I just kegged a smoked porter I brewed last week and let ferment for about seven days. Let’s say I wanted to know what the approximate alcohol by volume of this beer is. How do I figure that out? Time to remember your sophomore chemistry.
Equipment needed:
- A brewer’s hydrometer
- A brewer’s thermometer of some type (instant read electronics are plenty accurate.)
- Pad and pencil
NOTE: I use a hydrometer, but there are other methods. A refractometer is also a very accurate measurement tool. I just don’t have one, so I wouldn’t call myself experienced enough with their operation to comfortably explain it here. However, Christmas is coming and here’s a nice link to the Thirsty Brewer where any of you nice readers could purchase one for our mutual edification.
We’re going to measure the specific gravity, or liquid density of the wort.
First thing’s first: You’ve brewed your beer, turned off the burner and begun cooling your wort. When the wort is approximately 60˚ Fahrenheit, collect a sample of your cooled wort in the vessel that accompanies your hydrometer. Fill the cylinder enough to allow the hydrometer to float, but also leave enough room to allow for displacement so there’s no spillover.
You’ll notice that there are graduated numbers on the narrower end of the hydrometer. The higher number at the bottom of the narrow end usually reads 1.170, while the topmost number is .990.
When you drop the hydrometer into your sample, it should float with a certain portion of the graduated narrow end submerged, and a portion revealed.
Just like chemistry class, you’ll want to read the number that is closest to the middle valley of the meniscus, where the liquid ends, and the headspace begins.
This specific gravity reading — 1.047 in the case of my smoked porter — is referred to as the original gravity, or O.G. Ice-T would be proud. Write this number down. I often write the O.G. down on the side of the fermenter with a Sharpie.
Next step: Wait. You’ll want to let the fermenter sit for at least five days, I usually let it go a full week. Then you’ll want to test the gravity again. I recommend using a device called a wine thief to take a sample. This allows you to avoid shaking up your fermenter and disturbing the yeast cake. Thorough brewers will usually take a measurement, then wait a day or two, take another, and if there’s no continued reduction, they’ll consider the beer fully fermented/attenuated.
This last gravity reading is referred to as the final gravity or F.G. — Sorry, no pithy mnemonic device this time. My smoked porter sat at 1.000 today, pretty much fully attenuated, though of course it could go a little lower, and it probably will in the keg.
Last step is to calculate the ABV. The most simple formula is to subtract the F.G. from the O.G. and multiply that number by 131. So in this case, 1.047-1.000 = 0.047 X 131= 6.157 percent approximate alcohol by volume. Not too shabby.
How did we get the 131? It’s complex, and honestly not really worth delving into too much, when we can get a ton of information out of those two gravity numbers at brewcalcs.com. But if you insist — you dork — the same site will explain it all. There is a calculator that will correct your gravity reading if the temperature was higher or lower than the 60˚ standard, approximate alcohol by weight, apparent attenuation, actual attenuation, refractometer conversion, and for those of you watching your figure there’s even approximate calories per 12 ounce serving.
The same readings will work for wine, meads, ciders, etc. Geek out to your heart’s content.
Tags: Brewing, Brewing Science, How do I do that?, How to
[...] the last “How do I do that?” we learned how to use a hydrometer and convert the resultant specific gravity readings to [...]