IBU Calculation Help

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mcar1919

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I have tried searching for the answer to my question, but haven't come up with anything concrete, so I apologize in advance if this has been covered extensively already or is stickied or something.

I am looking for some help with equations for calculating the expected IBU contributions from hop additions--or perhaps more specifically, an explanation on how to use the Tinseth formula to accomplish what I want which is to use a "known" utilization percentage to calculate IBU contributions from late/whirlpool additions and subsequent hop stands. From my research, it seems that the larger-scale (relative to normal homebrew batch sizes anyway) brewers get somewhere around 15-16% utilization from their whirpool hop additions, but a reasonable estimate for homebrew-scale might be around 10% due to lesser thermal mass, lack of pump-circulated whirlpool, etc. How could one use the IBU = AAU x U x 75 / Vrecipe equation to account for IBU calculation in the aforementioned manner? I am hopeful there is some relatively easy way to convert utilization percentage into a utilization factor value comparable to those found in Tinseth's table so that I am still able to use that same equation. Hopefully that makes sense.

I have looked at the following equations as well:

IBU = weight of hops in ounces x alpha acid % x utilization % volume of final batch in gallons x 1.34*

Ounces x Alpha Acid x Percentage Utilization(boil time) divided by 7.25

While these equations give me the ability to easily input a utilization percentage, I don't like that they don't really account for factors like gravity or volume in the case of the second equation. Plus, in some cases, I get a 10 IBU or more difference when plugging the same AAU value into the Tinseth equation compared to one of the simpler ones listed above.

I am well aware there are calculators out there and tools like Beersmith to help with this kind of thing, but I am an analytical person and I like understanding the math and calculating values for myself sometimes rather than just plugging data into a calculator. I did see there is a Brewer's Friend calculator listed above the Brewing Science stickies that seemingly does what I want, so perhaps someone here is familiar with the equation(s) used there? I followed the link on that calculator to the FAQ page where it explained IBUs, but I didn't see where it got into the math side of it much.

Thanks in advance for any help and insight you can provide!
 
In order to do what you propose you would first have to calculate the average SG of the wort; which can be calculated from the pre-boil volume, OG and post boil volume of your recipe.

Once you know the average SG value you can then calculate the IBU contribution per hop addition based on the weight, AA%, average SG and boil time of each hop addition.

The utilization rate for each hop addition can also be calculated based on the average SG and the boil time for each hop addition.

After adding up the total IBU contribution of each hop addition you will then have the total IBU number.

Determining the average SG of the recipe, based on the OG of the grain bill, is a key piece of information for this calculation. As hop utilization rates will decrease as the sugar content of the wort increases.
 
To quote the ASBC MOA on beer bitterness measurement: "IBU adequately represent beer bitterness..." under certain conditions of hop treatment which I don't recall in detail but essentially it means hops stored in the traditional old way i.e. leaf or plug but not where extracts or pre-isomerized hops are used. I figured the best thing to do was to compare the Tinseth formula to measured IBU data and to find the parameters for the Tinseth formula that best fit the measured data. This I did and the result wasn't very pretty. From this I conclude that given the hops that are available to home brewers there really is no formula that is going to give you a very accurate prediction of IBU from the numbers that are on the packages. I think you will do as well by using any of the formulas to get a WAG at IBU's and then being guided from there by tasting and experience. You can find out the IBU of many commercial beers. Learn what 15 IBU, 20 IBU etc. taste like from commercial examples and then compare your beers to that.
 
Its true, the formulas out there today have been 'tweaked to work' when calculating the IBU values of pellet hops. While the Tinseth formula will get you very close to the IBU value of pellet hops, based on the AA% printed on the packaging, there is no guarantee the IBU value of your beer will match that.

I would think IBU predictions using the Tinseth formula for late whirlpool hop additions and hop stands would have further been 'tweaked to work'. Hopefully the tweaks to the formula were based on real world testing results instead of guesstimates. But either way the results should be good enough to provide a baseline to use for future beers.
 
The following is copied and pasted from elsewhere. Maybe it will help, maybe not, but I'll be damned if it doesn't work sweetly. Cheers.

The following is an estimation method that very closely emulates the results you would get from Tinseth. I like to call this “the Taylor formula”. Yes, I invented it. Give it a try and see how close you get to Tinseth. These rules are designed for pellet hops used in 5 gallons final boil volume, 60-minute boil, @ approximately 1.060 OG. And away we go...

3.6 * oz * AA% = IBUs from bittering hops added @ about 60 minutes left in the boil
1.6 * oz * AA% = IBUs from flavor additions @ 10-15 minutes
0.6 * oz * AA% = IBUs from aroma @ 5 minutes

Add all these together, and then add another +1.5 to the final total to get the final grand total IBUs.

For higher gravity worts (e.g., >1.075), the primary factors are reduced somewhat to about 3.0, 1.3, and 0.6 (stays same). For low gravity worts (e.g., <1.045), the factors are increased to around 4.0, 1.9, and 0.6 (this third factor always stays same). So yeah, you need to use a little swag and finesse. But it ain&#8217;t hard. (That&#8217;s what she said.)

For different batch volumes (V) other than 5 gallons, you need to multiply the result by 5/V. If you use whole hop cones, then multiply your final result by 0.9 (a.k.a., 90%). No big whoop.

P.S. I should add, Tinseth based his formula on whole hops, and on NOT chilling the wort quickly after the boil but a bit more slowly than most of us do today. Fortunately for us in the 21st century, these two effects tend to balance each other out, since pellets make the beer more bitter than whole hops would, but fast chilling reduces the bitterness compared to a longer chill time. Unfortunately, the balance between these two variables is not well enough understood at present, and no one seems to have developed a new formula yet as awesome as Tinseth's designed for use with fast-chilled pellets. If/when we do, well... maybe I should make this suggestion to Sean Terrill or something. I would do it myself, but I'm afraid in 1955, plutonium is a little hard to come by... err, I mean, a spectrophotometer. Yeah.
 
and no one seems to have developed a new formula yet as awesome as Tinseth's .....

As noted in #3 Tinseth's formula is not particularly awesome in the sense that it, even with it's parameters optimized to minimize rms error between predicted and measured (with a spec) IBU values over an ensemble of beers does not do a very good job of predicting bitterness. Now maybe if these beers were made the day after the hops were picked up at the packing plant loading dock the story might be different.
 
As noted in #3 Tinseth's formula is not particularly awesome in the sense that it, even with it's parameters optimized to minimize rms error between predicted and measured (with a spec) IBU values over an ensemble of beers does not do a very good job of predicting bitterness. Now maybe if these beers were made the day after the hops were picked up at the packing plant loading dock the story might be different.

But the fact remains, it's the best of what we have, and it will take a good bit of work to improve upon it in any way that is meaningful to a majority of brewers. Based on the time and place it was developed (1990s?), as well as its fairly long tenure as the king of formulae, I maintain my argument that it still is indeed friggin awesome.

All due respect (I do indeed respect you very much, sir). :)
 
I should have put more emphasis on the second sentence which implies that the Tinseth model is unimpressive because it is usually given bad data. GIGO applies to the best of models and I agree that the Tinseth is the best of any of which I am aware.
 
Using data from Malowicki and Shellhammer, I found the Tinseth formula is indeed the most accurate of the typical formulas (Rager, Garetz, Tinseth) for iso-alpha content (IBU) in unfermented wort. I'm assuming that AJ's discrepancies arise from looking at IBU's in finished beer. The fermentation process and yeast action throw a huge wrench into the IBU prediction. In my opinion, we can abandon any hope of predicting IBU's in beer, but we can reasonably model IBU's in wort.

I target certain IBU levels for my beer, but adjust those targets based on the actual beer perceptions in the glass.
 
Yes, I did look at finished beer and it is true that the yeast strip some of the bittering from the wort with, presumably, different strains doing this to different extents. I did not control for yeast strain (I wouldn't have had enough data). Even so i can't help but think that the biggest discrepancies between modeled and actual IBU's is probably caused by the fact that we have little knowledge or control of how/how long the hops we buy have been processed and stored so that if it say X % on the package it is really X ± Sigma_x and we have no idea how big Sigma_x is.
 
Funny that, I was just looking for information on tinseth on another forum and Graham Wheeler had this to say from a few years ago

"If the formula represents anything at all, I would say that it best matches ex-copper wort utilisation rather than finished beer utilisation. Tinseth as good as admits this in an e-mail given as a response to question posed by another member of this board and reproduced elsewhere on here. Unfortunately I do not have a viable alternative. I wish that I had, because I believe that hop utilisation is probably the last significant unsolved frontier in home brewing by numbers.
G.W.
"
 
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