A type of single-celled fungus, yeasts play a vital role not in brewing and winemaking. According to biologists Luisa Alba-Lois and Claudia Segal-Kischinevzky, the yeasts used by ancient cultures to make alcoholic drinks like wine may have been transferred accidentally from yeasts present in the environment, such as fungal infections on the feet of people pressing grapes. Today winemakers use specialized strains of yeast to create the specific qualities they want in their wines.

To survive, yeast needs sugar and other nutrients; all or most of these can be found in fruit juice. The yeast breaks down the sugar present in the juice and converts it to energy in a process called fermentation. As a byproduct of this process, the yeast generates both alcohol and carbon dioxide. A similar process occurs in baking; carbon dioxide from yeast is what makes dough rise. In sparkling wines, the carbon dioxide bubbles generated by the yeast are retained. In non-sparkling wines, however, they are allowed to dissipate.

Yeast in fruit juice continues to convert sugars into alcohol as long as they can. This process can stop naturally when the alcohol content of the solution becomes high enough to kill the yeast or when the sugar in the juice run outs, leaving the yeast without a source of food. A drop in temperature can also stop the fermentation process naturally. The brewer or winemaker at times adds chemicals to stop fermentation artificially to achieve a desired flavor.

Although the process of fermentation is simple, alcohol producers have developed a huge range of variations. Different strains of yeast are bred to survive at different levels of alcohol, while different combinations of juice and other materials (such as the grape skins that give red wine its color) produce different flavors and qualities. Adding sugar during fermentation, a process called chaptalization, can let fermentation go on longer, increasing the alcohol content of the resulting beverage.