Boil time effecting IBU

Not that it really matters much to me, but just curious if it is intentional and I am looking at things wrong..

Basic recipe, some grains and some hops. The single hop addition of 60 minutes changes It's IBU value if I change the length of the boil.



I am under the impression that the IBU would be the same for the 60 minute addition no matter how long I boil before the addition. The EST pre-boil gallon amount changes to reflect the 6 gallon batch size as per my boil-off rate. thanks

boil time effect the amount of alpha acids isomerized.

boil volume does the same.

the greater the value (for either) the more IBUs contributed, the lower the time/volume, the less IBU contribution.

this is why First Wort Hop additions add much more bitterness than 60 minute additions.

it is also why dry hopping (and flavor additions) have little bittering effect.

KipDM said:

this is why First Wort Hop additions add much more bitterness than 60 minute additions.

Click to expand...

Question: Compared to dry hopping at relative low temperatures, in what temperature range is first wort hopping starting to give bitterness to the wort?

R, Slurk

Braukaiser has an interesting chart on this.

http://braukaiser.com/wiki/index.php?title=How_pH_affects_brewing#Hop_utilization

jomebrew said:

Braukaiser has an interesting chart on this.

Click to expand...

Thanks Jomebrew! I forgot this one Love their site. It has been too long ago I visited their site 8)
R, Slurk

grathan said:

The single hop addition of 60 minutes changes It's IBU value if I change the length of the boil.

I am under the impression that the IBU would be the same for the 60 minute addition no matter how long I boil before the addition. The EST pre-boil gallon amount changes to reflect the 6 gallon batch size as per my boil-off rate. thanks

Click to expand...

My guess is that BSmith knows the longer boil time will evaporate more water, increasing the gravity present during the 60 minutes those hops are wet.  In a side-by-side, one wort boiled for 30 minutes first would now be at a different starting gravity than the other wort that has not yet boiled.  Higher gravity => lower utilization, as pointed out above. 

KipDM said:

this is why First Wort Hop additions add much more bitterness than 60 minute additions.

Click to expand...

In BSmith, there is an adjustment on the bittering acheived from FWH, which you can change.  The default is +10, but I actually changed mine to -10.  I have no idea what the actual IBUs are, but the perceived bitterness from FWH has always seemed lower since it is smoother and less harsh. 

AHh, good point Maltlicker, I probably should have mentioned that the IBUs are increasing and not decreasing though.