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Home Brew Forums > Home Brewing Beer > Beginners Beer Brewing Forum > IBU calculation
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Old 01-06-2010, 03:00 AM   #1
mojotele
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Default IBU calculation

Hey guys,

I have a question that I couldn't seem to find an answer for searching around the internets, possibly because it is a dumb question.

I've been using ProMash to formulate recipes based off other recipes I find. Basically, I'm only doing extract brewing and the occasional partial mash, so I converted a few all grain recipes to extract or partial mash recipes. I'm also doing partial boils, not full boils, and with extract I like to add some at flameout. So, I often have to adjust the amount of hops used as well.

My question is, do you calculate IBUs based off the pre-boil volume of wort or the volume of wort left after the boil is over? For instance, if you're boiling 3 gallons of wort and, after the boil, you're left with 2.55 gallons, would IBUs be calculated on the 3 gallon volume or the 2.55 gallon volume? My intuition tells me it would be the volume before the boil, but I just wanted to check. I suppose the answer would determine what number I put in the "Wort Size" field in ProMash.

Thanks!!

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Old 01-06-2010, 03:07 AM   #2
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Technically you should use the volume when the hops were added.

But, most recipe calculators are probably taking evaporation rate into account and applying the evaporation rate base on the time that the hop addition was made during the boil.

At least that is how I would do it being in IT and all.

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Old 01-06-2010, 03:21 AM   #3
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I think I may have just answered my own question.

Just read on the ProMash site that the "Wort Size" field is actually the post-boil wort volume. There's a "Pre-Boil" button which opens up a calculator that tells you what volume of wort you should have pre-boil to end up with the amount you put in the "Wort Size" field.

So, in order to get the IBUs I currently have in the recipe, I need to make sure I start with more wort than I put in that "Wort Size" field.

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Old 01-06-2010, 03:29 AM   #4
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It seems to me that IBUs are based off of three things: amount of hops, length of boil, and final volume, and that leads me to believe that the IBUs are calculated as the final volume sitting in the fermenter. So my beer math may be slightly flawed here but you should be able to see my logic anyway... so say you formulate a recipe to have 40 IBUs in the finished beer, but you are only boiling 3 gallons down to 2.5 over a 60 min boil and then dilute the beer to 5 gallons in the fermenter with plain water. I don't have all my spreadsheets immediately in front of me, but if I recall correctly, you need many more than just 80 IBUs in your 2.5 gallons to dilute down to 5 (i.e. the ratio is not linear) and still get 40 IBUs in your finished beer. Someone else may be able to correct my math a bit, but I believe in your 2.5 gallons of post boil, you would need somewhere in the ballpark of 120 IBUs to dilute down to 40 once you add another 2.5 gallons of water. Again, sorry if my math is off but I hope you get the picture.

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Old 01-06-2010, 03:37 AM   #5
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Quote:
It seems to me that IBUs are based off of three things: amount of hops, length of boil, and final volume
It's actually based off of more than that. Amount of hops, alpha acid % of hops, length of boil, gravity of wort, and batch size.

Alpha acid isomerization decreases in higher gravity solutions, which is why increasing your boil volume can increase the IBUs in a beer even if you end up with a higher final volume.
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Old 01-06-2010, 03:49 AM   #6
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It's actually based off of more than that. Amount of hops, alpha acid % of hops, length of boil, gravity of wort, and batch size.

Alpha acid isomerization decreases in higher gravity solutions, which is why increasing your boil volume can increase the IBUs in a beer even if you end up with a higher final volume.
In short, the higher the gravity of the wort the harder it is for the oil in the hops to disperse.
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Old 01-06-2010, 04:03 AM   #7
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Thanks for the info, guys. All this seems to jive with what I'm seeing in ProMash, especially with what RiverCityBrewer mentioned about diluting the concentrated wort.

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Old 01-06-2010, 04:07 AM   #8
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Originally Posted by mojotele View Post
Thanks for the info, guys. All this seems to jive with what I'm seeing in ProMash, especially with what RiverCityBrewer mentioned about diluting the concentrated wort.


It's more complex than that, though. You can just boil with more water to begin with and get more IBUs out of the same hops.
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Old 01-06-2010, 04:10 AM   #9
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Since you are doing extracts, add half the extract at first and the rest at flame out. This works out great and allows for excellent hop utilization.

However, if you ever switch to AG. You have to boil out off-flavors like tannins.

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Old 01-06-2010, 04:12 AM   #10
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And, the only dumb question is the one you do not ask.

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