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I see version of this question has been asked, so apologies for the repeat but its never quite answered in the way I need.

I made a batch last night and noticed half way through the process that lowering the boil size had seriously dropped my IBU's (40 vs 8). My pot size limits me to about 5L once the hops are in. Its a simple recipe using LME, DME, dextrose and fresh hops. The recipe didn't address when to add the sugars, only the hops so I had to wing it. I boiled 2/3 hops for 60 mins, added the LME at 40 mins into the boil, hops at 7 mins and the DME after flame out.

After reading around I am still left with a couple of questions:

  1. Can hopes add bittering without sugar? I've made hop tea before using only water.
  2. Is my IBU reduced to the same extent if I did not add LME until late in the boil?
  3. Is there a formula for figuring this out as I now have no idea what I just made. My only gauge is that the wort had a strong bitter aftertaste.

Thanks for your help.

Jamie

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Hops add bittering via isomerization. When you add the hops to the boil, the alpha acids are extracted, and eventually isomerize and provide bitter flavors. This happens with or without sugars. Ray Daniels's book Designing Great Beers has lots of useful formulae and equations, including how to calculate IBU. Basically it looks like this:

IBU = Woz * U% * A% * 7489 / Vgal / Cgrav

where Woz is the weight of the hops in ounces, U% is the utilization as a decimal, A% is the alpha acids as a decimal, V gal is the volume of wort when you pitch the hops, and Cgrav is the correction for gravities over 1.050. Utilization is a product of boil time, and you can find a chart for that here.

It sounds like you're doing a partial boil. The drastic reduction in IBU you're seeing is because the wort is more concentrated. This means that the alpha acids have a harder time going into solution. It also means that more of the acids get carried out with the break material.

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    Thanks for the quick response. So part 1 is answered, and losing more acid to the break makes sense, but I'm still unclear on the crux of part 2. The chart shows U% is a product of boil time and gravity; so if you change the gravity half way through the boil by adding LME how does that affect your U%? (Not to mention adding DME afterwards). What I mean is can you get a larger absorption of acids before the LME is added? And how does the final gravity once all sugar is added including DME relate to the actual ABU. Is that constant regardless of when the sugar is added?
    – JamieG
    Mar 8, 2013 at 10:25
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    Yes, you can get a better utilization by reducing the amount of sugar you add upfront. Add only enough to create a typical strength wort - say around 1.040. You can then add the remainder close to flameout. Once the isomerized alpha acids have dissolved, it doesn't matter if you add more sugar - the sugar doesn't affect the AAs already in solution.
    – mdma
    Mar 8, 2013 at 12:19

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