Sustainable microwave-assisted eutectic solvent extraction of polyphenols from vine pruning residues

This study describes the development of a laboratory-scale extraction method for polyphenols in vine pruning residues (VPR) using eutectic solvents and microwave heating. Grape VPR are an abundant waste biomass that is currently underexploited. VPR are rich in valuable extractives such as polyphenols, and their extraction with green and sustainable processes could constitute a promising valorization strategy. Microwave-assisted heating was used to obtain high extraction yields under relatively mild conditions. The performances of three eutectic solvents based on choline chloride and polyols (ethylene glycol, propylene glycol, and glycerol) were compared against a conventional water/ethanol solvent. These solvents were particularly effective at extracting stilbenoid polyphenols such as resveratrol, whose low water solubility usually requires high amounts of organic solvents. The solvent based on choline chloride and ethylene glycol provided the best results, and was chosen for further optimization by experimental design. Optimal performances were obtained under mild conditions (30 min, 85 degrees C). The effects of controlled amounts of water on the extraction efficiency of this solvent were also evaluated. Small amounts of water (approximately 10 %) led to an increase in the extraction yield, while higher water contents led to lower yields. Total polyphenol yields of up to 22 mg/g of substrate were obtained. The mild extraction conditions also allowed recovery of oligomeric stilbenoids such as epsilon-viniferin, which are often degraded when harsh conditions or water-based solvents are used. The developed method constitutes a fast and reliable technique for the recovery of polyphenols in VPR, and a promising laboratory-scale study of a potential valorization pathway for this biomass.