Tannin: what it actually is

Tannin is the most misunderstood word in wine vocabulary, because people look for it among the flavours and it is not there. It is a texture — dryness, grip, a puckering at the sides of the tongue — and it is happening to your mouth rather than in the wine.

The compounds involved are polyphenols, principally proanthocyanidins (also called condensed tannins) and, from oak, hydrolysable tannins such as ellagitannins. They are large molecules by the standards of wine chemistry, and their size is most of the story.

Because they are structural rather than aromatic, tannin is one of the few things about a wine you can assess with reasonable confidence without a trained nose.

Saliva contains proline-rich proteins that make the mouth feel lubricated. Tannins bind to these proteins and precipitate them out, and the resulting loss of lubrication is what registers as astringency. This binding is well demonstrated and is the accepted account of the sensation.

That is also why astringency builds across successive sips. Salivary proteins are depleted faster than they are replaced, so the third glass of a tannic red can feel considerably harder than the first — a fact about your mouth rather than about the bottle.

Tannins arrive from several places at once, and in different proportions. Skins contribute the largest share in most red wines; seeds contribute tannins often described as harder and more bitter, particularly if the fruit is crushed aggressively; stems contribute their own if they are retained; and barrels contribute hydrolysable tannins from the wood.

During ageing, tannin molecules react with one another and with anthocyanins to form larger polymers. Larger molecules bind salivary protein differently and, past a point, precipitate out of the wine altogether — which is what the sediment in an old red largely is. The wine typically reads as softer as a result.

How much tannin ends up in the wine is governed by extraction rather than by the grape alone: temperature, time on skins, alcohol level, and how vigorously the cap is worked all bear on it.

Tannin is one of the strongest predictors of which foods a wine will suit. Its interaction with protein and fat is the mechanism underneath the oldest pairing convention there is.

It is also the main structural component behind a red wine’s capacity to age: polymerisation gives it somewhere to develop over a decade, rather than simply oxidising.

And it is the single most common reason a beginner dislikes a red wine without being able to say why. Naming the sensation usually helps more than any amount of flavour vocabulary.