Antimicrobial Efficacy of Cinnamaldahyde, Carvacrol and Citral against Escherichia coli O157:H7 on Organic Leafy Greens Held at 4°C

Introduction: Several foodborne outbreaks of Escherichia coli O157:H7 have been linked to the consumption of fresh produce, including organic leafy greens. There is a need for alternative natural interventions for organic leafy greens since chemicals are not permitted in organic production. The essential oils of cinnamon, oregano, and lemongrass are known to effectively reduce E. coli O157:H7 in many food products. Further studies are therefore needed to determine if their main chemical constituents are responsible for antimicrobial activity.

Purpose: To determine the efficacy of assorted plant-derived compounds against E. coli O157:H7, during washing and storage of organically grown leafy greens.

Methods: Baby and adult spinach, and romaine and iceberg lettuce, were inoculated with a cocktail of E. coli O157:H7 at 5-log CFU/ml. Inoculated leafy greens were washed in different concentrations of the constituents (0.1, 0.3 and 0.5%, v/v in phosphate buffered saline, PBS) and refrigerated for three days. Hydrogen peroxide, PBS, and water were included as controls. The survivors were enumerated on days 0, 1 and 3 of storage.

Results: Significant reductions (P < 0.01) in E. coli O157:H7 populations were observed for all three compounds, at all concentrations and in all the leafy greens. The most effective being carvacrol, with a log reduction of 4.1, 4.9 and 4.5 on day 0 for 0.1, 0.3, and 0.5% concentrations, respectively. On day 0, citral had a log reduction of 2.5, 3.4, and 3.9 and cinnamaldahyde had a log reduction of 3.0, 3.4 and 4.4 for 0.1, 0.3, and 0.5% concentrations, respectively. An increasing log reduction pattern after each day was also seen for all concentrations of compounds and in all types of leafy greens.

Significance: Plant-derived compounds, carvacrol, cinnamaldahyde, and citral, are effective natural antimicrobials and can be used as a wash treatment for bagged leafy greens stored at refrigerated temperatures.