A quiet airlock is not proof of a stuck beer fermentation. First verify a corrected gravity trend, compare the result with the yeast's published attenuation and the recipe, and check the measured beer temperature. Only then choose a targeted intervention; adding sugar, nutrients, oxygen, or random yeast without a diagnosis can make the beer worse.
Define the problem with measurements
Wyeast describes a stuck fermentation as one that began normally and then stopped or became very slow partway through. That differs from a delayed start, a leak that hides airlock activity, and a fermentation that has reached its true terminal gravity. The remedy depends on which situation you actually have.
Take a representative, degassed hydrometer sample with sanitized equipment. Correct for the instrument's calibration temperature and known offset. Repeat after an appropriate interval for the recipe. A refractometer reading taken after fermentation cannot be interpreted as uncorrected Brix because alcohol changes refraction.
Calculate apparent attenuation from OG and the current gravity, then compare it with the yeast maker's current range. A value outside the range is a clue, not a verdict. Mash temperature, unfermentable ingredients, measurement error, wort composition, yeast handling, and process temperature can all change the finish.
Beer fermentation diagnosis table
| Evidence | Likely interpretation | Next safe check |
|---|---|---|
| Airlock quiet, gravity still falling | Fermentation is active; the vessel may leak gas elsewhere | Leave it closed and follow the gravity trend |
| Gravity stable and within a plausible attenuation range | Fermentation may be complete | Confirm recipe, sensory condition, and cleanup plan |
| Gravity stable and unexpectedly high | Possible stall, measurement error, or low-fermentability wort | Verify hydrometer, temperature, recipe, and yeast data |
| No gravity change after pitching | Delayed start, nonviable/insufficient yeast, or measurement problem | Check elapsed time, temperature, yeast handling, and OG |
| Gravity falls again after gentle warming within the strain range | Cool temperature likely contributed | Hold a supported temperature and monitor |
| Unexpected sourness, film, or phenolic character | Possible contamination rather than a simple yeast stall | Isolate the batch and assess conservatively |
A staged diagnostic workflow
1. Verify the gravity instrument and sample
Check the hydrometer in water near its calibration temperature, inspect it for a shifted scale, and degas the beer sample. Bubbles attached to the stem can make it float higher. If a corrected second reading is lower, the batch is slow, not stopped. Follow the detailedOG and FG measurement workflow.
2. Check beer temperature against the exact strain
Measure the vessel, not just the room. Compare that reading with the yeast maker's published range. White Labs warns that a large temperature drop can slow fermentation or make a culture dormant. If the beer is too cool, move it gradually into the supported range and wait for a gravity response.
3. Reconstruct the pitch
Record yeast product, package age or harvest history, storage, pitch amount, wort volume, OG, and pitching temperature. The American Homebrewers Association identifies low viability as a cause of incomplete or stuck fermentation and undesirable fermentation byproducts. Wyeast lists pitching rate among the principal causes of stalls.
4. Review wort fermentability and ingredients
A high finishing gravity is not always a failed fermentation. A warm mash, extract composition, crystal malt, lactose, maltodextrin, or other less-fermentable material can leave gravity that healthy brewing yeast will not reduce. Compare with the recipe design and ingredient specifications before pitching a more aggressive organism.
5. Look for contamination evidence
A stalled gravity reading alone does not establish contamination. Conversely, a restart will not correct an unintended microbial process. Inspect and smell without tasting a visibly suspect beer. Keep questionable equipment separate until it can be cleaned and sanitized appropriately.
Choose an intervention that matches the cause
Begin with the least invasive supported action. If measured temperature is below the strain range, warm gradually within the range. If a highly flocculent yeast dropped early, the yeast manufacturer's guidance may support gently resuspending it without splashing. If pitch viability or quantity was inadequate, use the maker's restart procedure or obtain advice from the yeast supplier rather than dumping in an arbitrary package.
Wyeast identifies viability, nutrients, malt quality, oxygenation, and pitch rate as principal causes. Most of these are best prevented before or at pitching. That list is not a recipe for adding all five after a stall. Nutrients have product-specific composition and timing; late additions can be ineffective or alter the beer. Oxygen is important for growth at the correct stage, but introducing air to partly fermented beer also creates an oxidation risk.
For a high-gravity beer near the yeast's alcohol tolerance, a restart is a different problem from warming a standard-strength ale. Alcohol, osmotic stress, and nutrient limitation can all affect cells. Use a strain and acclimatization process documented for restart work, or consult the manufacturer. Do not assume that more table sugar will “wake up” yeast; it changes the recipe without necessarily enabling maltose or maltotriose fermentation.
Worked example: high gravity does not automatically mean stuck
A beer started at OG 1.060 and now reads 1.018. Apparent attenuation is:
((1.060 − 1.018) ÷ 0.060) × 100 = 70%
The airlock is quiet. Before calling this a stall, the brewer degasses a sample, verifies the hydrometer, and repeats the corrected reading. The yeast maker lists 68–72% apparent attenuation, and the recipe contains a meaningful amount of crystal malt. A second 1.018 reading may therefore be a plausible terminal result, not proof that yeast must be added.
In a different batch with the same OG, the same strain, a simpler grist, and stable readings at 1.030 while the beer is below the manufacturer's temperature range, the evidence points elsewhere. The first action is to restore a supported temperature and watch gravity—not add sugar. Enter OG and the current reading in the Beer ABV Calculator to see the attenuation, but treat the number as a diagnostic input rather than a completion certificate.
What not to do to a stalled beer
- Do not diagnose from bubbles, krausen, or elapsed days alone.
- Do not splash or aerate partly fermented beer as a default restart step.
- Do not add sugar merely to make the airlock move.
- Do not add nutrients without checking their composition, timing, and maker guidance.
- Do not pitch a highly attenuative or mixed culture without accepting the flavor and packaging consequences.
- Do not package an unexpectedly high gravity just because it has been unchanged briefly.
Changing several variables at once destroys the diagnosis. If warming, rousing, nutrient, and new yeast are all added together, a successful batch teaches little and an unsuccessful batch becomes harder to troubleshoot. Record one justified action, its time, and subsequent gravity.
Know when the beer is ready to package
Packaging requires stable, plausible gravity plus a process assessment. It is not enough that the beer stopped moving today. Confirm the instrument, compare attenuation with the yeast and recipe, complete any planneddiacetyl cleanup, and make sure the sensory profile shows no credible contamination.
Active fermentation in a sealed package produces carbon dioxide. Priming an unfinished beer adds more fermentable material and can create excessive pressure. If completion remains uncertain, do not bottle. Get help from the recipe author, yeast supplier, or an experienced local brewer with the full batch record.
Safety note
Use sanitized tools for every sample and intervention. Avoid improvised heating near glass vessels, electrical hazards, and sealed fermenters without pressure relief. Do not drink beer with credible contamination signs. FermentWise does not provide a laboratory alcohol or microbial assessment; gravity-based ABV is an estimate and does not identify the organism in the batch.