IBUs affected by sugar added in fermenter?

[Matteo57]

So, I am brewing a bit stout and am trying to figure out IBUs. I think I heard/read that if I add a few pounds of brown sugar in the fermenter instead of during the boil it changes how the IBUs are calculated... How do i calculate what the final IBUs will be if I add sugar into the fermetor instead of the boil?

Thanks!

 

[user 103393]

Looking at Beersmith, it looks like post-boil addition of sugar does not affect IBUs. My guess (just thinking about this, no real research) is that it during/before the boil adds to the volume which affects the boil off which in turn changes how much bitterness gets released as a percentage. After the fact, you're just boosting the alcohol content.

 

[causeimthesquid]

If you add sugar post-boil you have higher hop utilization due to lower specific gravity during your hop additions.

You can accomplish the same thing by making sugar additions near flame out.

 

[Jeffro74]

If you add sugar post-boil you have higher hop utilization due to lower specific gravity during your hop additions.

You can accomplish the same thing by making sugar additions near flame out.

So hops will be more pronounced if sugar is added at flame out as opposed to during the boil?!?... interesting. This will be utilized for my Pliny clone this weekend...

 

[Hex23]

A slight simplification, but IBUs are a measure of the the isomerization of the alpha acids in hops. Isomerization requires heat. Different amounts of sugar during the boil affect the degree of utilization (isomerization) of the hops. More concentrated boils (more sugar, higher SG) reduce utilization. So starting with more sugar at the beginning of the boil or adding sugars during the boil will reduce IBUs.

Adding sugar after the boil (usually what post-boil means) has no effect on the IBUs, but it will make a stronger beer which will affect the balance of the beer and thus your perception of the IBUs. Sugar additions very near flameout will have very minimal affect on utilization because utilization also takes time.