Editing Hemicellulose (section)

== 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.<ref name="Gallina-2018">{{Cite journal|last1=Gallina|first1=Gianluca|last2=Cabeza|first2=Álvaro|last3=Grénman|first3=Henrik|last4=Biasi|first4=Pierdomenico|last5=García-Serna|first5=Juan|last6=Salmi|first6=Tapio|date=2018-03-01|title=Hemicellulose extraction by hot pressurized water pretreatment at 160°C for 10 different woods: Yield and molecular weight|url=http://uvadoc.uva.es/handle/10324/35166|journal=The Journal of Supercritical Fluids|series=Biomass Fractionation by Subcritical and Supercritical Water|language=en|volume=133|pages=716–725|doi=10.1016/j.supflu.2017.10.001|issn=0896-8446}}</ref><ref name="Li-2013">{{Cite web|url=https://bioresources.cnr.ncsu.edu/|title=Hot water extraction of hemicelluloses from aspen wood chips of different sizes :: BioResources|last=Li Z, Qin M, Xu C, and Chen X|date=2013|website=bioresources.cnr.ncsu.edu|access-date=2020-04-24}}</ref> 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.<ref name="Gallina-2018" /><ref name="Li-2013" /> The main advantage to hot water extraction is that it offers a method where the only chemical that is needed is water, making this environmentally friendly and cheap.<ref>{{cite journal |last1=Gianluca Gallinaa Álvaro Cabezaa Henrik Grénmanbc Pierdomenico Biasibd Juan García-Sernaa Tapio Salmi |title=Hemicellulose extraction by hot pressurized water pretreatment at 160 °C for 10 different woods: Yield and molecular weight |journal=The Journal of Supercritical Fluids |date=March 2018 |volume=133 |issue=Part 2 |pages=716–725 |doi=10.1016/j.supflu.2017.10.001 |url=https://research.abo.fi/files/25488060/Gallina-woods-160C-UVaDocs.pdf |access-date=7 October 2017}}</ref>

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.<ref name="Tunc-2008">{{Cite journal|last1=Tunc|first1=M. Sefik|last2=van Heiningen|first2=Adriaan R. P.|date=2008-09-17|title=Hemicellulose Extraction of Mixed Southern Hardwood with Water at 150 °C: Effect of Time|journal=Industrial & Engineering Chemistry Research|volume=47|issue=18|pages=7031–7037|doi=10.1021/ie8007105|issn=0888-5885}}</ref>  When xylose and furfural{{typo help inline|date=December 2020}} 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.<ref name="Tunc-2008" />

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.<ref name="Li-2013" /> This changes the structure from crystalline to amorphous. Hydrothermal pretreatment is another method.{{explain|date=July 2021}} 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.<ref name="Gallina-2018" />

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.<ref name="Li-2013" /> While the particle size decreases the yield production decreases as well.<ref>{{Cite journal|last=Ayrilmis N, Kwon J, Han T|date=October 2017|title=Effect of Wood Chip Size on Hemicellulose Extraction and Technological Properties of Flakeboard|journal=Turkish Journal of Agriculture & Forestry|volume=41|pages=331–337|doi=10.3906/tar-1704-63|doi-access=free}}</ref> This is due to the increase of cellulose.{{Cn|date=February 2021}}

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.<ref name="Tunc-2008" /> This process can take several minutes to several hours depending on the temperature and pH of the system.<ref name="Li-2013" /> Higher temperatures paired with higher extraction times lead to higher yields. A maximum yield is obtained at a pH of 3.5.<ref name="Gallina-2018" /> If below, the extraction yield exponentially decreases. In order to control pH, sodium bicarbonate is generally added.<ref name="Gallina-2018" /> The sodium bicarbonate inhibits the [[autolysis (biology)|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.<ref name="Gallina-2018" />

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.<ref>Vila, C., Romero, J., Francisco, J. L., Santos, V., and Parajó, J. C. (2012). "On the recovery of hemicellulose before kraft pulping," BioRes. 7(3), 4179-4189. https://bioresources.cnr.ncsu.edu/resources/on-the-recovery-of-hemicellulose-before-kraft-pulping/</ref>