Difference between revisions of "Enzymatic processes"

<onlyinclude>'''Enzymatic processes''' utilise enzymes (/ˈɛnzaɪmz/) which are proteins that act as biological catalysts (biocatalysts).<ref>{{Cite web|year=2021|title=Enzyme|e-pub date=|date accessed=24-09-21|url=https://en.wikipedia.org/wiki/Enzyme}}</ref> In terms of lignocellulosic biomass valorisation, enzymes find two main applications: i) biomass pretreatment, and ii) polysaccharides hydrolysis. Biomass enzymatic pretreatment falls under the category of "biological pretreatment". Other pretreatment methods for lignocellulosic biomass includes, physical (e.g., mechanical), [[Hydrolysis|chemical]] (e.g., acid and alkali), physico-chemical (e.g., [[steam explosion]] and AFEX), and a combination thereof<ref>{{Cite journal|author=E. Hosseini Koupaie, S. Dahadha, A.A. Bazyar Lakeh, A. Azizi, E. Elbeshbishy|year=2018|title=Enzymatic pretreatment of lignocellulosic biomass for enhanced biomethane production - A review|journal=Journal of Environmental Management|volume=233|page=774-784|doi=10.1016/j.jenvman.2018.09.106}}</ref>. polysaccharides hydrolysis is a concept mostly applied in biorefineries as part of the [[hydrolysis]] of plant cell wall constituents like cellulose, hemicellulose, and lignin. </onlyinclude>

Cellulose is a polysaccharide polymer of glucose disaccharide units, cellobiose, linked tightly by ß-1,4-glycoside bonds. Cellulose molecules are linked by hdyrogen bonds and have different orientations resulting in different levels of crystallinity. Its crystallinity plays a crucial role in the biodegradation of cellulose and in general the higher crystallinity level makes it harder to biodegrade the cellulose.

Lignin is a complex and large compound made out of phenylpropane units linked in a three-dimensional structure. The main monomers of lignin are p-hydroxyphenyl alcohol, coniferyl alcohol, and sinapyl alcohol. Lignin acts as a cementing material that links celluose and hemicellulose to from the rigid three-dimensional structure of the plant cell wall.

Biological pre-treatment systems rely on biological agents (e.g., enzymes) to delignify lignocellulose and make the process of enzymatic hydrolysis more convenient.  The effect of enzymes on the lignocellulosic biomass depends on the type of enzymes as well as the composition of the biomass being treated. This is due to enzyme specificity in terms of the type of the reactions that they catalyze. Laccase (Lac), manganese peroxide (MnP) and versatile peroxide (VP) are enzymes that are used extensively to treat the lignocellulosic substrate<ref>{{Cite journal|author=Baruah J., Nath B.K., Sharma R., Kumar S., Deka R.C., Kalita E.|year=2018|title=Recent Trends in the Pretreatment of Lignocellulosic Biomass for Value-Added Products|journal=Front. Energy Res.|volume=141|page=|doi=10.3389/fenrg.2018.00141}}</ref>. Biological pretreatment of LCB is often knows as a simple, inexpensive, selective, and environmentally-friendly technology. This is mainly due to the fact that biological pretreatment does not require high energy inputs or chemicals addition. Furthermore, enzymatic treatment has found success in the removal of toxic inhibitory compounds (i.e., complete removal of phenolic compounds). The limitations asociated with enzymatic pretreatment is its production cost, stability, shelf life, and reusability<ref name=":0" />.   

Enzymatic hydrolysis processes allow to produce monomeric sugars from (ligno)cellulosic biomass by using specific enzymes (e.g., cellulases and hemicellulases) able to break down the chemical bonds in cellulose and hemicellulose polymers. Several factors can affect the efficiency of this process: accessible surface area and crystallinity of the biomass, as well as pH, time and temperatures of the process<ref>{{Cite journal|title=Investigation of Enzymatic Hydrolysis of Coffee Silverskin Aimed at the Production of Butanol and Succinic Acid by Fermentative Processes|year=2019-06-01|author=Saverio Niglio, Alessandra Procentese, Maria Elena Russo, Giovanni Sannia, Antonio Marzocchella|journal=BioEnergy Research|volume=12|issue=2|page=312–324|doi=10.1007/s12155-019-09969-6}}</ref>. Enzymatic hydrolysis is gaining increased attention with respect to acid hydrolysis due to equipment corrosion, energy consumption, non-recyclability of reagents, fermentation inhibitors production during acid hydrolysis <ref>{{Cite journal|title=Enzymatic hydrolysis of lignocellulosic biomass: converting food waste in valuable products|year=2015-02-01|author=Gabriela Piccolo Maitan-Alfenas, Evan Michael Visser, Valéria Monteze Guimarães|journal=Current Opinion in Food Science|volume=1|page=44–49|doi=10.1016/j.cofs.2014.10.001}}</ref>. Different enzymes play different roles in the hydrolysis of lignocellulosic biomass:

The accessory enzymes are also a crucial part in the hydrolysis of LCB and enhance the hydrolysis yield and reduce the enzyme cost and dosages. The breakdown of hemicelluloses is further accompanied by the addition of ß-xylosidases which produces a final product of oligomer with different length as intermediates. Another important accessory enzyme is α-arabinofuranidase for breaking down of arabinose into monomer of furanose and pyranose.