Comparative structural and functional characterisation of lignin fractions isolated from grape stalks using green pretreatment technologies.

This study compares lignin-rich fractions recovered from grape stalks (GS) through different green biorefinery processing routes: alkaline pretreatment (AL), first and second deep eutectic solvent (DES) pretreatment cycles (DES-L1 and DES-L2), and non-thermal air plasma (NTP) pretreatment followed by enzymatic hydrolysis and DES extraction (DES-L6). The lignin fractions were characterized for purity, structure, morphology, solubility, molecular weight, and phenolic hydroxyl content using FT-IR, SEM, SEC, UV-Vis spectroscopy, and quantitative 31P NMR. DES-based processing increased lignin purity from 82.6% for AL to 89.5-91.0% for DES-L1 and DES-L2, while DES-L6 achieved the highest purity (91.5%). DES-L6 also exhibited complete dissolution in DMSO and dioxane/water, the highest guaiacyl OH content (2.19 mmol/g), and although it presented the broadest molecular weight distribution. Comparison of UV-Vis ionisation-difference spectroscopy with 31P NMR showed that UV-Vis is a useful tool for rapid screening of phenolic hydroxyl content, although its accuracy depends on lignin solubility and structural characteristics. Overall, this work provides a comparative evaluation of lignin fractions generated from GS biorefinery streams using alkaline, DES-based, recycled DES-based, and NTP-assisted DES processing routes. The results demonstrate that these green processing routes can produce lignin fractions with distinct purity, solubility, molecular weight distribution, and phenolic hydroxyl profiles, providing a sustainable basis for their further valorisation.