Need help in understanding hop additions in recipe

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Tam004

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I am preparing to brew a saison ale and the recipe is a bit confusing when it comes to the hops. It reads as so:

5.64AAU Hallertau hops
(1.41 oz/40 g at 4% alpha acids)

3.36 AAU Hallertau hops
(0.84 oz/24g at 4% alpha acids)

What exactly does all of this mean?
 
AAU stands for Alpha Acid Unit.

AAU = Alpha Acid % x number of ounces.

Example from above:
1.41 oz. x 4 = 5.64 AAU

With this equation, if the hops you have has a different Alpha Acid percent than the original recipe, you can use the equation to determine how large your addition should be based on the Alpha Acid of your hops.

Clear as mud?
 
So that would mean a total of 5.64 oz and then 3.36 oz. correct?

Correct, assuming the hops you have is 4% Alpha Acid.

Hops can vary in Alpha Acid % from batch to batch even for the same variety of hops.

For example, let's say that I buy some Hallertau from my LHBS, but it isn't 4% Alpha Acid, it's 3.8% Alpha Acid instead.

If I were to use 1.41 ounces of this hop, I wouldn't have as much bittering in my brew as the recipe calls for - it might not be a big difference, but it'd be a difference.

Since we know the AAU target, though, we can figure out how much of my 3.8% Alpha Acid hop is needed to get the same level of bitterness as the recipe author's 4% Alpha Acid hop.

5.64 AAU = ounces x 3.8 (the Alpha Acid of my batch of hops)

Then we just solve the equation:

5.64 / 3.8 = 1.48 ounces (as opposed to the original 1.41)
 

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