Boiling method

Jul 25, 2004 2 Replies

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K.J.Kristiansen 21 years ago

I am trying to determine the alcohol content in my wine using this method

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250 mL original SG=1,000 boiling down to half the volume and reconstitute to its original volume with water. Final SG=1,017.

On the reference web page the alcohol content shall be calculated like this:

SG(final)-SG(initial)/0,211*100=8,05 vol%.

On another web page

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there is a table saying that the alcohol vol% for the SG difference of

17 (actually 0,017 converted to "Spirit Indication by multiplying with
1000)should be 13,2 vol%. This table is also quoted on rec.crafts.winemaking FAQ (http://216.162.125/winemaking_faq.html). The alcohol % derived from the tables looks much closer to the expected value going by must initial gravity.

However, when I try to set up the equations for calculating the vol% alcohol I come to the same equation as the first one quoted and calculate 8,05% !!!

Does someone know which is the right calculation method and why there are two different methods around for the same procedures? And does somebody know how the numbers in the tables are calculated?

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Ben Rotter 21 years ago

The first calculation (from Monash Scientific) looks right to me.

I derive it as follows (and get the same answer as yourself and the Monash page):

The SG of the unboiled wine, SG1 = (Vw/Vt*Pw + Va/Vt*Pa)/Pw where: Vw = volume of water in sample Va = volume of alcohol in sample Vt = total volume of sample = Vw + Va Pw = density of water Pa = density of alcohol

When the sample is distilled, the alcohol will be driven off. This will leave the original water content but the Vt of this distilled sample will now be equal to Vw. (This assumes no reconstitution with water of course, which is different to the experiment, but the concept for calculation purposes is correct). Thus,

The SG of the distilled sample, SG2 = (Pw*Vw/(Vt-Va))/Pw

Skipping all the algebra in between leads to:

SG1 - SG2 = Va * (Pa/(Vt*Pw) - 1/Vt)

So Va = (SG1-SG2) / (Pa/(Vt*Pw) - 1/Vt)

In your case (units are litres & kilograms), Va = -0.017 / ((0.789 /

1*0.25) - 1/0.25)) = 0.020014 L

The alcohol content = Va/Vt * 100 = 0.02014/0.25*100 = 8.05 %

I don't know why the second reference

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is different, but I'm not sure I'd trust it.

Are you sure your distillation setup and procedure are correct?

Ben

Improved Winemaking

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K.J.Kristiansen 21 years ago

Yes, the destillasjon setup should be OK. Actually it is very simple since this method does not require any analysis of the distillate. Maybe I should try the full distillation method just to be sure. I have also done a Rayleigh (single pot) distillation calculation to verify that the remaining alcohol is negligible. And this was confirmed (less than 0.05 vol% before volume adjustment). I agree that I should use the calculation method which comes out of the analysis (yours and mine)until someone points to any error in them. But it is strange that this other correlation is around on several web pages. I have tried to send these people a mail asking them how it is derived or from where their source is. But no replies so far.

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