Fault
Stalled fermentation
Fermentation stopping well short of terminal gravity — leaving a sweet, underdeveloped beer and a package-pressure risk.
Also called stuck fermentation.
What it tastes and smells like
The important practical consequence is not flavour but safety of packaging. A beer that appears finished but is merely stalled will resume fermenting in the bottle, potentially producing enough pressure to burst it. Two identical gravity readings 48 hours apart is the only reliable test.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- Grisette — A low-strength Walloon farmhouse ale, historically brewed for miners — dry, wheaty and gently peppery.
- Sahti — A Finnish farmhouse ale filtered through juniper branches and fermented with baking yeast — banana, juniper and no hops.
- Saison — Dry, peppery and highly carbonated, with a bracing finish — the most refreshing thing strong beer does.
If you are drinking it
This is a brewing-side fault. For a homebrewer, the important point is never to package on a falling gravity — a stalled beer that restarts in the bottle can build dangerous pressure.
Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.
What causes it
| Cause | Stage | How often | Who can act |
|---|---|---|---|
| Temperature falling mid-fermentation, causing yeast to flocculate out before finishing. | fermentation | most common | brewer |
| Inadequate free amino nitrogen or zinc, common in high-adjunct or high-sugar worts. | fermentation | common | brewer |
| Too little viable yeast for the wort gravity, especially in strong beers. | fermentation | common | brewer |
If you are brewing it
- Brewer: Raise the temperature by a few degrees and rouse the yeast gently back into suspension. (mitigates)
- Brewer: Pitch an adequate starter, add yeast nutrient for high-gravity or adjunct-heavy worts, and oxygenate properly. (prevents)
Where in the process this comes from
Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.
- Mashing — Hold milled malt in hot water so the grain’s own enzymes convert its starch into sugars that yeast can ferment.
- Pitching — Add yeast to cooled, oxygenated wort at a rate matched to the volume, gravity and fermentation temperature.
- Primary fermentation — Convert wort sugars into ethanol, carbon dioxide and the esters, phenols and higher alcohols that give beer much of its character.
- Wort oxygenation — Dissolve oxygen into cooled wort before pitching, because yeast needs it to build the cell membranes required for healthy growth.
- Yeast harvesting — Collect yeast from a finished fermentation and pitch it into the next batch, generation after generation.
Often confused with
- Unintended sweetness — cloying sweetness, syrupy body
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
The risk in a stalled fermentation is not the flavour but the package. Sugar left unfermented at bottling ferments afterwards, and the CO₂ it produces raises internal pressure in a container that has a rated limit.
Checked against the source
Source: Thongon et al., "Mathematical modeling and optimization of pasteurization for the internal pressure and physical quality of canned beer" (2023), results, internal pressure and can bulging (Heliyon 9(11):e21493)