IMHO the calculations are a great tool.
Maybe you have an IBU calculator in your basement. If you don't, what is the point of bringing this up?
I think the calculation formulas are a helpful tool for a brewer. They give you a decent indication over multiple batches of what a beer calculated to "40 IBU's" (with a traditional bittering process) in your brewery tastes like, and if you find it too bitter/not bitter enough, you know that a 30 or 50 IBU calculation may be more to your liking. Also, they give us a common language to speak with. For those reasons, they are a nice tool.
My "point" of bringing this up is that the OP was told that his beer would not be bittered correctly because some BS formula said so. I simply wanted to show that the formulas are BS to begin with, and to tell someone his beer does not calculate to an appropriate IBU level is just perpetuating the BS.
Like it or not, IBU's
are a finite number, just as OG is a finite number. The difference is that we have tools in hand to correctly determine gravity, but the neccessary tools for measuring bitterness are unheard of in a homebrew set-up. So for anyone to say that someone's recipe won't give them enough bitterness because a formula says so is crazy. It is a formula, not a measurement. More so, a hop bill will shine differently from brewhouse to brewhouse.
The Pliny example was easy because it is such a well known hop bomb, but the actual IBU's are far less than the formula, or even Russian River, claim them to be. I'll give another more personal example. I got a recipe for an APA from a commercial brewer who had his beer measured to 40 IBU's. The recipe calls for .3 oz of Magnum at 60, and .8 oz each of Centennial, Cascade, and Amarillo in the 20 minute flameout whirlpool. Plug that recipe into a formula and it will give you about 20 IBU's, but we know (or at least we were told) it actually measures HIGHER. I brewed it as directed, and it has a very similar bitterness to SNPA, which I understand to also be measured to about 40 IBU's. It certainly has more bitterness than a Blonde ale I've tasted that has also been calculated to an identical 20 IBU's. In that case, even the formula's do not jive.
Unlike Pliny, this example shows a formula undercalculating measured bitterness. Formulas will give you some snap shot, and at times can be fairly accurate, but just as often, they are insanely inaccurate. To rely on them blindly is a big error, because there are a lot of regressions at play that the formulas do not know how to handle. As you admitted, things like a whirlpool and a hopback are complete crap shoots, but they are processes that have tremendous impacts on a recipe. I'm sorry, but any tool that is susceptible to such an inaccuracy is a crappy tool. To keep defending them is silly.
I use the formulas too, but I also have some techniques that circumvent their inaccuracy. Admittedly, in many ways, my techniques have the same flaws as the formulas. For instance, my whirlpool addition is always 20 minutes hot. For whatever reason, I always calculate the whirlpool as a 20 minute boil addition. It may be a coincidence, but if you apply it to the APA recipe I mentioned above, I get ~40 IBU's, just as the commercial beer was measured to be. Obviously it is not a measurement, but it is an adjustment to the formula that I can consistently make, and from there, I can adjust recipes as need be.
Again, my point is that the formula is a nice starting point, but each brewer has to dig a little deeper to learn how the formula they choose to use adapts to their system. That advice is most helpful when you are trying to emulate another person's recipe - the final product is much less of a surprise if you know a little more to the story going into brewday.
Joe
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