Multi-functioning deep eutectic solvents as extraction and storage media for bioactive natural products that are readily applicable to cosmetic products

As part of our ongoing efforts to apply deep eutectic solvents (DESs) as green versatile solvents, we investigated DESs as multi-functioning solvents using green tea as a model. Green tea (Camellia sinensis) is a valuable source of catechins such as epigallocatechin-3-gallate (EGCG), a potent antioxidant. We screened DESs composed of cosmetics-compatible components, selected effective components, and recombined these components to produce a series of ternary DESs. Examination of the solvents' physicochemical properties suggested that catechin extraction efficiency was partially associated with solvent pH. Among the many solvents with high yields, BGG-4 (betaine, glycerol, and D-(+)-glucose, 4:20:1) was selected as the final solvent after considering usage limits in cosmetic products and production cost. The extraction conditions optimized by response surface methodology were ultrasound-assisted extraction using 81% BGG-4 at room temperature for 6.5 min, resulting in significantly higher extraction yields than alternative methods involving high temperature and/or long extraction times. Scanning electron microscopy analysis of green tea powder before and after extraction under different conditions supported that our optimized method caused rapid exudation of catechins during rupture process and subsequent efficient dissolution of catechins, leading to superior extraction efficiency. Stability of EGCG, which is prone to undesirable alterations, was monitored at 60 °C after extraction. Over a three-week storage period, the fastest destabilization was observed in 70% MeOH, leaving 27% of EGCG intact. In contrast, almost 60% of EGCG remained unchanged in BGG-4. These suggest that BGG-4 could serve as a multi-functioning medium to produce a beneficial catechin-rich tea extract in which catechins remain relatively stable. Moreover, the solvent itself could function as an active ingredient, and the extract could be readily applicable to cosmetic or pharmaceutical formulations for skin.