IBUs and leaving trub behind in boiler

Hi,

I have a question about the IBU calculation.  

When I'm setting up my equipment and leave some trub behind, the IBUs become more concentrated in my final batch.

I understand that leaving beer behind with the hot/cold break and hops in the boiler effects the brewhouse efficiency, but why does it effect the IBUs?

Just because I've left beer behind doesn't mean the concentration of the alpha acids in the beer in my fermentor is any higher.   Is IBUs the concentration of AA in the solution?

I'll give you an example.   I brew 12 Litre batches and leave 2.5 litres in the boiler.   I hop to 28IBU.  If I then change the equipment so that I don't leave any beer/trub behind, the IBUs plummet to 22.6.   The IBUs should be the same.  

I work in a laboratory and from a scientific perspective I'm a little confused.  ???   If I have 15L of a 28M solution and add 12.5 L to a second bottle, the concentration of the 12.5L remains 28M.

I think the IBUs are being determined based on the final batch volume.   I would've thought that the post boil volume would've been the important volume for determining the IBUs.

Mind you, I am new to brewing so correct me if I'm wrong  ;D .

Thanks for any help,

James

James,
 IBU's are being determined by the boil volume.  The trick here is when you go from 2.5 liters to 0 liters of loss in the trub it does change your boil volume.  For example on a 12 Liter batch with 2.5L loss to trub and 10% per hour boil off you need 16.7 liters in the boiler.  The same batch without the trub loss requires only 13.9 liters in the boiler.

 Thus you are boiling more wort to account for the trub loss, and boiling more wort results in higher extraction from the hops because the wort has a much lower gravity.  (Hop extraction rates go down as you raise the gravity, or up as you lower the boil gravity).

 It sounds a little counterintuitive, but because of the effect boil gravity has on hop extraction rates you will actually get more IBU's extracted for a larger boil.  That is why (for example) all grain recipes generally require significantly less hops than equivalent extract recipes - as all grain recipes are usually done with a full boil as opposed to a concentrated one.

Cheers!
Brad

Hi Brad,

I thought it might have something to do with the extraction rate in different volumes.   That does make sense.

However Beersmith is telling me that the boil volume is the same.   The following two batches have the same pre- and post-boil volumes ( I just leave trub behind on one), but have very different IBUs from the same ingredients.

Batch 1
12L batch size + 2.5L trub loss

=14.5L post boil volume

= ~18.3L pre-boil volume

IBUs = 26.1

Batch 2
14.5L batch size + 0 trub loss

=14.5L post boil volume (nothing left in boiler)

= ~ 18.4L pre-boil volume

IBUs = 21.7

As you can see, both these batches have the same post boil volume, and a very similar pre-boil volume.  There is 100mL difference in the pre-boil volume, is that enough to lower the IBUs by ~5.   That's a lot for only a small volume.

This is why I'm getting the impression that the batch volume is effecting the IBUs.   If Beersmith is calculating the IBUs based on pre or post boil volumes, shouldn't these IBUs be the same?

The only thing that is changing significantly is the batch volume in the fermentor.   Is the batch volume what you end up with in the fermentor?   Maybe thats where my confusion is.

Sorry for sounding a bit thick, but I would've though these two batches above would have the same IBU because the pre- and post- boil volumes are the same  ???.

Am I explaining my concern clearly?

Thanks,

James.

James,
 The answer in this case is pretty simple.  All of the equations used for hop bitterness include the final volume as a term in the equation.  Therefore even if you manage to have two batches with the same boil volume and boil gravity but with two different final volumes the final result will be different.  

 Keep in mind that the commonly used IBU equations are all "best fits" of experimental data.  While they take into account the most often used parameters (boil time, boil volume, final volume, boil gravity), none of them is 100% perfect - nor do any of them attempt to account for all of the possible variables (i.e. certain equipment losses).  

 As others have noted simply changing from one equation to another (see the options command, bitterness tab) can result in a significantly different IBU number.  This does not necessarily mean that one equation is wrong and another is right.  It just means that the standard IBU equations are different and take into account different parameters.  Some equations may be better suited for certain brewing styles and/or equipment.

 The IBU numbers are primarily guidelines to aid you in formulating recipes and determining relative bitterness.  As long as you use a consistent method to estimate IBUs (and BeerSmith will let you choose the method if you like) I think you will be happy with your brewed results.

Cheers!
Brad

Hi Brad,

Thanks for your time and patience.   I guess consistency is the key then.  Find what I like and stick to it.

Thanks for your help,

James.