Isomerisation of whirlpool hops

A discussion on the IBU contribution of late kettle hops in a whirlpool

I often get a Beersmith printout of a recipe to prepare for customers. One such recipe earlier this week had a range of hop additions simplified as per below:

  • 30g Chinook 11.4% AA at 60 mins boil = 35.7 IBU
  • 30g Chinook 11.4% AA at 5 mins boil = 7.1 IBU
  • 30g Chinook 11.4% AA at 20 mins whirlpool = 10.8 IBU

The whirlpool hops are added at flame-out, with a vigourous stir and passive cooling for 20 minutes or until the wort reaches 80'C. This contributes a smoother hop flavour and aroma. (for more information, read this excellent article on byo)

Beersmith calculates the IBUs for this recipe at 53.6. However the IBU contribution of the 5 minute addition is clearly underestimated as it contributes bitterness for both the 5 minutes of the boil and 20 minutes of the whirlpool.

After a bit of investigation, I opined that the best method for calculating the extra IBUs would be to insert a theoretical extra whirlpool hop addition in order to put a value on the hop addition. You can estimate* the size of this addition using this formula:

Extra Whirlpool hops (g) = Amount 5 min hops (g) * (1 - (5 minute IBU / 90 minute IBU))

So in the example above, the additional theoretical whirlpool hop addition would be:

Extra Whirlpool hops (g) = 30 * (1 - (7.1 / 38.2)) = 24.42g = an extra 8.8 IBU.

Other contribution times can be calculated as well. For instance the whirlpool contribution of the 60 minute hops are calculated as follows:

Extra Whirlpool hops (g) = Amount 60 min hops (g) * (1 - (60 minute IBU / 90 minute IBU))

Extra Whirlpool hops (g) = 30 * (1 - (35.8 / 38.2)) = 1.88g = 0.7 of an IBU

*This is by no means an exact method. It is my opinion that this is vaguely accurate but I would welcome any suggestions or alternative methods. Neither is this article a critique of Beersmith - it's an excellent investment even if (like me) you only use 5% of its potential.

Cheers, beers, and Happy Christmas.