Physical and chemical evolution of sugar beet pulp during aqueous ammonia pretreatment and its impacts on the recalcitrance of cellulose

Recognition and deconstruction of biomass recalcitrance are essential steps in fuel ethanol production. Aqueous ammonia (AA) is an efficient pretreatment to improve the enzymatic digestibility of sugar beet pulp (SBP), but the precise mechanism is still ambiguous. Multidimensional methods, e.g., physical properties, wet chemistry compositions, and glycome profiling were integrated to elucidate the impacts of AA pretreatment on recalcitrance of SBP semi-quantitatively. Additionally, the variations in enzyme adsorption and cell wall morphology during the enzymatic hydrolysis were systematically observed. The results confirmed that the constitution, association, and distribution of glycan epitopes within cell walls significantly correlated to recalcitrance. Specifically, the arabinogalactan and rhamnogalacturonan epitopes negatively correlated to the recalcitrance. And xylans only bound to cellulose contributed to recalcitrance. Dislocation and fragmentation of cell walls through sequential removal of several glycan epitopes could facilitate enzymatic digestibility, in terms of enzyme adsorption, reducing sugar yields, and enzymatic saccharification behaviors. Whereas, cell corners seemed to be highly resistant to enzymatic digestion due to some gathered epitopes. The maximum reducing sugar yields ranging from 437.7 to 441.48 mg/g were obtained from the pretreated SBP (3, 5, and 7 h), which nearly increased by 87% compared to the control. Selecting a proper pretreatment to precisely strip out the vital glycan epitopes might be an economically feasible strategy in fuel ethanol production.