Effect of heating temperature of a novel wheat-derived surfactant on a mixture of thyme essential oil/surfactant and on the final emulsions

Abstract

Emulsions formulated with natural compounds are increasingly interesting in fields such as food, cosmetics or pharmaceutics. Surfactants play a significant role in the stability of emulsions. In this work, the influence of the temperature at which a mixture of thyme essential oil and surfactant is heated, on its physical stability and on properties and physical stability of emulsions formulated with this mixture as the dispersed phase, were investigated. A new bio-surfactant derived from wheat straw was employed as an emulsifier. It is an alkyl poly pentoside which is poorly soluble in water and solid at room temperature, thus, it is necessary to melt it prior to being used with the oil. As a previous step, both a physical characterization of thyme essential oil used in this work and a thermal study of surfactant were carried out. DSC technique applied to the surfactant showed that this was completely melted at 50 °C. From this information, several temperatures of heating (from 55 °C to 75 °C) for an oil and surfactant mixture in a ratio of 10:1, were applied in order to know the effect on the physical stability of the dispersion. Multiple light scattering technique was used for this purpose. It was found that slightly lower stability for the dispersion heated was at 55 °C and an optimum value at about 70 °C. For this reason, two thyme essential oil/water emulsions were prepared. One of them, from a dispersed phase previously heated at 55 °C and another one, from a dispersed phase previously heated at a higher temperature (70 °C). Then, their influence on the droplet size distribution, flow behaviour and physical stability was studied. No significant differences were observed between properties of both emulsions exhibiting similar values of mean droplet sizes, viscosity and stability.