Paper mills may use either coniferous or deciduous
trees as feedstock. Mature plant cells need to stiffen cell walls to
support the plant, and this is done by the production of a structure
containing linked aromatic rings, a material known as lignin.
The weight of a tree means a lot has to be produced. Lignin is also
involved in water transport.
Coniferous trees produce lignin with a different structure to deciduous
trees. The difference is small, but concerns the number of methoxy groups
present on the aromatic rings and so affects the amount of vanillin
that can be made.
Route to Vanillin
Specialist paper, with a higher content of fibrous
cellulose, is made using the sulphite process. After the paper pulping
process (which produces cellulose), a rather unpleasant black liquor
remains containing lignin, sugars and process chemicals. The sugars
can be fermented, and the remaining lignin is treated with alkali and
oxidising agents to break apart the tough lignin structure. The resulting
mixture contains vanillin, which is separated from other products by
solvent extraction and purified. Purity is important if the vanillin
is to be used in food products.