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Old 06-21-2010, 06:32 PM   #1
BrettV
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Default How does size of boil affect IBU?

**EDIT: As will sometimes happen, I posted this, and with more poking around found the answers I was looking for. Disregard this post.**

I will soon be brewing my second batch of beer, and this time I'll be going for an IPA. I was using http://beercalculus.hopville.com/ to help assist me in tweaking a recipe to make sure I was hitting my numbers for the style I was brewing. I'm doing a stovetop extract/steep, so I won't be doing a full 5 gallon boil, more like a 2-3 gallons and topping off the carboy with water. I noticed, though, that when I adjusted the size of my boil, it seemed to significantly affect the final IBUs. I get a 15 IBU swing between a 2 and a 3 gallon boil, (from the low 50s-high 60s.) Is this correct? Have I "saturated" the 2 gallon batch with hops, thus wasting leftover potential acids? I can't think of another reason why adding more water would yield more IBUs.
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Last edited by BrettV; 06-21-2010 at 06:39 PM. Reason: Found answer elsewhere.
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Old 06-21-2010, 06:39 PM   #2
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It is the boil size, but not for the reason you think - it's the specific gravity of your wort. As you add water, you're diluting the concentration of sugars in solution, and that changes your hops utilization.

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Old 06-21-2010, 06:43 PM   #3
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Let the arguments begin!!!!!

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Old 06-21-2010, 06:44 PM   #4
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Apart from your math seeming to be a little off... As I can't see where 15 numbers are from the low 50's to high 60s lol.

That's weird though, I'm interesting in knowing too.

*edit* My math is actually very wrong, and in my stupidty trying to laugh at you, I realized this. So I'll leave this here for the world to see my dumb-dumbness lol.

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Old 06-21-2010, 06:44 PM   #5
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Thanks, Bart. Basically as soon as I posted, I found other threads on this topic. This community is so huge, sometimes it's hard to find the right topic right away.

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Old 06-21-2010, 06:45 PM   #6
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This is probably the best source of information you'll find on the subject.

Thanks go to SumnerH for teaching me more and more about hop utilization every time this topic comes up.
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Old 06-21-2010, 06:46 PM   #7
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Bosox - 68-53 = 15. I don't recall specific numbers, but it was about that. Based on some more reading on this, I think I understand why now.

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Old 06-21-2010, 06:48 PM   #8
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Yeah, I made an edit on my post. I realized it was indeed my math that sucked. I always do stove top boiling with about 2-2.5 gallons

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Old 06-21-2010, 06:48 PM   #9
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Quote:
Originally Posted by bartyen82 View Post
It is the boil size, but not for the reason you think - it's the specific gravity of your wort. As you add water, you're diluting the concentration of sugars in solution, and that changes your hops utilization.
That's not entirely accurate. It's not the gravity so much as it is the concentration of break material.

So adding more extract or malt will to the same volume of wort will reduce hop utilization, but adding pure sugar won't. That is not how the brewing software works, though. Take a look at the thread I posted, I found it very enlightening!
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Old 06-21-2010, 06:54 PM   #10
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Bosox - My last boil was 2.5, based on the IBUs for that sized boil (a touch under 60) that's probably what I'll do.

Mojo - the way I've been trying to wrap my mind around it is by using an analogy (whereby I use sugar in place of hops.) If I tried to make a sugar solution with 4 ounces of water and a cup of sugar, I would saturate the water and have leftover "unused" sugar. However, if I increased my volume to 10 ounces of water, I could dissolve all of the sugar without leaving any behind. Mixing the second solution with another volume of water would clearly yield a sweeter liquid because I would have utilized all the sugar. So, more water in the sugar solution (aka brewpot) = sweeter final result (aka wort.) Is this analogy on the right track?

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