Hemicelluloses xylans

Endo-xylanases (1,4-p-D-xylan xylanohydrolase, EC 3.2.1.8), which hydrolyze mainly interior p-(l- 4)-D-xylosyl bonds in the plant hemicellulose xylan, have been isolated from many fungi and bacteria and occasionally from plants and invertebrates. Because xylan is one of the most common of all natural materials, and its removal or conversion is often desirable, the endo-xylanase family has high commercial as well as scientific interest. 

Thermal analysis of cottonwood and its major components (2), as shown in Figures 2 and 3, indicates that the thermal behavior of wood reflects the sum of the thermal responses of its three major components, cellulose, hemicellulose (xylan) and lignin. 

Table 15.8 presents the feedstock composition expressed in key components cellulose, hemicellulose (xylan, arabinan, mannan, galactan) and lignin. 

Cellulose pyrolysis in the presence of salts, acids, or bases may be used as a model of cellulose pyrolysis in a wood matrix. A series of differences are usually seen during pyrolysis of microcrystalline cellulose and wood, even considering only those components of wood that are generated by cellulose pyrolysis. The differences can be explained by the influence of inorganic components in wood, but also by the presence of chemical bonding of cellulose with lignin and hemicelluloses (xylans, etc.). 

Fig. 2 Hemicellulose (xylan backbone with arabinose, glucuronic acid and feruloyl groups) [32]... 

Enzymes that can be harnessed for the breakdown of hemicellulose in cereal crops, and crop fiber biomass are becoming increasingly important because of their pivotal role in the utilization of these renewable energy sources. Hemicelluloses (xylans, aiabinoxylans) are widely found as stmctmal components in plant cell walls, where they cross-link with lignin and are extensively hydrogen-bonded to cellulose [1]. Structurally, xylans are heteropolysaccharides consisting of a linear P-d-(1— 4)-linked xylopyranoside backbone that. 

Quantitative acid hydrolysis with 72% (w/w) H2SO4 [19] was used to characterize the BSG feedstock. The monosaccharides and acetic acid were determined by HPLC to estimate (after corrections for stoichiometry and sugar decomposition) the contents of glucan (cellulose) and hemicelluloses (xylan, arabinan, and acetyl groups) in the sample. The acid-insoluble residue after hydrolysis was recovered by filtration and considered as Klason lignin after correction for the acid-insoluble ash. Protein was determined by the KJeldahl method [20] using the Ax 6.25 conversion factor. 

Hemicellulose is one of the major components of lignocellulose. Depending on the nature of the raw material, the hemicellulose fraction contains varying levels of xylose-based hemicelluloses, xylans (Table 1). The xylan content is generally high in hardwood (wood from deciduous trees) and in agricultural residues, and somewhat lower in softwood (wood from coniferous trees). 

Xylans, and other hemicelluloses offer many properties that are similar to exudate gums. Because of the increased price of such gums and the great abundance of hemicelluloses, there is expected to be an increased industrial market for the hemicelluloses. Xylan was employed for a variety of reasons ... 

Yi G, Zhang Y (2012) One-pot selective conversion of hemicellulose (xylan) to xylitol under mild conditions. ChemSusChem 5(8) 1383-1387... 

Some chemicals obtained from hemicellulose (xylan). 

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