Thymol nanoemulsions formed via spontaneous emulsification: Physical and antimicrobial properties

- Spontaneous emulsification was exploited to create thymol based nanoemulsions.
- Stability screening show SLS & polysorbate 20 were most effective emulsifiers.
- Antimicrobial activity of emulsions was dependent on surfactant type & fraction.
- Best formulations highly antimicrobial against planktonic microorganisms & biofilms.

In this work, we prepared various sub-micron thymol emulsions with high hydrophilic-lipophilic balance (HLB) surfactants via spontaneous emulsification. Emulsion properties, such as size, polydispersity and charge, were assessed for each surfactant type and mass fraction. Emulsion stability was characterized by monitoring droplet size following exposure to physical (centrifugation) and thermal stressors (freeze, thaw cycling). Emulsions were subsequently screened against several challenge pathogens to evaluate antimicrobial efficacy. Based on these time-kill assays, exemplary formulations were further tested as sanitizing washes on lettuce and blueberries inoculated with food-borne bacterial biofilms. Antimicrobial data elucidate both surfactant and formulation specific antagonisms between thymol and the emulsifying agents. However, the best emulsion compositions were capable of reducing planktonic bacteria by >4 logs and biofilm bacteria by 1.5-2.5 logs in 60 s. These results are comparable to the efficacy of chlorine at ∼50-200 ppm. The experimental results have implications in emulsion formulations involving thymol and other terpenoids.

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