IBU Calculation Per Recipe

[stadelman]

From what I can gather, every IBU calculation formula has it's flaws.  Sometimes the calculated and actual results are vastly different.  I mean really, really different.

So, the only real reason to attempt to calculate IBUs is to come up with some sort of subjective standard or give a rough approximation.  Also, some formulas do better in certain conditions.  For instance, one formula might be better at calculating IBUs when a lot of late hopping is employed.

As such Beer Smith's global setting for IBU calculation is too restrictive.  I think there should be a system default, but the user should have the ability to change the formula on a per recipe basis.

[dhaenerbrewer]

You are absolutely right about IBU formula's. When I worked as a regional brewmaster for BJ's a few years back, I had the unenvious task of trying to come up with reasonable formulas that we could use to make our beers more standardized (We had 10 operating breweries at the time). I would take quarterly lab analysis and compare them with recipe sheets and different formulas I had for IBU's. I could NEVER get anything that was consistent. I eventually ended up with a hybrid of two formulas. It looks something like this:

 IBU = (((((AlphaAcid% * Quantity * 284) * (Utilization(BoilTime) * IBUFactor)) / (Volume / 31)) * 0.000117335) * (1.65 * (0.000125 ^ (Gravity / 250))))

Quantity is in Ounces, Gravity is in degrees Plato, Volume is in Gallons, and there is a table for utilization factors dependant on boil time. I had to add the IBU factor field in there because across different breweries they got different utilizations overall. For instance Boulder was always higher than calculated IBU's so they had an IBU factor of .89, but West Covina was always lower, so theirs was 1.23.

I have a spreadsheet with all the utilization factors and the formula on it so you can just put in the info and it will automatically calculate it for you. The one thing I have noticed about BeerSmith's IBU calculation is that it does not take Gravity into account. Utilization goes down exponentially as gravity increases. This formulas accounts for that. Feel free to e-mail me @ dhaenerbrewer@gmail.com if you would like a copy of the spreadsheet.

[UselessBrewing]

The one thing I have noticed about BeerSmith's IBU calculation is that it does not take Gravity into account. Utilization goes down exponentially as gravity increases.

Would this be the same thing as the bitterness ratio?

Cheers
Preston

[dhaenerbrewer]

No, it's not. And I do have to apologize and correct myself. BeerSmith DOES account for this. I wasn't making drastic enough changes in gravity for it to  have an effect on IBU's. I don't think it accounts for it as much as it should, but it does indeed account for it.

Darin

[MaltLicker]

http://www.realbeer.com/hops/research.html

It's more the hops utilization model than BSmith, but BSmith does default to Tinseth's model, and here's a small piece of his numbers.  For the same three minute boil-time, utilization in a 1.120 beast would be 50% of a 1.040 bitter (0.015 vs 0.031).  I've always been surprised at how little we supposedly get from an additional ten minutes:  in a 1.050 wort, going 90 mins from 45 mins only gets an additional 3%. 

Decimal Alpha Acid Utilization vs. Boil Time and Wort Original Gravity                        
                        
Boil   OG                     
Time   1.040   1.050   1.060   1.070   1.090   1.100   1.120   
(min)                        
 0   0   0   0   0   0   0   0   
 3    0.031     0.029     0.026     0.024     0.020     0.018     0.015    
 6    0.059     0.054     0.049     0.045     0.038     0.035     0.029    
 9    0.084     0.077     0.070     0.064     0.054     0.049     0.041    
12    0.106     0.097     0.088     0.081     0.068     0.062     0.052    
15    0.125     0.114     0.105     0.096     0.080     0.073     0.061    
30    0.194     0.177     0.162     0.148     0.124     0.113     0.094    
36    0.212     0.194     0.177     0.162     0.135     0.124     0.103    
45    0.232     0.212     0.194     0.177     0.148     0.135     0.113    
54    0.246     0.224     0.205     0.188     0.157     0.143     0.120    
60    0.252     0.231     0.211     0.193     0.161     0.147     0.123    
70    0.261     0.238     0.218     0.199     0.166     0.152     0.127    
80    0.266     0.243     0.222     0.203     0.170     0.155     0.130    
90    0.270     0.247     0.226     0.206     0.172     0.157     0.132    
120    0.275     0.252     0.230     0.210     0.176     0.161     0.134