From the HBD Archive
From: Frank Tutzauer <COMFRANK@ubvmsb.cc.buffalo.edu>
Subject: IBUs for a steam beer
Date: 1992-04-23 22:28:00 GMT

Bryan Gros asks about IBUs in steam beers.

Anchor bitters with 33 IBUs for their steam beer. In my own brews of interest
(4 extract-based steam beers), I have tried the high 30s and low 40s, and the
results were usually a little too bitter (although one of them, with a high
finishing gravity, was a little too sweet). Next time I try I'm going to
shoot for the low 30s.

As for Bryan's particular numbers, below are the answers I got. I've written
a short BASIC program that does the calculations for me. I use the equations
in Charlie II. As for the utilization rates, I don't use Charlie's table
directly; instead, I use a regression equation given to me by Tom
Hettmansperger. The equation explains 99% of the variation in Charlie's
table, and allows me to estimate utilizations for non-tabled times and
gravities. The assumptions I'm making for Bryan's problem are as follows.
First, I'm assuming a 2-3 gallon boil with a gravity of about 1.110. If you
do a larger boil (giving a lower gravity), the IBUs you get will be higher, so
you'll have to adjust the quantities/times down. Second, I'm assuming a 5
gallon batch. Third, I'm assuming hop pellets; if you use leaf hops, you'll
get lower utilizations, so you'll need to increase quantities/times. Finally,
I'm assuming Bryan's alpha of 8.5%.

First, Bryan considers 13-14 HBUs, i.e. 1.5 ounces. A sixty minute boil will
give almost 46 IBUs. To get the 35-40 IBUs Bryan talks about, you would need
to add fewer hops. 1.15 oz. gets 35 IBUs and 1.30 gives 40, assuming a sixty
minute boil. When Bryan uses the "formula in Papazian or the Zymurgy issue"
he gets 1/2 oz. I'm not sure exactly which formulae he's talking about, but
my program shows that 1/2 oz. in a 60 minute boil yields just over 15 IBUs.
Finally, Bryan's compromise of 3/4 oz. for 60 min., 1/4 oz for 30 min., and
1/3 oz. flavoring (1 min., say) yields 23 + 4 + 1 = 28 IBUs, which is probably
ok if the California Lager yeast is attenuative enough.

And then again, remember that a bunch of other stuff--kettle geometry, boil
vigor, pH, etc.--affect hop utilization, so these numbers are all
approximations anyway. If anyone wants to look at the program, or if anyone
wants me to redo the calculations assuming different times, alphas, or
gravities, let me know.

- --frank

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