A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis. Hemicelluloses are branched, shorter in length than cellulose, and also show a propensity to crystallize. They can be hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.

Composition

Diverse kinds of hemicelluloses are known. Important examples include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan.

Hemicelluloses are polysaccharides often associated with cellulose, but with distinct compositions and structures. Whereas cellulose is derived exclusively from glucose, hemicelluloses are composed of diverse sugars, and can include the five-carbon sugars xylose and arabinose, the six-carbon sugars glucose, mannose and galactose, and the six-carbon deoxy sugar fucose. Hemicelluloses contain most of the D-pentose sugars, and occasionally small amounts of L-sugars as well. Xylose is in most cases the sugar monomer present in the largest amount, although in softwoods mannose can be the most abundant sugar. Not only regular sugars can be found in hemicellulose, but also their acidified forms, for instance glucuronic acid can be present. In addition, hemicelluloses may be branched polymers, while cellulose is unbranched. Hemicelluloses are embedded in the cell walls of plants, sometimes in chains that form a 'ground' – they bind with pectin to cellulose to form a network of cross-linked fibres.

thumb|Section of a cell wall; hemicellulose in green|alt=|195x195px

Based on the structural difference, like backbone linkages and side groups, as well as other factors, like abundance and distributions in plants, hemicelluloses can be categorized into four groups as following: 1) xylans, 2) mannans; 3) mixed linkage β-glucans; 4) xyloglucans.

Xylans

Xylans usually consist of a backbone of β-(1→4)-linked xylose residues and can be further divided into homoxylans and heteroxylans. Homoxylans have a backbone of D-xylopyranose residues linked by β(1→4) glycosidic linkages. Homoxylans mainly have structural functions. Heteroxylans such as glucuronoxylans, glucuronoarabinoxylans, and complex heteroxylans, have a backbone of D-xylopyranose and short carbohydrate branches. For example, glucuronoxylan has a substitution with α-(1→2)-linked glucuronosyl and 4-O-methyl glucuronosyl residues. Arabinoxylans and glucuronoarabinoxylans contain arabinose residues attached to the backbonethumb|279x279px|Xylan in hardwood|alt=

Mannans

The mannan-type hemicellulose can be classified into two types based on their main chain difference, galactomannans and glucomannans. Galactomannans have only β-(1→4) linked D-mannopyranose residues in linear chains. Glucomannans consist of both β-(1→4) linked D-mannopyranose and β-(1→4) linked D-glucopyranose residues in the main chains. As for the side chains, D-galactopyranose residues tend to be 6-linked to both types as the single side chains with various amount. Two models explain their synthesis: 1) a '2 component model' where modification occurs at two transmembrane proteins, and 2) a '1 component model' where modification occurs only at one transmembrane protein. After synthesis, hemicelluloses are transported to the plasma membrane via Golgi vesicles.

Each kind of hemicellulose is biosynthesized by specialized enzymes.

Mannan chain backbones are synthesized by cellulose synthase-like protein family A (CSLA) and possibly enzymes in cellulose synthase-like protein family D (CSLD). This process was primarily applied to calcium sulfite brown liquors.

; Xylan

:The films based on xylan show low oxygen permeability and thus are of potential interest as packaging for oxygen-sensitive products.

; Mixed-linkage glucan

:Mixed-linkage glucans (industrially known as 'β-glucans') have an important role in food supplements, while they also show promise in treating health-related issues, especially in immune reactions and the treatment of cancer.

; Glucomannan and galactomannan

: Various (gluco/galacto)mannans such as konjac, locust bean gum, and guar gum, are frequently used as food additives, where they act as thickeners, stabilisers, and emulsifiers.

Hemicellulose is also abundantly found in cereal hull/husk, bran, and straw. A number of proposed processes aim to break it down into the above-mentioned parts for utilization.

Natural functions

thumb|Hemicellulose contribution to structural support within plant cells

As a polysaccharide compound in plant cell walls similar to cellulose, hemicellulose helps cellulose in the strengthening of plant cell walls.

Extraction

There are many ways to obtain hemicellulose; all of these rely on extraction methods through hardwood or softwood trees milled into smaller samples. In hardwoods the main hemicellulose extract is glucuronoxlyan (acetylated xylans), while galactoglucomannan is found in softwoods. Prior to extraction the wood typically must be milled into wood chips of various sizes depending on the reactor used. Following this, a hot water extraction process, also known as autohydrolysis or hydrothermal treatment, is utilized with the addition of acids and bases to change the yield size and properties.

The goal of hot water treatment is to remove as much hemicellulose from the wood as possible. This is done through the hydrolysis of the hemicellulose to achieve smaller oligomers and xylose. Xylose when dehydrated becomes furfural. When xylose and furfural are the goal, acid catalysts, such as formic acid, are added to increase the transition of polysaccharide to monosaccharides. This catalyst also has been shown to also utilize a solvent effect to be aid the reaction. This is due to the increase of cellulose.

The hot water process is operated at a temperature range of 160 to 240 degrees Celsius in order to maintain the liquid phase. This is done above the normal boiling point of water to increase the solubilization of the hemicellulose and the depolymerization of polysaccharides.

See also

  • Cellulose
  • Lignin
  • Polysaccharides

References

  • Structure and Properties of Hemicellulose /David Wang's Wood Chemistry Class