Enhanced biomass saccharification and bioethanol production by optimizing 1-ethyl-3-methylimidazolium acetate pretreatment for effective delignification of finger millet straw

Valorization of under explored millet agri-residues for bioethanol production is a sustainable approach towards energy recovery and waste management. In this study, bioethanol potential of finger millet straw (FMS) was explored through green pretreatment using 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]). Cell wall compositional analysis of destarched raw straw from five finger millet varieties estimated cellulose, hemicellulose and lignin in the range of 43–49 %, 20–25 % and 15–17.5 %, respectively and no significant difference was observed among varieties. [EMIM][Ac]/water (80:20) with 5 % biomass loading at 150 °C for 3 h, yielding 44.1 % delignification was found to be the optimum pretreatment condition. High decoumarylation (75 %) and deferuloylation (66 %) were also observed after pretreatment. Structural characterization of the pretreated biomass confirmed enhanced delignification. Recycled [EMIM][Ac] was effective up to three pretreatment cycles, making the process economic. Total fermentable sugar recovery was 676 mg/g pretreated biomass. Co-fermentation of glucose and xylose in the enzymatic hydrolysate of pretreated straw using Saccharomyces cerevisiae MTCC 36 and Pichia stipitis NCIM 3498 yielded 314.4 mg ethanol/g pretreated biomass at 18 h, accounting 91.2 % of theoretical maxima. Mass balance analysis showed 15.8 g ethanol from 100 g destarched FMS. This indicates that FMS could be used as a promising bioethanol feedstock contributing to sustainable energy demands.