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Michael Twelftree, the proprietor and managing director of Two Hands Wines in Australia’s Barossa Valley, describes the experience of tasting a red wine this way: “The fruit is the engine and rail cars; the acidity is the rails; and the tannins are the brakes.”

When we sip many a red wine, its tannins draw the taste to a close with an astringent, palate-puckering sensation. It’s as if our tongue were sandpapered.

“Tannins don’t have flavor,” says Joel Aiken, winemaker for Amici Cellars in Calistoga and Aiken Wines in St. Helena. “They’re tactile.”

Tannins are phenols. In wine they come from the skins, seeds and stems of the grapes. That’s why they are more prevalent in red wines than whites; red wines receive their color from their grapes’ skins during fermentation.

(White wines have tannin, too, but it’s far less perceptible because the tannin level is so low.)

You’re familiar with tannins even if you’re not familiar with red wine. They make over-steeped tea bitter and astringent; they’re what scrunch up your tongue if you chew too long on a Popsicle stick or wooden toothpick. They make walnut skin, raw eggplant and green olives rough and drying in your mouth.

And they are absolutely crucial to grapes growing up successfully on the vine, and to wine being made and being appreciated, especially over time.

“Tannin, organically, is a deterrent to organisms (such as birds or animals) that would go after the fruit before it’s ripened in its time,” says Chris Carpenter, winemaker for La Jota Vineyard Co. in Napa. “But, from a structural angle, they’re the true backbone of the wine. As much or more, even, than the concentration of fruit, they give a wine weight, chewiness and texture. You don’t get that with any other beverage.”

Adds Aiken, “They add a lot more to a wine than just bitterness or astringency. They give the wine body.”

Tannin is also crucial to the aging of red wine. Because tannins are antioxidant, “they help preserve the fruit character of a wine over time,” says Carpenter.

Tannins begin life in red wine as short-chain molecules, but over time polymerize or plasticize into long-chain molecules. They’re like Tinkertoys; when young, a couple “spools,” one dowel between them. As they age, these same molecules are chemically attracted to and bind to each other so that, later, they are many “spools” held together by several dowels.

Short-chain tannin molecules are more tactilely aggressive than long-chain molecules. In a manner of thinking, the bigger molecules cannot “fit” into the places on your palate where you sense touch. Long-chain tannins simply glide over your palate, and the wine seems smoother, softer and richer than when it was young and its short-chain tannin molecules could easily, tactilely attack those same sensory locales.

When opening a bottle of red wine, anyone can speed along this process by aerating or decanting the wine a couple of hours before it’s served. “When you decant, you cause an early polymerization,” says Carpenter. “You are binding those tannins; the bigger they become, the less they can be perceived.”

Tannins play another key role at table. By their very nature, they react or interact with proteins and fats such as any eater finds in animal or fish meats, or in the oils and fats of foods.

Long ago, when people wanted to soften buffalo or other animal hides so that they could fashion leather-based moccasins, skirts, pants or housing material, they would tan the hides with tannin obtained from the bark of trees.

The tannin bound with the protein of the hide and altered it, softening it and making it malleable.

Without tanning, hides would have become a sort of leather plywood.

When you eat a piece of steak with a rind of fat on it, and sip a tannic cabernet sauvignon, the tannin binds with the blood protein and the fat and causes them to seem less fatty, as they in turn bind with the tannin to make it seems less astringent or bitter.

Not only meat eaters benefit from this interaction; it works with any fat or oil, vegetable-based or otherwise, and any other protein such as casein as found in cheese.

Bill St John has been writing and teaching about wine for more than 40 years. He is based in Denver.

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