Isolation And Degradation Of Hemicelluloses For The Production Of Xylooligosaccharides From Lignocellulosic Biomass

Hemicelluloses, accounting for20to35%of the biomass, can be utilized in their native or modified forms in various areas. However, hemicelluloses have not been effectively exploited because of their structural diversity. Xylooligosaccharides (XOS) are sugar oligomers made up of xylose units, which are mainly produced during the hydrolysis of xylan-rich hemicelluloses and present important physicochemical and physiological properties. In order to promote the utilization of the hemicelluloses, the present work focused on isolation and characterization of hemicelluloses as well as hydrolysis of xylan-rich hemicelluloses to generate xylooligosaccharides from lignocellulosic biomass.Sequential treatments of dewaxed Caragana korshinskii with dimethyl sulfoxide (DMSO) and dioxane-triethylamine (9:1, v/v) at80℃for5h, saturated barium hydroxide,1M potassium hydroxide and1M sodium hydroxide at50℃for5h, and3M potassium hydroxide at50℃for4h released a total of95.0%of the original hemicelluloses in the cell wall. The four alkali-soluble hemicelluloses contained more xylose (79.0-88.0%) than DMSO-soluble and dioxane-triethylamine (9:1, v/v) soluble hemicellulose fractions. In comparison, the treatment with DMSO and dioxane-triethylamine favored to release much lower molecular weights than those of the other four alkali-soluble hemicelluloses. It was confirmed that the hemicelluloses from C. korshinskii were (1â†'4)-linked β-D-xylans with4-O-methyl-D-glucuronic acid attached based on both1H and13C NMR spectra analysis. The results from thermal analysis showed that the thermal stability increased with increasing molecular weights.Seven hemicellulose fractions were extracted with10%KOH from delignified sugarcane bagasse for10h at20,25,30,35,40,45, and50℃, respectively. Chemical composition and structural features of all the fractions were investigated by a combination of HPAEC, GPC, FT-IR,1D (1H,13C) and2D (HSQC) NMR spectra, and TGA-DTA. Notable differences in the molecular weights were observed that the fractions extracted at20,25, and30℃showed relatively lower molecular weights (68400～76900g mol-1) and the extraction at elevated temperatures from35to50℃yielded the hemicellulose populations of somewhat higher Mw (80400-93300g mol-1). However, the differences in the yield, chemical composition, structural features and thermal stability were much less pronounced. The results also suggested that all the hemicelluloses had a backbone of (1â†'4)-β-D-xylan and mainly substituted with (1â†'2) and (1â†'3)-linked arabinofuranosyl residues, and also with4-O-methyl-D-glucuronic acid linked to the backbone.The alkali-extractable hemicelluloses from C. korshinskii and Populus gansuensis were successively sub-fractionated by graded ethanol precipitation. Neutral sugars and molecular weight analyses revealed that the molecular weights and the distribution of branches along the xylan backbone were different among the hemicellulose fractions obtained in various ethanol concentrations. The less branched hemicelluloses with larger molecules were precipitated in lower ethanol percentages, while more branched hemicelluloses with lower molecular weights were obtained with the increasing ethanol concentrations. The thermal stability of the linear and larger molecular hemicellulose sub fraction EH30appeared higher than that of the branched subfraction EH75.Alkali solubilized hemicelluloses from sugarcane bagasse were subjected to microwave-assisted acid hydrolysis at mild temperature to produce XOS. The hydrolysis was performed with dilute sulfuric acid at90℃and the influence of acid concentration (0.1-0.3M) and reaction time (20-40min) on the XOS production was ascertained with a central composite design (CCD) of response surface methodology. The fitted models of XOS and xylose yields were in well agreement with the experimental results. Compared to hydrolysis time, acid concentration was a more significant factor in the production of XOS. A well-defined degree of polymerization (DP) of XOS and the monomer in the hydrolysates were quantified and no sugar-degraded byproduct was detected. The maximum XOS yield was290mg g-1, which was achieved by hydrolysis with0.24M H2SO4for31min. Results indicated that xylose together with the byproduct in microwave-assisted acid hydrolysis can be controlled by acid concentration and reaction time.XOS were prepared from xylan-rich hemicelluloses isolated from sugarcane bagasse by hydrolysis with crude xylanase secreted by Pichia stipitis. Hydrolysis for12h produced XOS with a maximum yield of31.8%, equivalent to5.29mg mL-1in the hydrolyzate. XOS with DP from2to4(xylobiose, xylotriose, and xylotetraose) were the major components in the hydrolysates, whereas the oligosaccharides with higher DP of5-6(xylopentaose and xylohexose) showed a constant low level. FT-IR and NMR (1H,13C, HSQC) analysis demonstrated that XOS contained Araf and4-O-Me-a-D-GlcpA residues. The2,2-diphenyl-l-picrylhydrazyl (DPPH) assay showed that the XOS exhibited concentration-dependent antioxidant activity.Crude cellulose prepared from alkali-extracted sugarcane bagasse was subjected to a rapid purification treatment with a mixture of80%acetic acid-68%nitric acid (10/1, v/v) at120℃for15min. The yields of the preparations decreased slightly from57.3-58.6%in the crude cellulose preparations to50.3-51.9%in the purified cellulose samples. The purification treatment removed a large quantity of resistant hemicelluloses strongly associated to the cellulose. XRD analysis revealed that the structure of both the crude and purified cellulose was cellulose â… . Compared to the crude cellulose, an increase in the crystallinity index of the purified cellulose was observed by FT-IR, XRD, and CP/MAS13C NMR analyses.Crude cellulose isolated from sugarcane bagasse was subjected to ionic liquid ([Emim]Ac) dissolution at a mild temperature (90℃) followed by regeneration in water and subsequently hydrolyzed by commercial cellulases. The original and regenerated cellulose was thoroughly characterized by XRD, FT-IR, CP/MAS13C NMR, and SEM. It was found that the original cellulose experienced an increase in glucose content from80.0-83.3%to91.6-92.8%, a decrease in DP from974-1039to511-521, a crystal transformation from cellulose â…  to cellulose â…¡, as well as an increment of surface area during the pretreatment. The results suggested that the changes of cellulose led to effective disruption of the cellulosic recalcitrance as evidenced by a high glucose conversion rate of95.2%.