Hemicellulose

Hemicellulose refers to a group of heterogeneous polysaccharides present in the primary and secondary cell walls of plants. Unlike cellulose, hemicelluloses are branched, amorphous polymers that bind to cellulose microfibrils and to lignin, forming part of the lignocellulosic matrix. They typically account for 15–35% of the dry weight of wood and vary by plant type. Hemicelluloses are composed of various sugars, including xylose, mannose, galactose, glucose, arabinose, and rhamnose, often with acetyl or uronic acid substitutions. Major classes include xylans (notably glucuronoxylans in hardwoods), mannans and galactoglucomannans in softwoods and some hardwoods, and glucomannans, galactans, and arabinans in various species. Their structures are more branched and less crystalline than cellulose, which makes them more susceptible to acid and enzymatic hydrolysis.

In plants, hemicelluloses function as matrix polysaccharides that cross-link cellulose fibrils and interact with lignin, contributing to wall porosity and mechanical strength. They are released during biomass pretreatment and can be hydrolyzed by enzymes such as xylanases and mannanases, or by dilute acids.

Industrial relevance is mainly as a source of fermentable sugars for biofuels and bioproducts, and as a component of dietary fiber in foods. In papermaking, removal of hemicelluloses alters pulp properties and process efficiency. The composition and properties of hemicellulose depend on species, tissue, and environmental conditions.