Also known as tannic acid, this compound is used by plants as a repellent to herbivores. In industrial environments, tannins are in the form of beige powder and are generally used in the textile industry (leather, dye, ink). They are also found in the food sector as they are used as a clarifier during the production of wine and beer. Finally, tannins are also used in water for industrial boilers to prevent scaling.

With a molecular weight ranging from 500 to 3000 Da, tannins bind to certain proteins and precipitate them. Tannins generally bind to hydrogen molecules.

In general, tannins can be classified into two chemical groups: hydrolyzable tannins and condensed tannins

Hydrolyzable tannins

Hydrolyzable tannins are a category of tannins that have the ability to decompose in water. Following their decomposition, water-soluble derivatives are formed. These derivatives can be gallic acid, protocatechics or sugars. The most common are gallic acids or gallotannin.

Condensed tannins

Generally coming from wood or bark extract, condensed tannins form insoluble precipitates known as phlobaphenes (tanner red).

Tannins and water

Typically, when water is highly concentrated in tannins, it will have much the same aesthetic problems as water concentrated in iron. This means an unattractive yellowish colour, a peculiar taste and odours.

In simple terms, the extraction of tannins can be done in two ways. The first one consists in a membrane filtration step. The second and most common way is to add an ion exchange step.

• In both cases, the filters used must have particularities in order to allow the extraction of tannins and it is the same for the resin used in the ion-exchange system.