Wine is one of the numerous products of natural fermentation. The ancient Greeks cultivated wine grapes and made wine long before Pasteur explained the process of fermentation. Wine can be made from a must (juice) of almost any fruit, vegetable, or flower that can be fermented. Fermentation is an anaerobic metabolic process that releases energy from a sugar (or other organic molecule), and uses an organic molecule as the final electron acceptor. In the production of wines, the biochemical conversion of juice to wine occurs when yeast (Saccharomyces cerevisiae var. ellipsoides) enzymatically breakdown the fruit sugars (fructose and glucose) first to acetaldehyde and then to alcohol, as illustrated in the figure below. The concentration of alcohol in wine is a function of the amount of sugar in the must and the tolerance of the yeast. The yeast will continue producing alcohol until the concentration inhibits their growth: most strains tolerate up to 12-14 % alcohol. The tastes of wines are due to the variety of grape, the amount of sugar they contain, and the strain of yeast used.
In the production of wine, grapes are crushed to release the juice (must). A pure culture of wine yeast is added as a starter culture and grown in sterilized or pasteurized grape juice. During the initial stages, air is present in the liquid and rapid aerobic growth of the yeast occurs; then as the air is used up, anaerobic conditions develop and alcohol production begins.