"Desired IBU" value in Mark Garetz equation in Ray Daniels book

[likeybikey]

I'm reading Ray Daniels' Designing Great Beers and on pages 85-86 he talks about augmenting the IBU calculation for a beer by including a "hop concentration factor," or HCF, as designed by Mark Garetz:

HCF = ((Final Volume/Boil Volume) * Desired IBU)/260 + 1

Main question here: I don't understand how I am supposed to come up with a "Desired IBU" value.

The HCF then goes in the denominator of the overall IBU calculation on page 79. So,

Numerator = (Hop Weight * Hop Utilization decimal value * Alpha Acid decimal value * 7489)

Denominator = Final Volume * Gravity Correction

And with HCF added into the original equation:

IBU = Numerator/(Denominator * HCF)

If the whole point of the equation is to calculate IBUs, what value would I be putting in for "Desired IBU" to come up with the HCF in the first place? If someone out there has used this equation, can you perhaps illustrate an example?

Thanks in advance.

 

[Talgrath]

Desired IBU is the IBU you want to hit for the beer. The issue with calculating how much hops you need to add is that it isn't linear because the wort can only absorb so much, the more you add the less effective each individual addition is. So, HCF's purpose is to tell you how effective your hop additions will be when you add them, it influences further IBU calculations because it tells you how effective each addition is. I don't have the book in front of me and I can't find the second formula you mention right now on the online copy, so I'm not sure about the second one. Personally, I just let a recipe calculator figure it out for me after I build the recipe.

 

[Weezy]

I wouldn't use that. A lot has changed since that book was written and we don't know where the magic 260 number comes from.

Besides we know wort density does not have a meaningful effect on hop utilization, until you get to around 100 ibus. So, if you do partial boils and, say you're brewing a 5 gal batch of beer with 75 ibus...but you only plan to partial boil with 3 gallons and top off later. Then you might have to worry, because at 3 gallons, your trying to pack >100 ibus in there.

5/3 x 75 = 125 ibus.

If you do full volume boils its moot.

Also, we use hops for more than bittering anymore. We use a ton of late hops for the flavor and aroma, all the while the equations tell use those hops are adding IBUs. The equations are not accurate, but they are our guide. They're a ruler to gauge against your taste buds. You just need to train your taste buds to know that "the equation says this is 30 ibus; this is what 30 ibus tastes like." and you can adjust recipes based on your palate.

 

[likeybikey]

Thanks, Talgrath. I think I see what Garetz and Daniels were getting at after your explanation. The numerical results seem a bit capricious (if I add 37 IBUs and desire 37 IBUs I will end up with 33 IBUs but if I add 37 IBUs and desire 20 IBUs I will get 35 IBUs), but I at least understand the main idea that if you have a lot of hops floating around they will inhibit each other from doing their thing.

Weezy--yeah, I have been using Brewtarget to calculate all this stuff for me, but I've been enjoying the Daniels book and was trying to grasp some of the math behind the magical numbers that Brewtarget and other software spits out. I agree the Daniels book is showing its age, in many ways. I heard an interview with him recently on the Beersmith website where he said his publisher wants him to update the book but he hasn't found the time. I have seen people here at homebrewtalk tout Glenn Tinseth's equations as the current preferred standard. I'll look into those as well. In the end I'm less concerned about IBUs than gravity, but that equation was frustrating me so I thought I'd turn to the homebrewtalk community to see what people thought.

Thanks to you both!

 

[Weezy]

Love that book. Anybody interested in building their own recipes can learn a lot from that book. The process he goes through and teaches you, in general, is timeless, even if some of the formulations and results relating to certain beer styles are dated.

Tinseth is heavily reliant on wort boil gravity, which is incorrect. Rager does not. But, like I said, the numbers these things spit out are just a gauge to use against your palette.

 

[Talgrath]

Glad to help, personally i don't worry about IBU's because they aren't necessarily the best indicator of flavor. They give you a rough estimate of bitterness, but there's so much more that goes into flavor from hops than just a straight bitterness measurement, my amber ale is on the high end of hoppy for the style, but you wouldn't know it thanks to Nelson Sauvin hops, for example.