Wine is fermented fruit juice. Fermentation occurs when microorganisms called yeast metabolize the sugars in fruit juice, creating alcohol and flavor compounds.
The yeast may occur naturally in the environment, or the winemaker might add commercially produced yeast.
Of all the fruits used to make wine, the fruit of the grape is the most widespread and successful for several reasons.
Reason One: Grapes are among the sweetest of fruits. This means their sugar content is sufficient to generate the level of alcohol consumers expect in a wine. When trying to make wine using the juice of apples, for example, producers must often add sugar from another source, because the apple juice simply is not sweet enough. Ironically, many times the sweetener they use is concentrated white grape juice.
Reason Two: Grapes have the right kind of acid. Tartaric acid maintains the chemical stability of the wine and contributes to its taste and color. You will notice from this chart of fruit acidity that grapes do not have the most acidity (there is more to a wine than acidity), but the right kind, tartaric acid.
Reason Three: Grapes bring flavor intensity, on their own and when fermented with yeast.
Reason Four: Red (black) grape skins, seeds and stems add tannins to wines. These are naturally occurring compounds that give the sensation of dryness or astringency in the mouth. We do not taste tannins – we feel them.
The right balance of tannins with alcohol, flavor and acidity makes for an enjoyable and balanced red wine.