What are tannins, and why can some wines age for decades?

Tannins are the astringent compounds in plants that bind to proteins, and in red wine they act partly as preservatives, which is why a few tannic wines can last for decades. The astringency of tannins is what causes the dry, puckery feeling in your mouth after unripe fruit, red wine or tea. Plants use tannins to protect themselves from predators.

Tannins are old friends of human craft. The word itself refers to the abundance of these compounds in oak bark, which was used to tan animal hides into leather. In wine, tannins come from the grape skins and also from oak: most wines aged in charred oak barrels have tannins absorbed from the wood.

Why would they help a wine last? Pigmented tannins and related compounds not only affect a wine's colour but also act as preservatives. In red wine a high level of phenolics, most notably tannins, makes it more likely that a wine can age. Cabernet Sauvignon, Nebbiolo and Syrah are high in phenols. In white wines, acidity plays a similar part, acting as a preservative in a role similar to that of tannins in reds.

As a tannic red wine ages, the harsh tannins of its youth give way to a softer mouthfeel, and its inky colour fades, turning orange at the edges and eventually brown.

But do not imagine that old always means better. It is a widespread misconception that wine always improves with age. Only about 10% of reds and 5% of whites taste better after five years than after one, and a wine aged too long declines, with hollow fruit and dominant acidity.