Big difference in IBU calculation

[liamw]

This is my first foray into recipe calculation, so I tried a couple different free tools (Brewtarget and Brewer's friend online). The feature levels were both great for me, but I noticed a huge discrepancy in the calculations. I put an extract recipe that I have brewed before into both just to see, and I got way different results in the IBU section. Here it is:

Fermentables:
9lb Gold LME (I selected amber in both recipes in the software)
8oz Victory malt (steeping grain)

Hops:
1oz Columbus 15%AA @60min
1oz Chinook 12.5%AA @10min
2oz Columbus @0min

Brewtarget gives me 100 Ibus!
Brewer's friend gives me 67, which I think is closer to the actual number.

Is there anything I'm doing wrong? I have the tinseth formula set on both. Anyone know what might be causing my discrepancy?

 

[ajdelange]

The Tinseth formula has a couple of parameters which need to be set for your equipment and practices. I expect the two calculators you are using assume different values for those parameters.

If I take the Tinseth model and find the parameter values which minimize the ms error between what the model predicts and measured IBU values for the actual beers the results aren't too impressive. Are you sure that 15% AA hop is really 15%. This years crop? Stored at low temperature? Shipped in a refrigerated truck? etc.

 

[liamw]

I'm not 100% sure I follow. I get that no, I'm not sure that the hops are 15% AA and bittering may be different based on storage conditions, etc. like you mentioned. These factors would explain the difference between my calculations and finished beer.

What I don't get is how the two calculators could arrive at such a huge difference with the same values input. Has anyone else had this problem with brewtarget (IBUs too high) or brewer's friend (IBUs too low)? From what I've read both products seem to be fairly well regarded on this forum, so I'm assuming it's user error on my part!

 

[Oginme]

There are a few calculations for determining IBU content. ALL of them are estimates based upon some experimental results. They all will give different results and each of them has their good and bad aspects.

How they relate to what you will get with your process is totally up in the air. My recommendation is to not worry about the number, per se, and choose one of the IBU models (I personally use Tinseth, but the others will work as well) and stick with it. When you make a recipe, your taste buds will let you know how much bitterness is present and you can use that as a benchmark to adjusting future recipes.

Just to give you an example, when I make a recipe I will target a slightly lower calculated IBU content for a given style. The reason for this is that with MY equipment and process I seem to get a higher perceived bitterness than the calculated value would suggest.

 

[dkevinb]

Basic Brewing Radio did a podcast on this topic:

July 5, 2007 - Measuring and Predicting IBUs
http://www.basicbrewing.com/index.php?page=basic-brewing-radio-2007

They brewed the beers, ran the IBU calculations through the different formulae, tasted them, then had a chemistry professor measure the actual IBU's. The formulae disagreed with each other wildly (600% sometimes) and were wildly inaccurate compared to the measured IBU's. The conclusion was to pick one formula, taste the beer brewed using it, see if you like it, and stick to that.

 

[ajdelange]

What I don't get is how the two calculators could arrive at such a huge difference with the same values input.

If you look up the Tinseth formula at http://www.rooftopbrew.net/ibu.php you will see that its value depends on the weight of the hops, the percent alpha acid, the volume of the wort, the time and the gravity of the wort. There are several constants in the formula, for example, the boil time is multiplied by -0.04 before the exponential is evaluated. The values of those constants have as much to do with the value computed by the formula as the boil time, AA% etc. The values of those constants depend on your system. If your kettle produces unusual levels of agitation relative to the average kettle perhaps you should use -0.05 instead of -0.04. The only way to determine those parameters is to do what I did: search for the values which minimize the mean square error between observed (measured) IBUs and what the formula returns. The values I got were not the ones found on any website. They are the ones that best describe my system.

Has anyone else had this problem with brewtarget (IBUs too high) or brewer's friend (IBUs too low)? From what I've read both products seem to be fairly well regarded on this forum, so I'm assuming it's user error on my part!

Even with the 'optimum' set of parameters for my setup I still get results that are way off predictions. Part of this is due to the fact that while I have the optimum version of the Tinseth model it is still not the optimum model and the rest is due to discrepancy between what it says on the package and the actual alpha acid content of the hops in them. So, yes, I and anyone else who uses this formula has the same problem with it. It doesn't predict bitterness very well. But as most don't have their beers measured they may be unaware of this beyond the 'this seems pretty bitter for 20 IBU' sort of observations.

 

[liamw]

Thanks for all the great advice! I guess I was just alarmed with the numbers I was getting. I actually brewed the beer I listed above before doing any calculations on it. It came in (my guess) at about 60ibu based on my taste. I was just shocked that the software calculated it at 100 and said it would be way over hopped for the style(it wasn't)