Alcoholic Strength And Measurement in beer is generated as a function of both the quantity of fermentable sugars originally present in beer wort and the extent to which those sugars are actually fermented by yeast. The initial original gravity of wort is a measure of the specific gravity (SG) of the wort at 20°C (where, simplified, water at a standard reference temperature has an SG of 1.0000) and is also known as the original extract. In the brewery this is often measured using a hydrometer. Brewers and winemakers take this as an expression of the sugar content in units of grams of sugar per 100 g of wort. This number is equivalent to a percentage weight/weight. In the brewing industry this measurement is denoted in degrees Plato (°P) and winemakers refer to it as degrees Brix. See balling scale, hydrometer, plato gravity scale, and specific gravity.

As fermentation continues, the sugars in the extract are consumed and the SG of the liquid drops. The brewer monitors the progress of fermentation by following the changing SG (or Plato) value until it reaches a terminal value (the maximum degree of fermentation). However, as sugars are consumed the alcohol content rises and the “extract” in the beer is not read correctly and is regarded as the apparent value (apparent extract). There is a drop in gravity caused by the conversion of the sugars to alcohol, but the gravity also drops because the newly generated alcohol is lighter than water. The real extract (the gravity value not compromised or “obscured” by alcohol) is an important value and can be computed or determined following the careful removal of alcohol from a known amount of the wort/beer. The real extract then represents the true final extract (containing residual sugars and dextrins—more complex carbohydrates, some protein, and the mineral content of the sample) in the beer expressed as grams/100 grams (or percentage terms).

The real degree of fermentation is a measurement of the percentage of the original gravity that was actually fermented, adjusted once again for alcohol content. Not all sugars in the wort will be fermented because wort contains non-fermentable elements, particularly complex sugars. These will be left behind and provide the beer with body and sometimes sweetness. See real degree of fermentation (rdf) and real extract.

The original gravity minus the final gravity (real extract or true final gravity) will produce a value that indicates the amount of fermentable sugars consumed; this, in turn, indicates the amount of alcohol produced (see below). These above values can be used together with equations known to the brewer and brewing chemist to determine the actual exact alcohol strength.

The average strength of beer is between 4.8% and 5.2% alcohol by volume (ABV). However, the range of strength in beer is far wider than that of wine. There are many beers on the market with only 3.5% ABV or below, whereas some beer styles, such as barley wine, commonly reach 12% ABV. Although it is possible to coax beer fermentations to produce beers with more than 20% ABV, these generally require special yeast strains and techniques. In this, they can come to resemble laboratory experiments rather than beers, although some very interesting beers have been produced in this manner. For normal worts and fermentations, alcoholic strength in beer tops out at about 15% ABV, with these beers normally requiring long aging before they become palatable. See aging of beer and ethanol- tolerant yeast strains.

Once alcohol is produced in beer, brewers need to measure it. The analysis of beer for alcohol content is an important part of brewing laboratory work both for quality assurance programs and for legal reporting purposes. Results, however, are subject to appreciable variation and, under official methods, the analyses are time consuming and expensive. The history and theory behind alcohol measurement are lengthy and complex and could not be presented in any appreciable detail here, but it is possible to explore the methods by which today’s brewing chemists best determine alcohol content in beer.

The production of 1 g of alcohol requires 2.0665 g of fermentable extract. (As originally determined, 2.0665 g sugar yields 1 g ethanol, 0.9565 g CO2, and 0.11 g yeast. Note: 0.9565 g and 0.11 g add up to a sum of 1.0665 extract not converted to alcohol.)

Predictions of alcoholic strength in beer can be made based on these numbers, but they will be approximate. For official and accurate determinations, the alcoholic strength of a beer was historically measured or originally reported in percentage alcohol by weight in the United States, with most of the rest of the world preferring its volumetric content (percentage by volume). As of 2011, the United States allowed reporting by volume for labeling and certification purposes. Alcohol by volume is a measure of alcohol content of a solution in terms of the percentage volume of alcohol per 100 mL total volume of beer (volume/volume). Alcohol by weight is a measure of alcohol content of a solution in terms of the percentage weight of alcohol per weight of beer (weight/weight, also expressed by mass as mass/mass in Europe).

Traditionally most brewers would use a carefully calibrated hydrometer to determine the alcohol content in their beer. Established tables of SG versus alcohol data were then used to compute the alcohol content. Distillations performed on precisely known volumes (or weights) of alcohol added to water led to the generation of an extensive set of tables of data by various agencies and academic laboratories, showing the interrelationships among specific gravity, density, and alcohol by weight and volume. These tables and formulas are now used by brewers and brewing chemists to accurately determine the alcohol content of beer.

Finally, it is to be noted that alcohol may be measured via the hydrometer, distillation, reference to alcohol and solute concentration tables, and sophisticated oscillating U-tube density meters; even refractometers are sometimes used with appropriate algorithms. Near-infrared instruments can measure the specific alcohol peak in a mixture, as can gas chromatography, which is now an approved method for the determination of the concentration of ethanol.See also chromatography.