Physical and Chemical Features of Pretreated Biomass that Influence Macro‐/Micro‐Accessibility and Biological Processing

Abstract

Lignocellulosic biomass containing about 55-65 wt% sugars in the form of polymeric structural carbohydrates can be a sustainable feedstock for production of a variety of sugars that can in turn be converted into fuels and chemicals [1][2][3]. For biological deconstruction of structural carbohydrates to sugars, tiny protein molecules called enzymes need to reach appropriate substrates to depolymerize them. In native plants however, carbohydrates are trapped in a complex matrix comprising non-sugar constituents and polymers such as lignin, which forms a strong and complex sheath around carbohydrates and presents challenges to effective and economical release of sugars from biomass [1,2,4,5]. Prior to biological conversion, some kind of treatment of biomass is therefore needed to make the carbohydrates accessible [6,7]. This pretreatment can be purely mechanical, thermal, chemical, biological, or combinations of these, as reviewed elsewhere [8][9][10]. Thermochemical pretreatments, however, are considered leading options due to their comparatively short reaction time, effectiveness, and lower energy requirements compared to mechanical options [11][12][13]. Nonetheless, thermochemical pretreatments typically need Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals, First Edition. Edited by Charles E. Wyman.

Assessment of the structural factors controlling the enzymatic saccharification of rice straw cellulose

“…One of the structural properties of the model samples, the "accessibility to cellulase," can be also deduced from the cellulase adsorption rate [13,14]. In particular, it is described as initial accessibility, because the accessibility is altered by the cellulase action known as amorphogenesis [15].…”

Examining the role of particle size on ammonia-based bioprocessing of maize stover

“…The differential increases may reflect the difference in accessibility between cellulose and hemicellulose, or the heterogeneity of cellulose fiber morphology. Given that several structural features such as fibril length and pore size distribution determine cellulose accessibility, the results indicate that they may influence cellulase binding and hydrolytic efficacy even at smaller particle diameters.…”

“…In recent times, the accessibility has been posited to consist of two components, “macroaccessibility” and “microaccessibility.” Macroaccessibility denotes the accessibility of the enzyme to the cellulose surface, and is reduced by the presence of hemicellulose and lignin, which bind the surface. Pretreatment‐based delignification is predominantly held to increase macro‐accessibility, bringing no further improvements beyond specific limits . Once access to the cellulose surface reaches critical mass however, the enzyme must bind productively to the reducing ends of cellulose fibrils, to generate soluble oligomers .…”

Parameter determination and validation for a mechanistic model of the enzymatic saccharification of cellulose-Iβ

“…This cellulose may appear as part of the amorphous population when measured by many of the crystallinity index measurement methods. Several studies have indicated that cellulolytic components not only catalyze cellulose hydrolysis but can significantly disrupt cellulose structure at the same time . Pu et al used the NMR methodology developed by Larsson and Westlund to probe cellulose crystallinity and defined a para ‐crystalline population that is less ordered than crystalline cellulose but more ordered than truly amorphous cellulose.…”