Optimizing extraction from cork and almond wastes: the role of water in deep eutectic solvents

Four deep eutectic mixtures (DES) with varying water concentrations (10 % to 50 %) were synthesized using naturally derived and biodegradable components: choline chloride:urea (ChCl:U), choline chloride:levulinic acid (ChCl:Lev), betaine:levulinic acid (Bet:Lev), and glycerol:phosphoric acid (Gly:Ph). These ternary mixtures, designed as greener alternatives to conventional solvents, were tested as substitutes of anhydrous DES to extract bioactives from agro-industrial residues, specifically cork and almond by-products and analyzed by HPLC-MS. As water content increased, conductivity rose while viscosity, density, and refractive index decreased. Water addition lowered melting temperature of DES, suggesting structural disruption or modification of molecular packing. FTIR-ATR and ¹H NMR confirmed hydrogen bonding in the ternary system. High concentrations of phenolics, including protocatechuic acid and vanillin, were identified in cork extracts (DES 2 and 3 with 50 % water). In almond extracts, protocatechuic acid, catechin, and isorhamnetin-3-O-rutinoside were the primary compounds identified (DES 2 and 3 with 50 % water). DES-H₂O mixtures demonstrated higher extraction yields, resulting in extracts with higher phenolic and flavonoid content and enhanced antioxidant activity compared to anhydrous DES.

These findings highlight the potential of DES-H₂O mixtures as sustainable, green and high-efficiency solvents alternatives, offering an environmentally friendly approach aligned with circular economy and green extraction technologies.