Lignocellulosic Biomass: Understanding Recalcitrance and Predicting Hydrolysis

Abstract

Lignocellulosic biomass (LB) is an abundant and renewable resource from plants mainly composed of polysaccharides (cellulose and hemicelluloses) and an aromatic polymer (lignin). LB has a high potential as an alternative to fossil resources to produce second-generation biofuels and biosourced chemicals and materials without compromising global food security. One of the major limitations to LB valorisation is its recalcitrance to enzymatic hydrolysis caused by the heterogeneous multi-scale structure of plant cell walls. Factors affecting LB recalcitrance are strongly interconnected and difficult to dissociate. They can be divided into structural factors (cellulose specific surface area, cellulose crystallinity, degree of polymerization, pore size and volume) and chemical factors (composition and content in lignin, hemicelluloses, acetyl groups). Goal of this review is to propose an up-to-date survey of the relative impact of chemical and structural factors on biomass recalcitrance and of the most advanced techniques to evaluate these factors. Also, recent spectral and water-related measurements accurately predicting hydrolysis are presented. Overall, combination of relevant factors and specific measurements gathering simultaneously structural and chemical information should help to develop robust and efficient LB conversion processes into bioproducts.

Multiscale analysis of lignocellulose recalcitrance towards OrganoCat pretreatment and fractionation

“…In addition to lignin-related factors, we could also show that polysaccharide specific parameters correlate with recalcitrance towards OrganoCat. Many cellulose-specific parameters like degree of polymerization and crystallinity index have already been shown to be important for pretreatment and enzymatic hydrolysis of lignocellulose [5]. However, cellulose-specific factors were not part of this investigation.…”

“…Lignocellulose recalcitrance depends on the heterogeneous multiscale structure of the wall materials and impacts the subsequent conversion process [3,4]. Despite many studies on lignocellulose recalcitrance, it is still unclear how the structural and chemical features hamper a successful and efficient biomass pretreatment [5]. Many factors have been related to the recalcitrance of the plant cell walls: Changes in the cellulose´s degree of polymerization, its crystallinity, and pore size can interfere with the hydrogen-bond network of the fibers and therefore are believed to facilitate accessibility of degrading enzymes [6].…”

“…Hemicelluloses-mainly xylan-can have chemical modifications such as substituents. Hemicelluloses are amorphous and are relatively well hydrolyzed by, e.g., dilute acids in pretreatment processes [5]. These hemicelluloses represent a physical barrier limiting accessibility of the cellulosic fiber.…”

“…O-acetylation might sterically hamper the productive binding of cellulases. For instance, a reduced acetyl content in corn stover seems to improve effectiveness of these enzymes [11], but might still depend strongly on the chemical structure of xylan and/or the interaction with other wall polymers [5].…”

Interaction of enzymes with lignocellulosic materials: causes, mechanism and influencing factors

“…Hemicellulose is a branched polysaccharide which fastens to the surface of each cellulosic micro-fibril (Hopkins and Huner 2017). In biomass, cellulose is generally 40-50% and hemicellulose is 20-40% of the material by weight (Brigham et al 1996;Pauly and Keegstra 2008;Saini et al 2015;Zoghlami and Paes 2019). Hemicellulose has five-carbon sugars while cellulose has 6-carbon sugars.…”