Brewers, especially home brewers, tend to talk a lot about gravity. Not the type of gravity that keeps us firmly planted on the Earth, but the gravity of their beer recipe. This includes the beer’s specific gravity, original gravity, and final gravity.
Brewers measure gravity throughout the brewing process to estimate how many sugars can be converted to alcohol. In the end, these calculations are used to determine the beer’s final strength (its alcohol by volume or ABV).
Gravity measurements, or ‘specific gravity readings’ are made at key points throughout the wort-making and fermentation process; prior to boiling the wort (“pre-boil gravity”), immediately prior to pitching the yeast (“original gravity”), and post-fermentation (“final gravity”). These readings are usually taken using a hydrometer or refractometer.
Sources: Oxford Companion to Beer, from Craft Beer & Brewing
Original Gravity (OG), sometimes called original extract, is a measure of the solids content originally in the wort, before alcoholic fermentation has commenced to produce the beer. OG is one of the major measurements used by brewers to determine the future alcohol content of a beer fermented from a particular wort. It is measured by a saccharometer, hydrometer, or refractometer as the density of the wort at standard temperature and pressure (STP; usually 20°C and 760 mm) at the final collection point before the yeast is added. Original gravity is expressed as the density above that of distilled water and in the UK is called the excess gravity. Water is deemed to have a density at STP of 1.000. If the wort density is 1.048, it will have 48° of excess gravity and an OG of 48.
Specific Gravity, sometimes called present gravity, is the density of beer or wort at standard temperature and pressure (20°C, 760 mm Hg) measured by saccharometer, hydrometer, or refractometer. A more accurate laboratory method is by weight using a specific gravity bottle. The units of specific gravity follow the practice of the country for original gravity, namely excess gravity, degrees Plato, degrees Balling, etc. In the case of unfermented wort, the specific gravity is the same as the original gravity. For fermenting wort or beer it is more correctly the apparent specific gravity that is measured by a saccharometer because the alcohol being lighter than water reduces the actual reading given by the floating bulb.
Apparent Extract is a direct measurement of the dissolved solids in brewers wort, gauged according to specific gravity. See specific gravity. A substantial portion of the dissolved solids will be removed from the wort during fermentation by the action of yeast, so the progress of the fermentation may be monitored by measuring the disappearance of these solids. However, the measuring method compares the weight of the dissolved solids with that of water. As the fermentation progresses the solute is no longer pure water but is a mixture of water and alcohol.
Alcohol has a specific gravity substantially lower than water. This means the “true” or “real” extract is greater than that measured directly, hence the term “apparent extract.”
Attenuation involves the removal of sugars and production of alcohol by yeast during fermentation such that the wort becomes less dense and viscous and is thus thinned-out or attenuated. The use of the hydrometer or saccharometer for measurement of attenuation during a fall in specific gravity of fermenting wort was introduced to brewing in late 18th-century England. Determination of temperature, brought in a few years earlier, and gravity measurement were the first quantitative process control measures made available to the brewer.
The extent of attenuation achieved in fermentation of wort to beer is often given as the difference between the starting gravity of the unfermented wort and the gravity of the finished beer, expressed as a percentage of the starting gravity. Thus, if the wort has a starting gravity of 15°Plato and the finished beer a gravity of 3°Plato, then the percent attenuation would be (15 – 3)/15 x 100 = 80%.
Because alcohol produced during fermentation has a lower density then that of water, measurement of the beer gravity overestimates the extent of sugar removal from the wort and the calculation above yields what is termed the “apparent attenuation.” To determine the “real attenuation,” it is necessary to remove the alcohol by distillation before measuring the beer gravity and use this figure in the determination of percent attenuation of the wort. Real attenuation is about 80% of the measured apparent attenuation in normal circumstances.
Real Extract (RE) is a precise calculation concerning the gravity of beer. It is related to a boiled wort’s gravity, a fermented wort’s attenuation, and a finished beer’s alcohol level. The specific gravity of wort—its density—is greater than that of water, which sets the mark at 1.000 because of the presence of sugars in solution. Gravity is commonly measured in terms of original gravity or degrees Plato (°P). A standard beer may have an original gravity of 1.048 or 12°P. As beer is being fermented—or attenuated—its gravity decreases because sugars are converted into carbon dioxide gas, which largely escapes, and alcohol, mostly ethanol, which mostly stays in the beer.
Attenuation is thus the extent to which wort sugar has been converted to alcohol. If a beer has plenty of residual sugar after fermentation, it has a lower attenuation value than a beer from the same-gravity wort with next to no residual sugars. If the difference in gravity between the starting wort and the finished beer is measured using a hydrometer, this attenuation value is misleading, because the wort gravity is measured based on sugar dissolved in a reference liquid of water only. The beer gravity is measured based on residual sugars dissolved in a reference liquid that is both water and alcohol. This is significant, because alcohol has a lower density (or gravity) than water. Thus, the difference between the hydrometer readings of wort gravity and of finished beer gravity lead to a value called apparent attenuation—“apparent” rather than actual, because it contains a small error.
Apparent Extract is a direct measurement of the dissolved solids in brewers wort, gauged according to specific gravity. See specific gravity. A substantial portion of the dissolved solids will be removed from the wort during fermentation by the action of yeast, so the progress of the fermentation may be monitored by measuring the disappearance of these solids. However, the measuring method compares the weight of the dissolved solids with that of water. As the fermentation progresses the solute is no longer pure water but is a mixture of water and alcohol.
Alcohol has a specific gravity substantially lower than water. This means the “true” or “real” extract is greater than that measured directly, hence the term “apparent extract.”
Real Degree Of Fermentation (Rdf) measures the degree to which sugar in wort has been fermented into alcohol in beer, defined as “attenuation.” A sweet beer has more residual sugar and lower attenuation. The RDF expresses the percentage of extract that was fermented. RDFs in the 50s represent full-bodied beers with over 40% of their original extract left unfermented, whereas RDFs in the 80s represent highly attenuated beers with less than 20% of their original extract unfermented. Mouthfeel is largely determined by RDF percentage; the higher the RDF percentage, the lighter and drier the beer. Conversely, a beer with a lower RDF percentage may have a round and even syrupy mouthfeel. The term “real” in this instance separates this measurement from those that are “apparent.” Because alcohol is lighter than water, a sample of beer may “appear,” when a hydrometer is used, to have lower residual sugar than it actually does.