How to substitute if Hop Alpha Acid different than recipe

[DeanRIowa]

I have a recipe that calls for 2 ounces of Hallertau at 2.9%, but I am going to purchase Hallertau at 4.1%.

Do I adjust by doing the math:

2 ounces X 2.9% / 4.1% = approx 1 1/2 ounces?

Is this the general rule?


thank you,
Dean

 

[mongoose33]

You're right on the general formula, but if it were me, I'd just toss in the 4.1 percent. It's not very high in AA regardless and unless you want to retain perfect fidelity to the original recipe--which you might depending on your goals--it's not going to matter all that much.

Further, it depends on when you're adding the hops. You'll get bittering at 60 minutes, but if you add at flameout or whirlpool you'll get much less bittering and more flavor/aroma.

 

[RPh_Guy]

I would adjust bittering hops based on IBU. Your math is right.

Cheers

 

[BrewnWKopperKat]

Do I adjust by doing the math:

2 ounces X 2.9% / 4.1% = approx 1 1/2 ounces?

Is this the general rule?

Yes. Higher AA hops generally require a smaller adjustment, so many people will reasonably not adjust the amount.

The basic equation for calculating IBUs is (from HtB, 1e, http://howtobrew.com/book/section-1/hops/hop-bittering-calculations)

IBU = AAU x U x 75 / Vrecipe

where AAU stands for Alpha Acid Units:

AAU = Weight (oz) x % Alpha Acids

Some simple algebra leads to the equation that you used to get 1.5 oz.