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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 [1,2]. 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 vastly change the yield size and properties[1,2]. The main advantage to hot water extraction is that it offers a method where the only chemical that is needed is water, making this enviormentally friendly and cheap.

The hot water treatment goal is achieve as much removal of hemilleculose from the wood as possible. This is done through the hydrolysis of the hemicellulose to achieve smaller oligomers and monosacccharie xylose. Xylose when dehydrated becomes furfural [3]. When xylose and fufural are the goal, acid catalysts, such as formic acid, are added to increase the transition of polysaccharide to monosaccharide. This catalyst also has been show to also utilize a solvent effect to be aid the reaction[3].

One method of pretreatment is soak the wood with diluted acids (with concentrations around 4%). This converts the hydroloze hemicellulose into monosaccharaides. When pretreatment is done with bases (for instance sodium or potassium hydroxide) this destroys the structure of the inherent lignin. This changes the structure from crystalline to amorphous. Another pretreatment method is to pretreat hydrothermally. 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 [2]. While the particle size decreases the yield production decreases as well [4]. 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 phrase. This is done above the normal bioling point of water to increase the solubilization of the hemicellulose and the depolymerization of polysaccharides [3]. This process can take several minutes to several hours depending on the temperature and pH of the system [2]. Higher temperatures paired with higher extraction times lead to higher yields. A maximum yield is obtained at a pH of 3.5 [1]. If below, the extraction yield exponentially decreases. In order to control pH sodium bicarbonates are generally added[1]. The sodium biocarbonates inhibits the autodyolysis of acetyl groups as well as inhibiting glycosic bonds. Depending on the temperature and time the hemicellulose can be further converted into oligomers, monomers, and lignin [1].

[1] Gallina G, Cabeza A, Grenman H, Baisi P, Garcia-Serna J, Salmi t. “Hemicellulose extraction by hot pressurized water pretreatment at 160 ºC for 10 different woods: Yield and molecular weight” The Journal of Supercrtiical Fluids 133.2 (March 2018). Accessed 2 April 2019

[2] Li Z, Qin M, Xu C, Chen X. (2013). "Hot water extraction of hemicelluloses from aspen wood chips of different sizes" BioRes. 8(4), 5690-5700.

[3] Goldman W M, Ahola J, Mikola M, Tanskanen J. "Formic acid adided hot water extraction of hemicellulose from European silver birch (Betula pendula) sawdust" Bioresource Technology 232 (May 2017). Accessed 2 April 2019

[4] Ayrilmis N, Kwon J, Han T. “Effect of Wood Chip Size on Hemicellulose Extraction and Technological Properties of Flakeboard.” Turkish Journal of Agriculture & Forestry 41.5 (October 2017).