Stuck fermentation — diagnosis and rescue
When fermentation halts before reaching target FG, what to check first, and how to restart safely.
Last updated 5 April 2026 · 8 min read
Confirm it’s actually stuck
Before you do anything, confirm fermentation has truly stopped. Take a hydrometer reading three days apart. If the gravity hasn’t moved for 72 hours, it’s stuck. If it’s moving slowly, it isn’t stuck — it’s just slow, and slow can be fine.
Common false-stuck causes:
- A sluggish lag phase being mistaken for finished fermentation (give it 72 hours after pitch before worrying)
- Reading SG off a refractometer post-fermentation without correcting for alcohol — your “FG of 1.020” might really be 1.012
The five-minute diagnosis
| Check | Why | Action |
|---|---|---|
| Is the temperature too low? | Yeast slow or hibernate at low temp | Raise to mid-range for the strain |
| Is the target FG realistic? | Expected ≠ achievable in some grist designs | A mash at 70 °C gives unfermentable sugars; that’s the FG you’re going to get |
| Was the pitch rate adequate? | Under-pitching causes early flame-out | Repitch fresh yeast |
| Is the wort within yeast’s alcohol tolerance? | Some strains tap out at 9-10% | Switch to a higher-tolerance strain to finish |
| Is there yeast in suspension? | Excessive flocculation can leave residual sugar | Gently rouse the yeast |
Recovery procedure (in order)
- Raise temperature by 2-3 °C and wait 48 hours. Most stuck ferments restart from temperature alone.
- Gently rouse the yeast by swirling the fermenter (don’t introduce oxygen).
- Pitch fresh yeast — a fresh sachet of US-05 will out-compete tired yeast and finish almost any stuck ale.
- For high-gravity stalls, pitch a high-tolerance strain (champagne yeast or a saison strain) to finish.
Prevention
- Pitch correctly. Underpitching is the #1 cause.
- Aerate well. Cold wort needs oxygen at pitch; warm wort gets oxidised, so aerate before pitch.
- Don’t mash too high. A 70 °C+ mash will produce wort that no yeast can fully ferment.
- Match strain to gravity. A 1.080 wort needs more cells and more tolerance than a 1.050 wort.
Related products
- Fermentis SafAle S-04A clean, fast-fermenting English ale strain in dry form — a workhorse for bitters, milds and porters.
- Fermentis SafAle US-05The dry-form American ale workhorse. Clean, neutral, high attenuation — the right choice when you want your malt and hops to do all the talking.
- Fermentis SafLager W-34/70The dry lager standard — a Weihenstephan 34/70 derivative producing clean, slightly sulphury, deeply attenuating ferments. Bohemian Pilsners and Helles start here.