Sustainable thermoresponsive whey protein- and chitosan-based oil-in-water emulsions for cosmetic applications.

Abstract

In this study, the biopolymers whey protein and chitosan were used to create a thermoresponsive emulsion. The impact of the inclusion of chitosan and inclusion of specific oils on the rheological properties and response to temperature were investigated by a stepwise build-up from simple solutions to oil-in-water (O/W) emulsions. Whey protein (WP) concentration and chitosan concentration were varied. The results may help develop strategies for incorporating thermoresponsive materials in stable and high-performing formulations for use in cosmetics. Solutions of whey protein concentrate (WPC) by itself, chitosan by itself and the combination of the two at various concentrations were tested with flow sweeps, temperature sweeps and frequency sweeps. Then, three different oils of jojoba, avocado and silicone were included to form emulsions and the tests were repeated to determine flow behaviour, response to temperature and structure. By comparing 15 wt. % and 20 wt. % WP solutions, it was found that 15 wt. % WP could provide good viscosities and modulus at a lower amount of material used. The solution composed of 15 wt. % WP, and 0.5 wt. % chitosan was found to have the greatest structural response to temperature compared to solutions with 1.0 wt. % and 1.5 wt. % chitosan. Compared to the addition of 10 wt. % silicone and 10 wt. % avocado oil to form emulsions, the addition of 10 wt. % jojoba oil further strengthened the gel network the most. The final emulsion with pigment added had improved viscosity and thermoresponsive behaviour. The WP and chitosan emulsions were shear thinning, elastically dominated and behaved as classical gels. The behaviour of the emulsions was dependent upon the hydrophobic interactions between the protein and the oil and the electrostatic interactions between the protein and the chitosan. An emulsion composed of 15 wt. % WP, 10 wt. % jojoba oil and 0.5 wt. % chitosan solution was found to have the greatest structural response to temperature. This study of an O/W emulsion containing whey protein concentrate and chitosan demonstrated that different oils and conditions can be used to tune thermoresponsive and rheological behaviour.