Kai, that would be a good question for Tinseth.
It must have been an exchange I had with him a while back where he pointed me to the procedure he used. I just looked here: http://realbeer.com/hops/
and couldn't find it.
An inherent problem for estimating beer IBU from AA amount and boil time (and maybe hop product type) is that beer IBU depends on many more factors. Some of the major factors that come to mind are:
- boil pH. It is known that higher boil pH extracts more bitterness
- bitterness lost during fermentation blow-off (I believe that this is a good thing)
- bitterness lost to yeast trub. I expect this to vary quite a bit
- non-linearity of bittrerness extraction. I.e one cannot extrapolate the bitterness extraction of a high AA concentration boil from low AA concentration experiments.
There is a reason why IBU calculators are largely a home brew thing. All the authors that came up with extraction formulas were home brewers. Without a doubt, there has been commercial research on that topic, but it has not yielded the extraction formulas that we are using. Commercial brewers do pilot brews and measure IBUs to correct the recipe or process.
If you know that a particular commercial IPA has 60 IBU, then this number came from actual IBU measurements and not from an IBU prediction based on the hop bill. If you like that bitterness and want to have it in your home-brew you may start aiming for 60 estimated IBUs, but you may have to correct the hops up or down after you tasted the beer.
I just think many, especially beginning brewers, trust these IBU calculators too much w/o knowing how they work and what their limits are.