Natural deep eutectic solvents for stilbenoid extraction from vine by-products: Solubilization and extraction...

Natural deep eutectic solvents (NADES) have emerged as sustainable alternatives to conventional solvents for polyphenol extraction; however, their rational selection remains a major challenge due to the vast number of possible component combinations and the lack of rapid screening approaches. This study aimed to develop a rapid and cost-effective ultraviolet-visible (UV-Vis)-based screening method to assess stilbenoid solubility in NADES without requiring pure standards, and to use this approach to guide the selection and optimization of a NADES-assisted extraction process. Twenty NADES were screened using a fast UV-Vis method applied to a stilbenoid-rich plant extract. The results showed moderate correlations with high-performance liquid chromatography with diode-array detection analysis (r = 0.49-0.61), enabling efficient preselection of 12 promising solvents. Among them, betaine/levulinic acid (1:2 molar ratio) exhibited the highest extraction efficiency. Process optimization using a Box-Behnken design identified the solid-to-liquid ratio as the most influential parameter, with optimal conditions corresponding to 20 min extraction time and 39% water content under ultrasound-assisted extraction. The optimized method was applied to Cabernet Sauvignon cane powders of different harvest years and particle sizes, yielding up to 2.6 g kg⁻¹ dry weight of (E)-resveratrol and 2.5 g kg⁻¹ of (E)-ε-viniferin in the most recent samples. Comparable extraction performance was obtained relative to previously reported NADES and a conventional ethanol/water system. Overall, this study demonstrates that solubility-based UV-Vis screening provides a rapid, scalable, and standard-free strategy for NADES selection, offering a practical alternative to conventional trial-and-error approaches and supporting the development of efficient and sustainable extraction processes.