The science of alcohol
The science of alcohol

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The science of alcohol

1.3 Hydrometry and specific gravity

Just as the early tax collectors did, people still use hydrometers to measure alcohol content today. If you are taking part in the homebrew experiment, you probably used a hydrometer in Week 1. You may well have taken another specific gravity reading (the ratio of the density of the fermenting liquid to the density of water) in Week 4, half way through the brewing experiment.

You can now check if your homebrew is complete by taking a final specific gravity reading as illustrated by Louise MacBrayne in the following video.

Download this video clip.Video player: soa_alcohol_boc_vid004.mp4
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Transcript

LOUISE MACBRAYNE
In order to measure the strength of the homebrew that you've produced, you're going to need to use your hydrometer again. If you recall at the start of your homebrew experiment, you took a reading of the specific gravity of your starting fermentation mixture. And we referred to this reading as the original gravity. In fact, you can use your hydrometer, as well, to determine when your fermentation has actually finished. If you take specific gravity readings of your final fermentation when you think it's finished, if the readings are constant over the period of two days, then that's a pretty good indicator that all of the sugars from your starting mixture have been used and your fermentation is complete. Now, in order to measure how strong your beer actually is that you've produced, you're going to need a sample of it. And you're going to use the hydrometer again to take another specific gravity reading that we refer to as the final gravity. So you're going to take your hydrometer and put it carefully back into a sample of your finished beer. Again, exactly the same as before, you just let the hydrometer settle, and then you take your reading. So you can see now it's just about settled down, and we have a reading of around 1.015. And this is fairly typical of a homebrew. This reading is referred to as the final gravity reading, and it's lower, if you note, than your starting gravity reading. The reason for that is that all of the sugars in your starting fermentation mixture have been used by the yeast in the fermentation reaction to produce carbon dioxide and alcohol. So the mass of this volume of liquid is now lower, and therefore our density and specific gravity readings are now lower. So now you're ready to take your original gravity and your newly measured final gravity reading, and we can work out just how strong a beer you've actually produced.
End transcript
 
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As you saw in Week 1, when the beer is undergoing fermentation the sugars in the liquid are converted by the yeast into alcohol and carbon dioxide. Alcohol in water is less dense than sugar in water and so this will result in a change in the specific gravity – the hydrometer will now sink in the liquid compared to the original gravity. It will now have a specific gravity closer to that of water.

The beer has finished fermentation when the hydrometer reading remains constant over a period of two days and does not decrease any further. This reading is called the ‘final gravity’ or ‘FG’ and is used, in conjunction with the original gravity, to work out the alcohol by volume content of the liquid. For beers the final gravity is around 1.015–1.005.

To convert the final gravity to ABV, a simple calculation is needed, as illustrated in Equation 1.

equation left hand side cap a times cap b times cap v equals right hand side 1.05 divided by 0.79 times open cap o times r times i times g times i times n times a times l postfix times cap g times r times a times v times i times t times y minus cap f times i times n times a times l postfix times cap g times r times a times v times i times t times y divided by cap f times i times n times a times l postfix times cap g times r times a times v times i times t times y close multiplication 100
(Eqn 1)

So, using the example from the video above:

Original gravity before fermentation       = 1.04

Final gravity after fermentation                 = 1.015

equation left hand side cap a times cap b times cap v equals right hand side 1.05 divided by 0.79 times open 1.05 minus 1.015 divided by 1.015 close multiplication 100
equation left hand side cap a times cap b times cap v equals right hand side 3.27 percent

Although this may seem a little on the low side, this is possible for a homebrew!

SOA_1

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