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A hemicellulose (also known as polyose) is one of a number of (matrix polysaccharides), such as , present along with cellulose in almost all embryophyte . 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 Cellulase enzymes.
Hemicelluloses are 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.
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.
The two most common types of xyloglucans in plant cell walls are identified as XXXG and XXGG.
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). Mannan synthase, a particular enzyme in CSLA, is responsible for the addition of mannose units to the backbone. The galactose side-chains of some mannans are added by galactomannan galactosyltransferase. Acetylation of mannans is mediated by a mannan O-acetyltransferase, however, this enzyme has not been definitively identified.
Xyloglucan backbone synthesis is mediated by cellulose synthase-like protein family C (CSLC), particularly glucan synthase, which adds glucose units to the chain. Backbone synthesis of xyloglucan is also mediated in some way by xylosyltransferase, but this mechanism is separate to its transferase function and remains unclear. Xylosyltransferase in its transferase function is, however, utilized for the addition of xylose to the side-chain. Other enzymes utilized for side-chain synthesis of xyloglucan include galactosyltransferase (which is responsible for the addition of [galactose and of which two different forms are utilized), fucosyltransferase (which is responsible for the addition of fucose), and acetyltransferase (which is responsible for acetylation).
Xylan backbone synthesis, unlike that of the other hemicelluloses, is not mediated by any cellulose synthase-like proteins. Instead, xylan synthase is responsible for backbone synthesis, facilitating the addition of xylose. Several genes for xylan synthases have been identified. Several other enzymes are utilized for the addition and modification of the side-chain units of xylan, including glucuronosyltransferase (which adds glucuronic (responsible for acetylation). Given that mixed-linkage glucan is a non-branched homopolymer of glucose, there is no side-chain synthesis, only the addition of glucose to the backbone in two linkages, β1-3 and β1-4. Backbone synthesis is mediated by enzymes in cellulose synthase-like protein families F and H (CSLF and CSLH), specifically glucan synthase. Several forms of glucan synthase from CSLF and CSLH have been identified. All of them are responsible for addition of glucose to the backbone and all are capable of producing both β1-3 and β1-4 linkages, however, it is unknown how much each specific enzyme contributes to the distribution of β1-3 and β1-4 linkages.
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.
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.
One method of pretreatment is to soak the wood with diluted acids (with concentrations around 4%). This converts the hemicellulose into monosaccharides. When pretreatment is done with bases (for instance sodium or potassium hydroxide) this destroys the structure of the lignin. This changes the structure from crystalline to amorphous. Hydrothermal pretreatment is another method. This offers advantages such as no toxic or corrosive solvents are needed, nor are special reactors, and no extra costs to dispose of hazardous chemicals.
The hot water extraction process is done in batch reactors, semi-continuous reactors, or slurry continuous reactors. For batch and semi-continuous reactors wood samples can be used in conditions such as chips or pellets while a slurry reactor must have particles as small as 200 to 300 micrometers. While the particle size decreases the yield production decreases as well. 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. This process can take several minutes to several hours depending on the temperature and pH of the system. Higher temperatures paired with higher extraction times lead to higher yields. A maximum yield is obtained at a pH of 3.5. If below, the extraction yield exponentially decreases. In order to control pH, sodium bicarbonate is generally added. The sodium bicarbonate inhibits the autolysis of acetyl groups as well as inhibiting glycosyl bonds. Depending on the temperature and time the hemicellulose can be further converted into oligomers, monomers and lignin.
Solid bits of wood remain after autohydrolysis, as the lignin is largely untouched. A proper degree of autohydrolysis can preserve the lignin well enough to be used for paper production. This is useful for the Kraft process, which normally does not recover wood hemicellulose into useful products.Vila, C., Romero, J., Francisco, J. L., Santos, V., and Parajó, J. C. (2012). "On the recovery of hemicellulose before kraft pulping,"
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