The science of alcohol
The science of alcohol

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The science of alcohol

6 Checking your homebrew experiment

If you set up your own homebrew experiment at the start of this course, your brewing should now have been progressing for four weeks, so should be roughly half-way there.

In the following video Danny Allwood demonstrates the use of a hydrometer to take another specific gravity reading from your homebrew.

Download this video clip.Video player: soa_1_w4_s6_vid_homebrew.mp4
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OK. So hopefully by now, your beer should be fermenting away happily. Now, it's just a case of leaving the yeast to do its job. What you'll notice is that the airlock on the top, there'll be a period where there's vigorous gas evolution. That's primary fermentation taking place. That's all of the fermentable sugar being converted into carbon dioxide, which is then leaving through the top of the barrel. So after the primary fermentation is complete and the gas evolution has stopped up here, you can then start to take gravity readings. So this involves using a sterile piece of equipment, usually something like a turkey baster that you've sterilised. You can then go into the bucket, lift the lid off the top, take a sample out, and put it into a trial tube. And then we're going to measure the specific gravity with a hydrometer. So here's a sample I pulled earlier. And if you get your hydrometer and just drop it into the beer, then it will tell you the specific gravity of the beer. And that's a measure of how much sugar is left in the beer. So most beers finish about 1.01 or something like that, depending on how dry or sweet your beer recipe was. But what you're looking for after the gas evolution stopped is consistent readings. Over three or four days, the reading doesn't change. And that's an indication that the fermentation is finished. So the yeast will continue to clean up after themselves and do secondary fermentation and conditioning, but they can also do that in the bottle, because we're going to keep the yeast in the bottles as well. The most important thing is that primary fermentation is finished before you bottle. Because if we produce extra gas during bottling, you can overcarbonate your beers, and there's a chance that they'll froth up everywhere. So it really is a question of patience at this stage. Just leave the yeast to do its job. Keep checking on it. And as soon as it's ready to package, stick it in some bottles.
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This is exactly what you did at the start of your brewing in Week 1, but you may wish to take another specific gravity reading now to compare with the final readings you will take in Week 8 when you calculate the strength of your finished beer.


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