[timberati]

The Jul/Aug 2010 issue of Zymurgy lists the hop bill below in the Pliny the Elder recipe. Beer Alchemy calculates the IBUs in PtL at 236. The recipe says the actual (not calculated) IBUs for PtL are 90-95. Why the vast difference between calculated and actual?

HOP BILL
3.50 oz Columbus 13.90% A.A. 90 min.
.75 oz Columbus 13.90% A.A. 45 min.
1.00 oz Simcoe 12.30% A.A. 30 min.
1.00 oz Columbus 8.00% A.A. 0 min.
2.50 oz Simcoe 12.30% A.A. 0 min.
1.00 oz Columbus 13.90% A.A. Dry Hop (12 to 14 Days Total)
1.00 oz Centennial 9.10% A.A. Dry Hop (12 to 14 Days Total)
1.00 oz Simcoe 12.30% A.A. Dry Hop (12 to 14 Days Total)
.25 oz Columbus13.90% A.A. Dry Hop (5 days to go in dry hop)
.25 oz Centennial 9.10% A.A. Dry Hop (5 days to go in dry hop)
.25 oz Simcoe 12.30% A.A. Dry Hop (5 days to go in dry hop)

[tschmidlin]

There is a limit to the solubility of isomerized alpha acids, and the limit is affected by wort gravity.  The yeast will also pull some out when it sediments.  The formulas are fine up to a point, and above that they're just wrong.  I wouldn't trust any calculation over 80 IBUs (and the higher it is the more it will be off), but have not tested it directly.  The taste is more important than a calculated number anyway.

[timberati]

Thanks. I'm new to all the calculations and their limitations and when I head to some of the books I get into the weeds pretty quick.

[hopfenundmalz]

Tom explained it.  I can only add that if you see a claim for anything over 110 IBUs or so, be very sceptical.   Small breweries and brewpubs don't have labs, or most don't bother with the testing.  Vinnie had his Pliny the Elder tested, so that is why he has the actual number.

[markaberrant]

We brewed a DIPA last year with 2lbs of hops in a 5gallon batch.  Calculated IBUs were well over 200, can't remember offhand.  Had it lab measured, and it came in at 105, which is actually very impressive.

[timberati]

There is a limit to the solubility of isomerized alpha acids, and the limit is affected by wort gravity.  The yeast will also pull some out when it sediments.  The formulas are fine up to a point, and above that they're just wrong. 

If the upper limit is ~ 90-105, then why add more hops than what a 90-105 calculation might give you? What's the payoff in adding more hops? (Don't get me wrong. I love hops. I want a hop flavored cologne.) Is it analogous to adding sugar in iced tea, where the saturation limit is reached and the additional sugar just ends up at the bottom of the glass or is there more flavor and aroma?

[dbeechum]

There's more flavor and aroma.

Remember when we talk about a practical limit of solubility, that's just for the alpha acids. All of those other oils and compounds have their own limits.

So yeah, you may not get more bitterness, but there's going to be more oomph.

[maxieboy]

I love hop "oomph"! Time to brew PTE again...