Sorry, Did We Say There Would Be No Math?
This is for extra credit then.
Parti-gyling is less intimidating if you are not fussy about target gravity, but I know some of you want to predict everything on paper and then nail it on brew day. You have my admiration, because I am the type to predict everything on paper then completely miss it on brew day, taking notes to record it with the hilarious notion of repeating it in the future.
We’ll go with Keeling’s example of two worts of 1.080 and 1.020, respectively. With those worts you’d like to make three beers of varying strengths—let’s say an IPA at 1.070, a mid-range pale ale at 1.050, and a saison of 1.040.
How do you make that happen? This is where the math comes in. The simplest way to do the math is to use the gravity points— 80 and 20—in per gallon terms. Let’s assume you draw 5 gallons of each wort, so your total points are 400 and 100 respectively (80 × 5 = 400; 20 × 5 = 100).
Are you with me so far? To get 2 gallons of 1.070 wort (for the IPA), we want 70 points per gallon or 140 points. The way to get there is to blend 1⅔ gallons of the stronger wort with ⅓ gallon of the weaker one. Okay, I’ll show my work:
1⅔ × 80 = 133⅓ and ⅓ × 20 = 6⅔
133⅓ + 6⅔ = 140
140 ÷ 2 = 70
You get the idea. The next one is easy. When you take 2 gallons from each wort you get a neat 200 points, divided by 4 gallons to make a tidy 1.050. There’s your pale ale.
That leaves 4 gallons—1⅓ gallons of strong stuff to blend with 2⅔ gallons of lighter stuff. You end up with 4 gallons worth 160 points, and there’s your 1.040 wort for your saison.
That’s an illustration. It’s not necessary to nail your 1.080 and 1.020 gravities in the first place, as long you’re ready to do some math with what you have. Also be aware that these are post-boil gravities, as the boil will concentrate the strength somewhat.
There are other ways to be flexible: For example, hit your targets on two of your beers but take whatever you get on the third; or, hit all three of your targets without using all of your wort, saving the rest for a yeast starter or some other use.