Author Topic: Calculating Bitterness  (Read 4687 times)


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Calculating Bitterness
« on: January 20, 2009, 11:56:33 AM »
My question pertains to the on-going discussions of IBU calculations.  I am fairly new to brewing and have been reading much about the various ways to estimate IBUs using the Rager, Tinseth, etc. equations.  It seems that many believe that Tinseth is more "accurate," but I've also see opinions that Tinseth is better for full boils while Rager is better for partial boils.  Using BeerSmith does it really matter whether you are doing a partial or full boil?  I ask because for Tinseth it appears from playing around with the program that BeerSmith calculates the hops utilization (not final IBU) based on the boil volume, not the final beer volume, and then simply ratios the IBUs calculated from the boil by the final dilution.  This would make sense to me since IBU is a measure of "bitterness concentration," but I want to confirm this is the case. 

I can illustrate what I'm trying to ask with an example (making up the numbers).  Say I want to boil 3 gallons with a final beer volume of 5 gallons.  From what I can tell BeerSmith first calculates the IBUs assuming that the 3 gallon boil is a full boil and performs all of the utilization calculations assuming a 3 gallon boil.  For my example the result would be 40 IBUs for this intermediate step.  It then dilutes the beer, which is not related to extraction or utilization, to account for the final volume and concentration.  In this case (ignoring losses to keep it simple) Final IBU = 40*3/5 = 24.  I want to make sure that this is what BeerSmith is doing.  If this is the case, then it shouldn't make any difference whether you are doing a full or partial boil.  While it is true that the Tinseth equations assume a full boil, this can be compensated for by accounting for the dilution after the boil separately as it appear BeerSmith does.  A bit of a ramble but I hope I'm being clear and that you can confirm my reasoning.

For my immediate problem I'm simply trying to scale an existing recipe to go from a full boil to a partial boil due to equipment limitations (my boil pot isn't big enough).  I presume the goal here would be to keep the calculated IBUs constant between the full and partial boils regardless which method I use (i.e., if BeerSmith tells me the IBUs are 77.5 for the full boil, then I still want 77.5 IBUs with the partial boil, which would be expected to use more hops for the partial).  I've done this assuming both Tinseth and Rager and am surprised that I am coming up with a quite different hop requirement depending on which method I pick (33 gm with the Tinseth vs. 38 gm with Rager).  My inclination is to go with Tinseth, but . . .

Thanks for any opinions.
« Last Edit: January 20, 2009, 12:01:06 PM by GottBeer »

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Re: Calculating Bitterness
« Reply #1 on: January 20, 2009, 09:27:47 PM »
  I'm a bit short on time tonight, but yes the hop utilizations are adjusted for boil size.

   My personal recommendation is to use Tinseth for full size boils.  Your utilization is higher with full size boils, and Tinseth does a better job taking this into account.

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Offline MaltLicker

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Re: Calculating Bitterness
« Reply #2 on: January 21, 2009, 06:22:20 AM »
Interesting...I would speculate that each hops utilization model ignores the "batch size" since no one can know how much the brewer will dilute the beer, (intentionally or not).  By that I mean Tinseth and Rager don't know whether I'm full-boil brewing 3 gallons of dopplebock, or partial-boiling 60% of a five-gallon bock that might have the same gravity in the pot. 

I thought the models were largely based on boil time and sugar density, which Tinseth calls a "bigness" factor that has an inverse relationship to utilization, i.e., the higher the sugar concentration, the less IBUs extracted, all else equal.  Below is a snippet of Tinseth's math:  for a 60-minute boil, he assumes half the IBU's in a 1.130 monster compared to a 'typical' 1.050.  But for that 1.050 wort, he assumes little is gained from 60 minutes to 90 or 120 minutes. 

The major difference then, between models, are their assumptions and math.  My ESB has a certain IBU that I could learn if I sent it to a lab.  The models "estimate" the IBUs at 52 (T), 47 (R), and only 35 by Garetz, all for the same beer, and they each could be wrong. 

My "Aha" moment was realizing these hop models are similar to economic models and all three could be wrong, and what matters most is your impression of "what's in your wallet?"  It is helpful, however, to know (or best guess) the hops model used in a recipe you are trying to copy.  If Rager, then build your recipe file using Rager to better copy the IBUs and the IBU:gravity ratio of the original brew. 

Boil   Original Gravity                           
Time   1.030   1.040   1.050   1.060   1.070   1.080   1.090   1.130
  0   0.000   0.000   0.000   0.000   0.000   0.000   0.000   0.000
  3   0.034   0.031   0.029   0.026   0.024   0.022   0.020   0.014
  6   0.065   0.059   0.054   0.049   0.045   0.041   0.038   0.026

 60   0.276   0.252   0.231   0.211   0.193   0.176   0.161   0.112

 90   0.295   0.270   0.247   0.226   0.206   0.188   0.172   0.120
120   0.301   0.275   0.252   0.230   0.210   0.192   0.176   0.123

Offline UselessBrewing

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Re: Calculating Bitterness
« Reply #3 on: January 21, 2009, 12:32:38 PM »
+1 for the Aha moment! Wait, Is that beer related, he's talking about whats in my wallet.. Is this another Capitol One commercial where the the barbarians jump out?

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