Grapes provide the substance of wine, and therefore determine many of its qualities. Their most important components are
- Sugars, which the yeasts feed on and convert into alcohol. Wine grapes are generally harvested with 20–30% sugar, mainly glucose and fructose.
- Acids, mainly tartaric and some malic, which help prevent the growth of undesirable microbes during fermentation, and are a major component of wine flavor.
- Tannins and related phenolic compounds, which contribute an astringent feeling and thereby a body and weightiness to wine.
- Pigment molecules that provide color, and sometimes contribute to astringency as well. Red grapes contain anthocyanin pigments, mainly in the skins. “White” grapes lack anthocyanins; their yellowish color comes from a different group of phenolic compounds, the flavonols.
- Aroma compounds, which may be generically grapey, or distinctive of a particular grape variety. Many aromatics are chemically bound to other molecules, often sugars, and so aren’t evident in the raw fruit; during wine-making, fruit and yeast enzymes liberate the aromatics and so make them available for us to enjoy.