How Brewers Measure Alcohol in Beer (Part 7)
Brewers measure the amount of alcohol in beer by comparing the density of the wort (liquid before fermentation) to the density of the beer (liquid after fermentation). The difference between the two is the result of the creation of CO2 and alcohol during the fermentation process.
To measure density, brewers use a measurement known as “specific gravity.” Wikipedia defines specific gravity well, as:
Specific gravity (SG) is a special case of relative density defined as the ratio of the density of a given substance, to the density of water when both substances are at the same temperature. Substances with a specific gravity greater than 1 are more dense than water, and those with a specific gravity of less than 1 are less dense than water.
To measure this, brewers use a hydrometer. A hydrometer, which contains a weight to float upright, works by floating and displacing some of the liquid it its container (usually a cylinder). The level of the surface of the liquid is noted where it falls on the scale of the hydrometer.
A hydrometer in water should read about 1.000, depending on temperature and mineral content. A hydrometer in wort will read higher because of the increased density due to all of the dissolved sugars. A hydrometer in beer, will be higher than 1.000, but lower than the initial reading (original gravity) because some of the sugar has been turned into ethyl alcohol (final gravity).
How Stuff Works goes on to explain the details of how to calculate the amount of alcohol present by weight. I will let you read all about molecular weight and chemical equations of sugar getting split into alcohol and CO2 on your own. For the brewer, the hydrometer not only has the scale for specific gravity, but also includes the scale for percent of potential alcohol by volume, already calculated out for you. (the conversion of alcohol by mass to alcohol by volume is already considered on the potential alcohol scale of a hydrometer) Potential alcohol is the amount of alcohol that would be produced if all of the sugar present in the wort is fermented into alcohol, but not all will be.
For an example using approximations with a custom photo of my hydrometer, a wort with an original specific gravity of 1.069 has the potential alcohol of about 9%. If the beer then has a final gravity of 1.032, measuring a potential alcohol of about 4%, the difference gives us a result of 5% alcohol by volume.
