Changing recipe - hop utilization calcs

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Frothy

Changing recipe - hop utilization calcs

Post by Frothy » Tue Oct 03, 2006 9:47 pm

hallo

I recently made an "English Bitter" recipe from Palmers How to brew. The recipe uses extract and a partial boil of 11.3L which is then diluted to 19L(5 gal US.) I really like the beer but now I'd like to repeat this same recipe but using the all grain method. I now must use a full boil method

Q: is it worth adjusting the quantities of hops used in the recipe to allow for the increased boil volume and changed boil gravity?

The original partial boil recipe was supposed to provide 35IBU.
Using the calculation below the only alteration I have made is for the new full boil gravity and the affect this has on utilization "U"
IBU = AAU x U x(75/5)
at 60, 30 & 15 minutes of boil time the total is now 32IBU

Am I right or am I left? I thought that increased boil volume actually increased utilization? although the equation doesn't take this into account.
Matt

tribs

Post by tribs » Tue Oct 03, 2006 11:08 pm

Partial boil
IBU = AAU x U x (75/5)
35 = AAU x U x 15
U = 2.3333 x AAU

Full boil
32 = AAU x U x 15
U = 2.1333 x AAU

Since AAU is constant and the utilisation increases with a lower gravity boil (i.e. full boil) either the original 35 IBU is incorrect or you have not got the correct value for U.

Frothy

Post by Frothy » Tue Oct 03, 2006 11:58 pm

Ah fair play
Looking at the recipe it uses only 1/2 the malt extract during the partial boil so making the boil gravity ~ 1.040 which is actually lower than the full boil version @ ~ 1.047. Thus my confusion - thanks Tribs. Is this the equation that is commonly used? I've encountered some confusion as apparently changing the conversion factor to 10 allows the use of Metric measurements (IBU is in g/L anyway)

IBU = AAU x U x (10/V)

U has no units, V is now in litres but how then do we work out the AAU's? which is typically
AAU = %alpha acid x hop weight (oz)

Matt

tribs

Post by tribs » Fri Oct 06, 2006 3:10 pm

AAU is now AA% x hop weight (grams)

IBU is mg/L of AA

/100 for the AA% as a decimal, x1000 (mg -> g) = 10 (factor used above)

Hope that makes sense.

Frothy

Post by Frothy » Mon Oct 09, 2006 12:40 pm

thanks mrTribs I worked with this today and it turned out just great - feels so much better to finally be metric after so much bother about UK & US gallons. I swear they only invented US gallons to be difficult.

Matt

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