Tunable Natural Deep Eutectic Solvents-Driven Fractionation of Olive Pomace in an Integrated Biorefinery: Linking Lignin Structural Tailoring to Carbohydrate Valorization and Biopolymer Production

The development of sustainable biorefineries requires integrated fractionation strategies capable of preserving carbohydrate streams while generating high-value lignin and bioactive fractions. In this study, olive pomace was valorized through a cascade approach combining optimized microwave-assisted extraction (MAE) of phenolic compounds, natural deep eutectic solvents (NADES)-enabled pretreatment, and chemo-enzymatic modification. MAE enabled recovery of a phenolic-rich fraction and reduced inhibitory compounds in the residual biomass, improving its suitability for downstream processing. The dephenolized substrate was subsequently subjected to NADES-based fractionation using acidic, alkaline, and neutral systems, including a laccase-assisted variant. Pretreatment performance depended on solvent chemistry. The acidic system (ChCl:lactic acid) achieved high delignification (≈68%) but produced restructured lignin with extensive β-O-4 cleavage. The alkaline NADES (K₂CO₃:ethylene glycol) combined high lignin removal (≈75%) with partial preservation of native linkages and solvent-induced functionalization. The neutral NADES (betaine:glycerol) preserved both polysaccharides and lignin structure, while laccase induced selective oxidative modifications. These differences yielded lignin fractions with distinct structural features suitable for different downstream valorization routes. In parallel, alkaline NADES enabled efficient saccharification with balanced glucose and xylose release, supporting polyhydroxyalkanoate production by Haloferax mediterranei. Overall, this work demonstrates a tunable cascade strategy integrating phenolic recovery, DES fractionation, and lignin structural design for multiproduct lignocellulosic biorefineries.