Hemicellulose content

Table 1. Approximate Hemicellulose Content of Selected Vegetable Materials and Their Residues...

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. 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

Lignocellulosic biomass is a valuable and plentiful feedstock commodity and its high cellulose and hemicellulose content (about 80% of total) provides considerable potential for inexpensive sugars production. However, enzymatic deconstruction of these polysaccharides remains a costly prospect. Strides in cellulase cost reduction have been made, yet further improvements are needed to reach the goal of 0.10/gal of EtOH expected to enable this new industry. Strategies to reach this goal will combine reduction in the cost to produce the needed enzymes as well as efforts to increase enzyme efficiency (specific activity). As this work proceeds, the more easily attained achievements will be made first, and thus the overall difficulty increases with time. 

C and the hemicellulose content decreased by 76% (23). For calculation of the amount of hemicellulose, the amount of mono- and disaccharides (xylose and arabinose) in the liquid was measured by HPLC following the strong acid hydrolysis by Hagglund. 

The total hemicellulose content of plant materials is 15-30% (Norman, 1943), depending on species, cultural practices, and maturation. Collectively, they are second only to celluose in natural abundance. 

The amount of hemicelluloses of the dry weight of wood is usually between 20 and 30% (cf. Appendix). The composition and structure of the hemicelluloses in the softwoods differ in a characteristic way from those in the hardwoods. Considerable differences also exist in the hemicellulose content and composition between the stem, branches, roots, and bark. 

Activation energy Preexponential factor logAi Preexponential factor log A Preexponential factor log As Content of hemicellulose Content of cellulose Content of lignin Constant in Antoine Eq, for KCl Constant in Antoine Eq. for... 

Holocellulose. Holocellulose is the total polysaccharide (cellulose and hemicelluloses) content of wood, and methods for its determination seek to remove all of the lignin from wood without disturbing the carbohydrates. The procedure generally used (34) was adopted as Tappi Standard T9m (now useful method 249), and as ASTM Standard D 1104. Extracted wood meal is treated alternately with chlorine gas and 2-aminoethanol until a white residue (holocellulose) remains. The acid chlorite method is also used (3). The... 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Lignin is present in plants for which water conduction is important. Of greatest interest is its presence in trees. The lignin content depends on the type of tree about 28% for softwoods and 20% for hardwoods. The cellulose content is approximately 45% in the wood of both types, while the hemicellulose content is roughly 17% in softwoods and 25% in hardwoods. Lignin structure can vary within the same plant,... 

A 10% wheat bran suspension in water was autoclaved for 30 min at 115 °C and the resulting soluble (liquor) and insoluble components (residue) were separated by filtration. The liquor and residue fiactions were then used alone or mixed with 1 % MCC as carbon sources for P. decumbens growth. The biomass in cultures grown on wheat-bran liquor plus MCC was less than that on wheat-bran residues plus MCC, but the cellulase activity released was higher (Fig. 4). The liquor apparently contained a factor that stimulated cellulase synthesis and/or secretion. In contrast, the xylanase activity in the supernatants of cultures supplemented with wheat-bran liquor was lower than that in cultures supplemented with the wheat-bian residues, which were in consistent with the residues having higher hemicellulose content. 

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. 

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