Gravity (alcoholic beverage)
Gravity, in the context of fermenting alcoholic beverages, refers to the specific gravity (abbreviated SG), or relative density compared to water, of the wort or must at various stages in the fermentation.
Specific gravity is measured by a hydrometer, refractometer, pycnometer or oscillating U-tube electronic meter.
By monitoring the decline in SG over time the brewer obtains information about the health and progress of the fermentation and determines that it is complete when gravity stops declining.
For example, for a typical strength beer, original gravity (abbreviated OG) could be 1.050 and FG could be 1.010.
By considering the original gravity, the brewer or vintner obtains an indication as to the probable ultimate alcoholic content of their product.
The OE (Original Extract) is often referred to as the "size" of the beer and is, in Europe, often printed on the label as Stammwürze or sometimes just as a percent.
The bigger the difference, the greater the amount of alcohol present and hence the stronger the beer.
The pressure is always considered (in brewing) to be 1 standard atmosphere (1,013.25 hPa) and the temperature is usually 20 °C (68 °F) for both sample and water but in some parts of the world different temperatures may be used and there are hydrometers sold calibrated to, for example, 16 °C (60 °F).
Mathematically This formula gives the true specific gravity i.e. based on densities.
True readings are easily obtained from apparent readings by However, the ASBC table uses apparent specific gravities, so many electronic density meters will produce the correct °P numbers automatically.
, is the °P obtained by inserting the FG into the formulas or tables in the Plato scale article.
The use of "apparent" here is not to be confused with the use of that term to describe specific gravity readings which have not been corrected for the effects of air.
The amount of extract which was not converted to yeast biomass, carbon dioxide or ethanol can be estimated by removing the alcohol from beer which has been degassed and clarified by filtration or other means.
This is often done as part of a distillation in which the alcohol is collected for quantitative analysis but can also be done by evaporation in a water bath.
If the residue is made back up to the original volume of beer which was subject to the evaporation process, the specific gravity of that reconstituted beer measured and converted to Plato using the tables and formulas in the Plato article then the TE is See the Plato article for details.
but they can be calculated simply from p This formula is fine for those who wish to go to the trouble to compute TE (whose real value lies in determining attenuation) which is only a small fraction of brewers.
This lies in Tabarie's Principle[1]: 428 which states that the depression of specific gravity in beer to which ethanol is added is the same as the depression of water to which an equal amount of alcohol (on a w/w basis) has been added.
Inserting this into the alcohol formula the result, after rearrangement, is Which can be solved, albeit iteratively, for
Most brewers and consumers are used to having alcohol content reported by volume (ABV) rather than weight.
corresponds to an OE of 12 °P which is 0.4187, and 1.010 can be taken as a typical FG then this simplifies to With typical values of 1.050 and 1.010 for, respectively, OG and FG this simplified formula gives an ABV of 5.31% as opposed to 5.23% for the more accurate formula.
Formulas for alcohol similar to this last simple one abound in the brewing literature and are very popular among home brewers.
Formulas such as this one make it possible to mark hydrometers with "potential alcohol" scales based on the assumption that the FG will be close to 1 which is more likely to be the case in wine making than in brewing and it is to vintners that these are usually sold.
The drop in extract during the fermentation divided by the OE represents the percentage of sugar which has been consumed.
The real degree of attenuation (RDF) is based on TE and the apparent degree of fermentation (ADF) is based on AE Because of the near linear relationship between (SG − 1) and °P specific gravities can be used in the ADF formula as shown.
A much more accurate (mean average error less than 0.02°P) conversion can be made using the following formula:[2] where the specific gravity is to be measured at a temperature of T = 20 °C.
Software tools are available to brewers to convert between the various units of measurement and to adjust mash ingredients and schedules to meet target values.
The resulting data can be exchanged via BeerXML to other brewers to facilitate accurate replication.
