It may be defined as incrusting material of the plant which is built up mainly of phenyl propane building blocks. It is a polymeric non carbohydrate material. It is woody part of plant called as lignin and combination with cellulose is called ligno-cellulose.
Occurrence of Lignin:
1. Lignin occurs in woody tissues of plants.
2. Occurs only in secondary cell wall material in wood: 25-40%, straw: 15- 25%.
Chemical Nature of Lignin:
1. Lignin mostly consists of phenyl propane ring i.e. C6-C3 of the type represented by conferral alcohol, p-hydroxyl cinnamyl alcohol and cinapyl alcohol.
2. It also contains large proportion of aromatic ring, with high content of methyoxy group (OCH) e.g. Syringaldehyde e.g. Vaxillin
Properties of Lignin:
1. Amorphous and high molecular weight with, yellow brown colour.
2. Soluble in hot alkali and bisulfite and readily condensed with phenol and thiol compounds, unhydrolyzable by acid.
3. Insoluble in water and all organic solvents.
4. on partial oxidation yield aldehydes and acids.
Physiological Role of Lignin:
1. It reduces the digestibility of feeding stuff.
2. It imparts biological strength to cellulose.
3. Affects the hydrophilic bonding components of middle lamella and of primary cell wall.
4. Cementing and anchoring cellulose fiber together and to protect cellulose from chemical, physical and biological attack.
Uses of Lignin:
1. Used in paper industries
2. Preparation of hard binding and plywood.