Chemical substances from the large group of phenols (polyhydroxyphenols), which are found in the bark of many trees, shrubs and bushes as well as in the skins of fruits such as grapes, especially of red wine varieties. They ultimately serve the plants as a defence against herbivores or predators and also as protection against microorganisms. Tannins can inactivate a variety of viruses and therefore have an antitoxic effect. They also have a very strong tanning effect, which is why they have been used for thousands of years in the production of leather. For this reason, they are often colloquially referred to as tanning agents, although this term has a rather negative or pejorative connotation compared to tannins.
All tannins are odourless, easily oxidisable and soluble in water and ethanol (alcohol). They react with proteins. For this reason, protein clouding is rather rare in red wines, as the insoluble tannin-protein compounds precipitate and are already removed with the tankard. Tannins are an essential characteristic component of red wines in particular and to a much lesser extent of white wines. They are responsible for the characteristic astringent (mouth contracting) effect, which, by the way, is not a taste sensation but a trigeminal one. The bitter taste on the other hand comes mainly from the tannin-like catechin.
The tannin content in wine depends mainly on the vinification(fermentation and ageing) and the grape variety. Among the varieties rich in tannin are BlaufränkischCabernet Sauvignon, Cot (Malbec), Mandilaria, Monastrell (Mourvèdre), Nebbiolo, Petit Verdot, Sangiovese, Syrah and Tannat. Varieties with low tannins or those with "soft tannins" are Carmenère, Cinsaut, Gamay, Merlot, Pinot Noir, St. Laurent, Schiava (Vernatsch, Trollinger) and Zweigelt. The longer the duration of the mash fermentation for red wines or the more strongly the mash is pressed, the more tannins and anthocyanins (colouring agents) reach the wine through extraction.
The tannin content in white wines is 0.3 g/l, but in red wines it is up to 2.5 g/l (see a list of all wine ingredients under total extract). The tannin index was developed for an objective measurability. The low proportion in white wines is mainly due to the method of vinification, as there are usually no combs and grape seeds involved in fermentation. Only with a pomace maceration a higher tannin content is also achieved in white wines. In the course of time, the tannin content in the wine moderates in a natural way, as polymerisation leads to the formation of larger molecules, which are precipitated as a deposit and sink to the bottom of the bottle.
A distinction is made between two groups according to the origin of the tannins. The "grape-born tannins" come from the seeds, combs (centaury, stems) and skins of grapes; only a very small part also comes from the pulp. They are also called flavonoid tannins, condensed tannins or proanthocyanidins. These in turn consist of prodelphinidins (only in skins and stems) and procyanidins (also in grape seeds). These make up the main component in wine. The second group of "wood-born tannins" with a much lower proportion comes from the wood during barrel ageing or barrique ageing. These are also known as hydrolysable tannins or ellage tannins. Through oxidation and acid hydrolysis they decompose to ellagic and gallic acid. These tannins promote the polymerization of the anthocyanins to more intensely colored pigments. They contribute positively to colour stabilisation and ageing ability in red wine.
Since the mid-1980s, various techniques have been used in red wine making to improve the taste towards "mild" on the one hand and to enhance the positive effect of the tannins during ageing or bottle ageing on texture and shelf life on the other. These are, especially in the case of barrique maturation, the proportion of new barrels, the type of oak and the extent of toasting, regulating the tannin content by fining, as well as a targeted, dosed supply of oxygen by means of micro-oxygenation. New is the use of oenological tannins (especially wood-born ones), which are used individually in the winemaking process: during maceration, after BSA, before barrel ageing or before bottling. The not undisputed use has in any case a taste-altering effect. Two negative terms for the tannin taste are green tannins and tannin to loose. See also under Wine appeal and wine evaluation.
oenological tannin: By Simon A. Eugster - Own work, CC BY 3.0, Link
Barrique barrel: By Gerard Prins - Own work, CC BY-SA 3.0, Link
Grape varieties: Ursula Brühl, Doris Schneider, Julius Kühn Institute (JKI)