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!!

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.

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.

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.

It seems to me that IBUs are based off of three things: amount of hops, length of boil, and final volume

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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.

weirdboy said:

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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In short, the higher the gravity of the wort the harder it is for the oil in the hops to disperse.

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.

mojotele said:

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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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.

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.

And, the only dumb question is the one you do not ask.

I had this question a couple of months ago when I was creating my recipe spreadsheet. I know for certain that using the Tinseth method, you use the average gravity of the wort for the boil. Here's the link that I was pointed to:
http://www.realbeer.com/hops/research.html

About halfway down the page
"The Bigness factor accounts for reduced utilization due to higher wort gravities. Use an average gravity value for the entire boil to account for changes in the wort volume.

Bigness factor = 1.65 * 0.000125^(wort gravity - 1)"

I haven't found anything on the Rager method, but when I do my calculations, I use the average gravity.

weirdboy said:

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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Sorry if I didn't come across clearly with my explanation as I was trying to keep it simple. Weirdboy pretty much summed up the point I was trying to make.