Here's a winemaking brainteaser question I get asked a lot this time of year:
Could you check my math on making some port?
I have 4.63 tons of Petite Sirah, ~810 gals
It came in at 28.5 Brix
Hi-Proof = 84.5% Alc
Target Port Alc = 18.50 - 18.75
Target Residual Sugar 8 - 10%
Add ______ gals of Hi-Proof at ______ Brix
Preliminary calcs gave me 114 gals at 13.5 brix ??
You won’t get 8-10% RS if you stick at 13.5 brix (which is really around 14.5 RS) and dilute 810 gals with 114 gallons. You’d technically get 14.5% x 810/924 = 12.7 RS.
The tricky bits here are 1) how much more sugar will soak out of those skins, 2) the actual liquid volume and how much high proof hangs up in the skins, and 3) how long does it take the mass to actually stick.
Assuming your 810 gals and 28.5 brix are spot on, let’s model it at 10% RS and take whatever continuing fermentation we get, ending between 8 and 10% RS.
Now we use an iterative trick, guessing the answer to find the answer. Assuming the alcohol addition answer is around 100 gallons, the dilution of the RS would be about 810/910 or 89%. Thus your RS before addition should be 10%/.89 = 11.2%, which is about 10.1 brix given an alcohol at the time of (28.5 – 11.2) x 0.6 = 10.4%.
Using Vinoenology’s calculator, you add 99 gallons of high proof at that time.
My guess is that you’ll get some raisins soaking out and end up a little sweet doing this. It’s tricky to check alcohol content on fermenting must, because the sugar affects the boiling point. I use a cash still to steam distill an equal volume into a volumetric flack and do hydrometry or ebuliometry on that. Otherwise it just means you might end up with 19.5 alcohol instead – no biggie. Just remember it's always easier to add sugar or alcohol than to remove it.