Biomass recalcitrance. Part I: the chemical compositions and physical structures affecting the enzymatic hydrolysis of lignocellulose

Lignocellulosic biomass is recalcitrant to biodegradation due to the rigid and compact structure of plant cell wall. The recalcitrance of biomass is mainly constructed by its chemical compositions that build a spatial network as a protective bulwark. Generally, the factors affecting the accessibility of biomass cellulose can be divided into direct and indirect factors. The direct factors refer to the accessible surface area, and the indirect factors include biomass structure-relevant factors (pore size and volume, particle size, and specific surface area), chemical compositions (lignin, hemicelluloses, and acetyl group), and cellulose structure-relevant factors (cellulose crystallinity and degree of polymerization). Pre-treatment is actually the process to alter indirect factors and improve direct factors thus enhancing the accessibility of cellulose. In this review, we summarize the effects of chemical compositions and physical structures on the enzymatic digestibility of lignocellulosic biomass. We suggest that future work should be focused on but not limited to the molecular mechanisms of biomass recalcitrance by investigating the microscale and nanoscale features as well as hydrogen bonds network of lignocellulosic biomass. (C) 2012 Society of Chemical Industry and John Wiley & Sons, Ltd