How does fermentation turn grapes into wine?

Fermentation converts grape juice into wine through the metabolic activity of yeast, primarily Saccharomyces cerevisiae. Yeast consumes the natural sugars in grape juice, mainly glucose and fructose, and breaks them down through glycolysis. This process produces ethanol (alcohol) and carbon dioxide as byproducts, along with small amounts of other compounds that contribute to flavor and aroma. The process begins when yeast, either naturally present on grape skins or added by winemakers, contacts the sugar-rich juice. Temperature, yeast strain, and sugar content all influence the outcome. Fermentation continues until the sugar is exhausted or alcohol levels rise high enough to kill the yeast, typically around 12 to 15 percent alcohol by volume.