Alcohol is produced by yeast during the process of fermentation. The amount of alcohol in the finished liquid depends on how much sugar there was at the beginning for the yeast to convert into alcohol. In beer, the alcohol is generally 3% to 12% (6 to 24 US proof) and usually about 4% to 6% (8 to 12 US proof). Depending on the strain of yeast, wines top out at about 14% to 16% (28 to 32 US proof), because that is the point in the fermentation process where the alcohol concentration denatures the yeast. Since the 1990s, a few alcohol-tolerant 'superyeast' strains have become commercially available, which can ferment up to 20%. [3]
Fortified wines, such as sweet and dry vermouths become fortified by adding alcohol. These added alcohols are typically distilled wines such as brandy, port, sherry, marsala, and madeira. This second source of alcohol is added before the wines are finished fermenting, as the added alcohol kills the yeast in fermenting wine, leaving an abundance of sugar.
Stronger liquors are distilled after fermentation is complete to increase their alcohol content. This way, the alcohol content can be raised up to a maximum of 95.6% (191 US proof), the concentration of the water/alcohol azeotrope. When 100% ethanol (absolute alcohol) is required for scientific, commercial, or industrial purposes, other methods must be used. 100% ethanol is hygroscopic, and if it is left open, it will absorb water out of the air, reducing the concentration.
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