Purpose: The antimicrobial activity of three essential oil (EO) compounds (thymol, eugenol, and trans-cinnamaldehyde) alone and in combination with three naturally-occurring apple aroma (AA) compounds (hexanal, trans-2-hexenal and 1-hexanol) were evaluated to identify the minimum inhibitory concentrations necessary to inhibit Escherichia coli O157:H7. Additionally, three commercial apple juice flavoring mixtures (natural apple cinnamon, natural apple spice and natural red apple) were tested alone for antimicrobial activity against E. coli O157:H7.
Methods: Minimum Inhibitory Concentrations (MIC) were determined using the standard agar dilution (SAD) method. Antimicrobial compounds at concentrations of 0.2 to 51.2 mg/ml were dissolved into separate containers of molten TSA, containing 0.5% Tween-20. Molten agar solutions were mixed for one minute then poured into individual petri dishes and allowed to solidify. Individual plates were then divided into thirds and spot inoculated with 2.5 μl of E. coli culture (diluted to log 106 CFU/ml). Plates were incubated at 35°C and evaluated after 24-48 hours for presence or absence of growth.
Results: Among EO compounds, cinnamaldehyde exhibited the highest degree of antimicrobial activity against E. coli (MIC = 0.2 mg/ml). Thymol and eugenol inhibited E. coli O157:H7 at an MIC value of 1.6 mg/ml. Among AA compounds, trans-2-hexenal exhibited the highest degree of antimicrobial activity (MIC = 1.6 mg/ml). Hexanal and 1-hexanol were significantly less active, with MICs of 12.8 and 6.4 mg/ml, respectively. Of the apple juice flavoring mixtures tested, apple cinnamon exhibited the highest degree of antimicrobial activity (MIC = 12.8 mg/ml), followed by apple spice with an MIC of 51.2 mg/ml. Combinations of essential oil and apple aroma compounds resulted in an indifferent effects.
Significance: Essential oil, apple aroma and apple flavoring compounds were shown to inhibit E. coli O157:H7.