Lignocellulosic BiomassLignocellulosic biomass is defined as a plant, or plant-derived, material that is mostly composed of cellulose, hemicellulose, and lignin. Lignocellulosic feedstocks are highly abundant, covering many biomass types including grasses, wood, energy crops (e.g. Miscanthus and coppices), agricultural residues (e.g. straws and corn stover), and municipal wastes.
Hydrolysis of LignocelluloseA major pathway by which many lignocellulosic feedstocks are processed is known as hydrolysis, where monomeric sugars are released from the lignocellulosic polysaccharides (i.e. cellulose and hemicellulose). Typically, these polysaccharides are hydrolysed by acid or, more commonly, by enzymes. The hydrolysis of cellulose will yield monomeric glucose (as cellulose is a hompolysaccharide, i.e only containing one type of sugar), whilst the hydrolysis of hemicellulose will yield a variety of different sugars covering the hexoses (6-carbon sugars) glucose, galactose, and mannose, and the pentoses (5-carbon sugars) xylose and arabinose, depending on the type of hemicellulose. Hydrolysis of hemicellulose can also yield uronic acids and acetyl groups.
Early Work (Acid Hydrolysis)The potential of lignocellulosic biomass as a source of fermentable sugars was recognized as early as the late 19th century, with notable work by researchers such as Charles Tanret. However, the robust structure of lignocellulose—comprising cellulose, hemicellulose, and lignin—posed significant challenges. Acid hydrolysis emerged as an early method for biomass conversion, making use of sulphuric acid to cleave glycosidic bonds in cellulose and hemicellulose.
Early Enzymatic HydrolysisIn the mid-20th century, a paradigm shift occurred with the introduction of enzymatic hydrolysis. Enzymes produced by microorganisms, such as Trichoderma reesei, were found to efficiently convert cellulose into glucose. This biological hydrolysis was more environmentally friendly and yielded higher sugar concentrations. Despite its promise, the cost of producing cellulase enzymes was prohibitive, which led to a research emphasis on reducing enzyme costs and improving their efficiency.
Research AdvancesIt was recognised that the recalcitrance of lignin remained a major challenge in developing efficient biological hydrolysis processes. As a result, pre-treatment processes were introduced to enhance the accessibility of cellulose and hemicellulose to enzymes. These processes include dilute acid pretreatment, steam explosion, and organosolv methods. Each of these methods disrupts the lignocellulosic structure in various ways, increasing the efficiency of subsequent enzymatic hydrolysis.
Ongoing ResearchResearch continues for developing improved pretreatments and more robust enzymes. Additionally, the early 21st century has seen a particular focus on the development of consolidated bioprocessing (CBP). This approach consolidates enzyme production, saccharification, and fermentation into a single step. CBP employs engineered microbes, such as Clostridium thermocellum, that can both produce cellulolytic enzymes and ferment sugars into ethanol. This process further simplifies the biomass-to-product conversion process and reduces costs.
Dilute-Acid HydrolysisThis involves the use of acids, in relatively low concentrations and at elevated temperatures, to hydrolyse the biomass polysaccharides. The dynamics and outputs of the process are highly dependent on its severity, based on factors such as the type and concentration of acid and the temperatures used. Since cellulose requires more severe hydrolysis conditions than hemicellulose, the dilute-acid approach usually involves two stages. The fist stage focuses on the hydrolysis of hemicellulose and the second stage then involves more severe conditions that allow for cellulose to be hydrolysed.
Concentrated Acid HydrolysisText.
Separate Hydrolysis and Fermentation (SHF)This process involves two distinct stages: hydrolysis, in which the cellulose and hemicellulose (if not already removed in the pretreatment) are broken down into simple sugars; and fermentation, where these sugars are transformed into biofuels.
Simultaneous Saccharification and Fermentation (SSF)SSF involves the concurrent breakdown (hydrolysis) of cellulose (and hemicellulose, if present) into monomeric sugars (saccharification), and the conversion of these sugars into products via fermentation. Unlike in SHF, in SSF both stages take place in the same reactor.
Simultaneous Saccharification and Co-Fermentation (SSCF)SSCF is a modification of the Simultaneous Saccharification and Fermentation (SSF) method, incorporating a step to ferment pentose sugars alongside the conventional hexose sugars.
Consolidated Bioprocessing (CBP)Consolidated Bioprocessing (CBP) is an even more integrated approach in which the enzyme production, hydrolysis, and fermentation steps all occur in one step and one reactor.
Global Recognition as Bioprocess Experts
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