Calculating IBUs using Trub loss

Hi.  I'm confused and hope someone can enlighten me....

My recipe starts with 7.5 Gal of water, 1.25G boil off and 1.0 Gal Trub Loss, so I end up with 5 Gal in the fermenter (6 Gal in the kettle considering the 1 Gal of Trub).  Beersmith says the IBUs are 34.7.

Same recipe, change trub loss to zero, so it appears I have 6 gal going into the fermenter (still 6 gal in the kettle)...  Now Beersmith says the IBUs are 37.6.

I would think the IBUs would be the same either way since there is 6 gal left in the kettle at the end of the boil in both cases.....

Thank You!

I'm not getting the same variation that you are, but I do see a lowering of utilization.

I don't know the exact calculations used in BeerSmith, but there are published papers showing that kettle proteins absorb alpha acids very effectively. Larger protein levels = lower hop utilization.

That might not be exactly what's being predicted here, but it's close, even if unintentional.