Partial Boil - Math Check Please

[Krrazy]

After two years of brewing 3 gallon batches, I've started upgrading my equipment with two 6 gallon glass carboys. I still have the same kettle so far and I'd like to ask for a double check on my first partial boil recipe. Thank you very much in advance!

I typically boil 4 gallons in a 5 gal kettle to get to 3 gal of wort at the target gravity.

Beer Style: Mild
Target OG: 1.038
Batch Size: 5.5 gallons
Plan: 3 gal wort + 2.5 gallons pure H2O (boiled and cooled)

"SG"=3 gallon specific gravity target to add to pure water
1.038 = (3gal*SG + 2.5gal*1.000)/5.5gal
(1.038*5.5-2.5)/3 = 1.069
Target SG for 3 gal batch = 1.069

Efficiency: 75%
Grain Bill:
6.0lb Maris Otter
0.75lb Crystal 60L
0.5lb Pale Chocolate Malt
0.5lb Crystal 12L

Hops:
.75 East Kent Goldings @ 60minutes

Thanks for your review! I hope I get a shiny new brew kettle soon, but for now I'd like to get my partial boil math down!

 

[kgg_033]

Math is right on, in both grain yeild and total wort conversion to 5.5 gal.
Nice! You didn't list intended IBUs. Assuming 5% Alpha onthe hops, I have it at 21 IBUs

 

[Krrazy]

Thank you! I appreciate the feedback.

 

[Krrazy]

Just thinking about the hops...would someone please go through the IBU calculation for me on a partial boil? I've read utilization is less...do most folks just estimate?

 

[kgg_033]

Correct: Utilization is less. It's not as simple as accounting for dillution when adding your 2.5 gallons of water. The higher the gravity of the boil, the lower the utilization.
Answering this question has proved more helpful to me than it will be to you! The 21 IBUs i gave you was based on an online calculator at Tastybrew.com, but because you asked, I went through the hands-on formulas. YOUR IBUs WILL BE 11 FOR THIS RECIPE. The complete recipe calculator at tastybrew.com has a formula error, but if I use the 'IBU only' calculator from the same site, it confimrs my math below to be 11 IBUs (and my math is based on John Palmer). So considering i always used the complete recipe calculator, I can see why my bitterness often tastes 'off'
Step 1: AlphaAcid Units (AAU) = weight x percent AlhpaAcid (do not convert to decimal)
Step 2: IBU = AAU x Utilization (see chart below) x 75 (constant) / Recipe's Volume
So for your Recipe (again, assuming AA% of 5.0, check your bag of EKG):
0.75 x 5.0 = 3.75 AAU
3.75 x .231 x 75 / 5.5 = 12 IBUs (note the 0.231 is based on the starting gravity of the boil, which is based on 4 gallons)
Link to chart: http://www.howtobrew.com/section1/chapter5-5.html
**Can someone else weigh in on the IBUs and confirm? I'd be interested to see what other people arrive at**

 

[dcp27]

**Can someone else weigh in on the IBUs and confirm? I'd be interested to see what other people arrive at**

you did it correctly. although I believe it'll actually be closer to ~10IBUs, cuz I use the average boil gravity. the difference is insignificant tho

 

[Krrazy]

Great information kgg_033 and dcp27! Thank you for your replies and the decomposition of the IBU formulas for partial boils. I think 10 IBUs is at the bottom of the style guidelines for a Mild so I may bump up the addition a bit just to have a more balanced product based on your help.

I've also seen the concept of AAU expressed as HBU (Home Bittering Units) in the Beer Capture Book for what it's worth.