How the size of the boil affects the IBU

[Smiffy]

I've done lots of kits, but so far only one brew using extract and hop pellets. I intend to do some more, and was looking for a calculator to work out the hop quantity and boil time for the desired IBU.
I found a few on the www, but for have been trying some figures in this one:
http://members.cox.net/boxelderbrewers/tools/IBU.html

The strange thing is, with everything else the same, the boil size makes a big difference to the result.
e.g. total 5.5 gallons, gravity 1.040, boil time 90m, hop weight 1.5oz, alpha acids 7.2%
If I put in 5.5 gallons as the boil volume, the IBUs is 50
If I change the boil volume to 1.1, the IBUs drops to 28

That suggests that the concentration of the alpha acids in the boil reduces the extraction from the hops.

However, if I double the amount of hops and leave everything else the same, the IBUs doubles, which suggests that the above is not true.

Can anyone shed any light on this apparent discrepancy.

Smiffy

[MrC]

By simply reducing the size of the boil in a calculator like that the gravity figure will greatly increase. It's the increased gravity that will affect the hop utilisation and resulting IBUs.

Check this out:
http://www.howtobrew.com/section1/chapter5-5.html

If you want to have a smaller boil an still get the same hop utilisation as a full boil you could adjust the fermentables so that the gravity of the boil is the same as it would have been for the full boil and then add the remaining fermentables at the end of the boil.

Here's a rough example using two cans of extract and a 17L pot:
- Bring to boil 17L of water
- Add one can of extract, stir and bring back to boil
- Begin 60 min boil and hop as desired
- After 60 min boil turn off heat and add second can of extract and stir
- Cool and add to fermenter with 17L of cold water

In this scenario the gravity of the boil is the same as it would have been for a full boil with all fermentables added up front. This means that the hop schedule is the same for smaller boil as it would be for a full boil.

Hope that helps.

[Smiffy]

Thank you. From the link you gave: "As you will see in the next section, hop utilization decreases with increasing wort gravity. The higher concentration of sugars makes it more difficult for the isomerized alpha acids to dissolve."

That was what I was missing.

Now that I know what the relationship is I can demonstrate this with the calculator, basically if the boil is more than 1.050, the end result has less IBUs.

So if I boil the hops in plain water, then add to the boiled extract and top up with cold water, I can get the IBUs which I would get with a full boil (of less than 1.050).

Thanks again

Smiffy