Help with beersmith: why does boil volume affect IBUs?

i have a recipe all set-up in beersmith. it happens to be my first AG. i'll be mashing in a 5gal converted cooler, and boiling in a 15gal kettle. however, the equiptment i had put into the software was just a 5 gal kettle for boiling, which gave me a 4.08gal boil volume. when i entered the proper equiptment in recipe view and changed the batch size back to 5 gallons the boil volume jumped to 6.82 and the IBUs for the recipe jumped from 56.4 to 71.5!?!? i also noticed that if i changed the kettle size, the boil volume changed with it. im going to have a pretty large volume mash with my 12lbs of grain and i expect to need a boil volume of 6.25 - 6.5 gallons. can anyone explain why the IBUs go up with boil volume? is this a software fluke or does this really happen in a boil? should i set my expected boil volume and then adjust my hopping schedule so i get the IBUs im looking for. im really confused here!

thanks for the help!

hop utilization is a function of volume and concentration i.e. if your boil volume increases there is more room for alpha acids in the wort and as you concentrate the wort there is less room for more alpha acids.

When I went from partial boils to full boils (in extract brewing) I did an experiment. My Dead Guy Ale was about twice as bitter to taste it when I did I full boil! After that I downloaded Beersmith and the IBUs were indeed nearly twice as high! In AG, I usually start with 6.41 gallons to make a 5 gallon batch, and beersmith seems to be right on the mark with IBUs.
Erik is right about the basic chemistry of it. The short is- the higher the sg (concentration) of the wort, the less hops utilization you get. That's why for late extract additions, it's recommend to decrease your bittering hops.

Yooper Chick said:

That's why for late extract additions, it's recommend to decrease your bittering hops.

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So THAT'S why the witbier I have in secondary is hoppier than I expected!!

Thanks for your help as usual!

What you are seeing in beersmith is a real effect, not a software fluke.

The gravity of the wort in your boil pot is a function of the volume of liquid and the amount of fermentables present (either from an AG mash or adding extract). For a given amount of fermentables, the gravity will decrease as volume increases. That is, the more water you have in the pot, the lower the gravity of the boil.

Hop utilization is an inverse function of wort gravity. Lower gravity worts have higher hop utilization.

Therefore, for a given amount of fermentables, hop utilization increases with boil volume.

See Table 7 at the link below (scroll to the bottom of the page). It shows this clearly.

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

Kettle size changes your boil-off rate, which changes your starting volume, which changes utilization.

True, concentration of malt to water is ther factor.

With my HW, late addition and 1.5 gal boils I only use 3% AA for 60 mins.

david_42 said:

Kettle size changes your boil-off rate, which changes your starting volume, which changes utilization.

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thanks for mentioning the bit about kettle size. should i trust the numbers that beersmith gives me for boil volume when i check "Set Boil Volume Based on Equiptment", and then adjust my mash volumes to meet that number?

SenorWanderer said:

thanks for mentioning the bit about kettle size. should i trust the numbers that beersmith gives me for boil volume when i check "Set Boil Volume Based on Equiptment", and then adjust my mash volumes to meet that number?

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Well, maybe. I did use those default values originally but tweaked it later on after a few batches. Your evaporation rate may vary from the "default" values so you might need to adjust that later but for the first batch or two, it'll give you pretty good results.

Yooper Chick said:

Well, maybe. I did use those default values originally but tweaked it later on after a few batches. Your evaporation rate may vary from the "default" values so you might need to adjust that later but for the first batch or two, it'll give you pretty good results.

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thanks yooper chick, that's sort of what i was thinking. beersmith is telling me 6.82g boil volume, but i think im going to shoot for 7g because 1) measuring is easier that way, and 2) i'm guessing that boiling water evaporates a bit faster at 5345ft and 10% humidity.

what do you think of my mash schedule for 12 lbs of grain to reach that volume:

mash in with 4 gallons (1.25quarts/lb), lose 1.25 gallons in absorption (.1g/lb, 2.75g left), then sparge with 4.25g to reach 7 gallons. that seems like a lot of sparge water to me, what about you?

I think the absorption rate might be too high- I lose about .30 gallon to MLT dead space and absorption. Others can chime in with their "lost" amount. I usually sparge with 3 to 4 gallons, depending on how many pounds of grain I'm using and how much I still need in the kettle. I assume you're batch sparging for these figures (that's what I do).