No, there’s nothing wrong with the Earth’s gravity, I’m only referring to Specific Gravity, the ratio of the density of a liquid to the density of water. In this case, the specific gravity I’m concerned with is the specific gravity of my two fermenting batches of beer.
The alcohol content of beer can be calculated based on two measurements of the beer’s specific gravity. The first, called Original Gravity (OG) is measured before fermentation starts. The last, called Final Gravity (FG) is generally taken at bottling or kegging time. Between these a brewer can of course take interim measurements. The reason for the change in specific gravity is that the amount of dissolved sugar in the beer at the start of fermentation makes for a higher density liquid than water, but as the yeast converts sugar into low-density alcohol, the density of the beer drops.
Basically, if the alcohol content at the start of fermentation is zero, then as the yeast creates alcohol the density of the beer drops accordingly, and a measurement of density makes it possible to estimate how much alcohol is present.
The alcohol content is estimated based on the difference between the original and final gravity measurements. I typically use software or a lookup table to estimate the alcohol content by volume (ABV) of my beer. The equations most software use are empirical and therefore approximate. I know of two, one simpler, and one more complex1.
ABV = (OG - FG) * 131.25
ABV = (76.08 * (OG - FG) / (1.775 - OG)) * (FG / 0.794)
Right now I have two 5 gallon batches fermenting. One is a Belgian Strong Golden Ale, which spent 3 weeks in the first fermenter, and has been in the second fermenter for about 3 weeks so far. The target time for secondary fermentation for this beer is approximately 2 months, so I’m still 5-6 weeks out from bottling unless I decide to bottle early.
The second batch is a simpler American-style (really, California-style) pale ale which is less complex. I expect to only do a single stage 4 to 5 week fermentation. This beer has been fermenting for 2 weeks so far.
Tonight I took interim gravity measurements of both beers to estimate how far along in the brewing process they are, and approximately how strong they will be when ready to drink. So far things appear to be going well, and I’m getting the itch to bottle these brews so I can start another.
Belgian Strong Golden Ale
Original Gravity (OG): 1.072
Gravity at transfer to secondary fermenter: 1.014
Gravity on April 14: 1.010
By the first equation, the percent alcohol by volume when I transferred it to the secondary fermenter was approximately 7.6%. By the second equation it was estimated at approximately 8.0%. As of today, the specific gravity has dropped further, to 1.010. By the first equation the beer is estimated to be about 8.1% alcohol, and by the second about 8.5%. Either way, this looks like it will be a very strong beer, and since it is so golden and used a Belgian yeast, I think Belgian Strong Golden is an apt description.
As part of the measuring the gravity of my beer I have to extract a sample. Some people sanitize all their equipment and return the sample to the fermenter, but I tend to only sanitize the thief2 used to extract the sample, and then drink it to gage how the flavor of the beer is developing. As of today, the Belgian ale is coming along nicely; it is pleasantly sweet with no off flavors. I’ll let it continue in the fermenter for a while longer, but I think it would be fine to bottle this before too long.
Original Gravity (OG): 1.048
Gravity on April 14: 1.008
By the first equation, the percent alcohol of my pale ale is estimated to be about 5.3 percent. By the second equation, the alcohol content is also estimated to be about 5.3 percent. The beer is a very pretty pale golden, about what would be expected of the style.
As the fermentation is only two weeks in, it is expected that the beer is still a little rough around the edges. The sample I extracted for the measurements tasted generally good, but a bit watery and yet alcoholic. This could be a byproduct of higher than desired fermentation temperatures in the first two days of fermentation, but it may very well just be a young beer that needs a little more time to condition.
Right now I plan to bottle this brew at the end of April, and I’ll sample it again at that time. If the flavor seems good then, I’ll call it ready. If not, I’ll give it a couple more weeks.
A beer (or wine) thief. Basically a simple plastic tube to extract a sample of liquid from a fermenter. Some are based on creating a vacuum above the sample like mine (you stick it into the beer and let it fill, then cover a hole with your thumb like a straw), while others use a simple gravity based one-way valve. ↩
Posted on 14 April 2012