hemicelluloses

Hemicelluloses are a heterogeneous group of polysaccharides that form a major part of the plant cell wall along with cellulose and lignin. Unlike cellulose, which is a uniform polymer of glucose, hemicelluloses are diverse and usually branched, comprising a range of monosaccharides such as xylose, mannose, galactose, glucose, arabinose, and rhamnose. They are typically associated with cellulose microfibrils and help determine wall porosity, flexibility, and overall mechanical strength. Hemicelluloses are usually soluble in alkaline conditions but are insoluble in water, and they can be extracted from biomass under specific chemical treatments.

The principal classes of hemicelluloses include xylans, mannans, glucomannans, galactans, and arabinans. Xylans have a β-1,4-xylosyl backbone with various side chains such as arabinose, acetyl groups, or uronic acids; they are common in hardwoods and cereals. Mannans and glucomannans feature backbones of mannose and glucose/mannose, with galactose branches in galactomannans (e.g., guar gum, locust bean gum). Galactans and arabinans are more heavily substituted with galactose or arabinose residues. The exact composition and branching depend on the plant source, with hardwoods rich in glucuronoxylans, softwoods containing glucuronoxylans and glucomannans, and legumes contributing galactomannans.

Functionally, hemicelluloses provide structural support and contribute to wall matrix properties, hydration, and porosity. In industry, they are sources of fermentable sugars for biofuel production and serve as hydrocolloids (thickeners and stabilizers) in food and other products. Enzymatic breakdown by xylanases and mannanases is important in biomass conversion and paper processing.