Alcohol is a term applied to a large group of hydroxyl derivatives of paraffin hydrocarbons. They contain a hydroxyl (OH) group in place of one of the hydrocarbon's hydrogens. Examples of common alcohols include methanol, ethanol, isopropanol, butanol, vinyl, cetyl, amyl, and lauryl alcohol. Although many of the alcohols have important commercial or industrial applications, most biological interest is focused on ethanol, a product of biological fermentation. Ethanol is sometimes called ethyl alcohol, grain alcohol, or simply alcohol. It is the intoxicating agent found in wine, beer, and other fermented and distilled liquors and has the molecular formula C₂H₅OH. It has been produced for millennia, mostly by fermentation of fruit juices. Primitive humans discovered that the fermented juice could be stored in seal containers for extended periods of time, and provide a safe drink when other fluids were not readily available. Grapes and many other natural products can provide the sugars and starches needed for the production of alcohol beverages, although much of the present day industry depends on grains such as corn, wheat, rye, and barley. Ethanol production from grains involves conversion of grain starches to sugars that are then converted to ethanol with a complex of enzymes called zymase. The ethanol content of fermented beverages is limited to about 12% by the inability of the biological fermenting system to tolerate higher concentrations of alcohol. Distillation processes are used to further concentrate ethanol in beverages such as brandy, whiskey, and liquors.