Glucuronoarabinoxylan Research Articles

Structure of KOH-soluble polysaccharides from сoniferous greens of Norway spruce (Picea abies): The pectin-xylan-AGPs complex. Part 2

Polymers containing arabinoglucuronoxylan, fucogalactoxyglucan, pectin and arabinogalactan proteins were obtained from PAK isolated from Norway spruce with 7 % KOH. The pectin core of PAK-I2-F-1 and PAK-I2-F-2 was dominated by RG-I, as treatment with 1,4-α-D-polygalacturonase resulted in almost complete removal of homogalacturonan. Interestingly, the above has not affected the co-fractionation of arabinoglucuronoxylan (AGX), arabinogalactan proteins and rhamnogalacturonan I (RG-I). Since pectin was mainly represented by RG-I, we concluded that xylan is specifically associated with RG-I. Correlations in the HMBC spectrum demonstrate intermolecular interactions between the α-L-Rhap (RG-I) and the Xyl (xylan), indicating a covalently bound AGX:RG-I complex via the Xyl-(1→4)-Rha bond: …→2)-[(2,4-β-D-Xylp)-(1→4)]-[(α-D-GalpA-(1→2)]-α-L-Rhap-(1→4)-α-D-GalpA-(1→…. In PAK-H1-1-F-1 and PAK-H1-1-F-2, parts of RG-I and xylan were removed by enzymolysis. Part of the xylan was probably attached to the above-mentioned RG-I blocks. The removal of part of RG-I, xylan and the disappearance of the signal in the HMBC spectrum indicating the bond between RG-I and xylan confirms that part of the arabinoglucuronoxylan is covalently bound to RG-I. The observed glycosidic linkage contradicts the dominant PCW model in which pectin and hemicellulose polysaccharide networks are considered as independent components. It can be concluded that alkali-soluble xylan from Norway spruce was detected both in the free state and covalently bound to pectin.

A novel substitution pattern in glucuronoarabinoxylans from woody bamboos

(1 → 4)-β-D-Xylans are the second most abundant plant biopolymers on Earth after cellulose. Although their structures have been extensively studied, and industrial applications have been found for them and their derivatives, they are still investigated due to the diversity of their structures and uses. In this work, hemicellulose fractions obtained previously with 1 M KOH from two species of woody bamboos, Phyllostachys aurea and Guadua chacoensis, were purified, and the structures of the glucuronoarabinoxylans (GAX) were studied by chemical and spectroscopic methods. In both cases, major amounts of α-L-arabinofuranose residues were linked to C3 of the xylose units of the backbone, and also α-D-glucuronic acid residues and their 4-O-methyl-derivatives were detected in minor quantities, linked to C2 of some xylose residues. Methylation analysis of the carboxyl-reduced derivative from GAX from P. aurea indicated the presence of terminal and 5-linked arabinofuranose units. NMR spectroscopy showed the presence of disaccharidic side chains of 5-O-α-l-arabinofuranosyl-L-arabinofuranose for the GAX from P. aurea, while for those of G. chacoensis, only single side chains were found. To the best of our knowledge, this disaccharide was not found before as side chain of xylans.

CHANGES IN HEMICELLULOSE STRUCTURE ASSOCIATED WITH THE TRANSITION FROM EARLYWOOD TO LATEWOOD AT JUVENILE WOOD IN CRYPTOMERIA JAPONICA

The chemical composition and variations in chemical structure of hemicellulose in earlywood (EW) and latewood (LW) of two individual Japanese cedar trees (C-Boka and T-Boka) were investigated. The trees were cultivated under different growth conditions: C-Boka grew slowly in a forest, while T-Boka grew rapidly in a location rich in nutrients and sunshine. For the chemical structure of hemicellulose, arabinoglucuronoxylan (AGX) showed varied side-chain substitution rates with glucuronic acid and different molecular weights in the transition between EW and LW. In contrast, the fundamental composition of glucomannan/galactoglucomannan (GM/GGM) was relatively unchanged between EW and LW. The modification of AGX and GM/GGM from EW to LW differed between C-Boka and T-Boka and might be influenced by the growth rate of the trees.

Influence of hydrothermal treatment on hemicellulose structure in Cryptomeria japonica

The influence of hydrothermal treatment (HTT) on the hemicellulose, i.e. glucomannan/galactoglucomannan (GM/GGM) and arabinoglucurono-xylan (AGX) in Japanese cedar was examined using mild temperatures (95 °C and 120 °C) for its kiln drying. Based on infrared spectra, only a slight change due to HTT was observed in the hemicellulose chemical structure. The quantitative changes of the major chemical components including lignin, cellulose, and hemicellulose also exhibited slight differences among samples. However, the molecular chain-length composition of GM/GGM and AGX in the cell walls decreased clearly with HTT, as well as with an increase in temperature. Also, it was found that the strength of the interaction between cellulose and hemicellulose molecules such as GM/GGM and AGX in cell walls decreased depending on the increase in HTT temperature. These results showed that the polymeric structure of GM/GGM and AGX was degraded by hydrolysis at 95 °C of HTT and proceeded further at 120 °C of HTT.

Grass xylan structural variation suggests functional specialization and distinctive interaction with cellulose and lignin.

Xylan is the most abundant non-cellulosic polysaccharide in grass cell walls, and it has important structural roles. The name glucuronoarabinoxylan (GAX) is used to describe this variable hemicellulose. It has a linear backbone of β-1,4-xylose (Xyl) residues that may be substituted with α-1,2-linked (4-O-methyl)-glucuronic acid (GlcA), α-1,3-linked arabinofuranose (Araf), and sometimes acetylation at the O-2 and/or O-3 positions. The role of these substitutions remains unclear, although there is increasing evidence that they affect the way xylan interacts with other cell wall components, particularly cellulose and lignin. Here, we used substitution-dependent endo-xylanase enzymes to investigate the variability of xylan substitution in grass culm cell walls. We show that there are at least three different types of xylan: (i) an arabinoxylan with evenly distributed Araf substitutions without GlcA (AXe); (ii) a glucuronoarabinoxylan with clustered GlcA modifications (GAXc); and (iii) a highly substituted glucuronoarabinoxylan (hsGAX). Immunolocalization of AXe and GAXc in Brachypodium distachyon culms revealed that these xylan types are not restricted to a few cell types but are instead widely detected in Brachypodium cell walls. We hypothesize that there are functionally specialized xylan types within the grass cell wall. The even substitutions of AXe may permit folding and binding on the surface of cellulose fibrils, whereas the more complex substitutions of the other xylans may support a role in the matrix and interaction with other cell wall components.

High-performance arabinoglucuronoxylan-based biosurfactants for oily sludge separation

Arabinoglucuronoxylan (AGX), an important carbohydrate polymer in lignocellulosic biomass, contains many functional groups. It has excellent amphiphilic modification properties and has potential application in the separation of oily sludge. In this study, a simple strategy for producing high-performance AGX-based biosurfactants was studied and the raw AGX was extracted from bamboo by hydrothermal treatment. AGX-based biosurfactants with amphiphilic structure were produced by AGX and dodecanal succinic anhydride (DDSA) using rapid homogeneous esterification reactions in deep eutectic solvents (DES). This resulted in a significant reduction in the surface tension of the water from 72.32 to 29.76 mN·m−1. These parameters are similar to those achieved using sodium dodecyl sulfate (SDS), a widely employed synthetic surfactant. Other physicochemical properties, including foamability, emulsification activity, stability, solubilization, and detergency were significantly improved compared to the unmodified AGX, demonstrate that AGX-based biosurfactants are promising detergents for oily sludge remediation and oil recovery. The results provide a new pathway for high value utilization of arabinoglucuronoxylan.

Production of xylose through enzymatic hydrolysis of glucuronoarabinoxylan from brewers’ spent grain

Xylose is an abundant bioresource for obtaining diverse chemicals and added-value products. The production of xylose from green alternatives like enzymatic hydrolysis is an important step in a biorefinery context. This research evaluated the synergism among four classes of hydrolytic purified enzymes—endo-1,4-β-xylanase, α-l-arabinofuranosidase, β-xylosidase, and α-d-glucuronidase—over hydrolysis of glucuronoarabinoxylan (GAX) obtained from brewers’ spent grain (BSG) after alkaline extraction and ethanol precipitation. First, monosaccharides, uronic acids and glycosidic-linkages of alkaline extracted GAX fraction from BSG were characterized, after that different strategies based on the addition of one or two families of enzymes—endo-1,4-β-xylanase (GH10 and GH11) and α-l-arabinofuranosidase (GH43 and GH51)—cooperating with one β-xylosidase (GH43) and one α-d-glucuronidase (GH67) into enzymatic hydrolysis were assessed to obtain the best yield of xylose. The xylose release was monitored over time in the first 90 min and after a prolonged reaction up to 48 h of reaction. The highest yield of xylose was 63.6% (48 h, 40 ℃, pH 5.5), using a mixture of all enzymes devoid of α-l-arabinofuranosidase (GH43) family. These results highlight the importance of GH51 arabinofuranosidase debranching enzyme to allow a higher cleavage of the xylan backbone of GAX from BSG and their synergy with 2 endo-1,4-β-xylanase (GH10 and GH11), one β-xylosidase (GH43) and the inclusion of one α-d-glucuronidase (GH67) in the reaction system. Therefore, this study provides an environmentally friendly process to produce xylose from BSG through utilization of enzymes as catalysts.Graphical

Covalently linked pectin-arabinoglucuronoxylan complex from Siberian fir Abies sibirica Ledeb.

In this study, by NMR spectroscopy, first was proved that the pectic polysaccharides of Abies sibirica are covalently bound to arabinoglucuronoxylan (AGX). Namely, AGX is attached at the 4th position to the 1,2,4-α-L-Rha residues of the RG-I main chain, indicating the following fragment: …→2)-[(2,4-β-D-Xylp)-(1→4)]-[(α-D-GalpA-(1→2)]-α-L-Rhap-(1→4)-α-D-GalpA-(1→…. We present direct evidence of covalent attachment between C1 of 1,2,4-β-D-Xylp from arabinoglucuronoxylan and С4 of 1,2,4-α-L-Rhap from RG-I of pectin by detailed 2D NMR analyses. It can be concluded that the 1,2,4-α-L-Rhap residues, were substituted at the 4th position by 1,5-α-L-arabinan or 1,4-β-D-galactan or AGX. The discovery of this interglycosidic linkage between xylan and pectin contradicts the classical model of the cell wall, which describes networks of binding glycans and pectic polysaccharides as separate and independent of one another. We can conclude that pectin undoubtedly plays a more important supporting and cross-linking role in the cell wall than is commonly thought.

Structural characterization of the family GH115 α-glucuronidase from Amphibacillus xylanus yields insight into its coordinated action with α-arabinofuranosidases

The coordinated action of carbohydrate-active enzymes has mainly been evaluated for the purpose of complete saccharification of plant biomass (lignocellulose) to sugars. By contrast, the coordinated action of accessory hemicellulases on xylan debranching and recovery is less well characterized. Here, the activity of two family GH115 α-glucuronidases (SdeAgu115A from Saccharophagus degradans, and AxyAgu115A from Amphibacillus xylanus) on spruce arabinoglucuronoxylan (AGX) was evaluated in combination with an α-arabinofuranosidase from families GH51 (AniAbf51A, aka E-AFASE from Aspergillus niger) and GH62 (SthAbf62A from Streptomyces thermoviolaceus). The α-arabinofuranosidases boosted (methyl)-glucuronic acid release by SdeAgu115A by approximately 50 % and 30 %, respectively. The impact of the α-arabinofuranosidases on AxyAgu115A activity was comparatively low, motivating its structural characterization. The crystal structure of AxyAgu115A revealed increased length and flexibility of the active site loop compared to SdeAgu115A. This structural difference could explain the ability of AxyAgu115A to accommodate more highly substituted arabinoglucuronoxylan, and inform enzyme selections for improved AGX recovery and use.

Development of a thermophilic coculture for corn fiber conversion to ethanol

The fiber in corn kernels, currently unutilized in the corn to ethanol process, represents an opportunity for introduction of cellulose conversion technology. We report here that Clostridium thermocellum can solubilize over 90% of the carbohydrate in autoclaved corn fiber, including its hemicellulose component glucuronoarabinoxylan (GAX). However, Thermoanaerobacterium thermosaccharolyticum or several other described hemicellulose-fermenting thermophilic bacteria can only partially utilize this GAX. We describe the isolation of a previously undescribed organism, Herbinix spp. strain LL1355, from a thermophilic microbiome that can consume 85% of the recalcitrant GAX. We sequence its genome, and based on structural analysis of the GAX, identify six enzymes that hydrolyze GAX linkages. Combinations of up to four enzymes are successfully expressed in T. thermosaccharolyticum. Supplementation with these enzymes allows T. thermosaccharolyticum to consume 78% of the GAX compared to 53% by the parent strain and increases ethanol yield from corn fiber by 24%.

Glucuronoarabinoxylans and other cell wall polysaccharides from shoots of Guadua chacoensis obtained by extraction in different conditions

Guadua is a genus of woody bamboos, native to the American continent, which comprises economically important species. Cell wall polysaccharides from young shoots of Guadua chacoensis showed glucuronoarabinoxylans (GAX) as the major hemicellulosic components (65%, of the recovered carbohydrates by extraction with aqueous solutions), which were obtained in major quantities with 1 M KOH (molar ratio Xyl:Ara:GlcA, 100:28:8). AGP and pectin polymers, as well as mixed linkage glucans and xyloglucans were present in smaller amounts (16% and 15%, respectively). Alternative extraction of GAX with solutions of DMSO allowed the characterization of alkali labile substituents, in particular, acetyl groups, which were linked mainly to C3 or, to a lesser extent, to C2 of some of the xylose residues. Besides, small amounts of phenolic compounds, released by saponification, were detected, linked to C5 of some α-l-arabinofuranose units. These results could be relevant for their use as a new resource for the food industry.

Characterization of hemicelluloses in Phyllostachys edulis (moso bamboo) culm during xylogenesis

Hemicelluloses are β-(1→4)-linked backbone polysaccharides found in plant cell walls that include xyloglucans, xylans, mannans and glucomannans, and play important roles in plant tissue configuration. In this study, hemicelluloses were isolated from the apical, middle and basal segments of 6 m Phyllostachys edulis culm using KOH and DMSO extraction procedures, respectively. Chemical composition and structural characterization of hemicellulosic fractions were comparatively investigated by a combination of HPLC, GPC, FT-IR, 1H-, 13C-, HSQC NMR and TGA techniques. Our results show that the main chain of hemicellulose in P. edulis consists of glucuronoarabinoxylans (GAXs) with backbone 1, 4-β-d-Xyl, and side chain arabinose, glucuronic acid and acetylation. Hemicellulose content and molecular weight increased with culm xylogenesis in P. edulis. Our results provide new insights on the dynamics of hemicellulose structure in culm xylogenesis in P. edulis.

Development of an oligosaccharide library to characterise the structural variation in glucuronoarabinoxylan in the cell walls of vegetative tissues in grasses

BackgroundGrass glucuronoarabinoxylan (GAX) substitutions can inhibit enzymatic degradation and are involved in the interaction of xylan with cell wall cellulose and lignin, factors which contribute to the recalcitrance of biomass to saccharification. Therefore, identification of xylan characteristics central to biomass biorefining improvement is essential. However, the task of assessing biomass quality is complicated and is often hindered by the lack of a reference for a given crop.ResultsIn this study, we created a reference library, expressed in glucose units, of Miscanthus sinensis GAX stem and leaf oligosaccharides, using DNA sequencer-Assisted Saccharide analysis in high throughput (DASH), supported by liquid chromatography (LC), nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Our analysis of a number of grass species highlighted variations in substitution type and frequency of stem and leaf GAX. In miscanthus, for example, the β-Xylp-(1 → 2)-α-Araf-(1 → 3) side chain is more abundant in leaf than stem.ConclusionsThe reference library allows fast identification and comparison of GAX structures from different plants and tissues. Ultimately, this reference library can be used in directing biomass selection and improving biorefining.

Experimental and Theoretical Evaluation of the Solubility/Insolubility of Spruce Xylan (Arabino Glucuronoxylan).

The molecular solubility of softwood arabinoglucuronoxylan (AGX) has been thoroughly investigated, and it has been shown that the chemical and physical structures of the extracted hemicellulose are not significantly influenced by different purification steps, but a transient molecular solubility of AGX was observed in aqueous media at low concentrations (1 g/L) when the dissolved macromolecules had a hydrodynamic diameter of up to 10 nm. A phase separation was detected when the concentration was increased to 15 g/L leading to an association of the smaller molecules into fractal structures with a considerably larger diameter, even though the dispersions were still transparent to ocular inspection. Dynamic Light Scattering and Cryo-Transmission Electron Microscopy showed dimensions in the range of 1000 nm. The phase separation of the sample was further characterized by estimating the χ-interaction parameter of AGX in water using the Flory-Huggins theory, and the results supported that water is a poor solvent for AGX. This behavior is crucial when films and hydrogels based on these biopolymers are made, since the association will dramatically affect barrier and mechanical properties of films made from these materials.

Wood-Derived Dietary Fibers Promote Beneficial Human Gut Microbiota.

Woody biomass is a sustainable and virtually unlimited source of hemicellulosic polysaccharides. The predominant hemicelluloses in softwood and hardwood are galactoglucomannan (GGM) and arabinoglucuronoxylan (AGX), respectively. Based on the structure similarity with common dietary fibers, GGM and AGX may be postulated to have prebiotic properties, conferring a health benefit on the host through specific modulation of the gut microbiota. In this study, we evaluated the prebiotic potential of acetylated GGM (AcGGM) and highly acetylated AGX (AcAGX) obtained from Norwegian lignocellulosic feedstocks in vitro In pure culture, both substrates selectively promoted the growth of Bifidobacterium, Lactobacillus, and Bacteroides species in a manner consistent with the presence of genetic loci for the utilization of β-manno-oligosaccharides/β-mannans and xylo-oligosaccharides/xylans. The prebiotic potential of AcGGM and AcAGX was further assessed in a pH-controlled batch culture fermentation system inoculated with healthy adult human feces. Results were compared with those obtained with a commercial fructo-oligosaccharide (FOS) mixture. Similarly to FOS, both substrates significantly increased (P < 0.05) the Bifidobacterium population. Other bacterial groups enumerated were unaffected with the exception of an increase in the growth of members of the Bacteroides-Prevotella group, Faecalibacterium prausnitzii, and clostridial cluster IX (P < 0.05). Compared to the other substrates, AcGGM promoted butyrogenic fermentation whereas AcAGX was more propiogenic. Although further in vivo confirmation is necessary, these results demonstrate that both AcGGM and AcAGX from lignocellulosic feedstocks can be used to direct the promotion of beneficial bacteria, thus exhibiting a promising prebiotic ability to improve or restore gut health.IMPORTANCE The architecture of the gut bacterial ecosystem has a profound effect on the physiology and well-being of the host. Modulation of the gut microbiota and the intestinal microenvironment via administration of prebiotics represents a valuable strategy to promote host health. This work provides insights into the ability of two novel wood-derived preparations, AcGGM and AcAGX, to influence human gut microbiota composition and activity. These compounds were selectively fermented by commensal bacteria such as Bifidobacterium, Bacteroides-Prevotella, F. prausnitzii, and clostridial cluster IX spp. This promoted the microbial synthesis of acetate, propionate, and butyrate, which are beneficial to the microbial ecosystem and host colonic epithelial cells. Thus, our results demonstrate potential prebiotic properties for both AcGGM and AcAGX from lignocellulosic feedstocks. These findings represent pivotal requirements for rationally designing intervention strategies based on the dietary supplementation of AcGGM and AcAGX to improve or restore gut health.

Arabinose substitution effect on xylan rigidity and self-aggregation

Substituted xylans play an important role in the structure and mechanics of the primary cell wall of plants. Arabinoxylans (AX) consist of a xylose backbone substituted with arabinose, while glucuronoarabinoxylans (GAX) also contain glucuronic acid substitutions and ferulic acid esters on some of the arabinoses. We provide a molecular-level description on the dependence of xylan conformational, self-aggregation properties and binding to cellulose on the degree of arabinose substitution. Molecular dynamics simulations reveal fully solubilized xylans with a low degree of arabinose substitution (lsAX) to be stiffer than their highly substituted (hsAX) counterparts. Small-angle neutron scattering experiments indicate that both wild-type hsAX and debranched lsAX form macromolecular networks that are penetrated by water. In those networks, lsAX are more folded and entangled than hsAX chains. Increased conformational entropy upon network formation for hsAX contributes to AX loss of solubility upon debranching. Furthermore, simulations show the intermolecular contacts to cellulose are not affected by arabinose substitution (within the margin of error). Ferulic acid is the GAX moiety found here to bind to cellulose most strongly, suggesting it may play an anchoring role to strengthen GAX-cellulose interactions. The above results suggest highly substituted GAX acts as a spacer, keeping cellulose microfibrils apart, whereas low substitution GAX is more localized in plant cell walls and promotes cellulose bundling.

Fractionation and Structural Characterization of Hemicellulose from Steam-Exploded Banana Rachis

Banana production in tropical countries generates significant quantity of waste. Biorefinery of food waste biomass into cellulose, hemicellulose, lignin or pectin macromolecules have grown interest in recent scientific literature. In this paper, hemicellulose from banana rachis (Musa cavendish) was extracted by steam explosion at three severity levels (2.97, 3.57 and 3.78) and was further fractionated by graded ethanol precipitation method (15%, 60% and 80%). The recovered hemicelluloses sub-fractions (H1, H2 and H3) were characterized for their chemical composition and structural features by HPSEC, TGA/DTG, FTIR, 1H and 2D NMR techniques. The hemicellulose extraction yield increased with the severity level and treatment duration. The average molecular weight of the extracted hemicellulose macromolecules decreased from H1-60% ethanol hemicellulose sub-fraction with 143 790 g/mol, followed by H3-60% ethanol hemicellulose sub-fraction with 110 841 g/mol and finally H2-60% ethanol hemicellulose sub-fraction with 61 404 g/mol. The H1-60% ethanol hemicellulose sub-fraction extracted during the steam explosion at the lowest severity level showed the largest molecular weight and exhibited rather a high arabinose/xylose ratio and uronic acid content. Structural analysis revealed that hemicellulose from the 60%-ethanol hemicellulose sub-fractions were mainly arabino-glucuronoxylan (AGX). However, chemical analysis also revealed significant contents of co-extracted residual lignin. Although the ethanol fractionation helped at lowering the lignin content in the 60%-ethanol hemicellulose sub-fractions (20.1% in H2-60%, 24.0% in H1-60% and 28.0 in H3-60%) relatively to 80%-ethanol hemicellulose sub-fractions, additional purification step was still required to improve the quality of the extracted hemicellulose sub-fractions (purity and coloration). Nevertheless these results proved that steam explosion was an effective technique for the extraction of high molecular mass AGX hemicellulose macromolecules from banana rachis residues.

Hemicellulose characterization of deuterated switchgrass

This work describes the structural characterization of hemicellulose isolated from hydroponically grown switchgrass in H2O medium (protiated) or 50% D2O medium (deuterated) through compositional analysis, GPC, FTIR, 13C and 1H/13C HSQC NMR. 4-O-methyl glucuronoarabinoxylan (GAX), the major hemicellulose in switchgrass isolated from deuterated switchgrass, had structural properties similar to hemicellulose isolated from protiated switchgrass. Both had comparable arabinose to xylose ratio (0.25) and molecular weight (47–50 kDa). Structural similarities show that deuterated switchgrass hemicellulose can be used as a model carbohydrate polymer in neutron scattering, or pharmaceutical studies due to their immunomodulatory activity and gastroprotective effects.

Structure of acid-extractable polysaccharides of tree greenery of Picea abies

The polysaccharide PAA was isolated from tree greenery of spruce Picea abies by the extraction with HCl solution. Monosaccharide analysis PAA revealed high content of uronic acid residues (45%), arabinose (19.5%) and galactose (8.6%). It was shown that this polysaccharide contained polymers of various structures. The major component of which were pectic polysaccharides represented mainly by partly methyl-esterified and partly acetylated 1,4-α-D-galacturonan and by minor parts of partly 2-O- and/or 3-O- acetylated rhamnogalacturonan-I (RG-I). The side carbohydrate chains of the branched region of RG-I were represented predominantly by highly branched 1,5-α-L-arabinan and minor portions of 1,4-β-D-galactan.In addition to the dominant pectins, the isolated polysaccharide also contained arabinogalactan proteins (AGP) and binding glycans of the glucuronoarabinoxylan (GAX), glucomannan (GM) and xyloglucan classes, which indicated a close interaction of these polysaccharides in the cell walls. It can be concluded that, at least a part of the pectin is strongly associated with the AGP and binding glycans of the GAX and GM classes. However, neither GM nor GAX were associated with RG-I.

Carbohydrate content of black liquor and precipitated lignin at different ionic strengths in flow-through kraft cooking

Abstract The influence of sodium ion concentration [Na+] on the dissolution of carbohydrates in black liquor (BL) during flow-through kraft cooking of Scots pine wood meal (Pinus sylvestris) was studied. Fractions of BL were collected at different times and the carbohydrate content of the various fractions was analysed. Lignin was precipitated from the BL by lowering the pH, and the carbohydrate content of the precipitated lignins (Lprec) was also examined. The molecular weight distribution (MWD) of the Lprec samples was analysed. Xylose (Xyl) was found to be the most predominant sugar in BL aside from arabinose (Ara) and galactose (Gal), while the amounts of these sugars decreased with increasing levels of [Na+] in the cooking liquor. The minor amounts of mannose (Man) found in BL was not influenced by the [Na+]. The effects of NaCl and Na2CO3 on the carbohydrate dissolution were similar, but slightly lower concentrations of Ara and Xyl were found in the case of NaCl application. All of the Lprec samples contained some carbohydrate residues, the contents of which increased with increasing cooking time and decreased with higher [Na+]. It can be concluded that arabinoglucuronoxylan (AGX) along with arabinogalactans (AG) and arabinan, are covalently linked to lignin. The glucose (Glc) residue detected in Lprec may originate from 1,3-β-glucan linked to lignin.