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:
- Check that the specific gravity of your tap water is exactly 1.000. If not you will need distilled or rain water.
- 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.
- 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.
- Allow to cool (hydrometers are normally calibrated for room temperature)
- Pour back all the liquid back into the graduated cylinder. In my case this amounted to 45ml.
- Top up with water back to the original 100ml mark.
- 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]?
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.
