Stuck Fermentation & High Final Gravity
A stuck fermentation is one that stops before reaching its expected final gravity (FG). Confirm it first with stable hydrometer readings 2–3 days apart — don’t trust the airlock. The most common causes are cold temperature, tired or under-pitched yeast, and too-high a mash temperature (which leaves unfermentable sugars). Fixes, in order: warm it up, rouse the yeast, add yeast nutrient, and if all else fails re-pitch fresh yeast.
You expected the gravity to drop to, say, 1.012, but it’s parked at 1.030 and not budging. That’s a stuck (or stalled) fermentation. First, make sure it’s actually stuck and not just finished or slow.
Confirm it’s really stuck
Take a hydrometer reading, wait 2–3 days, and take another. If the numbers are the same but the gravity is higher than expected, it’s genuinely stuck. If the readings are still dropping, it’s just slow — leave it alone. Remember the airlock tells you nothing reliable here; only gravity does.
Why fermentations stall
- Too cold — the simplest and most common cause. A temperature drop puts yeast to sleep and it settles out. A fermenter in a cold garage is a classic culprit.
- Under-pitched or unhealthy yeast — too few viable cells to finish, especially in higher-gravity beers.
- Mash too hot (all-grain) — mashing high leaves long-chain dextrins the yeast can’t ferment, so a high FG here isn’t stuck at all; it’s baked into the wort.
- Low nutrients / oxygen — under-aerated or nutrient-poor wort stresses yeast into quitting early.
How to restart it (in order)
- Warm it up. Move the fermenter to 68–70 °F. In most cases this alone wakes the yeast and restarts fermentation. Give it a day or two.
- Rouse the yeast. Gently swirl the fermenter (or stir with a sanitized spoon) to put settled yeast back into suspension.
- Add yeast nutrient and rouse — this gives the existing cells fuel to finish.
- Pitch fresh yeast. If it’s truly dead, add an active yeast starter (a 2-quart starter for a 5-gallon batch is plenty).
Prevent it next time
Pitch enough healthy yeast (use a pitch-rate calculator), aerate the wort before pitching, hold a steady fermentation temperature, and — for all-grain — mash at the temperature your recipe specifies. Getting these right up front prevents the large majority of stuck fermentations.
Frequently asked questions
How do I know if fermentation is stuck or just finished?
Take two hydrometer readings 2–3 days apart. If they are identical but higher than your expected final gravity, it is stuck. If they match and are near your target, it is simply finished. Airlock activity is not a reliable indicator.
What is the first thing to try for a stuck fermentation?
Warm the fermenter to about 68–70°F and gently rouse the yeast back into suspension. Cold is the most common cause, and warming plus rousing restarts most stalled batches within a day or two.
My final gravity is high but readings are stable. Is that a problem?
Not necessarily. If you mashed at a high temperature (all-grain), the wort contains unfermentable sugars, so a higher stable FG is expected and the beer is finished. Taste it — it may simply be fuller-bodied than planned.