What is tannin in wine?

Tannin sits mainly in the skin, seeds and stems of the grape, and is released from them during winemaking. Iowa State University measures in grapes of the species Vitis vinifera about 1.5 milligrams of tannin per gram of berry. In the hybrid Marquette it is about 0.5 milligrams per gram, so a third of that.

The composition differs per place in the berry. Seeds are rich in epicatechin, catechin and epicatechin gallate. Skin and stems also contain epigallocatechin and gallocatechin. With white wine the juice usually comes off the skins straight away, with red wine it stays on them for days. That difference in method explains most of the difference in tannin content between red and white.

Tannin binds to proteins in your saliva, which then precipitate, so the lubrication in your mouth disappears and everything feels rough. The UC Davis laboratory describes that binding as an interplay of hydrogen bonds and hydrophobic interaction.

Iowa State University summarizes it as a process in three steps: first the tannins bind to the saliva proteins, then these complexes clump together, then they precipitate. Astringency is therefore not a taste but a tactile sensation. Bitterness is a taste. The small tannin molecules from the seeds are mainly perceived as bitter, the larger molecules from the skin mainly as astringent.

According to an overview article in the trade journal Molecules, white wine contains 10 to 50 milligrams of tannin per litre, against about 593 milligrams per litre in red wine. The same article gives an average of 333 mg per litre of total polyphenols in white wine and 2,674.5 mg per litre in red wine, a difference of roughly eight times.

The Australian Wine Research Institute measured in Australian red wines from 2004 to 2006 an average of 1.85 grams of tannin per litre in shiraz, with a range from 0.36 to 4.8 grams. Cabernet sauvignon came to 1.97 grams (0.12 to 4.17) and merlot to 1.54 grams (0.31 to 2.98), expressed in epicatechin equivalents. Iowa State University gives a range for vinifera wines from under 100 to 1,600 milligrams per litre, from pinot noir at the bottom to cabernet sauvignon at the top.

Tannin from the skin forms long chains of 3 to 83 building blocks, tannin from the seeds stays small with 2 to 16 building blocks. UC Davis links the sensory difference to that: small and bitter versus large and astringent.

Iowa State University gives as a practical upper limit about 50 building blocks, which comes down to a molecular mass of around 14,500 daltons. The average chain length, called mDP in the specialist literature, also determines how strong the astringency turns out. A wine with the same milligrams of tannin can therefore still feel different, because the molecules are built up differently.

Oak yields far less tannin than the grape itself, namely a maximum of about 250 milligrams per litre, against 2 to 4 grams per litre from skin, seeds and pulp. Cask and grape therefore do not contribute in the same order of magnitude.

It is also a different chemical type. Cask tannin is hydrolysable, grape tannin is condensed and consists of repeated flavan-3-ol units such as catechin, epicatechin, epigallocatechin and epicatechin gallate.

Tannin binds to protein, and that is exactly what happens as soon as a tannic wine comes into contact with a dish containing a lot of protein. UC Davis and Iowa State University describe that binding for salivary proteins, where the complexes clump together and precipitate.

If part of the tannin is bound in this way, less is left to make your mouth feel dry. In a red wine with 1.5 to 2 grams of tannin per litre that difference is clearly noticeable. In a white wine with 10 to 50 milligrams per litre there is little to bind and it hardly matters.