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As title really - I just noticed a few different recipes, different sites and authors, that specify AAU for bittering and finishing hops, but dry hops just by mass.

Is it because the amount is typically so much less that the variation in alpha-acid% for a given variety isn't going to make much difference? Or perhaps alpha acids just don't come into play without boiling?

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Dry-hopping does not impart bitterness.

The bitterness in the beer originates in the isomerization of hop alpha acids in the heat of the boil.

Since dry hopping is done cold, no AAU units of bitterness are generated.

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  • Actually... see: scottjanish.com/increasing-bitterness-dry-hopping and scottjanish.com/dry-hopping-effect-bitterness-ibu-testing These articles cite and summarize a fairly well known (at least in my circles) study done by J.P. Maye (of Hopsteiner), which kind of goes a long way to upending the conventional wisdom about bitterness in dry-hopped beer. Really fascinating stuff, it's sort of the cutting-edge of practically-based beer research (if you ask me).
    – Franklin P Combs
    Commented Sep 9, 2021 at 1:45
  • @FranklinPCombs: I know, but these are a different kind of bitterness. Make a hop tea at 70°-80° C, and you will notice how bitter this is. However, it is not possible to assign IBU, because it is not caused by the chemical reactions which create the bitterness in boiling. When you would dilute the hop tea to the same volume as a brewed beer, there will not be much left of this bop tea bitterness anymore.
    – chthon
    Commented Sep 9, 2021 at 5:20
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    True, I take your point that it's not feasible, given commonly available information, to assign a quantitative value to the bitterness imparted (although new research seems to indicate it's possible to make a fair estimation). In particular, though, I was responding to the statement that '[d]ry-hopping does not impart bitterness'. It very much can. In my opinion, the posts I link to (and the studies they cite) do more to undercut the trustworthiness of the IBU test as the general measure of sensory bitterness than anything else (especially with regard to dry-hopped beers).
    – Franklin P Combs
    Commented Sep 9, 2021 at 19:28
  • @FranklinPCombs: True, one should probably start using terms as isomerized bitterness (which can be measured) and "taste bitterness" or "raw hop bitterness" or "herbal bitterness" maybe.
    – chthon
    Commented Sep 10, 2021 at 6:11
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Its the overall oil content in hops that you're interested when dry hopping. Alpha-acids don't always reflect those amounts and the diversity. The data is available to some extent but you need to know the lot and manufacturer of the hops you have and track it back. Most homebrew shops don't track/offer that info with repackaged hops in-house.

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