Wine-making, Biology of
Wine is made by fermenting fleshy fruits, principally the cultivated grape Vitis vinifera (family Vitaceae). Grapes are grown on farms called vineyards. Grapevines are trellised to improve air circulation and access to sunlight. When they are ripe, wine grapes are usually harvested by hand and then mechanically destemmed and crushed. The juice is then inoculated with yeast ( Saccharomyces cerevisiae ), which ferments the sugars in the grape juice to form ethyl alcohol, at a final concentration of 12 to 14 percent. White wine is made without the grape skins, whereas red wine uses the skin to give color to the wine. After fermentation, the wine is usually transferred to oak casks for finishing, or flavor development. Sparkling wines, such as champagne, are bottled before fermentation is complete, thus trapping some carbon dioxide in the bottle.
The flavor of wine is influenced by many factors, beginning with the soil and climate. Although most varieties of grapes are frost-hardy, the best wines come from regions with more moderate climates. Grapes grow well on rocky hillsides, but more level and richer soils also produce fine wines, as long as the ground drains well. The deep roots of the grapevine minimize the need for irrigation, and fertilizer is kept to a minimum to prevent overproduction of lesser-quality grapes. Weather during the growing season is critical to fruit set, grape development, and wine quality. The best harvests occur in years with plentiful rainfall early in the season, followed by a warm but not excessively hot summer and a dry harvest season.
Johnson, Hugh. World Atlas of Wine, 4th ed. New York: Simon & Schuster, 1994.