I've realized that my simple post on grapes and wine has evolved into a rather lengthy essay on the subject, so to avoid being dry and long-winded I decided to break up the conversation into smaller, easier to digest -- pardon the pun -- morsels.
First up on our plate is tannins.
Chemically, tannins are polyphenolic compounds, which will bind with other compounds and molecules to create proteins and other organic compounds. Tannins are responsible for giving red wine that bitter, mouth-puckering taste. This is why highly-tannic wines (such as Cabernet Sauvignon) are typically paired with red meat. Meats high in protein minimize the astringency introduced by tannins.
Where do tannins come from?
Tannins are found in the stem, seed, and skin of the grape plant. They are produced by enzymes during the metabolic processes of the plant. If you think back to Biology class you'll remember that enzymes were responsible for breaking down proteins, which explains why tannins are so quick to link up with amino acids again.
The tannins found in wine are known as proanthocyanidins and are the reason doctors say a glass of red wine is good for the heart. These proanthocyanidins suppress the body's production of endothelin-1, which constricts and hardens blood vessels, leading to vascular disease. Wines from the south of France and Sardinia have the highest concentration of proanthocyanidins.
Don't be mistaken though. White wines have tannins as well, but in a lower concentration. Winemakers may ferment their grapes with the skin, seed, and stems present in order to up the amount of tannins in the wine.
The age of a grape will also play a part in its taste for obvious reasons. The longer a grape is on the vine, the longer the enzymes have had to break down the tannin, creating a softer, less acidic taste. Immature grapes will have higher acidity.
Further Reading: Wikipedia
Photo Credit: Tannic Acid