Long-term stability of food-grade nanoemulsions from high methoxyl pectin containing essential oils

Nanoemulsions have shown potential advantages over conventional emulsions due to their large active surface area, but are also susceptible to destabilization. Therefore, the purpose of this work was to assess the long-term stability (56 days) of nanoemulsions containing EOs (oregano, thyme, lemongrass or mandarin) stabilized by high methoxyl pectin and a non-ionic surfactant (Tween 80). The initial droplet size of nanoemulsion was below 50 nm regardless the EO type, which was confirmed by Transmission Electron Microscopy (TEM). Lemongrass and mandarin nanoemulsions remained optically transparent over time (56 days) and their droplet sizes were in the nano-range (between 11 and 18 nm), whereas the droplet size of oregano and thyme nanoemulsions increased up to 1000 nm probably due to Ostwald ripening. This fact induced creaming and a higher whiteness index in the latter nanoemulsions. The electrical charge (ae-potential) of nanoemulsions was negative due to the anionic nature of pectin molecule adsorbed at the oil-water interface, ranging between -6 and -15 mV depending on the EO type. However, lemongrass and mandarin nanoemulsions exhibited a more negative ae-potential than thyme or oregano EO indicating a stronger adsorption of pectin at the oil surface, and therefore a higher stability. The viscosity of nanoemulsions remained practically constant between 20 and 24 mPa s, during storage for all EOs. This work represents the starting point for future applications of nanoemulsions containing EOs to be incorporated in food products due to their high long-term stability. (C) 2015 Elsevier Ltd. All rights reserved.