Recent Trends in the Pretreatment of Lignocellulosic Biomass for Value-Added Products

Abstract

Lignocellulosic biomass (LCB) is the most abundantly available bioresource amounting to about a global yield of up to 1. 3 billion tons per year. The hydrolysis of LCB results in the release of various reducing sugars which are highly valued in the production of biofuels such as bioethanol and biogas, various organic acids, phenols, and aldehydes. The majority of LCB is composed of biological polymers such as cellulose, hemicellulose, and lignin, which are strongly associated with each other by covalent and hydrogen bonds thus forming a highly recalcitrant structure. The presence of lignin renders the bio-polymeric structure highly resistant to solubilization thereby inhibiting the hydrolysis of cellulose and hemicellulose which presents a significant challenge for the isolation of the respective bio-polymeric components. This has led to extensive research in the development of various pretreatment techniques utilizing various physical, chemical, physicochemical, and biological approaches which are specifically tailored toward the source biomaterial and its application. The objective of this review is to discuss the various pretreatment strategies currently in use and provide an overview of their utilization for the isolation of high-value bio-polymeric components. The article further discusses the advantages and disadvantages of the various pretreatment methodologies as well as addresses the role of various key factors that are likely to have a significant impact on the pretreatment and digestibility of LCB.

Pretreatment methods for lignocellulosic biofuels production: current advances, challenges and future prospects

“…Milling is another commonly applied method to pretreat biomass, so as to increase the specific surface area for enzymatic hydrolysis, increase the degree of polymerization, and reduce cellulose crystallinity. Milling can reduce the particle size down to 0.2 mm (Baruah et al, 2018). Smaller size of lignocellulosic biomass provides greater surface area.…”

“…The pretreatment process could be carried out at high temperatures (>180°C) for 1-5 min, or mild temperatures (<120°C) but at longer residence times (30-90 min) (Kumar & Sharma 2017). This method could also be performed at low temperatures (<100°C) for higher concentrations of acids (30-70%) and at higher temperatures (100-250°C) for dilute acids (<10%) (Baruah et al, 2018). Dilute sulphuric acid is commonly applied to pretreat switchgrass and corn stover.…”

“…Calcium hydroxide is the most economic alkaline pretreatment when compared to ammonia, sodium hydroxide, and potassium hydroxide. Biogas yielded from corn cob residues was two times higher than untreated corn cob, as lime pretreatment accelerated the digestion process by removing lignin (Baruah et al, 2018). Optimization strategies like time and chemical concentrations are important factors to enhance the reaction rates.…”

“…Steam explosion applies high pressures of steam followed by sudden reduction of pressure, so as the biomass undergoes explosive decompression. This quick depressurization involves an initial temperature of 160 to 260°C for seconds and minutes in saturated steam prior to exposure to atmospheric pressure (Baruah et al, 2018;Kim, 2018). The method is known to be effective to disintegrate various lignocellulosic biomass feedstock, forest residues, and wastes at low cost and fast rate.…”

“…al., 2017;Koupaie et al, 2019). White-rot, brown-rot, and soft-rot fungi are known to be effective microorganisms in pretreating biomass by producing lignin-degrading enzymes, while whiterot-fungi are more commonly used as they are able to provide higher sugar yields(Baruah et al, 2018;Waghmare et al, 2018). Lignin-degrading…”