The easiest way to calculate the amount of alcohol required for fortification is to use a Pearson’s Square.
First determine the alcohol strength of the wort or must that you want to fortify. You can do this by measuring the specific gravity before and after fermentation. Once you have the alcohol strength, place that number in position A.
Next note the alcohol strength of the fortifying spirit in position C. This is the alcohol strength of the brandy, whiskey, or Everclear that you are adding.
Decide the final alcohol strength you are targeting for your fortified drink. This number should be between the values chosen for A and C. This number should be placed in position X.
Calculate B and D. B is the difference between C and X (C – X). D is the difference between X and A (X- A).
Finally take the ratio of B over D (B / D). This is the ratio you will use to add to fortify.
Here are the steps with an example:
1) Determine A – Assume your initial alcoholic strength is 10%.
2) Determine C – Assume you are using 180 proof alcohol which is 90%
3) Determine X – Assume we are targeting a 25% beverage
4) Calculate B – (B = 90 – 25 = 65)
5) Calculate D – (D = 25 – 10 = 15)
6) Calculate the ratio B/D – B/D = 65/15 =13/3
13/3 is your final ratio. This means for every 13 parts of your initial beverage, you need to add 3 parts of your fortifying spirit. For example if you had 39 liters of wort, you would add 9 liters of your fortifying spirit.
[Edit 5-19-2008: Corrected values]
[Edit 5-17-2010: Corrected result]
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