Composition of hemicellulosic

In contrast to cellulases, the hemicellulases encompass a much broader suite of activities. In addition to analogue versions of endo-, exo-, and glycosidase cellulase activities, multiple debranching activities are needed to handle the high complexity of the heterogeneous hemicelluloses (Table 33.3). The varied backbone composition of hemicelluloses also adds complexity. Xylans, xyloglucans, man-nans, and numerous other minor polysaccharide chains form the backbone for different hemicelluloses. 

Hemicellulose HemiceUulose is predominantly found in the primary and second cell walls in wood. Compared to cellulose, hemicellulose usually exhibits lower molecular weight and lower degree of polymerization, which may be tens of hundreds of repeating units. Hemicellulose comprises of polysaccharides, e.g., arabino-xylans, gluco-mannans and galactans, of which xylans are predominant [21]. Unlike cellulose, the composition of hemicellulose varies between trees and species. Besides, hemicelluloses are more hydrophilic than cellulose and are extremely susceptible to alkali and acid hydrolysis because of their branched and amorphous nature [22]. The combination of cellulose and hemicellulose is known as holocellulose. 

Fig. 26.3. The molecular structure of a cell wall. It is a fibre-reinforced composite (cellulose fibres in o matrix of hemicellulose and lignin).

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

Rha, Ara and Gal are the neutral sugar components from all the fractions. Xyl is not present in Fla and is significantly present in the hemicellulose fractions, indicating that this monosaccharide is component of hemicellulosic polymers. Chemical composition of the water fractions were determined (Table V). High protein contents and the presence of O-acetyl-groups were observed in four aqueous fractions. Neutral sugar and uronic acid composition points to inclusion of these polymers in the class of pectic polysaccharides. 

Changes in molecular weight and carbohydrate composition of cell wall polyuronide and hemicellulose during ripening in strawberry fruit... 

Figure 2. Sugar composition of polyuronides (I) and hemicelluloses (II) extracted from strawberry tissues at different stages of ripeness. Galacturonic acid, [Q Rhamnose, S Arabinose, Galactose, 0 Glucose, Cl Xylose, ED.

Modifications of the cell wall composition of released cells occured during the maceration (Fig. 5). In pectic polysaccharide, galactose increased whereas galacturonic acid and arabinose decreased. Cellulose and hemicellulose compositions were not modified. 

Wood consists mainly of cellulose, hemicelluloses and lignin in various proportions. The amounts and compositions of these component groups depend primarily on the wood species [38]. 

Wood cell wall, 21 4 cellulose in, 21 5-8 chemical composition of, 21 5-16 hemicelluloses in, 21 8-10 lignin-carbohydrate linkages in, 22 14-15... 

Since the terms xylan, pentosan, hemicellulose and hemicellulose-A often refer to the same substance or to substances which differ only slightly, the literature is sometimes confusing. In this review the term xylan will be used to signify preparations which are believed to be reasonably pure, the term pentosan will be used to designate material the composition of which is based on furfural estimation, and the term hemicellulose will be used to denote polysaccharide preparations of uncertain composition. 

Hardwoods are less thermally stable than softwoods and this is attributable to differences in the hemicellulosic content and composition. Pentosans (which are found in higher proportions in hardwood hemicelluloses) are more susceptible to thermal degradation than hexosans (Fengel and Wegener, 1989). Additionally, hardwoods, in general, have a higher proportion of hemicellulose, and the hemicelluloses of hardwoods also have a higher acetyl content compared to softwoods. 

For monitoring the extent of polysaccharide hydrolysis, l.c. methods that sepeu ate and analyze the non-fermentable oligosaccharides (d.p. 3-30) derived from cellulose, hemicellulose, and pectins are useful, and have already been described (see Section III,l,c). For determination of the monosaccharide composition of completely hydrolyzed, plant polysaccharides, l.c. is especially useful and has been applied to the compositional analysis of hydrolyzed plant fiber,wood pulps,plant cell-walls,and cotton fibers.In these representative examples, the major sugars of interest, namely, glucose, xylose, galactose, arabinose, and mannose, have traditionally been difficult to resolve by l.c. The separa-... 

Solid wood material is built up of two major organic polymers (macro molecules) (1) polysaccharides and (2) polyphenylpropane [61,62], The polysaccharides consist of two groups - cellulose and hemicellulose, and make up around 65-75 % of the wood on dry basis. The polyphenylpropanes are more commonly termed lignins and constitute around 18-35 % of the wood on dry basis. In Table 9 we can see that wood fuels consist of extractives, minerals, and nitrogen as well. The chemical composition of wood of Sweden s most commonly wood species [63], the spruce, the pine and the birch are different, see Table 9. 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

Substrate and Pretreatment. Sweet corn (hybrid Lingodor) of W.H. Perron Laval, Quebec was grown in well prepared soil in a plot of 3 x 2 meters. Corn stalks were ground to 20 mesh to be used as a substrate. It was pretreated with 1.5% sodium hydroxide (NaOH) wt/vol with substraterwater ratio of 1 10 at 121 C for 60 minutes. The substrate was not washed after the pretreatment, and all the solubilized polymers (hemicelluloses and lignin) were retained along with the insoluble polymer (cellulose) in the fermentation medium. The composition of corn stalk is presented in Table 1. 

Cellulose is found in nature in combination with various other substances, the nature and composition of which depend on the source and previous history of the sample. In most plants, there are three major components cellulose, hemicelluloses, and lignin. Efficient utilization of all three components would greatly help the economics of any scheme to obtain fuel from biomass. Hemicelluloses, lignocellulose and lignin remaining after enzymatic degradation of the cellulose in wood would require chemical or thermal treatment - as distinct from biochemical - to produce a liquid fuel. 

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