Mechanism of lignocellulose modification and enzyme dis-adsorption for complete biomass saccharification to maximize bioethanol yield in rapeseed stalks
Rapeseed stalk provides enormous biomass resource for bioethanol production, but its characteristic recalcitrance decides an inefficient cellulose hydrolysis with lower bioethanol yield relatively to other major crop straws. Based on our previous analyses of large population of rapeseed samples, this study selected three rapeseed stalks that showed distinct cell wall composition, and then performed steam explosion followed with mild chemical pretreatments to reduce the lignocellulose recalcitrance. As a result, three typical pretreatments were established to distinctly extract wall polymers (hemicelluloses, lignin, pectin) and also to specifically degrade cellulose degree of polymerization (DP), leading to remarkable increase of biomass porosity and cellulose accessibility in rapeseed stalks. Notably, the steam explosion with mild CaO pretreatment (50 °C) could generate the optimal lignocellulose substrate effective for almost complete enzymatic saccharification while 1% Tween-80 was supplied to block lignin adsorption with cellulases enzymes. As a consequence, their bioethanol yields were achieved from 18.8% to 20.5% (% dry biomass) due to relatively high sugar-ethanol conversion rates from 90%-93%, which were much higher than those of the previously-reported in rapeseed stalks. Furthermore, this study proposed a mechanism model to highlight why the optimal lignocellulose modification could cause complete biomass saccharification for maximum bioethanol yield in rapeseed stalks, and how the surfactant plays an enhancement role in enzymatic hydrolysis of diverse lignocellulose substrates. Hence, this study has demonstrated an effective strategy to maximize bioethanol production under a low-cost and green-like biomass process in rapeseed and beyond.