Extraction of sugarcane bagasse arabinoxylan, integrated with enzymatic production of xylo-oligosaccharides and separation of cellulose

Leila Khaleghipour,Hamid Rashedi,Seyed Omid Ranaei Siadat,Andrius Jasilionis,Javier A Linares-Pastén,Eva Nordberg Karlsson,Said Al-Hamimi,Roya R R Sardari

doi:10.1186/s13068-021-01993-z

Leila Khaleghipour, Hamid Rashedi + Show 6 more

Open Access

https://doi.org/10.1186/s13068-021-01993-z

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Abstract

Sugarcane processing roughly generates 54 million tonnes sugarcane bagasse (SCB)/year, making SCB an important material for upgrading to value-added molecules. In this study, an integrated scheme was developed for separating xylan, lignin and cellulose, followed by production of xylo-oligosaccharides (XOS) from SCB. Xylan extraction conditions were screened in: (1) single extractions in NaOH (0.25, 0.5, or 1 M), 121 °C (1 bar), 30 and 60 min; (2) 3 × repeated extraction cycles in NaOH (1 or 2 M), 121 °C (1 bar), 30 and 60 min or (3) pressurized liquid extractions (PLE), 100 bar, at low alkalinity (0–0.1 M NaOH) in the time and temperature range 10–30 min and 50–150 °C. Higher concentration of alkali (2 M NaOH) increased the xylan yield and resulted in higher apparent molecular weight of the xylan polymer (212 kDa using 1 and 2 M NaOH, vs 47 kDa using 0.5 M NaOH), but decreased the substituent sugar content. Repeated extraction at 2 M NaOH, 121 °C, 60 min solubilized both xylan (85.6% of the SCB xylan), and lignin (84.1% of the lignin), and left cellulose of high purity (95.8%) in the residuals. Solubilized xylan was separated from lignin by precipitation, and a polymer with β-1,4-linked xylose backbone substituted by arabinose and glucuronic acids was confirmed by FT-IR and monosaccharide analysis. XOS yield in subsequent hydrolysis by endo-xylanases (from glycoside hydrolase family 10 or 11) was dependent on extraction conditions, and was highest using xylan extracted by 0.5 M NaOH, (42.3%, using Xyn10A from Bacillus halodurans), with xylobiose and xylotriose as main products. The present study shows successful separation of SCB xylan, lignin, and cellulose. High concentration of alkali, resulted in xylan with lower degree of substitution (especially reduced arabinosylation), while high pressure (using PLE), released more lignin than xylan. Enzymatic hydrolysis was more efficient using xylan extracted at lower alkaline strength and less efficient using xylan obtained by PLE and 2 M NaOH, which may be a consequence of polymer aggregation, via remaining lignin interactions.

Highlights

Xylan is extracted from Sugarcane bagasse (SCB), with separation of lignin and cellulose, allowing retrieval of the different materials in separate fractions
The present results clearly show that repeated extraction with high concentration of alkali, is an efficient way to release xylan from sugarcane bagasse (85.6 ± 1.10% of the xylan was released), which by precipitation separates it from the major part of the lignin stream, and that this gives the possibility to obtain a clean cellulose fraction of high purity (95.8 ± 0.59% of cellulose) after acid purification of the remaining residue
Lignin co-extracts with xylan at high concentration of alkali, but if high pressure was applied, lignin appeared easier to release than xylan, at lower alkali concentration (0.1 M NaOH)

Summary

Introduction

Sugarcane bagasse (SCB) is an important lignocellulosic residue that is generated in large amounts by sugar and alcohol industries. SCB consists of 14–30% lignin, 35–50% cellulose and 22–36% hemicelluloses with limited amounts of extractives and ash [10,11,12]. The chemical composition, with high carbohydrate content and low ash content, makes SCB a suitable raw material for manufacturing of high value-products [11]. By fractionation of SCB into core chemical components (e.g., cellulose, hemicelluloses and lignin), each individual component can be used for the development of a variety of products, as demonstrated for other types of lignocellulosic byproducts [13]. Xylans consists of a backbone of xylose residues that are linked via β-1-4 glycosidic bonds, substituted to varying degrees with, e.g., arabinose, glucuronic acid, 4-O-methyl and acetyl esters. The type and degree of branching are determined by the xylan source, resulting in sub-categories of xylans; including homoxylan, arabinoxylan, glucuronoxylan and arabinoglucuronoxylans [14]

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