Getting the right alcohol levels in your ale: Cheers Physics


Uploaded by InstituteofPhysics on 14.12.2012

Transcript:
So we’re going to do our final gravity check on it to see
and then we’ll be able to compare where the gravity was at first
so the original gravity and where the final gravity is
and then we can work out exactly how much alcohol we’ve produced
Rik: So to clarify, the gravity is, what you call specific gravity
Specific gravity, density I guess
Josh: So at this point we’ll just take a sample.
And this is a refractometer.
This is not the most accurate tool
we use hydrometers later
but for this process it makes it really easy,
we don’t waste a lot of stuff
and when you work with the hydrometer you have to calculate what the temperature of the sugar
Rik: because that changes the density?
Right, right. But with a refracometer you don’t have to worry about the temperature.
Rik: Is that because it’s measuring the light?
It’s how the light passes through the sugar.
So you just put a little sample over the glass piece
Push it down to get all the air out of it
so when you look through there
then you’ll see how many degree breaks are on there
It’s about 14.2
So that’s how we measure our beer, we take our starting point
how much sugar have we got at the start and how much sugar have we got at the end
and then the difference we can convert to give us the alcohol by volume.
So first what we need to do is, there’s a temperature adjustment so we need to make sure and measure the temperature first.
This is 7 degrees.
Rik: That’s because the colder the liquid gets, it gets more dense but as it heats up, it gets less dense.
This is a saccharometer from a scale of 10:10 to 10:20.
The yeast basically will eat the sugar and in eating the sugar it will then turn it into alcohol
and carbon dioxide so that’s where our bubbles come from, the carbon dioxide from the natural fermentation process
and so as you can imagine as the yeast turns some of the sugar into alcohol the density will get less and less and less as
Rik: alcohol is less dense than
Andy: Exactly, alcohol is less dense than sugar
4.5% is bang on the alcohol we’re aiming for this beer so we had the right starting gravity,
we had the right final gravity and the beer is consistent with what we’re trying to brew, a 4.5% best bitter.