Fortified

No matter how much sugar one puts in juice, there is a limit as to how much sugar the yeast can convert into alcohol before the alcohol kills the yeast. Therefore, to produce a higher alcohol product, some people add spirits such as brandy to the wine. This is known as fortification, and this is how port wine is made.

Fortification also has an added benefit. At lower alcohol levels, one has to go through great lengths such as pasteurizing, filtering, or chemical addition to make sure that the yeast is killed, removed, or subdued so that it does not start fermenting again if the wine is not dry. If one yeast cell has access to any sugar, it can start fermenting again, which builds pressure on the bottle and can cause the cork to pop or the bottle to shatter. However, if the alcohol level is increased enough though fortification, it will kill all the yeast with no fear of it ever eating the residual sugar. And, the fortification can be done before fermentation is complete, leaving some of the original juice sugars in their natural state instead of having to add table sugar to increase sweetness.

The process in which wine is fortified is done though a controlled manner using Pearson’s Square. Pearson’s Square says that if you have a liquid A with a given percentage of alcohol, and liquid B with a different percentage of alcohol, and you want to blend the two into a specified percentage of alcohol, then use so much of liquid A and liquid B. It looks like this:

Pearson's Square

For example, maybe I’ve made a wine that was 12% abv (B), but I really wanted it closer to 18% (C). I find some brandy that is 40% to add to it (A).  You need to know how many parts of A you need (D) and how many parts of B you need to get (E) a liquid of your desired alcohol percentage of C.

A = 40%
B = 12%
C = 18% desired
D = amount of A needed = C – B = 18% – 12% = 6 parts
E = amount of B needed = A – C =  40% – 18% = 22 parts

Please note that this is a total of 28 parts, but can be reduced to 3 parts brandy to 11 parts wine, for a total of 14 parts. It is up to you to decide what units the parts are – liters, pints, gallons, teaspoons, etc – so long as they are added in a 3 to 11 ratio and you have enough material to do so for your given unit.

Say I was experimenting, trying to find the right blend, and I used 4 cups of cider at 7% and 1 cup of calvados at 40%, but I don’t know what the resulting alcohol content is. The formula to find it looks like this:

A = 40%
B = 7%
C is unknown
D = 1 cups
E = 4 cups
% alcohol = C
C = [ (A x D) + (B x E) ] / (D + E)
C = [ (40 x 1) + (7 x 4) ] / (1 + 4)
C = [ 40 + 28 ] / 5 = 68 / 5 = 13.6%

At 13.6%, it is a pretty stable drink that should not get infected, but yeast could survive to eat sugar. 15% would probably be a better target, if it tasted correct.

Further Reading: Making Port Wine