Effect of boil size on IBUs

Question

I have the following recipe on beercalculus:

http://hopville.com/recipe/551364/belgian-pale-ale-recipes/wedding-belgian-ipa-2011-03-06-version

beercalculus (along with most IBU calculators), ask for both a target batch size (which for me is 5 gallons), and a boil size. I'm not sure what to put in for the boil size, because I do a partial boil of 2 gallons (I'm brewing with extract), then mix with water to bring the batch size up to 5 gallons. The boil size has a big effect on the final IBU's, so I'm trying to determine 2 things:

• For partial boils, what would I put into one of these calculator for boil size?
• Why does the boil size effect the IBUs? I would have thought only the final batch size would matter, because the same amount of alpha acids are being dissolved regardless.

Thanks in advance!

Answer 1

Put in the volume of the water you're boiling. Hopville knows how to handle partial boils.

As for your second question, I think that's still up for debate. Beer bitterness and hop utilization is not fully understood. The original theory was that the high concentration of sugar in a partial boil (or any high-gravity boil) would prevent the alpha acids from entering into solution with the wort. The idea there being that all the sugars just took up too much room.

But in a recent episode of Brew Strong, John Palmer talks a bit about how that might not be the the case. So, like I said, it's up for debate.

And, just to make this a bit more complex, the different IBC calculation formulas (Rager, Tinseth, etc.) handle boil volume and gravity differently. My advice there is to pick a single formula and stick with it.

Answer 2

Your calculations should use the amount of the partial boil. The calculated IBUs change because the alpha acids are not utilized as effectively when the gravity of the wort is higher. Higher gravity readings result in lower isomerization of the alpha acids. When doing a partial boil, the sugars are concentrated and your hop utilization rate drops.

Answer 3

It's not the boil volume per se that matters, it's the boil gravity. Of course, boil volume can affect that, but you can also have 2 boils of the same volume with different gravities.

Answer 4

There is a good article available to download for free on the Zymurgy website that outlines the different formulas. I believe the title of the article is "what is your IBU" and is found by clicking the free downloads link on their home page

Answer 5

Interesting answers. So if the answers given are correct then couldn't one just add a little malt to boil at first to keep the gravity somewhat low to increase the utilization of the hops. Then after hops are done being boiled take them out and add the rest of the malt so you get the IBU's your hoping for?