IBU calculation and DME

[ccfoo242]

I've read that hop utilization changes depending on the gravity of the wort being boiled. If that's correct then it should follow that adding DME to the boil at 60 minutes should result in different IBU than adding at 5 minutes. When I add DME to a recipe in Beersmith it does change the calculated IBU but it doesn't seem to matter when that is added...60 minutes or 5.

Are my assumptions and/or understanding of hops and IBU's just incorrect?

To reproduce what I'm talking about I took one of the sample recipes (NRB's All Amarillo APA) and added 10 lbs of Light Dry Extract at 1 minute left in the boil. This dropped the IBU's to 25.9.  I then changed the time to 60 and the IBU's stayed at 25.9.

I'm new to all of this so please be gentle if I'm just being stupid.

[jomebrew]

This is true.  If your have read John Palmer's book, you might recall him going over this in chapter 5. 

I don't know the recipe but it sounds like you are boiling hops and water  for 60 minutes and adding DME at 5 minute versus hops and DME for 60 minutes.

The question is how much it affects the IBU and if it can be detected, so it is worth it?

For this example, I have 1 oz of Amarillo Gold at 10%aa
This gives me a total of 10 AAU
10lbs DME at 1.073 gravity

John's calculation using the Tinseth formula is as follows
Example 1 - DME all at 5 minute
IBU(60) = 10 x .36 x 75 / 5 = 54  (.36 is the approx % of hop utilization in water)
IBU(1) = 10 x .04 x 75 / 5 = 6 (.04 is the approx % hop utilization of 1.073 wort for 5 minutes)
total IBU = 60

Example 2 - DME 60 minutes
IBU(60) = 10 x .196 x 75 / 5 = 29.4  (.196 is the approx % hop utilization of 1.073 wort for 60 minutes)

So, yea, the math says it makes a difference.  Beersmith 1.4 seems to use the hop boil time and total estimated gravity.

There are other reasons to only boil the hops with wort and for longer times.

[ccfoo242]

I noticed this because I'm trying to make a recipe that calls for more grain/water than my mash tun can hold, so I was going to use a smaller amount of grain and make up for the lost gravity with dme.  I wasn't sure when the dme should be added to I was playing around with the times and saw it didn't change the calculated ibu.  (I realize now that to achieve the same bitterness of the original recipe I should add it at the start of the boil since that would mimic the gravity of the original recipe)

[jomebrew]

You should probably just add it in the last 15 minutes and if you think you will be too bitter, just reduce the amount of hops a little.  You can use beersmith calculator or a simple online one like this http://realbeer.com/hops/bcalc_js.html to adjust the estimated IBU.

I would not change anything and see how I like it.