Sensory and instrumental characterization of fast inverting oil-in-water emulsions for cosmetic application

The aim of this study was to perform short-term sensory testing and instrumental (conductivity and rheological) characterization of a fast inverted oil-in-water (o/w) emulsion base, also known as a SWOP (Switch-Oil-Phase) emulsion, and reference o/w and water-in-oil (w/o) emulsion bases under various testing conditions: in the presence of ions and at different temperatures. SWOP emulsions are known as metastable o/w emulsions, which invert into w/o emulsions on application of mechanical energy, while rubbing it onto the skin and due to their properties SWOP emulsion are especially suitable as a cosmetic vehicle in, for example, sun-protection products.

Sensory testing, which included the evaluation of twenty attributes of the investigated emulsion bases, was performed by a panel of 20 healthy assessors experienced in the evaluation of cosmetic products. Rheological characterization of the investigated emulsion bases included continuous flow testing and oscillatory measurements under various testing conditions. Additionally, conductivity measurements were combined with rheological characterization to monitor stability changes of investigated emulsions. The instrumental and sensory results were analysed statistically and compared.

The obtained results indicated that the investigated emulsions behaved differently in the presence of ions (originating from artificial sweat solution) and at different temperatures (under storage and application conditions). Namely, the SWOP emulsion showed similar behaviour to the reference o/w emulsion under storage conditions, but in the presence of ions and at skin temperature, the SWOP emulsion was followed by re-establishment of a stable w/o system, whereas reference o/w emulsion was irreversibly destroyed. The statistical analysis of chosen sensorial attributes indicated that the reference w/o emulsion was significantly different in comparison with the reference o/w and SWOP emulsions, mainly, standing in good agreement with the results of rheological characterization.

The study showed that rheological measurements potentially could be related to certain sensory attributes and used for faster development of SWOP emulsions in the future. Finally, SWOP emulsions should be considered for further investigation as suitable vehicles in cosmetic products due to their favourable physicochemical and sensory characteristics which could be partially predicted with instrumental characterization.