Vegetable tannins are natural products contained in various parts of trees (e.g. bark, wood, pods etc.). The chemical constituents of vegetable tannins are composed of polymeric polyphenolic molecules. The molecules of vegetable tannins cover a wide range of molecular mass ranging from 500 to 3000 units. The tanning action of polyphenols is dependent on the molecular mass (particle size) and the number of phenolic –OH hydroxyl groups. Mimosa tannins have a molecular mass of on average 1250 units and generally have a good tanning action.
Natural Mimosa (Wattle) vegetable extract meets the requirements of a polymer in accordance with Article 3(5) of REACH Regulation.
As a naturally occurring polymer it is exempt from any registration provisions under Title II, since this substance has not been chemically modified and it meets the polymer definition of a naturally occurring substance ( according to article 3(39)) plus it does not meet the criteria for classification as dangerous in accordance with Directive 67/548/EEC.
Vegetable tannins are classified according to their chemical structure:
(i) Pyrogallol or hydrolysable tannins, such as Chestnut and Myrabolam extract.
(ii) Catechol or condensed tannins, such as Mimosa (or Wattle) and Quebracho extract.
In South Africa, Mimosa extract is obtained from the bark of Black Wattle (Acacia mearnsii) trees grown in plantations. It takes 7 to 10 years for the tree to grow large enough to be ready for cutting down and stripping of the bark, which is then chipped and extracted under controlled industrial conditions to extract the optimum amount of tannin with the lightest of colours. Mimosa or wattle bark contains about 30% tannin. The wood has other industrial uses.
Wattle bark is best extracted immediately after stripping to give extract of light colour; the older the bark, the darker is the colour.
Sometimes the bark has to be dried out before the extraction process, but this gives a dark coloured product. There is a definite season for stripping the bark depending on the weather and rainy season in the country where it is grown.
At the NTE factories, chopped bark is extracted, using a counter current principle, in autoclaves under pressure at temperatures above 100 degrees Celsius. The liquid extract so obtained is then concentrated by evaporation and either poured into Hessian bags, in which it slowly solidifies and becomes a solid, or the hot viscous concentrated liquid is spray dried as powder and the powder placed into bags, which are stitched closed and carefully stacked.