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Home Brew Forums > Home Brewing Beer > Brew Science > IBUs and Boil Gravity
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Old 07-22-2009, 09:25 PM   #1
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Default IBUs and Boil Gravity

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Originally Posted by ChshreCat View Post
A lower boil gravity increases utilization of your hops. So a smaller amount of hops in a low gravity boil can give you the same bittering as a larger amount of hops in a higher gravity boil.
*sigh*

Hop utilization is independent of wort gravity. This is something that John Palmer has said that he regrets getting wrong in even the most recent edition of How to Brew, but he's been trying to correct this misunderstanding lately. Listen, for instance, to the March 20 podcast "What is an IBU, Really?" here for his input:
Basic Brewing Radio - 2008

It's something that home brewers have gotten wrong for a long time (because of that, most IBU calculation algorithms get it wrong as well, so most brew software messes it up), but has been well known to commercial brewers for quite a while; the earliest publicly available link I've found is to an American Society of Brewing Chemists abstract from 1989 which reads, in part,
Quote:
The variations are mainly due to differences in equipment but are also influenced by the hop product used. In the range 10.5-13.5° P, no relationship between hop utilization and original gravity was found
ASBC Journal 1989 - Hop Utilization in the Brewery-An Interbrewery Comparison.

Though more recent journals have extended that gravity range significantly.
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Old 07-22-2009, 09:28 PM   #2
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Quote:
Originally Posted by SumnerH View Post
*sigh*

Hop utilization is independent of wort gravity. This is something that John Palmer has said that he regrets getting wrong in even the most recent edition of How to Brew, but he's been trying to correct this misunderstanding lately. Listen, for instance, to the March 20 podcast "What is an IBU, Really?" here for his input:
Basic Brewing Radio - 2008

It's something that home brewers have gotten wrong for a long time (because of that, most IBU calculation algorithms get it wrong as well, so most brew software messes it up), but has been well known to commercial brewers for quite a while; the earliest publicly available link I've found is to an American Society of Brewing Chemists abstract from 1989 which reads, in part,

ASBC Journal 1989 - Hop Utilization in the Brewery-An Interbrewery Comparison.

Though more recent journals have extended that gravity range significantly.
You know, you say that often. I can only tell you my experience- when I use a late extract addition, the beer is much more bitter. My Dead Guy clone tasted about 100% more bitter when I did a late extract addition. When I calculated it out, much later, the IBUs indeed changed from 15 to 32.
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Old 07-22-2009, 09:44 PM   #3
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You know, you say that often. I can only tell you my experience- when I use a late extract addition, the beer is much more bitter.
Late extract helps the final bitterness because of physical phenomena. It's not caused by the wort gravity. If you make a 1.050 wort with pale malt and another with wheat malt, the wheat one will be far less bitter because of the same physical phenomena, despite identical gravities.

Palmer addresses this extensively in the podcast.
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Old 07-22-2009, 09:53 PM   #4
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Originally Posted by SumnerH View Post
Late extract helps the final bitterness because of physical phenomena. It's not caused by the wort gravity. If you make a 1.050 wort with pale malt and another with wheat malt, the wheat one will be far less bitter because of the same physical phenomena, despite identical gravities.

Palmer addresses this extensively in the podcast.
Yes, he does. He talks about the old definition of 1 mg isomerized alpha acids per 1 L of wort as an IBU is determined to be incorrect.

However, even though the beginning definition was arbitrary, knowing the IBUs can can give you an idea of perceived bitterness. If you then need to "label" an amount of bitterness to your taste, giving an IBU guestimate equivalent is helpful. IBUs is still a very useful scale.
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Old 07-22-2009, 10:11 PM   #5
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Quote:
However, even though the beginning definition was arbitrary, knowing the IBUs can can give you an idea of perceived bitterness. If you then need to "label" an amount of bitterness to your taste, giving an IBU guestimate equivalent is helpful. IBUs is still a very useful scale.
Absolutely.

But it's very useful to distinguish between wort gravity and physical effects as a home brewer. That will let you know, that, e.g.:

* Making the same gravity wort in a full boil with AG vs. extract will give you higher IBUs in the extract wort (which has had many of the proteins precipitated out already)
* Making the same gravity wort with wheat or rye will give you lower IBUs than with barley malt
* Adding the same amount of alpha acids with 5 oz of low-alpha hops vs. 1 oz of high-alpha hops will give slightly lower amounts of alpha acids with the former
* Beta acids contribute to IBUs, so using balanced hops like Saaz give more IBUs than the AAs alone would indicate, albeit of a different qualitative flavor; similarly, something like Galena is going to maintain a fairly high beta proportion compared to some of the other high-alpha hops that might be 4:1 or even higher.
* Equal-IBU beers with equal sugars may not taste equally bitter, depending on the age of the hops, the age of the beer, the alpha:beta ratio, and other properties of the hops used.

and so forth. Some of this is useful only in niche categories (roggenbiers, or lambics that use aged hops, or whatever), but some of it is just good basic useful information for almost any home brewer. Phrasing it as "hops utilization varies with wort gravity" isn't just pedantically wrong in some theoretical sense, it also fails to capture a lot of factors that are applicable to real-world brewing.
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Old 07-22-2009, 10:13 PM   #6
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Yes, he does. He talks about the old definition of 1 mg isomerized alpha acids per 1 L of wort as an IBU is determined to be incorrect.
One more aside, what he really points out is that that was never the actual definition of an IBU--it's a simplification that home brewers have heard for 30+ years, but when commercial brewers (or others) actually talk about IBUs and measure them, that's not what they've ever meant.
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Old 07-23-2009, 01:58 AM   #7
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If you listen to the podcast linked to, it's very unscientific and just basically explains that the isomerization occurs, and that then due to kinetics, they "cling" to everything. This carries the isomerized acids out of solution, and affects the IBUs. So, what he's saying is that spectromometry is the only accurate way to determine the IBUs.

Still, he concedes that using the current formulas are a very good guestimate.
Ok so he's only saying that spectrometry is the only accurate way of determining IBU's. That I'm very ok with. It is also true of the color of a beer. Our SRM calculations are really just a good guesstimate. HOWEVER if you have a unit and that unit compares to itself then you can tell from experience what said unit will taste like. After all, all unit's are arbitrary numbers assigned to something to reference against anyways.

On the utilization thing though, I'd really like to see more to the contrary of the generally accepted knowledge.
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Old 07-23-2009, 04:00 AM   #8
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36:40 is where the discussion on this happens, and he says the it's a function of break material being greater in a high gravity beer taking the iso-alpha-acids with it out of solution (as the cling to the break). I finally understand what sumner was saying.

So in theory

I can go with this, but I'd still love to see a chart in iso-a-acid solubility vs gravity. I have one for pH. And then maybe one for iso-a-acid concentration vs break mass. The later would be particularly interesting.

In fact after listening to the whole thing, we(as homebrewers) basically can not measure IBU in any accurate way at all without spectrometry.

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