Hardwood Glucomannans

General Characteristics of Glucomannans from Arborescent Angiosperms 

Unlike their softwood relatives, hardwood glucomannans do not contain any galactose residues and are thus true glucomannans. Their diheteropolymer character is indicated (but not proved) by the fact that they give only one peak on free-boundary (Tiselius) electrophoresis - and retain a constant composition on repeated fractionation. More-direct evidence is furnished by the isolation and identification of 4-0- 8-D-glucopyranosyl-D-mannopyranose and 4-0- 3-D-mannopyranosyl-D-glucopyranose on partial hydrolysis.  

The nature of the hexose residues and inter-residue links is evident from the results of methylation studies, the preponderant sugars obtained after methylation and hydrolysis being 2,3,6-tri-O-methyl-D-glucose and 2,3,6-tri-O-methyl-D-mannose. The low specific rotation of the glucomannans indicates that the hexose residues occur in the jS-D modification. This is also clear from the nature of the above two disaccharides, as well as from 

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Softwoods galactoglucomannan with 1.3 (%) acetyl hardwoods glucomannan. 6Softwood arabino-4-O-methylglucuronoxylan hardwoods 4-O-methylglucuro-noxylan with 4 (%) acetyl. 

Figure 5.2 Structures and typical structural elements of hemiceUuloses (A) hardwood glucoronoxylan (B) softwood glucoronoxylan (C) hardwood glucomannan (D) softwood glucomannan.

Both types of xyloglucans exhibit monolayer sorption onto cellulose (116) and tamarind xyloglucan exhibits maximum specific sorption onto cellulose less than that of coniferous xylan. By inference with other data, this is also less than that of glucomannan and hardwood xylan, but similar to many additives used in the paper industry. 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

The main polyoses of hardwoods are 4-O-methylglucuronoxylan, with an average acetyl content of 4% (based on wood), glucomannan, with an average ratio of glucose to mannose of 1 2, and arabinogalactan. 

Other Polysaccharides In addition to xylan and glucomannan minor amounts of miscellaneous polysaccharides are present in hardwoods, partly of the same type as those occurring in softwoods. They might be important components for the living tree, although of little interest when considering the technical applications. 

Other Reactions In addition to the direct stabilizing effect of the uronic acid groups, the relatively high yield of hardwood kraft pulp is also due to the readsorption of xylan on the fibers (Fig. 7-35). After kraft pulping of softwood, the glucomannan remaining in the pulp still contains traces of... 

Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material of wood. In hardwoods the primary hemi-cellulose is a xylan (polymer of xylose), whereas in softwoods the primary hemi-cellulose is a glucomannan, although both of these polysaccharides occur to some extent in both types of wood. The DP of the hemicelluloses is much less than that of cellulose, in the range of 100-200. 

Glucomannan is a minor fraction of hardwood hemicelluloses (about 3-5%), and is a P-(l-4)-linked linear copolymer of glucose and mannose at the ratio of 1 1-2. 

In this study, thus, we selected sugi as softwood and buna as hardwood species. Just for comparison, cellulose and hemicellulose (glucomannan, xylan) and lignin (milled wood lignin) were also studied for their reaction in supercritical water. 

Glucomannan is present in hardwoods but is of minor significance compared to the more abundant xylans. It is a linear 1,4 copolymer with no substitution on the C2 and C3 positions (degree of polymerization of 60-70). The glucose to mannose ratio varies from 1 1 to 1 2. 

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