Hemicelluloses hardwood

Maloney, M.T., Chapman, T.W., and Baker, A.J. (1986) An engineering analysis of the production of xylose by dilute acid hydrolysis of hardwood hemicellulose. Biotechnol Progr., 2, 193. 

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. 

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. 

Further studies on hemicelluloses of hardwoods have shown that these may exhibit a considerable range in the content of xylan and 0-methylhexuronic anhydride. In a recent review of hemicelluloses, Wise suggests that hardwood hemicelluloses may have a rather simple, architectural pattern, consisting predominantly of n-xylose units and smaller amounts of 0-methyluronic acid units. Softwood hemicelluloses also contain this fundamental structural unit consisting of a mono-O-methyluronic acid in combination with n-xylose units. In the case of pine, this fundamental unit may be associated with a part of the maiuian. ... 

C) Partial chemical structure of O-acelyl-4-O-methylglucuronoxylan (a hardwood hemicellulose). 

IV. Other Hardwood Hemicelluloses 1. Polysaccharides Extractable from Wood with Water... 

Hardwoods also contain minor amounts (2-5%) of a glucomannan, which contains one to two mannose units for every glucose. In general, hardwood hemicelluloses are characterized by high concentrations of xylose and acetic acid structural units, whereas softwood hemicelluloses are relatively rich in mannose. 

In the cell wall, hemicelluloses are thought to surround and sheath microfibrils, forming a chemically bonded cellulose-hemicellulose network. Whereas the composition of cellulose is consistent for softwoods and hardwoods, the chemistry and structure of softwood and hardwood hemicelluloses differ. Even within a single tree, different parts will contain varying amounts of certain hemicellulose species (9). Given this variability, hemicellulose contents for hardwoods and softwoods are generally reported as ratios, rather than as exact quantitative measurements. 

Hardwood hemicelluloses are represented by xylans and a small proportion of glucomannans. Hardwood xylans are linear polymers, constituted of [l,4]-linked xylanopyranosyl units that constitute the main skeleton. Every tenth D-xylanopyranosyl unit is substituted by a 4-0-methyl-D-glucuronic acid residue, linked to the birch xylan chain by [1,2] linkages, that has been found to retard the alkaline peeling reaction. Analysis of partially hydrolyzed xylan indicates that 4-0-methyl-D-glucuronic acid units linked to the C2 position are randomly distributed along the birch xylan backbone [9]. 

Fermentation of the lignoceliulosic hydrolyzates is more difficult than the well-established processes of ethanol production from e.g. molasses and starch. Hydrolyzates contain a broader range of inhibitory compounds, where the composition and the concentration of these compounds depend on the type of lignoceliulosic materials and the chemistry and nature of foe pretreatment and hydrolysis processes. Secondly, foe hydrolyzates of hemicelluloses contain not only hexoses but also pentoses, where xylose is foe dominant sugar in foe hydrolyzates from hardwood hemicelluloses. Therefore, foe fermenting microorganism should be able to produce efoanol from foe hydrolyzates with a... 

Figure 3.8 Principal structure of the main softwood hemicellulose type, galactoglucomannan, and of the main hardwood hemicellulose type, 4-Omethyl glucuronoxylan.

Convert , A., P. Perego, J.M. Dominguez. 1999. Xylitol production from hardwood hemicellulose hydrolysates by Pachysolen tannophilus, Debaryomyces hansenii, and Candida guilliermondii. Appl. Biochem. Biotechnol. 82 141-151. 

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