Fermentation is what turns ordinary grape juice into wine. It's a completely natural process in which yeast—one-celled microorganisms that are naturally present on the skin of grapes—convert the grape's sugars into alcohol and carbon dioxide. The process can take anywhere from a few days to several months, during which time carbon dioxide and heat are released as a by-product. For dry wines, fermentation ends when all the sugar is converted into alcohol. For sweeter wines such as port, the vintner will halt the process prematurely by adding more alcohol in the form of a neutral grape brandy, which kills the yeast at levels above 16% and allows some sugars to remain unfermented. Fermentation was traditionally done in wooden vats, barrels, or neutral tanks. Today, for wines that do not require barrel fermentation, many wineries use huge stainless steel tanks that allow precise temperature control. See also carbonic maceration and malolactic fermentation.