in all the leafy greens.
Significance: Plant-based essential oils can not only act as effective natural antimicrobials, but also have a lasting effect on E. coli O157:H7 in organic leafy greens.