Physical Properties and Antimicrobial Efficacy of Thyme Oil Nanoemulsions: Influence of Ripening Inhibitors

Thyme oil-in-water nanoemulsions (pH 3.5) were prepared as potential antimicrobial delivery systems. The nanoemulsions were highly unstable to droplet growth and phase separation, which was attributed to Ostwald ripening due to the relatively high water solubility of thyme oil. Ostwald ripening could be inhibited by mixing thyme oil with a water-insoluble ripening inhibitor (>= 60 wt % corn oil or >= 50 wt % MCT in the lipid phase) before homogenization, yielding nanoemulsions with good physical stability. Physically stable thyme oil nanoemulsions were examined for their antimicrobial activities against an acid-resistant spoilage yeast, Zygosaccharomyces bailii (ZB). Oil phase composition (ripening inhibitor type and concentration) had an appreciable influence on the antimicrobial activity of the thyme oil nanoemulsions. In general, increasing the ripening inhibitor levels in the lipid phase reduced the antimicrobial efficacy of nanoemulsions. For example, for nanoemulsions containing 60 wt % corn oil in the lipid phase, the minimum inhibitory concentration (MIC) of thyme oil to inhibit ZB growth was 375 mu g/mL, while for nanoemulsions containing 90 wt % corn oil in the lipid phase, even 6000 mu g/mL thyme oil could not inhibit ZB growth. This effect is also dependent on ripening inhibitor types: at the same concentration in the lipid phase, MCT decreased the antimicrobial efficacy of thyme oil more than corn oil. For instance, when the level of ripening inhibitor in the lipid phase was 70 wt %, the MICs of thyme oil for nanoemulsions containing corn oil and MCT were 750 and 3000 mu g/mL, respectively. The results of this study have important implications for the design and utilization of nanoemulsions as antimicrobial delivery systems in the food and other industries.