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

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 [1–3]. 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 [8–10]. Thermochemical pretreatments, however, are considered leading options due to their comparatively short reaction time, effectiveness, and lower energy requirements compared to mechanical options [11–13]. Nonetheless, thermochemical pretreatments typically need