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Old 02-07-2012, 03:14 PM   #1
Feb 2012
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What does it mean and how is it done?

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Old 02-07-2012, 03:23 PM   #2
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Gravity, in the context of fermenting alcoholic beverages, refers to the specific gravity, or relative density compared to water, of the wort or must at various stages in the fermentation. The concept is used in brewing and wine making industry. Specific gravity is measured by a hydrometer, pycnometer or oscillating U-tube electronic meter.

The density of a wort is largely dependent on the sugar content of the wort. During alcohol fermentation, yeast converts sugars into carbon dioxide and alcohol. The decline in the sugar content and the presence of ethanol (which is appreciably less dense than water) drop the density of the wort. The percentage of alcohol can be calculated from the difference between the original gravity (abbreviated OG) of the wort and the current specific gravity (abbreviated SG) of wort. 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. If the fermentation is finished, the specific gravity is called the final gravity (abbreviated FG). For example, for a typical strength beer, OG could be 1.050 and FG could be 1.010.

Several different scales have been used for the original gravity. For historical reasons, brewing industry largely uses Plato (°P), which is essentially the same as Brix used by wine industry. For example, OG 1.050 is roughly equivalent to 12°P.

By considering the original gravity, the brewer or vintner obtains an indication as to the probable ultimate alcoholic content of his product. The OE 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. In the Czech Republic, for example, they speak of "10 degree beers", "12 degree beers" which refers to the gravity in Plato of the wort before the fermentation.
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Old 02-08-2012, 04:45 AM   #3
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Revvy has posted the definitive explanation, but to precis, a hydrometer calibrated for brewing. Plus if your fermenter would make it hard to read and/or retrieve it, then a test jar as well. A sanitised turkey baster is good for taking a sample and as long as you sanitise everything before sampling, the you can return the sample to the ferment. I do that routinely, but a lot of the beer brewers might not, as they can be a little more obsessed with hygiene matters - understandably do, apparently, need mega care with beers......
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Old 02-08-2012, 05:11 AM   #4
Apr 2011
Sacramento, CA
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I saw this link posted a while back and bookmarked it. I found it to be extremely useful.

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