Tuesday, April 23, 2013

Measuring the alcohol content of homebrew wine

A little over a week ago I started a batch of 3 week wine (my first in about 15 years).  According to the instructions (Cabernet Merlot "California Connoisseur" brand)  most of the fermentation takes place in the first week. But I was surprised that all airlock activity ceased after day 6. I really thought the airlock bubbling would decay exponentially over the 21 days or so.

Checking a sample with a hydrometer the specific gravity was 0.992 which is well in the range of a fully fermented wine. It also smelled and tasted like wine, and after drinking the 100ml sample... felt like fully fermented wine also!

However to gauge alcohol content I needed the pre-fermentation specific gravity reading, which I never bothered to take.

Instead I came across an alternative method:

  1. Check that the specific gravity of your tap water is exactly 1.000. If not you will need distilled or rain water.
  2. Take a small sample of wine (I used a 100ml graduated cylinder) and measure the specific gravity, SG1. This was 0.992 ± 0.001 in my case.
  3. Then transfer to a pot and boil off about half of the liquid. This ensures that all the alcohol is boiled off. I noticed a distinct change in odor after a few minutes of boiling... from a strong alcohol smell to a sweet odor.
  4. Allow to cool (hydrometers are normally calibrated for room temperature)
  5. Pour back all the liquid back into the graduated cylinder. In my case this amounted to 45ml.
  6. Top up with water back to the original 100ml mark.
  7. Measure the new specific gravity, SG2. In my case it was 1.008 ± 0.001.
To calculate alcohol content, use the following formula [1] :

SI = (SG2 – SG1)*1000

%ABV = (0.008032927443 * SI^2) + (0.6398537044 * SI) – 0.001184667159

So in my case that's SI = 16 ± 2, %ABV = 12.3% ± 1.8% which is a reassuring result, except the error bar is very large. Must see if I can get myself a narrow range hydrometer and repeat with more accurate specific gravity measurements.

Now, here is the interesting bit for me: where does this formula come from? The source at [1] is vague on that. When I try to derive a formula from first principles, I get %ABV = 100*(SG2-SG1)/0.211 which is also mentioned in [1] and said to be wrong. I'm also suspicious of the large number of significant digits of the coefficients. I'm wondering is this just a curve fit of the table in [2]?

If anyone has any insights, please post a comment below.

Update: after some more googling, I see that my attempt at deriving a formula from first principles was assuming that say 80ml water + 20ml ethanol would result in 100ml of liquid. This is not the case with water + ethanol. This may explain the quadratic term.