The natural process that turns grape juice into wine, fermentation is actually a chain reaction of chemical responses. During this process, technically called the primary fermentation, the sugars in the grape juice are converted by the enzymes in yeasts into ALCOHOL (55 to 60 percent) and CARBON DIOXIDE (40 to 45 percent). In addition, fermentation generates minor amounts of numerous incidental by-products that affect the aroma and taste of wine including ACETALDEHYDE, acetic acid (see ACIDS), ETHYL ACETATE, GLYCEROL, and alcohols other than ETHANOL. One of the potential problems winemakers must avoid is a stuck fermentation. This occurs when the yeast stops converting the sugar into alcohol and carbon dioxide, thereby prematurely leaving undesirable RESIDUAL SUGAR in the wine. As more is learned about fermentation, techniques are evolving to manage the process in order to produce optimum wines. For example, managing the temperature during the fermentation—cooler temperatures (45 to 60°F) for white wines, warmer temperatures (70 to 85°F) for heavier red wines—leads to superior wines. Red wines are usually fermented with their skins, seeds, and pulp to extract color and tannins—something not desirable in white wines. YEAST strains are also being experimented with to determine which ones work best for different wines under various conditions. Many winemakers believe BARREL FERMENTATION adds flavor and complexity to some white wines. CARBONIC MACERATION is a specialized fermentation process for producing light fruity red wines. See also BOTTLE FERMENTATION; FERMENTATION CONTAINERS; MALOLACTIC FERMENTATION; SECONDARY FERMENTATION; WHOLE BERRY FERMENTATION.