longer boil for extract = lower IBUs

i'm an extract brewer and have been experimenting with late extract additions to my boil. this evening i was messing around with beer smith and noticed that the IBUs increases significantly when i add my extract at 30 min as opposed to 60 min. i was just curious as to why this was. is there science behind this or is it just a quirk in the software?

thanks

The effeciency by which boiling extracts alpha acids (the stuff in hops that adds bitterness to beer) is dependent on a number of factors. One of these is the specific gravity of thw wort (i.e. how much sugar is in the boiling water). Lower sugar levels = better hop utilization (i.e. more of the alpha acids get extracted). In your example, if you boil for 30min without extract, you have no sugars, and thus get good extraction. If you boil with extract for the whole period, your hop utilization is lower.

Bryan

All of the common algorithms for forecasting IBUs take the gravity of the wort into account. The higher the gravity, the less the IBUs.
There is some debate about whether or not this is true, and whether the reduction in IBUs is caused by just gravity, or the amount of break material, or possibly some other factors.
Beersmith does not report the actual IBUs. It reports the estimated IBU's according to the algorithm being used.
If you design a recipe, and using that recipe, switch the bitterness formula between Garetz, Rager, and Tinseth, you will probably get 3 different IBU values reported. Obviously, the actual IBUs achieved will not depend on some setting in Beersmith.
You could do an experiment by brewing the same beer twice, once using late extract additions, and once adding all the extract early, and seeing if you can detect any change in bitterness..

-a.

I've noticed an increase in apparent bitterness when shrinking recipeis. I always assumed that this was due to this sugar factor - I always loose ~7L during a boil, meaning for a half-batch (12L) I'm loosing ~ 1/2 the final volume, comparer to a full-sized 23L batch with the same loss. I.E. the pre-boil SG would be much lower in the half batch compared to the full (but both end with the same OG). I think a more controlled experiment may be in order...

Bryan