Upcycling Spent Coffee Grounds into Bioactive Extracts Using New Natural Deep Eutectic Systems for Sustainable Topical Formulations

Millions of tons of spent coffee grounds (SCG) are produced worldwide and are typically discarded. However, SCG is a valuable source of bioactives, such as antioxidant phenolic compounds, making them potential ingredients in topical formulations. A sustainable and ecofriendly alternative for extracting these compounds from SCG is the use of Deep Eutectic Systems (DES). This study aimed to prepare extracts from SCG using a novel natural DES (NADES) for incorporation into sustainable topical formulations. Thirteen DES were produced using different natural metabolites and evaluated for their effectiveness in extracting bioactive compounds from SCG. Based on a cytotoxicity evaluation, five out of the 13 DES were selected to prepare SCG extracts. The total phenolic content of the extracts was determined, and two extracts obtained with DES containing proline were chosen for further studies. Finally, one extract and the corresponding DES, proline: glycerol: water (molar ratio 2:5:11.5), were selected based on cytotoxicity studies, stability assays, antioxidant capacity, elastase inhibition, and water content. This extract and the corresponding DES were then incorporated into oil-in-water emulsions developed according to the Eco-Label regulations. The characterization of these formulations showed they had suitable physicochemical, rheological, and droplet size properties and were stable over time, biocompatible, and capable of increasing skin hydration for at least 4 h. The emulsions resulted in semisolid emulsions, with shear thinning behavior, with the G ′ superior to G ″, and a stable bimodal distribution. The study suggests that DES are effective sustainable alternatives to conventional solvents to obtain SCG extracts with valuable properties for the cosmetic industry.