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Changing Equipment profiles changes the IBU's???


Dec 30, 2015
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I am working on building an APA extract recipe and was tinkering with the IBU's.  Then I noticed my equipment profile was not set, and when I changed it the IBU's jumped from 74 to 89.

The only difference in the equipment profile was the amount of boil volume & top up water (added to fermenter), so i'm not sure why it changed at all. The initial setting was 5.7 Boil volume & 0 Top up, and the setting I changed to was 4.14 Boil volume & 1.5 Top up water.

I use the second option due to kettle size, and then add water to the ferementer to bring up to 5 gallons and then mix vigorously.

Does boiling the full volume really make a difference in IBU's?
The boil volume doesn't make a difference, but the specific gravity of the boiling wort does make a difference.  It is why many extract brewers hold 1/3 to 1/2 of their extract until the last fifteen minutes of the boil.  With a lower gravity wort being boiled, the extraction of the Alpha Acids in the hops is more efficient.  You get more IBU's from the same amount of hops, when boiling a lower gravity wort.

However, I noticed that your IBU's went up, when you reduced your boil volume, which would have raised your wort specific gravity in the boil.  I would have expected your IBU's to go down, not up.

I'm a little puzzled by this.
I didn't realize that, thanks for the input. Still not sure, like you said, it went up and not down.

I assume you can trial it on any recipe and see it do the same thing though.

My main concern is I want to make a beer in the 60-65 IBU range, and want to make sure I have the right amount of hops at the right time. This equipment profile is throwing me for a loop though.
My bad, my brewhouse efficiency wasn't the same, now it makes sense and shows just as you said, the lower wort gravity has a higher hop utilization (higher IBU for same amount of hops).

Thanks again for your help!
I'm glad that you were able to figure it out.  Good luck with the APA and let us know how it turns out.