Potential Alcohol in Wine
It’s winemaking season, and that can mean only one thing: Fussing over the actual potential alcohol (PA) in your must. Oh, why do we need to pull our hair out on such trivial matters?
I see debates and discussions on winemaking chat and discussion forums where folks argue points about why the different versions of Brix/SG-to-PA conversion tables, and getting really frustrated over whether the must measures 25.5 Brix (B°) vs 25.0 B°.
Let me just start by saying that you should use your hydrometer as your tool to determine Brix/SG and PA and to guide your winemaking.
Make sure to properly clarify the must before taking a reading, and also make sure that the measurement is taken at the hydrometer’s calibration temperature. Then, if the Brix/SG and PA are within your desired range, you’re all set to go.
Now whether your PA is 13.4% alc/vol or 13.1% really doesn’t matter because your actual, final alcohol content can be very different from your measured PA. The final alcohol content should be measured by ebulliometry if you want to know with some accuracy what the alcohol content is; don’t assume that if you measured 13.4% PA and your wine has fermented dry that you, in fact, have 13.4%.
Let’s look at what happens during fermentation and do some calculations based on the knowledge of the rate of sugar conversion into alcohol (ethanol, or ethyl alcohol).
The chemical reaction and molecular weights of (MW) the reactants (sugar) and products (ethanol and CO2) are as shown below:
C6H12O6→ 2 CH3CH2OH + 2 CO2
MW (glucose/fructose) = 180.16
MW (ethanol) = 46.07
MW (carbon dioxide) = 44.01
From the above equation, we see that for every mole of glucose or fructose, 2 moles each of ethanol and carbon dioxide are produced, and that approximately 180 g of glucose or fructose results in approximately 92 g of ethanol and 88 g of carbon dioxide. So, in theory, alcohol is produced at a rate of 92/180, or 51% of fermentable sugar. The density of ethanol at 20°C is 0.789 g/mL and, therefore,92 g represents a volume of 92/0.789 or 117 mL. Now let’s assume that we are starting with 1 liter (1000 mL) of solution (grape juice) and that all of the carbon dioxide dissipates into the atmosphere during and after fermentation.The density of carbon dioxide at 20°C is 1.815 g/mL, and so, we expect a volume loss of 88/1.815 or 48 mL. Therefore, the final alcohol concentration will be117/(1000–48) × 100%, or 12.3% alcohol by volume. Since a small amount of sugar, up to 5%, is actually metabolized into other by-products such aspyruvates, acetate, acetaldehyde, and glycerol, and to allow for volume contraction of wine in the presence of alcohol, the alcohol content is closer to 10.8%. Assuming that 95% of the total solids content of must is sugar, then180/.95/108, or 1.75 B° produces 1% alcohol by volume. Putting this into equation, we can estimate the amount of alcohol produced as follows.
% potential alcohol by volume = 0.57 × Brix
For example, must measuring 23.0B° would yield 0.57 × 23.0, or 13.1% alcohol if all the sugar were to metabolize.
Some have calculated that the potential alcohol is closer to 0.55 × Brix while other shave determined experimentally that the actual conversion is lower due to conversion into other by-products and ethanol evaporation, and that it is closer to the following equation.
% potential alcohol by volume = 0.55 × Brix – 0.63
Then, 23.0 B° would yield 0.55 ×23.0 – 0.63, or 12.0% alcohol if all the sugar were to metabolize. That’s more than 1% alc/vol difference from the previous calculation. But again, the results depend on your winemaking and your analysis skills and equipment.
Personally, I use this last conversion equation as it suits my winemaking best, and so I know I have to chaptalize if my must measures 23 B° and 12.0% PA and I want 13% alcohol.
So don’t fuss, because the Brix/SG/PA you calculate will be different from what you will end up with.