Written by: Christin Zimmer – Product and Service Manager at Lignopure
Lignin from the Latin word lignum, which means “wood,” refers to the importance of the molecule in the structure of wood and plants. Lignin, the second most abundant polymer in the world, is the supporting material and hardening polymer of plants and can be described as the cell wall matrix, which acts as both a bonding agent and a stiffening agent especially in wood and bark, therefore making it particularly important in the formation of cell walls. Another important function of lignin is its key role in the nutrition of plant stems. Lignin tends to be more hydrophobic, while other components of plant cell walls are highly hydrophilic. This allows lignin to conduct water and aqueous nutrients efficiently. Last, but not least, lignin provides resistance against diseases by making the plant cell wall less prone to degradation.
For many years, lignin has been a waste product of pulp and paper production as well as the biorefinery industry and has been used to produce energy by burning it, but in recent years, the interest in high-value-added lignin products has been on the rise. The antioxidant, antimicrobial and UV absorbent properties found in lignin have led to an increased interest in new applications, especially in high-value industries like cosmetics, pharmaceuticals and medicine.
Chemically, lignin is a complex biopolymer , and its chemical structure depends on the lignin’s source of origin, type, and extraction process, and each lignin is thus incredibly unique. Lignin consists of phenylpropane units generated from the three basic precursor alcohols (monolignols): p-coumaryl alcohol (H), sinapyl alcohol (S) and coniferyl alcohol (G). The radical form of each monolignol prefers coupling at the β-positions, creating β-O- β, β- β, and β-5 dimers and resulting in a highly complex polymer. During biosynthesis, the monomeric radicals turn into p-hydroxyphenyl (H), syringyl (S) and guaiacyl (G) units. The connection and proportions of the precursors in lignin are highly variable and do not follow a structured pathway. There are inherent proportions of monolignols in a lignin’s biomass of origin: softwood lignin primarily contains G-units and low levels of H-units, hardwoods consist of G-units and S-units and annual plants (e.g., wheat straw) contain all three: H-units, G-units, and S-units. In addition to the lignin structure possibilities during plant growth, the extraction process leads to more structural variation, which has resulted in a wide variety of versatile lignin functionalities.