Beer tastes or smells like butter, butterscotch or movie-theatre popcorn
mediumDiacetyl is a fermentation byproduct produced by all yeast strains. Healthy fermentation reabsorbs it; truncated fermentation leaves it in the finished beer.
Likely causes
| Cause | Process article | Ingredient | Notes |
|---|---|---|---|
| Fermentation ended too early — primary terminated before the yeast cleaned up | Fermentation basics | — | Hold the beer at fermentation temperature for an extra 2-3 days after gravity stabilises (a "diacetyl rest"). |
| Yeast underpitched, leaving cells overworked | Fermentation basics | Fermentis SafAle S-04 | Pitch a fresh sachet at the correct rate. Stale or rehydrated-and-shocked yeast underperforms. |
How to confirm it’s diacetyl
Warm a sample to room temperature, swirl it, and smell deeply. Diacetyl is unmistakable — buttery, slightly slick on the tongue.
Fix in the fermenter
The simplest fix is the diacetyl rest above: raise temperature 2-3 °C for 48 hours once primary fermentation finishes. The active yeast will reabsorb diacetyl back into precursor compounds.
Fix in the glass
Once packaged, diacetyl is here to stay. Future batches: pitch fresh yeast at the right rate, ferment at the lower end of the yeast’s range, then warm at the end.