behaviour
Diacetyl rest
A deliberate warm hold near the end of fermentation that lets yeast reabsorb the diacetyl it produced earlier.
The full record
Diacetyl rest has its own catalogue entry with the detail this page summarises — mechanism, temperatures and what goes wrong.
Diacetyl rest — Raise a fermentation’s temperature near its end so the yeast reabsorbs and reduces the diacetyl it produced earlier.
Yeast produces alpha-acetolactate during growth, which oxidises outside the cell into diacetyl — the butterscotch compound. The same yeast will then take diacetyl back up and reduce it, but only while it is still active and warm enough.
Chilling a lager too early traps the diacetyl permanently, because the yeast drops out before it finishes the job. Raising the temperature by a few degrees for a day or two near terminal gravity is the standard fix, and it is why lagers are not simply fermented cold from start to finish.
Behaviour
| Fermentation temperature | 16–20°C |
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