In scientific terms, alcohol is the term for any one of a group of organic chemical compounds. Alcohols, specifically ethanol, are what makes fermented beverages intoxicating. The abbreviation ABV (or abv) means '(percent of) alcohol by volume'.
Types of alcohol
There are many known alcohols, but only a few that will ever concern the home fermenter:
Ethyl alcohol, or ethanol, is what people almost always mean when they say "alcohol". It's the substance in fermented beverages, created by yeast, that causes intoxication in humans.
Methyl alcohol, or methanol, is similar in chemical composition to ethanol, but very different in effect. While the body breaks down ethyl alcohol into harmless substances, the results of breaking down methyl alcohol are highly toxic formic acid and formaldehyde. Drinking methyl alcohol in significant amounts can cause nervous system damage, including blindness, and eventually death.
Methanol is present in small amounts in most fermented beverages; it is created primarily by the fermentation of pectin in wine production. However, in these amounts it is not a significant health risk, and there is no way a home fermenter can create dangerous levels of methanol during fermentation through contamination or any other means.
Methanol is concentrated along with ethanol during distillation. However, even in distillation the health risks are small and easily dealt with by removing the "distillation head", which contains most of the methanol. Stories about people going blind during Prohibition were common not because home distillation was inherently dangerous, but because unscrupulous illegal producers sometimes "cut" their product with methanol or methylated spirits to increase their profits.
Denatured alcohol refers to ethyl alcohol intended for industrial use that has been treated to make it unsuitable as a beverage; this allows the manufacturer to avoid the tax on alcoholic beverages. Alcohol may be denatured either by adding a strong, unpleasant flavorant, usually a bitterant. It may also be denatured by adding a small but dangerous amount of methanol; the resulting undrinkable alcohol is called methylated spirits.
There are many other forms of alcohol, but none of them are likely to appear in fermented beverages.
Expressing alcohol content
There are a number of ways of expressing the alcohol content of a fermented beverage.
Alcohol by volume and weight
The most commonly used measure of alcohol among home fermenters is alcohol by volume', abbreviated abv. This reflects the percentage of the volume of liquid that consists of alcohol. Some commercial brewers measure alcohol by weight instead; for the same beverage, the measurement of alcohol by weight will always be slightly lower than the measurement by volume.
For stronger drinks, alcohol is usually measured by proof. A proof is roughly twice the amount of alcohol by volume; thus a 15% Barley Wine would be considered 30 proof.
Calculating alcohol content
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The most common method used by home brewers to calculate the alcohol content of a finished beverage is to calculate a likely alcohol level based on the specific gravity before and after fermentation.