Hydrothermal Pretreatment of Lignocellulosic Feedstocks to Facilitate Biochemical Conversion

Abstract

Biochemical conversion of lignocellulosic feedstocks to advanced biofuels and other bio-based commodities typically includes physical diminution, hydrothermal pretreatment, enzymatic saccharification, and valorization of sugars and hydrolysis lignin. This approach is also known as a sugar-platform process. The goal of the pretreatment is to facilitate the ensuing enzymatic saccharification of cellulose, which is otherwise impractical due to the recalcitrance of lignocellulosic feedstocks. This review focuses on hydrothermal pretreatment in comparison to alternative pretreatment methods, biomass properties and recalcitrance, reaction conditions and chemistry of hydrothermal pretreatment, methodology for characterization of pretreatment processes and pretreated materials, and how pretreatment affects subsequent process steps, such as enzymatic saccharification and microbial fermentation. Biochemical conversion based on hydrothermal pretreatment of lignocellulosic feedstocks has emerged as a technology of high industrial relevance and as an area where advances in modern industrial biotechnology become useful for reducing environmental problems and the dependence on fossil resources.

Comparison of solid and liquid fractions of pretreated Norway spruce as reductants in LPMO-supported saccharification of cellulose

“…In addition to that, substances in the PL might inhibit cellulolytic enzymes. Such substances include both sugars (causing feedback inhibition of cellulases) and aromatics, such as phenolics (Zhai et al, 2016;Martín et al, 2022). The total monosaccharide concentration (~67 g/L) of the PL used in this work was similar to the total monosaccharide concentrations (64 g/L-77 g/L) of the PLs studied by Wang et al (2018a).…”

“…Enzymatic deconstruction of cellulose and hemicelluloses in lignocellulosic biomass to sugars offers a renewable alternative to the refining of fossil resources, which are associated with environmental problems and problems concerning energy security ( Kaparaju et al, 2009 ). Biochemical conversion typically includes hydrothermal pretreatment, enzymatic saccharification of cellulose, microbial fermentation of sugars, and valorization of the lignin-rich solid residue, often referred to as hydrolysis lignin ( Martín et al, 2022 ). The challenges of biochemical conversion are highly dependent on achieving high sugar yields from cellulose at low cost, thus making it more competitive ( Østby et al, 2020 ).…”

“…Other reasons for studying lignin and lignin derivatives as electron donors for LPMO include that the lignin content and type of lignin of lignocellulosic biomass differ depending on the biological origin, and that the structure and the fraction of lignin in the raw material are affected in different ways by different types of pretreatments and pretreatment conditions ( Martín et al, 2022 ). The lignin content (DW, dry-weight) in biomass differs from below 20% in grasses and many agricultural residues ( Ralph et al, 2004 ) to around 30% in softwood ( Sjöström, 1993 ).…”

“…The fractions of different interunit linkages also vary in different forms of biomass, although the β-O-4-aryl ether linkage is the most common ( Ralph et al, 2004 ). Although it is hemicellulose rather than lignin that is the target of hydrothermal pretreatment under acidic conditions ( Galbe and Wallberg, 2019 ; Martín et al, 2022 ), the lignin is also affected. A minor fraction of the lignin is degraded and ends up as phenylic substances (i.e., phenolic and non-phenolic aromatic substances) in the liquid phase after the pretreatment ( Ko et al, 2015 ; Jönsson and Martín, 2016 ).…”

Surfactant-assisted alkaline pretreatment and enzymatic hydrolysis of Miscanthus sinensis for enhancing sugar recovery with a reduced enzyme loading

“…Lignin is an important obstacle to the efficient saccharification of lignocellulosic biomass since it impedes its availability to enzymes and microorganisms (Qing et al, 2010;Martin et al, 2022). Alkaline pretreatment, which produces an effective delignification and chemical swelling of fibrous cellulose, is considered among the most streamlined and efficient techniques (Li et al, 2016a, b;Shimizu et al, 2018).…”