Tannins are usually divided into hydrolyzable tannins and condensed tannins (proanthocyanidins).

Occurrence

Tannins are distributed all over the plant kingdom. They are commonly found in both gymnosperms as well as angiosperms. In terms of location of the tannins in a plant, they are mainly located in the vacuoles or surface wax of the plants. These sites are where tannins do not interfere with plant metabolism, and it is only after cell breakdown and death that the tannins are active in metabolic effects. Tannins are found in leaf tissues, bud tissues, seed tissues, root tissues and stem tissues. An example of the location of the tannins in the stem tissue is that they are often found in the growth areas of trees, such as the secondary phloem and xylem and the layer between the cortex and epidermis. Tannins may help regulate the growth of these tissues. They are also found in the heartwood of conifers and may play a role in inhibiting microbial activity, thus resulting in the natural durability of the wood. However, there may be a loss in the bioavailability of tannins in plants due to birds, pests, and other pathogens.

The leaching of tannins from the decaying leaves of vegetation adjoining a stream may produce what is known as a blackwater river.

Hydrolysable tannins

At the center of a hydrolysable tannin molecule, there is a carbohydrate (usually D-glucose). The hydroxyl groups of the carbohydrate are partially or totally esterified with phenolic groups such as gallic acid (in gallotannins) or ellagic acid (in ellagitannins). Hydrolyzable tannins are hydrolyzed by weak acids or weak bases to produce carbohydrate and phenolic acids.

Examples of gallotannins are the gallic acid esters of glucose in tannic acid (C₇₆H₅₂O₄₆), found in the leaves and bark of many plant species.

Condensed tannins

Condensed tannins, also known as proanthocyanidins, are polymers of 2 to 50 (or more) flavonoid units that are joined by carbon-carbon bonds, which are not susceptible to being cleaved by hydrolysis. While hydrolyzable tannins and most condensed tannins are water soluble, some very large condensed tannins are insoluble.

Tea

Wine

Modern winemakers take great care to minimize undesirable tannins from seeds by crushing grapes gently when extracting their juice, to avoid crushing the seeds. Pressing the grapes further results in press wine which is more tannic and might be kept separately. De-stemming is also widely practiced. Wines can also take on tannins if matured in oak or wood casks with a high tannin content. Tannins play an important role in preventing oxidation in aging wine and appear to polymerize and make up a major portion of the sediment in wine.