Maximum Iso-Alpha Acid that can be absorbed during boil?

[binaryc0de]

I realize that IBU calculations are just mathematical models to give a reasonable approximation. What are the assumptions and boundaries of these equations? For example my IIPA calculates to ~150 IBUs. I've heard that the limit for wort is 100 IBU, but I haven't heard the reason why or what the boundaries of our IBU calculations are.

On a related note how were these different equations obtained... Through interpolation or statistical inference, ANOVA ect...?

 

[ajdelange]

I believe they were obtained by curve fitting to actual bitterness measurements but if one does that he finds that chi-squared isn't that impressive. IOW if you did the same you'd find quite a bit of scatter. Measurement of hops bittering principal in hops is a little iffy, the potency of the bittering substances varies over time, the degree of isomerization depends on lots of things (including wort density which the formulas try to model) and finally the BU is a unit found to "express the bitter flavor of beer adequately".

There is certainly a limit as to how much of the bittering acids can dissolve in boiling wort but I strongly suspect that the oft stated limit of 100 is based on the fact that BU are calculated as BU = 50*A275 and that lots of the older spectrophotometers run out of steam at around A= 2.0.

 

[binaryc0de]

Most of the equations I saw looked like curve fitting so I'm not surprised. I'd love to take a shot at an experimental design myself but I don't have access to a spectrophotometer. I assume commercial breweries use a spectrophotometers to measure iso-alpha acids and in turn calculate IBU? So there would be no need for more accurate models for them.

 

[ajdelange]

Yes, that's how they do it either in house or by sending samples off to a lab that usually does it that way. But they still need the models in order to be able to calculate hop charges. These "models" may be based as much on experience as math but they do need them.

 

[hector]

Listen to the August 11th edition of "Basic Brewing Radio" .

Dr. Brad Sturgeon of Monmouth College shares an experiment to try to find the top limit of how many IBUs can fit into a beer :

http://www.basicbrewing.com/index.php?page=radio

There is also a pdf file that fully covers the experiment report .

Hector

 

[mabrungard]

The maximum iso-alpha acids that can be dissolved in beer is about 80 ppm which correlates to 80 IBU's. Numerous studies and tests have confirmed that finding. Vinnie with Russian River Brewing confirms that his testing of Pliny the Elder typically only reaches about 75 IBU eventhough the amount of hops added give a calculation of like 150 IBU. This is the reason that Pliny is actually a fairly balanced beer...the gravity is high but the bittering is as high as it can get. Imperial India Pale Ales and Barleywines all display the same response in that they are more balanced even when the brewer tries to sock a lot of bittering in the recipe. To make a really bittered perception in a beer, you need to reduce the malt content while keeping the IBU's up at the 75 to 80 IBU maximum.

 

[Yooper]

I'm glad to see this thread, as Walker and I were just "discussing" this issue in another thread. I couldn't remember my source (perhaps it was ajdelange, even though I thought it may have been Palmer?) but I recall hearing the 100 IBUs maximum before.

The reason we were discussing it was because of the problem I see making IPAs/IIPAs with an extract beer and a partial boil. My thought was that if you can get a maximum of 100 IBUs in the wort, no matter how many hops you use, and you boil 2.5 gallons and then top up with water at the end of the boil to make a 5 gallon batch, that would mean the highest IBU beer you could make would be 50 IBUs. Perhaps that is why many extract/partial boil brewers complain about their IPAs ending up a bit too sweet, even with using plenty of hops and adding the extract late in the boil?

I realize that 100 IBUs might even be generous as in the above post mabrungard says the maximum iso-alpha acids is actually closer to 80 ppm.

 

[ArcaneXor]

Listen to the August 11th edition of "Basic Brewing Radio" .

Dr. Brad Sturgeon of Monmouth College shares an experiment to try to find the top limit of how many IBUs can fit into a beer :

http://www.basicbrewing.com/index.php?page=radio

There is also a pdf file that fully covers the experiment report .

Hector

Measured IBUs well above 50 aren't unusual at all. Something is screwy with his method or his beer.

See http://www.whitelabs.com/QC Day/2011/regional2011.html

 

[ajdelange]

I'm reasonably confident that one can make beers of over 100 IBU (not, I would think, that anyone would want to) but am also pretty sure that boiling hops is not a good way to do it. Adding pre-isomerized extract to the finished beer would be the way to go I'd guess. IOW I don't think it is a solubility limitation but rather the difficulty and time required to get isomerization to take place that would be the limiting factor in the traditional method. All you Vegemite/Marmite eaters know how bitter that stuff is. IOW yeast pull out a fair amount of bittering principal when they flocculate and I expect that's also a factor that is surmounted by post fermentation bittering with extract.

 

[binaryc0de]

I found those 2 episodes of basic brewing very interesting... However I don't think he really did a lot to answer the base question of the absorption limit. This is because there are (as ArcaneXor mentioned) lots of beers with measured IBUs well over 50. In fact in his own pdfs he list Founders Rye IPA with reported measure of 70 IBUs. His measure of that beer was 53.1 but as he talks about in the 2nd episode that could be accounted for with the age of the beer. Also sounds like he measured after fermentation. I'm sure that's standard practice but I'd be interesting in seeing difference in IBU in wort and the fermented product.

 

[Skrimpy]

I'd like to see more studies before I decide that Brad did something wrong or his instrumentation is off. In the second episode where he analyzed a beer with 2 lbs of aromatic hops they seem to confirm his instruments aren't all that off.

 

[Reno_eNVy]

Here's a fun chart from BYO magazine

 

[Skrimpy]

Looks pretty asymptotic there. It only makes sense that there will be a limit to how much of the oils and acids wort can absorb...plus it also makes sense that different malts will have different limits.