Physicochemical characterization and antimicrobial activity of food-grade emulsions and nanoemulsions incorporating essential oils

Coarse emulsions containing essential oils (lemongrass, clove, tea tree, thyme, geranium, marjoram, palmarosa, rosewood, sage or mint) and stabilized with tween 80 and sodium alginate were prepared by high shear homogenization. Nanoemulsions were obtained by microfluidization of coarse emulsions. In general, the average droplet size of coarse emulsions was dramatically reduced after microfluidization down to a few nanometers, with the exception of palmarosa and rosewood oil emulsions, which were already in the nano-range before being treated. The ζ-potential of nanoemulsions exhibited values more negative than −30 mV, indicating a strong electrostatic repulsion of the dispersed oil droplets in the aqueous phase. The viscosity of nanoemulsions significantly decreased after microfluidization, with at least a 30% drop in their initial values. The whiteness index of nanoemulsions diminished after being treated. In fact, nanoemulsions containing tea tree, geranium or marjoram essential oils became completely transparent after microfluidization. Lemongrass, clove, thyme or palmarosa-loaded nanoemulsions were those with a higher in vitro bactericidal action against Escherichia coli, as they achieved 4.1, 3.6, 2.8 or 3.9 log-reductions after 30 min of contact time. In addition, a faster and enhanced inactivation kinetic was observed in the case of nanoemulsions containing lemongrass or clove essential oils in comparison with their respective coarse emulsions. Thus, the present work evidences the promising advantages of using nanoemulsions as delivery systems of flavoring and preservative agents in the food industry.