IBU as a function of gravity ?

[guevedoche]

The What is an IBU really? brewcast with John Palmer

Brew Your Own: The How-To Homebrew Beer Magazine - BrewCast

has been mentioned numerous times on this board. The message that seems to be taken away from the brewcast is that the quantity of IBUs in the final beer is not related to the gravity of the boiled wort as suggested in How to Brew/Beersmith's or anyone else's IBU calculator.

I've listened to it a couple times now (admittedly each time under the influence of our barley inflected muse) and my impression is that Palmer says that:

(1) The solubility of alpha acids is not appreciably affected by wort gravity.

(2) The level of imparted IBUs is indirectly proportional to the amount of break material formed during the boil, which co-precipitates bittering compounds (i.e. more break equals fewer IBUs and more gravity equals more break). It is implied that this is the phenomenon that leads to the inverse relationship between gravity and bitterness rather than changes to bittering compound solubility as had been assumed.

My interpretation is that this has little impact on the originally postulated bitterness/gravity relationship.

That said, it is left as an open question as to whether extract brews with their reduced break formation follow the same IBU/gravity relationship. And the entire definition of IBU is challenged in the 1st half of the program (basically saying that we rely too heavily on Alpha Acid concentration and that the notion of the IBU has changed over time as fresher hops have become available).

What are other peoples' impressions of these relavations as it relates to homebrewers' customary use of IBU calculators, particularly with regards to partial boils/late extract additions?

 

[jkarp]

Heh, I gave up carrying that torch as I kept getting blasted every time I brought it up. Tinseth is LAW on HBT.

I do think Palmer cops-out by saying the old formulas are still "good enough". If gravity doesn't matter and break material does, then new tables should be developed to reflect that. How does one easily "measure break" though? Yes, higher gravity often equates to greater break, but clearly not always. Some grains produce more proteins than others. I've had 1.080 worts with almost no visible break and a 1.040 (6-row) that looked like it had a loaf of bread dissolved in it. I'm sure I'm not unique.

I find the whole concept rather intriguing, but I also think homebrewers put waaaay too much confidence in their IBU numbers. Should be interesting to see how this shakes out over time...

 

[Opherman47]

welp i didnt respond to this initially because i wanted someone to articulately answer a very well posed question....... i hope some others take it up, another related question, is what if you dont add hops till after the hot break? better utilization? i dunno im gonna try to conduct some measurements with my buddy who has access to a lab and a spectrometer, with reguards to what kind of IBU's we are getting on some of our more popular beers that we regularly brew....... hmmm i wonder what kind of experiments i could do with reguards to density, types of grain break etc? a spectrometer would measure everything in suspension..... thoughts comments.....

 

[McKBrew]

I honestly don't care much about the numbers vs reality. If my beer software tells me I'm getting 65IBUS's in a beer and someone asks me how many IBU's it is, I'll say 65. There is no way I am going to know the truth unless I spend money to have my sample analyzed at a lab.

I understand how some people really enjoy the exact science of brewing and want to know how many IBU's, how their water might be affecting the brew, etc... If that's you, enjoy it. I also think on the other hand that some new brewers see all of this stuff and worry about it more than they should.

Either way, the beer still turns out good.