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Hemicelluloses is a heteropolysaccharide found in plant cell walls. It constitutes about 20-30% of the total dry weight of plants. It is a second most abundant polysaccharide, the first most abundant polysaccharide is cellulose. It contains mainly D-xylose, D-galactose, L-arabinose, D-mannose, D-glucose etc. Hemicelluloses has low molecular weight than cellulose. There are many types of hemicelluloses like xylans, mannans, arabinogalactans, glucans,
Hemicelluloses are mostly found in plant cell walls. Xylans are found in the secondary cell wall of hardwoods while Xyloglucans are found in the primary cell wall of higher plants. Mannans or glucomannans are the primary constituents of secondary cell walls of softwoods. Mixed-linked β-glucans is found in poales (member of grass family) and pteridophytes
Hemicellulose provides physical and or structural strength to the cell wall together with cellulose. It is amorphous in nature, highly soluble due to H-bonding, hygroscopic and more reactive than cellulose. Hemicelluloses are more susceptible to hydrolysis, with 200 degrees of polymerization. There are different types of side group substitutions. The main pentoses in hemicelluloses are D-xylose and L-arabinose while the main hexoses in hemicelluloses are β-D glucose, D-galactose, D-mannose.
Hemicellulose is a fairly long chain with frequent branches on the backbone. There are many types of linkages like 1-3, 1-4, 1-6. Each 1-3 link is separated by two continuous 1-4 linkages. Most of the side group substitutions occur at C2, C3 and C6 In hemicelluloses monosaccharides linked are of different configurations. Mostly are in α-D conformation and some are β-D and some are of L conformations like α-D-mannose, α-D-xylose, α-D-galactose, β-D-glucose, α-L-arabinose.
Hemicelluloses are biodegradable and highly susceptible to hydrolysis. It is degraded through synergistic action of few enzymes of some bacteria and fungi. There are different enzymes involved in hemicelluloses degradation there are; chain degrading glycosidases and other carbohydrate esterases like acetylesterase, methylesterase which are mainly found in bacteria and fungi. Chain degrading glycosidases are β-xylosidase, α1 arabinofuranosidase, endo-xylanase.
- Hemicelluloses are used as films and/or gel for packaging. They can also be used as edible films to coat food stuff, to maintain texture, taste and mouth feel because hemicellulose is biodegradable, nontoxic. It is highly useable as a dietary fiber.
- Mannans (gum) helps to lower the blood and liver cholesterol and also have immunomodulatory activities.
- Xyloglucans are used as a thickening, stabilizing and gelling agent in foods.
- Xylans are used In paper making due to its adhesive property.
- Β-glucans, as thickening agent ice-cream formulation, stabilization of emulsion and foams. Hemicelluloses are converted to sugars for chemical fuels, source of heat energy etc