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.

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.

Let the arguments begin!!!!!

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.

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.

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.

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.

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

bartyen82 said:

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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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!

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?

BrettV said:

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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I've thought about the same thing, but that's actually different than what I was talking about earlier. But, while we're on the topic...

I'm not chemist, but I assume the same thing can occur with bittering compounds (basically isoalphas and oxidixed beta acids) that occurs with the sugars in your analogy. The question is - what is the saturation point? Can it reasonably be reached with your average 5 gallons of wort? I imagine those are complex questions to answer since it depends on so many variables - the particular bittering compounds involved, other things already suspended in the wort, etc. I think it could happen, though, as beers like Pliny apparently have massive amounts of IBUs on paper but end up with far less IBUs in reality.

But, for the majority of beers, the main thing that will affect your bitterness is the amount of break material in the wort and possibly the flocculation of your yeast (I think Palmer said you can lose some due to yeast, at least). Turns out that bittering compounds really like to stick to other particles (adsorption) and when those fall to the bottom of your kettle/fermenter, your IBUs go with them. This is different than things precipitating out because the wort is saturated, like the sugar in your analogy. It infers that you could still boost the IBUs in your beer by boiling hops in water for 60 minutes and adding the water to your beer after fermentation is complete.

I see. The science of beer-making gives me a headache sometimes. God I love it, but there's a reason I got a BFA in college, and not a BS.

Haha, yeah! It seems that hops and their contributions to beer aren't completely understood yet. Objectively measuring bitterness, hop flavor, and hop aroma is seriously something that just isn't possible right now and is one of the places where the science gives way to art, I think.

BrettV said:

**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?

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Not really. Brewing software uses algorithms that are based on the erroneous belief that boil gravity affects hop utilization.

That's incorrect, and the commercial brewing world has known for years that hop utilization is independent of boil gravity--but it's something that the home brewing world really believed until recently. John Palmer among others has apologized for getting this wrong as recently as the most recent edition of How to Brew (he's far from alone--almost every brewing book got this wrong, and he was one of the first to go to American Society of Brew Chemists meetings, learn the truth, and start spreading the word).

A smaller boil does have more concentrated break material which has some effect on final IBUs, but it's a far, far less dramatic effect than brewing software predicts.

Basic Brewing Radio actually brewed the same recipe with a full boil, a regular partial boil, and a partial boil with late extract additions, using the same hops in each one. The software predictions gave the same kind of wildly different IBU estimates that you're getting from Hopville (unsurprising, since there are about 4 algorithms that pretty much all brewing software uses), but when they sent them off to be tested they found that the actual IBUs in each were basically identical.

IMO, you're best off lying to your brewing software and telling it that you're doing a full boil regardless of circumstances. If you're doing a partial boil with all grain (which has substantially more break material than extract), you might want to adjust the hops up 10% or so, but even then not nearly as dramatically as any software will tell you.

http://www.basicbrewing.com/index.php?page=radio has the BBR tests in the March 4, 2010 episode if you want to listen. The link mojotele posted upthread has tons more info and discussion: https://www.homebrewtalk.com/f128/estimating-bitterness-algorithms-state-art-109681/

Thanks, Sumner. Great info.

SumnerH said:

IMO, you're best off lying to your brewing software and telling it that you're doing a full boil regardless of circumstances.

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Wow, I put in 5 gallons as my boil size, and it shot it up to 86.9 IBUs. So this is more like what I should expect regardless of boil size, huh? Maybe I should decrease the hops. I like hops, but I was shooting for more in the range of 60, basing it on the bitterness of the DFH 60 Min.

BrettV said:

Thanks, Sumner. Great info.



Wow, I put in 5 gallons as my boil size, and it shot it up to 86.9 IBUs. So this is more like what I should expect regardless of boil size, huh? Maybe I should decrease the hops. I like hops, but I was shooting for more in the range of 60, basing it on the bitterness of the DFH 60 Min.

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If you've brewed the recipe before and liked it, leave it alone--and if someone else has given it to you with that hopping schedule for the partial boil, leave it alone. But if you're devising it yourself, I'd lean more toward the full-boil IBU estimate (devise a hopping schedule that works out based on the full boil), and maybe tweak it up a bit if you're doing all grain like I said.


Remember all those algorithms are just approximations, anyway--for a typical beer with a full boil, they're likely to be within about 30% of the final number (if it says 60 IBU, you can probably expect the final product to be between 42 and 78 IBU), but they're not dead-on accurate. If you have a weird grain bill (especially lots of wheat/rye) or a strange continuous hopping schedule or something, then they can be even farther off.

The important thing is to pick one formula (Tinseth is great if you don't have a favorite already) and one way of using it, and stick with it. That'll give you a standard baseline that you'll learn to tweak against according to taste.

I've never made it before, and it's just a recipe I found online, tweaked somewhat. The algorithm hopville uses is tinseth, so without even realizing it, that's what I've been using. It's funny because the recipe claims the IBUs to be 31.9 with a 2.0 gal boil, which isn't even high enough to be considered an IPA by BJCP, so the poster of the recipe must be using a different algorithm.