Physical and antimicrobial properties of self-emulsified nanoemulsions containing three synergistic essential oils

The multiantibiotic resistant characteristic of Staphylococcus aureus and the prevalence of foodborne illnesses due to foods, particularly dairy products, contaminated by the pathogen call for alternative preservation technologies. Essential oils (EOs) have shown activities against various bacterial pathogens. However, EOs are water-insoluble and have unpleasant sensory properties, and strategies are needed to prepare colloidal systems for dispersion in liquid products and increase their antimicrobial activity and therefore reduce their usage level. In the present study, the overall objective was to fabricate and characterize self-emulsified nanoemulsions containing multiple EOs with synergistic antibacterial activities. Cinnamon, lemongrass, oregano, and tea tree EOs were observed to have synergistic antibacterial activities when used in combinations, especially for their triple combinations showing the fractional inhibitory concentration index of 0.625 and below. When triple EOs were emulsified by the combination of sunflower lecithin and Tween 20, mixing EOs with corn oil enabled the preparation of translucent oil-in-water nanoemulsions by simply stirring the mixtures. Three formulations were chosen for the preparation of self-emulsified nanoemulsions as these systems remained translucent, the droplet diameter remained to be smaller than 200 nm, and the polydispersity index remained to be smaller than 0.3 after 180-day ambient storage at 21 °C. The oil: surfactant mass ratio in these nanoemulsions (1.16 or higher) was higher than that of EO microemulsions (up to 0.77) prepared with a similar method. The minimum inhibitory concentration (MIC) of the nanoemulsions remained to be 0.5 or 1.0 mg/mL against S. aureus, while the minimum bactericidal concentration (MBC) remained to be 1.0 or 2.0 mg/mL after the 180-day storage. A higher EO: surfactant mass ratio of nanoemulsions resulted in a lower MIC and MBC. When tested in whole milk with 3.3% fat at 21 °C, no inhibition of S. aureus was observed for nanoemulsions at overall EO levels of 2.5 and 5.0 mg/mL; at an overall EO concentration of 7.5 mg/mL, no inhibition was observed for one nanoemulsion with the lowest EO:(corn oil + surfactant) mass ratio, while the gradual reduction of S. aureus by ~1 or 2 log CFU/mL in 120 h was observed for the other two nanoemulsions. The formulations and nanoemulsion preparation method in the present study may be significant to the utilization of EOs as natural preservatives to improve food safety.