Microwave-assisted deep eutectic solvent pretreatment of macadamia nut shells and pinewood shavings for effective saccharification and bioethanol production

Due to their unique heating properties, deep eutectic solvents in combination with microwave irradiation in pretreatment are now widely researched for achieving effective biomass fractionation and high sugar yields. In this study, optimisation of the pretreatment of macadamia nut shells (MNSs) and pinewood shavings (PWSs) using choline chloride-lactic acid DES in a microwave was attempted via the response surface methodology approach, to enhance fermentable sugar yield for bioethanol production. A central composite design was employed to optimise microwave power, biomass loading, and pretreatment time. The analysis of variance revealed that microwave (MW) power had the most significant influence on the examined responses (solid yield, cellulose recovery, and delignification). High cellulose recovery (>70 %) was achieved in both biomass although with higher delignification (73.68 ± 1.39 %) in softwood PWSs at the optimised conditions (81.24 % MW power, 8.94 % biomass loading, and 181.43 s) than in hardwood MNSs with lower delignification (39.01 ± 1.15 %) (at 82.55 % MW power, 6.88 % biomass loading and 251.35 s pretreatment time). Structural characterisation revealed a reduction in peaks linked to hemicellulose and lignin, severe pore formation in pretreated PWSs, and a higher crystallinity index (78.41 %) compared to pretreated MNSs (57.99 %). Pretreated PWSs also showed higher cellulose digestibility (60.33 %) and glucose yield (67.03 %) than MNSs (51.01 and 56.67 %, respectively) on hydrolysis with cellulase at 24 mg/g biomass. PWSs hydrolysate supplemented with glucose yielded 4.21 g/L ethanol (44.01 % theoretical yield) using Saccharomyces cerevisiae during fermentation. Overall, the MW-assisted CL pretreatment strategy significantly enhanced sugar yields, which could be beneficial for bioethanol production.