Synergistic antimicrobial activities of plant essential oils against Listeria monocytogenes in organic tomato juice

The objective of this study was to identify combinations of plant essential oils (EOs) that have synergistic antilisterial effects in raw organic tomato juice. Among the EOs approved for addition to organic foods in the Republic of Korea, twenty-four EOs were screened for antilisterial activity using a well diffusion assay. Six EOs (cinnamon bark, lemon, lemongrass, oregano, peppermint, and thyme thymol) showed relatively strong inhibitory activities (diameter of zone of inhibition ≥ 35.0 mm) against Listeria monocytogenes. The minimal inhibitory concentrations (MICs) and minimal lethal concentrations (MLCs) of these EOs against L. monocytogenes in a laboratory medium were determined. Oregano and thyme thymol EOs had the lowest MIC (0.3125 μL/mL), and cinnamon bark, oregano, and thyme thymol EOs had the lowest MLC (0.6250 μL/mL). The synergistic antilisterial activities of combinations of EOs in a laboratory medium were determined using a modified checkerboard assay. Combinations of cinnamon bark and lemongrass EOs, cinnamon bark and thyme thymol EOs, and lemongrass and thyme thymol EOs showed synergistic antilisterial activities (fractional inhibitory concentration index = 0.5). A combination of cinnamon bark and thyme thymol EOs was tested for synergistic antilisterial activity in tomato juice incubated at 10 °C and 25 °C for up to 48 h. The number of L. monocytogenes in juice treated with combined EOs at a total concentration of 0.3125 μL/mL decreased significantly (P ≤ 0.05) compared to numbers in juice treated with 0.3125 μL/mL of either cinnamon bark EO or thyme thymol EO alone. These observations show the potential for using combinations of EOs for controlling L. monocytogenes in juice released from cut or bruised raw organic tomatoes intended for consumption in food service or home settings.