Dealing with Stuck Fermentation

When Fermentation Stops Short

A stuck fermentation occurs when yeast stops working before reaching the expected final gravity. The beer remains sweeter, lower in alcohol, and higher in gravity than intended. It is one of the most frustrating brewing problems.

Is It Actually Stuck?

Before diagnosing a stuck fermentation, verify that fermentation has actually stalled:

Take gravity readings over 3 consecutive days. If gravity is stable, fermentation has stopped.
Check that your expected FG is realistic. Not all worts attenuate to the same degree — highly dextrinous worts (high mash temperature, low-attenuating yeast) legitimately finish at higher gravities.
If using a refractometer, apply the alcohol correction factor. Uncorrected refractometer readings in fermented beer read artificially high.

Common Causes

Temperature too low: Yeast slows dramatically when temperature drops below its optimal range and can go dormant at cold temperatures.

Under-pitching: Insufficient yeast cells for the wort gravity. The small population ferments until exhausted, leaving sugar behind.

Poor nutrition: Low free amino nitrogen (FAN) or zinc deficiency in the wort can limit yeast growth and fermentation capacity.

High gravity stress: Worts above 1.080 OG put osmotic stress on yeast. As alcohol rises above 8-10%, it becomes toxic to many strains.

Flocculation: Highly flocculant yeast strains may drop out of suspension prematurely, especially in conical fermenters.

Corrective Actions

Raise the temperature: If the beer is too cold, warm it to the upper end of the yeast's range (68-72 F for ales). This often restarts activity within 24-48 hours.

Rouse the yeast: Gently swirl or rock the fermenter to resuspend settled yeast. Do not aerate — introducing oxygen at this point would cause oxidation.

Add yeast nutrient: If nutrition is the issue, adding yeast nutrient (DAP, Fermaid-O) can help. Dissolve in a small amount of sterile water and add to the fermenter.

Re-pitch fresh yeast: If the existing yeast is exhausted or dead, pitch a fresh, actively fermenting starter. Use a neutral, high-attenuating strain (US-05 or champagne yeast for extreme cases).

Enzyme addition: For beers that finished high due to unfermentable dextrins, adding amyloglucosidase enzyme can break dextrins into fermentable sugar. Use cautiously — it can over-attenuate.

Prevention

The best cure is prevention:

Pitch adequate yeast (use a calculator)
Oxygenate wort properly
Control fermentation temperature
Use yeast nutrient for high-gravity worts
Choose an appropriate yeast strain for the target gravity
Mash at the correct temperature for desired attenuation

When to Accept It

Sometimes a beer finishes a few points above target FG and still tastes good. If gravity is stable, the beer is clear, and there are no off-flavors, it may simply be what this batch wants to be. Package it and enjoy.