A Modicum of Mathematics
Now that fermentation is under way, it's time for some mathematical fun. We need to calculate the point to stop fermentation in order to achieve a nice balance between sugar and alcohol. Most Icewines fall in a range of 11-13% alcohol by volume, with residual sugar between 10-20%.
First, let's do a little pre-calculation to see how much sugar it will require to generate this range of alcohol. We'll use the inverse operation of calculating potential alcohol and divide the desired ABV by 0.55. This will give us the Brix level to aim for...
Low end (11% ABV): 11 ÷ .55 = 20° Brix
Upper end (13% ABV): 13 ÷ .55 = 23.6° Brix
So to produce a 11% ABV wine, take the starting Brix level (36°) and subtract the Brix needed to achieve the target:
36° - 20° = 16° Brix
And we'll perform the same calculation for the upper range:
36° - 23.6° = 12.4° Brix
So we want to stop fermentation between 12.4° and 16° Brix. This also tells us how much residual sugar will remain in the finished wine (this is why I prefer Brix over SG). For my Icewine, I decided to head for the middle ground and stop fermentaion at 15°. This will give an alcohol level of 11.6%, with residual sugar around 15%. Sounds like a good plan.
Now we just need to float a hydrometer in the fermenter and keep an eye on it.
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