Grapes

By far the most common species of cultivated grape is the wine grape (Vitis vinifera), probably native to southwest Asia, possibly in the vicinity of the Black Sea. This species may have been cultivated for as long as 7,000 years. The wine grape now occurs in hundreds of cultivated varieties and is planted in temperate climates in all parts of the world. The fruits of this species can be blue, yellow, or green in color, and they contain one to four seeds. Ripe wine grapes typically contain 70% of their weight as juice and 20-24% as sugar. This species is widely grown in warm-temperate regions of Europe, especially in France and Italy, and to a lesser degree in Germany, Spain, and elsewhere. Other notable centers for the cultivation of wine grapes are California, Chile, Australia, Portugal, Russia, Algeria, and South Africa.

Two North American species of grapes are also cultivated for the production of wine. These are the fox-grape (V. labrusca) and, less commonly, the summer-grape (V. aestivalis). The skin of the fruits of the fox-grape separate quite easily from the interior pulp, which makes it easy to distinguish agricultural varieties of this species from the wine grape.

Grapes are often eaten fresh as a tasty and nutritious table fruit. Grapes can also be crushed to manufacture a highly flavorful juice. Grapes can also be preserved by drying, usually in the sun. Most dried grapes are called raisins, but dried seedless grapes are known as sultanas.

Wine is an alcoholic beverage that is produced by a careful fermentation of grape juice. The fermentation is carried out by the wine yeast (Saccharomyces ellipsoides), a microscopic fungus that occurs naturally on the surface of grapes. However, specially prepared strains of the wine yeast are generally used by commercial vintners in order to help ensure a constant, predictable fermentation and final product.

Wine yeast ferments the sugar content of the juice of pressed grapes into carbon dioxide and ethanol, a type of alcohol. The yield of alcohol is about 1% for every 2% of sugar in the juice, but the final alcohol concentration cannot exceed 12%, because this is the upper limit of tolerance of the yeast to alcohol in its growth medium. (Actually, there are wines with an alcohol concentration greater than 12%, but these are prepared by adding pure ethanol, a process known as fortifying.) The initial grape juice is prepared by pressing the ripe grapes. Originally, this was done by barefoot people stomping about in large wooden tubs. Today, however, the grapes are usually pressed using large machines. Red wines are obtained when the skins of blue grapes are left in with the fermenting juice. White wines are obtained when the skins are removed prior to the fermentation, even if the juice was pressed from red grapes.

The quality of the resulting wine is influenced by many factors. The variety, sugar content, and other aspects of the grapes are all important, as is the strain of wine yeast that is used. The soil conditions and climate of the growing region are also highly influential. The incubation temperature during the fermentation is important as is the sort of container that is used during this process. In addition, once the fermentation is stopped, the period of time during which the wine is stored can be important. However, a storage which is too long can be detrimental because the alcohol in the wine may be spoiled by a further metabolism of the ethanol into acetic acid, or vinegar.