Efficient delignification of wheat straw for microbial lipid production enabled by a novel ternary deep eutectic solvent containing ethylene glycol

Abstract

Lignocellulose pretreatment often faces the problem of lignin condensation with less attention to biofuel production from total sugars, which significantly hinders biomass fractionation and energy utilization efficiency. The aim of this work was to design a novel ternary deep eutectic solvent (DES) (citric acid: choline chloride: ethylene glycol in a molar ratio of 1:1:2) for pretreatment of wheat straw with a solid/liquid ratio (S/L) of 1:20 (w/w) to achieve non-condensed lignin and high-yield fermentable sugars at 100 °C for 12 h. The introduction of ethylene glycol into ternary DES could capture the formed benzyl carbocation to protect lignin structure, inhibit excessive degradation of hemicellulose but to fermentable sugars (e.g., xylose), and improve the enzymatic digestibility of cellulose (98.73%, w/w). The mechanisms of both lignin fractionation and the in-situ lignin protection during the pretreatment of wheat straw enabled by ternary DES were systematically elucidated. Importantly, Trichosporon cutaneum cultivated in undetoxified DES waste liquid (hemicellulose-rich) and residue hydrolysate (cellulose-rich) could totally afford 8.7 g microbial lipid per 100 g wheat straw, which is superior to previous reports. This green in-situ lignin protection and fractionation strategy provides a new avenue for upgrading lignocellulose to microbial lipids.