Diverse and complex group of chemical compounds that occur in the bark of many trees and in fruits, including the grape. Strictly speaking, a tannin is a compound that is capable of interacting with proteins and precipitating them; this is the basis of the process of tanning animal hides (hence the name tannin) and is also a process that is believed to be responsible for the sensation of astringency. Tannins in wine come predominantly from the grapes and, to a much lesser extent, from the wood in which the wine is aged. See also oak flavours.
The natural tannins of grapes, or condensed tannins, also called proanthocyanidins since they release red anthocyanidin pigments when heated in acidic media, are flavonoids consisting of oligomers and polymers of catechins. Formation of proanthocyanidins occurs under the control of enzymes as part of the metabolism of the grape but they may rearrange to longer or shorter molecules in the acidic wine medium. Other catechin polymers can be formed in wine as a result of enzymatic or chemical oxidation reactions. These polymeric flavonoids that can range from colourless through light yellow to amber, as well as pigmented tannins resulting from reactions of anthocyanins with catechins and tannins, may also be regarded as tannins. Wine may also contain hydrolysable tannins, deriving from gallic acid and ellagic acid, extracted from oak cooperage in the course of barrel ageing, from oak chips, or added as oenological tannins.
Tannins play an important role in the ageing of wine, particularly red wines, where pigmented tannins are crucial to the colour and sensory properties. Handling tannins during red winemaking is one of the most critical steps in optimizing the quality and character of a red wine, yet the process is based almost totally on experience and intuition because our understanding of the principles involved is still incomplete.
The tannins in grapes are predominantly in the skins and seeds of each berry and also the stems, the amount of tannins in grape pulp being much lower. Thus, the more skins, seeds, and stems are involved in the winemaking process, the higher the possible resultant level of tannins. Tannin levels in white and rosé wines, which are made largely by excluding or minimizing these grape components, are therefore lower than in reds. Although white wines contain structures similar to the pigmented tannins of a red wine, the absence of anthocyanins condensed into the tannins of white wines accounts for how different they look.
Tannins are most often encountered by the human palate in over-steeped tea, and by wine drinkers in young reds designed for a long life in bottle and in whites made with prolonged or excessive skin contact. They produce the taste sensation of bitterness and the physical tactile ‘drying’ sensation of astringency. Catechins and small tannins are said to be responsible for bitterness, while larger ones elicit the astringency sensation, presumably by interaction with the proteins of the mouth but also by the adherence of the tannins to the oral mucous membranes (see below).
Traditional methods for measuring tannins report them as if they were all gallic acid, and such analyses, including the widely used Folin Ciocalteu method, are popular because of their analytical convenience. Alternative methods for measuring the phenolic compounds of grape tannins more directly and as other than gallic acid are time consuming and require considerable analytical expertise. Gallic acid or GA equivalent concentration averages about 300 mg/l in white wines, but 1,800 mg/l in reds. The tannin types and their extraction rates vary considerably with vine variety and winemaking methods. Varieties notably high in tannins include cabernet sauvignon, nebbiolo, syrah, and tannat.
Since the late 1980s, much research into red winemaking has been aimed at minimizing the bitter and astringent impression made by tannins on the palate while enhancing the texture and ageing properties which they confer on a wine. These studies have involved, among other variations, ever more refinement of maceration techniques and deliberately controlled exposure to oxygen at various points during the winemaking process (see micro-oxygenation, for example). It is also widely recognized that the influence of such viticultural factors as grape ripeness and grape composition on the properties of tannins is not yet understood.
Different wood types contain different sorts of tannins, but these have most effect on wine when the cooperage is new. The tannins of the various species and varieties of oak, the most common wood used in winemaking, vary among themselves, and according to how the oak was seasoned (see barrel making). Oak tannins differ in significant ways from grape tannins, although the consequences of such differences on the stability of wine colour and on the sensory properties (including mouthfeel) of barrel-matured red wines in particular are yet to be scientifically rationalized. For more details, see oak flavour. Wine consumers may experience a certain amount of wood oak flavour in a wide range of wines, including some relatively immature wines, both red and white, whether the result of genuine barrel maturation or the use of oak chips. They are therefore often exposed to the effects of tannin on the palate, which can be considerably mitigated by the right choice of accompanying food.
Winemakers can adjust excessively high tannin levels by fining with casein, gelatin, or albumin, which selectively precipitate large-sized astringent tannins. Formation of soluble complexes with macromolecules such as proteins may also prevent tannins from interacting with salivary proteins and eliciting astringency. Given sufficient time, tannins are removed naturally, however, during wine ageing. The tannins polymerize and form aggregates that eventually precipitate as sediment so that they no longer have any bitter or astringent effect on the palate. Depending on the wine composition and ph, reactions of tannins can also yield smaller tannins and pigmented tannins, thus resulting in lower astringency.