Distinct mechanisms of enzymatic saccharification and bioethanol conversion enhancement by three surfactants under steam explosion and mild chemical pretreatments in bioenergy Miscanthus

Abstract

Miscanthus is a leading bioenergy crop that represents an enormous lignocellulose resource for biofuels and bioproducts. However, as lignocellulose recalcitrance leads to financially inviable bioethanol production and the potential of secondary wastes into the environment, it becomes crucial to explore green-like and cost-effective biomass processing technologies. To address these issues, low doses of chemical surfactants have been added to enhance biomass enzymatic hydrolysis and bioethanol conversion, but much remains unknown about the mechanism of enhancement. For first time, in this study, a novel chemical surfactant (1% Silwet L-77) was applied to enhance the enzymatic hydrolysis of raw Miscanthus straw, and 40% cellulose digestion was achieved, which is 1.2- and 4.5-fold higher than that of two well-known surfactants (PEG-4000 and Tween-80), respectively. Using lignocellulose substrates obtained from Miscanthus biomass samples that were pretreated by green-like steam explosion followed by mild chemical (NaOH or H2SO4) pretreatments, supplementation with the three surfactants led to significantly enhanced enzymatic saccharification. The 2% Tween-80 supply resulted in a hexose yield of 99% (% cellulose) from enzymatic hydrolysis, followed by 95% with 0.5% PEG-4000 and 71% with 1% Silwet L-77. Despite the slightly lower hexose yield, Silwet L-77 resulted in consistently higher sugar-ethanol conversion rates in all lignocellulose substrates examined. Furthermore, based on the enzyme profiling of mixed cellulase adsorption on lignocellulose and the chemical analysis of wall polymer features and lignocellulose accessibility, this study proposed multiple hypothetical models to interpret the distinct enhancement roles of three surfactants in the enzymatic hydrolyses of diverse lignocellulose substrates. These models also provide a powerful strategy for low-cost bioethanol production with the potential for high-value bioproducts by using desirable surfactants in Miscanthus and other bioenergy crops.