Your quick reference guide to understanding tannins, their role in wine and how to identify and describe them.
What are tannins?
Tannins are a type of bitter and astringent chemical compound that belongs to a larger group called polyphenols. They occur abundantly in nature, namely in the bark of many trees and in numerous leaves, legumes and fruits, including grapes.
Tannin molecules are usually much larger than those found in other types of polyphenols, and they have a unique ability to easily combine with other molecules, especially proteins, causing them to precipitate. This is the basis of leather production, in which the structure of animal hide is changed (tanned) by using various tree barks.
What do they do?
Because tannins bind with other proteins, including those in human saliva, they create a characteristic astringent, mouth-coating sensation in the mouth.
Their primary role in nature is to make unripe fruits and seeds unpalatable, thus dissuading animals from eating them when not yet ripe. Ripening changes the structure of the tannins through polymerization – increase of their overall length due to the combination of smaller molecules – and interaction with pectins (sugar compounds whose presence increases with maturation) and anthocyanins (red colouring compounds). Different harvest dates therefore mean different tannin structure and, ultimately, different tannic structure of the final wine, provided no corrections are made in the cellar.
Where do tannins in wine come from?
In wine, they come primarily from the skin, seeds and, to a lesser extent, the stems of the grapes. During alcoholic fermentation, the juice, skins and pips (and stems if the winemaker decides to do full or partial whole-cluster fermentation) macerate together. As sugar is processed and alcohol produced, colour and tannins are released into the wine. Tannins are soluble in alcohol, but not water; the longer the skins and pips macerate during and after alcoholic fermentation the more tannic the final wine will be. This is why pre-fermentation and carbonic maceration help to add colour (from anthocyanins) and flavour compounds to wine without the simultaneous addition of tannins.
This also explains why the colour of the wine somewhat correlates with the amount of tannins. Some tannins are pigment and/or precursor of pigment release – more tannins therefore contribute to deeper colouring.
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