Influence of surface-active phenolic acids and aqueous phase ratio on w/o nano-emulsions properties; model fitting and prediction of nano-emulsions oxidation stability

- Olive oil polyphenols were incorporated in olive oil based w/o nano-emulsions.
- Aqueous phase ratio affects emulsions properties.
- Increasing aqueous phase ratio led to lower colloidal and oxidation stability.
- Vanillic, caffeic and syringic acid presented emulsifying and antioxidant abilities.

The purpose of this study was to explore the influence of emulsion composition (with regards to aqueous phase ratio and incorporated antioxidant compounds) on the formulation and stability of water in extra virgin olive oil nano-emulsions. Two aqueous phase ratios (2% w/w and 3% w/w) and three olive oil endogenous phenolic compounds (vanillic, caffeic and syringic acid), incorporated in the aqueous phase, were tested in order to evaluate their effect on emulsion properties and colloidal and oxidation stability. Additionally, nonlinear regression method was employed to generate a model, as well as to predict and test hypotheses of the emulsions mean droplet diameter effect on their oxidation index. The experimental results proved that the aqueous phase ratio plays a crucial role in nano-emulsion formulation, with increasing ratio leading to degradation of all the examined emulsion properties. Moreover, the addition of the olive oil endogenous phenolic compounds in the aqueous phase resulted in improved kinetic and oxidation stability of the water-in-oil (w/o) nano-emulsions. From the tested phenolic compounds, caffeic acid appeared to be the most effective. In fact, the nano-emulsion with 2% w/w aqueous phase with incorporated caffeic acid showed the highest emulsion stability (92%), oxidation stability (33.6 h) and the lowest mean droplet diameter (251 nm), polydispersity (0.32) and turbidity (172 NTU).