It all comes down to tannins.
Tannins are compounds that are found in the skins, stems and seeds of grapes. Tannins are the chemicals in the wine that causes the "astringent" mouth feel you sometimes feel when you drink wine – it will dry out your mouth by reacting to the proteins in your saliva. (This is why it’s often recommended that very tannic wines be served with high protein meals, like steak.)
Some grapes, like Cabernet Sauvignon, tend to be more tannic than other grapes. A process called maceration increases the tannin levels in the wine (and also affects the color) by leaving the juice in contact with the skins for a longer period of time. Wines that are meant to be held onto for longer periods of time go through this process. With less tannic grapes like Pinot Noir, sometimes winemakers will even leave stems in the barrels as the wine ages to increase the tannins.
If you opened a bottle of Chateau Pontet Canet Pauillac 2006 tomorrow to have with your dinner, you might not be too pleased with the taste. Bordeauxs like that one are designed to be laid down to rest for a number of years. With wine, sometimes patience is a virtue, and if you pick it up in 10, 20 or even 30 years or more, you’ll be rewarded with a smooth, easy-drinking, delicious wine with bold fruits and immense richness.
Why is this? Because as a tannin-rich wine ages, the tannin molecules come into contact with oxygen and become larger. Oxygen is found in the open space in the bottle, as well as let in and out in small amounts by the porous cork. Also, the tannin molecules will bind together to form long chains, which also improves drinkability. In addition, sometimes winemakers will add chemicals that will bind themselves to the tannins and cause them to settle as sediment, lessening the astringency of the wine.
So, how do you ensure your wines will age properly? It comes down to two things: Temperature and humidity. You can’t just take a bottle of the Pontet Canet, stick it in your pantry for 20 years, and expect it to supple and delicious. Aging wine is a precise art.
While 45 – 65 degrees is considered acceptable, the ideal temperature for aging wine is 55 degrees Fahrenheit. A wine that is aged in temperatures too high will have accelerated aging, that will spoil the wine and turn it into vinegar. Too low a temperature will delay aging.
More important than the temperature itself, is the fact that the temperature must remain as constant as possible. A wine aged for 15 years at a constant 45 degrees will do a lot better than a wine aged for the same length of time, with 5 – 10 degree temperature fluctuations.
Humidity, although less crucial than temperature, is also important in wine storage. The ideal humidity is somewhere in the neighborhood of 80%. Too high a humidity is mainly a concern because it causes damage to the bottle labels, which will affect the resale value of the wine. Too low of a humidity level will dry out the cork, affecting the oxygen levels in the wine.