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I tested 'brewing' a cider for the first time a few weeks ago. Since this process was so extremely different from what I am used to with brewing beers I completely forgot to take an OG reading. To be honest the amount of alcohol in the brew usually doesn't matter but since the first question from people always is "How strong is it!?" I thought it might be nice to know.

This is a pear cider where the pears are 'centrifuged', liquid is then heated, cooled and then topped off with english cider yeast.

My first thought was to make a new smaller batch and test this, but don't have any more of the same types of pears (from own tree).

Is there any way to calculate the ABV without having taken the OG reading?

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I would think no, not with some kind of laboratory analysis of a sample, but maybe someone here will have an idea. –  Graham Oct 28 '13 at 12:12
For the curious, here's some details of the lab analysis - themadfermentationist.com/2009/07/… –  mdma Oct 28 '13 at 12:19
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2 Answers

up vote 3 down vote accepted

The combination of hydrometer and refractometer readings can be used to estimate the ABV % of a finished fermentation. See the section titled Measurement of ABV in this BYO article.

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Nice - I will try this with a few beers I have to see how the numbers stack up against what I know from measured OG/FG. The Brix2 and Brix3 in the formula is presumably Brix^2 (Brix*Brix) and Brix^3. Would be great if you could show a worked example. –  mdma Oct 28 '13 at 20:52
To be honest, I never do the math by hand. Brew Smith has a calculator for this. Here's an online version. It must use a different formula than Beer Smith as the output is slightly different. –  Tobias Patton Oct 29 '13 at 1:12
Nice! I will probably use this, getting the exact freezing point is a bit of a hassle. –  Sander Oct 29 '13 at 15:18
I'd assumed that Brix2 was Brix^2, etc. Why else have them as separate terms? But looking at the Javascript code behind the Northern Brewer calculator, they don't use any powers. Looks like a mistake to me. I can't see the code for the calculator I linked to, above, since the calculation is done in a post-back. Here's what I entered in Excel to do the calculation. B1 is Brix. B2 is gravity: =(277.8851 - 277.4*B2 + 0.9956 *B1 +0.00523 * B1*B1 + 0.000013*B1*B1*B1) * (B2/0.79). –  Tobias Patton Oct 29 '13 at 22:12
I have now tried this feature with great success! Or at least I got a value from it, a nice pear cider of 5.6%. Thanks very much! –  Sander Nov 10 '13 at 20:53
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Alcohol (ethanol) freezes at -114°F (-84°C) so you could freeze-distill the beer which will freeze the water, but not the alcohol, so you can separate out the alcohol and measure that. Although I believe you have to do this slowly for the alcohol to separate out, so I don't think it's workable in practice.

The freezing point of beer is related to the abv. There's a calculator here that computes abv from OG and FG and also shows the freezing point. Now, you don't know the OG, but you can measure the FG (degass cider by shaking and measure), and you can measure the freezing point.

You can then enter the FG into the calculator, and guess different values of the OG until the freezing point matches what you measured for your beer. That will then give you an estimate for the OG and the abv.

It's not going to be very accurate, but better than a wild guess.

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Very nice! I have read about different methods of trying to boil off the alcohol (ethanol) but with difficulties since the boiling point of alcohol and water are relatively close. Calculating the OG by having freexing point and FG was a very clever idea and I will try to see if I can test this in theory! –  Sander Oct 28 '13 at 12:43
Here's also a table listing abv against freezing point. I see you're new here - if you like an answer you can upvote it. Later after collecting hopefully several answers, you then choose the one that you feel best answers your question. –  mdma Oct 28 '13 at 12:46
The link does not seem to be working... Thanks! I will have to wait for 15 rep first before I can upvote :) –  Sander Oct 28 '13 at 13:48
Fixed the link - strange you cannot upvote answers to your own question. –  mdma Oct 28 '13 at 14:26
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