Beer Science 101: Measuring Alcohol Content

measure how much alcohol is beer

Contrary to popular belief, the amount of alcohol in beer has nothing to do with its color. Brewers have several tools for measuring the alcohol content in beer as well as some advanced math to make sure they get it right.

Ethyl alcohol, or ethanol, helps fuel most of the cars in the United States. Ethanol also fuels your brew. Beer ran on ethanol long before your car did — at least five and perhaps nine solid millennia before. That said, although gas is cheaper than gose, don’t get any ideas, kids.

Ethanol, grain alcohol, pure alcohol and drinking alcohol are but different terms for ethyl alcohol, one of the primary byproducts of the fermentation of sugar. It has no color, it’s flammable and, to the delight of countless throughout recorded history, it boasts a rather impressive psychoactive capacity. 

How Much Alcohol is in Beer?

The amount of alcohol in beer varies according to the variety of the beer and decisions made during the brewing process. Modern brewers rely on tools developed in the field of chemistry to achieve their goals and provide us with pints consistent with the characteristics of their category and, too, with their own production from batch to batch.

Tools used to measure the alcohol content of beer, for example, will help ensure that your India Pale Ale doesn’t develop alcohol levels more suited to an Imperial IPA, and thus compromise the balance of the brew; tools borrowed from the chemist also ensure that the next bottle of IPA from a given brand will taste and feel the same as the one you enjoyed yesterday.

Measuring Specific, Original, and Terminal Gravity

Specific Gravity is the ratio of the density of one substance compared to the density of a reference substance. Pure water is the normal reference substance, assigned a specific gravity of 1.00 at 15.5C (60F). The simplest method for measuring Specific Gravity is to use a hydrometer.

A brewer is interested in Specific Gravity as well as Original Gravity, which is the measure of Specific Gravity taken on the wort prior to fermentation. Calculating Original Gravity reveals the level of sugar dissolved in the wort before fermentation. The brewer, then, knows how much sugar is available for conversion into alcohol during fermentation, and he also has a rough idea of how much alcohol the final brew will contain, although several other variables, such as temperature, can influence the amount of alcohol produced, too.

Specific gravity is measured post fermentation, too. Brewers refer to this as Terminal Gravity. Ethanol is always less dense than sugared water, such as the wort for beer. Terminal Gravity, therefore, will always be lower than Original Gravity, because Terminal Gravity measures a liquid that contains ethanol (which is less dense than water) and less sugar. The difference between Terminal Gravity and Original Gravity gives the brewer enough information to estimate the quantity of alcohol created as the maltose was converted into ethanol and carbon dioxide during fermentation.

Alcohol by Volume

Once the brewer knows his Original and Terminal Gravity, he can calculate the amount of alcohol in the beer he’s made. It’s a matter of arithmetic.

C.J.J. Berry developed the simplest method:
Alcohol by Volume = (Original Gravity _ Terminal Gravity) / 7.36

A sample calculation for beer will look like the following, based on the assumptions that 1.05= the amount of CO2 produced for every gram of ethanol produced and .79 is the density of ethanol alcohol:

ABV = ((1.05 * (Original Gravity – Terminal Gravity)) / Terminal Gravity) / 0.79 * 100

Why would a brewer be busy with hydrometers and pycnometers and calculators? Checking the decline in Specific Gravity during the brewing process gives the brewer information regarding the status of the fermentation. When the gravity stops declining, for example, fermentation has ceased, and it is time for the next phase of activity. If the gravity has declined too slowly, perhaps the yeast was inferior. To achieve his flavor goals, the brewer may feel inclined to permit a level of residual sugar in the wort.

Alcohol Content in Beer is Important for Other Reasons

Ethyl alcohol is not just for kicks in beer — its production triggers numerous other events chemical events that can influence the flavor, aroma, and texture of the brew, and beer makers keep a close eye on alcohol levels as a tool by which they can gauge the health of qualities of their work.

image: Alcohol by Volume Glass from ThinkGeek

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