What Are Tannins? The Compounds That Make Your Mouth Feel Dry

Tannins are plant compounds that bind strongly to proteins, and the sensation they produce is astringency, which is a tactile effect rather than a taste. Saliva contains proteins that lubricate the mouth, and tannins bind them and precipitate them out of solution, which removes the lubrication and leaves surfaces rubbing against each other, producing the characteristic dry rough feeling. That is why astringency builds over successive sips rather than registering immediately and why it takes time to clear, since saliva has to be replenished. It is also why fat and protein reduce the effect, giving the compounds something else to bind, which is the reason milk in tea softens it and why tannic red wine is served with rich food. The effect is not detected by taste receptors at all, which is why it is not one of the basic tastes.

The compounds are widespread in plants and concentrated in particular places:

• •Bark and wood, where they deter insects and fungi, which is why oak barrels contribute them to wine and spirits
• •Leaves, particularly tea, where processing determines how much survives into the drink
• •Grape skins, seeds and stems, which is why red wine made with skin contact has them and white wine largely does not
• •Unripe fruit, where they deter consumption before the seeds are ready and decline as the fruit ripens
• •Nuts, pulses and some cereals, where they reduce digestibility and are part of the plant's defence of its seeds
• •Galls formed on oak by wasps, which are exceptionally concentrated and were harvested for that reason

The protein-binding property gave these compounds two major industrial uses long before anyone understood the chemistry. Tanning leather works by soaking hide in a tannin solution so the compounds bind to the collagen, which stops it rotting and converts a material that would decay into one that lasts for centuries, and the process gives the compounds their name. Vegetable tanning using bark and galls was the standard method for millennia and is still used for some leathers, having been largely displaced by faster mineral methods. Iron gall ink was made by combining a tannin extract with iron salts, producing a dark permanent ink that bonded into the paper rather than sitting on it, and it was the standard writing ink in Europe for well over a thousand years. Its acidity eventually eats through the paper it was written on, which is a significant conservation problem for archives.

The compounds exist as defence and the mechanism follows directly from the protein binding that makes them astringent. An animal eating tannin-rich tissue finds the compounds binding proteins in its digestive system, including its own digestive enzymes and the proteins in the food, which reduces how much nutrition it extracts and makes the meal less worthwhile. That effect is strong enough to be measurable in livestock and is a genuine constraint on which plants browsing animals choose. Some animals have countermeasures, including saliva proteins produced specifically to bind tannins harmlessly before they reach the gut, which several browsing mammals possess and which is a direct evolutionary response. Plants increase production in response to being eaten, sometimes within hours, and there is evidence of neighbouring plants increasing theirs too, which is one of the better-documented cases of plants responding to damage nearby.

Cooks and producers manipulate the compounds deliberately. Tea is brewed at controlled temperature and time, since hot water and long steeping extract more and produce bitterness and astringency that a shorter brew avoids, which is why the instruction about steeping time is not arbitrary. Wine makers control skin and stem contact during fermentation to set the level, and ageing softens the effect as the compounds combine into larger molecules that bind saliva less effectively. Soaking and cooking pulses reduces their content, which improves both taste and nutrient availability, since the compounds also bind minerals and reduce their absorption. Ripening fruit converts them. And pairing is the everyday management, since serving an astringent drink with fat or protein gives the compounds something to bind before they reach the saliva.

Astringency is a tactile effect rather than a taste, produced when these compounds bind and precipitate the proteins that lubricate the mouth. Fat and protein reduce it by giving them something else to bind. The same property tans leather by bonding to collagen and made iron gall ink, which was standard in Europe for a millennium and eventually eats through the paper.