Enhancing Enzymatic Digestibility and Lignin Production of Oil Palm Empty Fruit Bunch (OPEFB) by Green Deep Eutectic Solvent

Oil palm empty fruit bunch (OPEFB) is an abundant lignocellulosic residue whose high lignin content restricts its bioconversion into sugars and value-added products. Deep eutectic solvents (DESs), particularly choline chloride–lactic acid, offer a green and tunable platform for selective delignification and biomass fractionation. This study investigates the effects of ChCl:LA (1:2) DES pretreatment under varying temperatures (100–140 °C) and reaction times (3-6 h) on the chemical composition, structural modification, delignification kinetics, and enzymatic digestibility of OPEFB. A modified combined delignification factor (CDF) was developed to unify temperature, time, and DES acidity into a single severity descriptor. Delignification followed a biphasic pattern successfully captured by the CDF-based kinetic model (R² = 0.9961), with activation energy of 63.5 kJ.mol⁻¹. Increasing pretreatment severity enhanced hemicellulose and lignin removal (up to 95.5% and 84.4%), while cellulose remained largely preserved. SEM, XRD, and FTIR analyses confirmed progressive disruption of the lignin–carbohydrate matrix, increased cellulose exposure, and removal of amorphous domains. As a result, enzymatic hydrolysis yield improved by more than twofold relative to untreated biomass, reaching 75.5% at 140 °C for 6 h. Mass-balance evaluation demonstrated that from 100 g OPEFB, DES pretreatment yielded 21.6 g glucose and 24.7 g recoverable lignin under optimal conditions. Compared to other pretreatment strategies, the ChCl:LA DES system achieved a balanced co-production of sugars and lignin in significantly shorter processing time. Overall, this work provides mechanistic, kinetic, and mass-balance insights into DES-assisted fractionation of OPEFB and highlights its potential in integrated multiproduct biorefineries.