I was interested to read recently in The New York Times that adding milk to black tea reduces the effect of the antioxidants, vitamins and other healthful compounds inherent in tea. This happens because the proteins in milk bind to and neutralize the tannins in the tea. Likewise, when certain cheeses are paired with red wine the proteins in the cheese soften the wine’s 
tannins. Black tea and wine are the world’s most tannic beverages. Since I am one of those who like to add a splash of milk to my daily infusions of tea, and because I like wine (and cheese) too, I find this liaison between tannin and protein fascinating.
But what exactly are tannins? This is among the most frequently asked questions asked by wine education students, and it is one of those lessons most easily demonstrated by actual tasting rather than explaining. It’s hard to understand the sensation produced by tannins until one actually tastes–or, more accurately, feels–that specific tactile impression of astringency in the mouth while drinking red wine. (The perception of tannin is not to be confused with the tart sensation of acidity, or the mouth-puckering sharpness produced by underipe grapes).
With wine, what happens is that the tannins bind the mouth’s own lubricating mechanisms, rendering them hors de combat as it were. But if the wine is accompanied by certain kinds of cheese the milk protein in the cheese acts like a soothing balm to calm aggressive tannins. This dance between tannin and proteins is what makes high-protein foods such as steak so delicious with wine.
In grapes, tannins exist principally in the fruit’s skin, seeds and stems, and they play an important role in the fruit’s development. Combined with the high acidity of unripe grapes, the tannins makes the fruit unpalatable to birds, which allows the grapes time to develop color and sugars and to lower acidity and astringency.
In the wine, tannins undergo many different physical and chemical changes, and are important 
factors affecting the wine’s color and aging potential as well as its mouth-feel. They can help add structure to otherwise thin and/or weak wines. One way vintners add tannin to wine is by aging it in oak barrels, fermenting it with oak chips, or using commercial tannin additives. Since many grape varieties are naturally low in tannins, vintners often add whole grape clusters to fermenting wine to increase the tannin content of the finished wine. The downside of this is that if the stems are not fully ripe the wine may taste unpleasantly green and astringent. (Whole cluster fermentation can affect the wine in a host of other ways as well, but that, as they say, is another story.)
Because tannins are such an important, and generally pleasing component in the overall experience of drinking red wine, wine critics have developed an evocative vocabulary to try and describe the general effect of tannin on the palate. Common adjectives include velvety, dry, sandy, coarse, silky, smooth, green, and even splintery. I just took a quick look at a group of current reviews in Wine Review Online, noting that my colleagues’ descriptions of tannins include “mild,” “not intrusive,” “grainy,” “dusty,” “drying.” Few of these particular adjectives, by the way, are meant as a negative. Some wine drinkers prefer smooth red wines with only the subtlest of tannic intrusion, while many others, myself included, relish the good firm, well-balanced grip of tannin.
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