Structural characterization of Populus variants following pretreatment and biological digestion by fungal enzymes and CBP

The production of renewable fuels from lignocellulosic materials is restricted by the plant’s recalcitrance to hydrolyzing polysaccharides to sugars. Biomass recalcitrance is associated with several factors such as enzyme accessibility, lignin content, cellulose crystallinity, the presence of hemicellulose, and lignin structure etc. In this study, BESC 876 and SKWE 24-2 poplar variants were selected because they showed lower recalcitrance from a screening of 21 natural Populus compared to BESC standard poplar control. The biomass was processed through hydrothermal (HT) pretreatment, fungal enzymatic hydrolysis (EH), and CBP fermentation. The raw materials, pretreated biomass, and enzymatic hydrolysis and fermentation residues were characterized to elucidate the structural differences and to understand the effects of HT pretreatment and biological reactions on the biomass recalcitrance. After pretreatment, the lower weight-average degree of polymerization (DP_w) of cellulose in the two natural variants (1000-1100) compared with BESC standard poplar (1600) indicated that DP_w can be one of the contributing factors for lower recalcitrance of natural vairants. In addition, the natural variants demonstrated remarkably lower abundance of p-hydroxybenzoate (PB) unit after HT, EH, and CBP. The removal of PB and cleavage of β-O-4 resulting in a more hydrophilic and lower molecular weight of lignin could facilitate cell wall disruption which contributes to the lower biomass recalcitrance. Our results suggest that the structural differences of biomass may be associated with lower recalcitrance of BESC 876 and SKWE 24-2 poplar variants.