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Batch Volume to Fermetner - Why do IBUs change?

enricocoron

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I read the blog on understanding Brewhouse efficiency, and have understood that this is the total volume of wort that is transferred into the fermenter.  I understand how to calculate mash efficiency, which is more useful in my mind than brewhouse (which seems designed more for production breweries with profit margins in mind).  What I can't understand is why if you change the batch volume which is downstream of cooling loss and loss to trub and chiller in equipment profiling does it effect your IBU calculation...IBUs should be calculated based on OG, pre boil volume, boil off rate, and boil time (and therefore post boil volume)...in no way should changing the amount of wort and trub one chooses to leavin the kettle effect IBU calculations, whether Rager or Tinseth.  I'm thinking IBU calculation would be better to be done in BS based on post boil volume as your batch size.
 
Hello all

I am not sure why no one else ever replied to this post.

The poster is absolutely correct. I have a particular brew that if you calculate the bitterness manually on your post boil volume you get around 52 IBU's. However if you calculate on the volume into fermentor it's a whopping 64 IBU's.

This seems like a pretty important thing for recipe design and something that I think needs clarification.
 
enricocoron said:
.............in no way should changing the amount of wort and trub one chooses to leavin the kettle effect IBU calculations..............

I would disagree, since IBU's, like PPG from the grain, is a measure of concentration.  If everything goes perfectly and the boil isomerizes the optimum AA% from the hops, the total amount of bitterness is diluted across the volume of liquid in which it sits, right?  That's batch volume, which includes the bittered wort some brewers choose to leave in the boiler. 

In order for the "chosen gallons" to be the desired IBU, all the liquid has to be the desired IBU, right?
 
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