How to Fix a Stuck Fermentation

What Is a Stuck Fermentation?

A stuck fermentation is when your beer stops fermenting before it has reached its expected final gravity — meaning there are still fermentable sugars left in the wort that the yeast hasn't converted. If you bottle a stuck fermentation, you risk over-carbonated or even exploding bottles. If you leave it, you'll end up with a sweeter, lower-alcohol beer than intended.

The good news is that stuck fermentations are usually fixable. Here's how to identify one and what to do about it.

How Do You Know If Fermentation Is Stuck?

The only reliable way to know is with your hydrometer. If your gravity reading is well above the expected final gravity (FG) for your recipe, and the reading hasn't changed over 48 hours, fermentation may be stuck.

Don't rely on the airlock — a lack of bubbling doesn't necessarily mean fermentation is stuck; it may simply have finished, or there could be a small leak in the seal.

Common Causes

Temperature Too Low

Yeast becomes sluggish or dormant if the temperature drops too low. Ale yeast typically needs at least 15–18°C to ferment actively. If your fermentation area has cooled down — a common issue in UK winters — this is often the culprit.

Fix: Gently warm the fermentation vessel (a heat mat or moving it to a warmer location often does the trick) and give the yeast a gentle swirl to rouse it back into suspension.

Underpitching Yeast

Using too little yeast for the volume or gravity of your beer can result in an incomplete fermentation.

Fix: Pitch additional yeast. Gervin GV7 Restart Yeast is specifically formulated to re-start stuck fermentations. Sprinkle it directly onto the beer and maintain a warm fermentation temperature.

Lack of Yeast Nutrients

In high-gravity beers or fruit wines, the wort may lack the nutrients yeast needs to complete fermentation.

Fix: Add a teaspoon of yeast nutrient to provide the nitrogen and trace elements the yeast needs to keep going.

High Alcohol Level

Yeast has a natural alcohol tolerance — once the ABV reaches a certain level, it becomes inhibited. This is more common in wine and high-gravity beer recipes than standard ales.

Fix: Use a high-tolerance yeast strain, or accept that the fermentation has reached its natural limit.

Poor Yeast Health

Old or poorly stored yeast may not have enough viable cells to fully ferment the wort.

Fix: Always use yeast within its best-before date and store it correctly (cool and dry). When in doubt, pitch fresh yeast.

Step-by-Step: Rescuing a Stuck Fermentation

1. Confirm it's actually stuck with two hydrometer readings 24 hours apart
2. Check and correct the temperature — aim for 20–22°C
3. Gently swirl or rouse the fermenter to re-suspend the yeast
4. If no improvement after 24 hours, pitch Gervin GV7 Restart Yeast
5. Add yeast nutrient if brewing a high-gravity or fruit-based beer
6. Wait and monitor — fermentation should resume within 24–48 hours

Find restart yeast, yeast nutrient, and hydrometers at Brewco.uk to keep your fermentations on track.