Glossary
Isomerisation
The heat-driven rearrangement of hop alpha acids into iso-alpha-acids, which unlike their precursors are both bitter and soluble in beer.
Pronounced eye-SOM-er-eye-ZAY-shun.
Also called isomerization, iso-alpha-acid, iso alpha acids.
This is why hops must be boiled to bitter beer. Raw alpha acids are barely soluble and barely bitter; the isomerised form is both. The reaction is slow, which is why bittering hops go in at the start of a 60-minute boil and aroma hops added at the end contribute almost no bitterness.
It is also why dry hopping adds aroma but essentially no bitterness — there is no heat to drive the rearrangement.
Related
- Alpha acid — The precursor.
- IBU — The measure of the product.
- Dry hopping — The technique that deliberately avoids it.
More vocabulary
Arrested fermentation · Barrel ageing · Batch sparging · Biotransformation · Blending · Bottle conditioning · Bottom fermentation · Burtonisation · Cask conditioning · Centrifugation
See every term on the glossary index.
Last reviewed 2026-08-16.