Just read this Beer and Brewing article on iso-alpha acids in hops and am trying to understand something.

The 3rd paragraph in that article states:

"Iso-alpha acids are formed typically during wort boiling, when hops are added, and alpha acids are extracted from the hops’ lupulin glands (or from the hop resin in hop extracts). Isomerization is the result of the effect of heat on the alpha acids. The process is both time and temperature dependent. The longer the alpha acids are exposed to the rolling wort boil, the more alpha acids are converted into iso-alpha acids and the more bitterness is created in the wort and beer. See boiling. Hops that are added late in the boil, on the other hand, still release their aroma oils—and these will not all evaporate into the kettle stack—but a much smaller proportion of their alpha acids will become iso-alpha acids."

So it sounds like:

  • the hops' lupulin glands produce these alpha acids which in and of themselves are simply aromatics; they only contribute hops aromas/smells
  • when hops are added to the boil, their alpa acids leach into the boil and produce hop aromas
  • if they stay in the boil for a certain length of time, the temperature isomerizes these hop alpha acids (or is it resins?) which is what creates the hallmark bitter hop taste
  • and if the isomerized acids ("iso-alpha acids") stay in the boil for another certain length of time, they end up boiling out altogether, leaving neither bitter hop taste nor hop aroma behind

Can someone begin by clarifying/correcting me if any of my understanding from above is incorrect? Particularly on the difference between what alpha acids and iso-alpha acids contribute vs hop resins.

And once I'm set straight on that, my real question here is: what are the "certain points" mentioned above? In other words, at what point in the boil (minutes-wise) do hops isomerize? At what point do all hop flavors & aromas boil out altogether? Thanks in advance!

1 Answer 1


The hops' lupulin glands do not only produce alpha-acids. They produce also other volatile substances (oils, resins) which give flavor and aroma to beer. And the hops also contain tannins which also enter into the beer upon boiling.

The alpha-acid are primarily responsible for the isomerized alpha-acid bitterness in beer. This is influenced by the alpha-acid percentage of the hop, the amount of hop used and the duration of the boil.

The isomerized alpha-acids, or the hop bitterness, will not be removed from the boil by longer boiling.

The tannins (akin to the vegetal material of the hops) will also contribute to the flavor and mouthfeel of the beer. Brew to beers with the same recipe, only a bittering addition but from two different hops, they will taste different. This flavour will also remain in the beer.

And then there are the volatile compounds in the lupulin, aromatic oils, resins, etc. These have all different evaporation temperatures and resistance against boiling. These are indeed the ones that will disappear during the boil. That is why, to get an effect from these compounds in the beer, hops are added at the end (or the last quarter, or different other times, or even during fermentation and conditioning: dry hopping) of the boil. They will contribute different substances towards flavor and aroma of the beer.

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