Stuck Beer Fermentation: Diagnosis and Fixes

A stuck fermentation announces itself quietly: bubbling stops, the airlock goes still, and the hydrometer keeps returning the same final gravity - several points above where the recipe promised. Before reaching for fixes, confirm it is actually stuck (two readings, days apart, identical) and compute where you stand: the attenuation calculator turns OG and FG into an apparent attenuation, and the ABV calculator tells you what alcohol is already in the keg. The gravity fundamentals are in the gravity and ABV guide.

1. Temperature: the most common stall

Ale yeast slows dramatically as the ferment cools - fermentation generates its own heat, so a batch that started brisk at 20 °C may drift down to 15 °C as activity fades, right when the yeast needs warmth to finish the last sugars. Raise the fermenter 2-3 °C and give it two quiet days; a large share of stalls end here. The same temperature-compounds-time rule the winter guide covers for every other ferment applies to beer yeast too.

2. Yeast: population and health

Underpitching - one fresh pack into a strong wort - leaves too few cells to finish, and old packs lose viability in storage. Gently swirling the fermenter to rouse settled yeast (without splashing) often restarts a stall by itself. If the culture is truly exhausted, pitching a fresh, rehydrated sachet is the reliable move - the pitching rate calculator tells you how many cells the wort actually needs.

3. Fermentability: what is left to eat

Sometimes the yeast is fine and the sugar is the problem. A mash that ran hot leaves more unfermentable dextrins behind, and the beer stalls not because yeast quit but because the remaining sugar is not food for them. That kind of "stall" at high attenuation may simply be the beer's real final gravity - taste it: sweet and heavy suggests unfinished work, dry but dense points to dextrins. If a genuinely fermentable stall resists all else, a champagne or wine yeast restart is the last resort: it is flavor-neutral but strips the beer dry.

4. Prevention beats repair

Pitch the right cell count at the right temperature, aerate the wort before pitching, and hold fermentation temperature steady in the yeast's comfort range - the three habits prevent most stalls before they start. Keep notes: the batch that stalls at the same gravity two brews in a row is telling you something systematic about your mash or your yeast handling, not about luck.