Troubleshooting Stuck or Sluggish Alcoholic Fermentations

Last updated: 6/2021

Applies to: Winemakers dealing with slow or stuck alcoholic fermentations. It is recommended to read this guide before attempting to restart a stuck fermentation.

Many factors influence the success of a fermentation. If a fermentation becomes sluggish or you suspect it might be stuck, it is wise to get a complete picture of the wine before jumping into a full restart. Sometimes a simple adjustment or a mixing can be enough to get a fermentation back on track. If a restart is necessary, having all the details will help you determine the best course of action.

Identifying A Troublesome Fermentation

Recognizing that a fermentation is sluggish or stuck is the first step in rectifying a challenging situation. A fermentation can be viewed as stuck if the sugar has not dropped for >48 hours. A warning sign that a sluggish fermentation may become stuck is when the fermentation approaches ~1 °Brix and fermentation slows to <0.25 °Brix per day (and the temperature is reasonable).

If you suspect a troublesome fermentation, we recommend measuring or sending out for some lab analysis to get a full picture of your wine and to help determine next steps. Important parameters include:

  • Alcohol
  • Glucose & fructose
  • Malic acid
  • Volatile acidity/acetic acid


If you’ve detected a stuck or sluggish fermentation, consider the following factors that may have contributed to the problem.

The juice/wine might be too clear.

If your juice is clear (<50 NTU), there may not be enough solids to keep the yeast in suspension during the early phases of fermentation. Increasing the turbidity is advised. This can be achieved through an addition of RESKUE™, in addition to stirring. A side benefit of using rehydration nutrients like GO-FERM™ and GOFERM PROTECT EVOLUTION™ is that they also help to increase the turbidity of the juice. To learn more about juice clarification, see our Best Practices Guides for Juice Clarification via Static Settling and Flotation.

The yeast may not have been properly acclimatized.

Rehydrated yeast must have time to acclimatize to the changes in sugar concentration and temperature of juice/must during inoculation. Improper acclimatization can delay the start of fermentation after inoculation. See our Saccharomyces Yeast Rehydration protocol for proper timing and steps for acclimatization.

There may not be enough yeast.

If you didn’t inoculate at the appropriate rate it is possible that you don’t have a sufficient population of yeast. In this case a re-inoculation might be necessary.

The yeast population is not healthy.

You may have an adequate population, but the cells may not be healthy.

The yeast might need additional nutrients.

For a fermentation to be happy and healthy it is essential that the yeast have enough nutrients. Even when starting with high nutrient levels (>300ppm YAN) additional nutrients are almost always needed at 1/3 of the way through the fermentation. If you’re at the tail end of the fermentation with just a small amount of sugar left, an addition of RESKUE and a mixing can be beneficial.

The temperature of the wine is problematic.

Each yeast strain has an optimal temperature range for fermentation. Anything outside of this range can cause stress for the yeast. If your temperature is too low, try warming the wine slowly. If the temperature is too high, or if it spiked at any point, it’s possible that the yeast are no longer viable and a re-inoculation will be necessary. To increase success rates at the end of fermentation, try adjusting the temperature of the wine to 20-22°C (68-72°F).

The alcohol may be too high for your selected yeast strain.

Certain yeast strains can tolerate more alcohol than others. If you have exceeded the tolerance level of your selected strain, you may need to re-inoculate with a strain that has a higher tolerance. Based on your starting sugar, you should always select a strain that will be able to handle the potential alcohol level.

The wine may need a detoxification.

When wine yeast become stressed, they can produce certain compounds that impede fermentation. RESKUE™ can be extremely beneficial in this situation. If possible, RESKUE should be added and racked after 48 hours. If you are unable to rack, adding RESKUE and leaving it in the wine can still help.

There may be other microbial populations you need to address.

Check your malic acid and volatile acidity (VA). If malic acid has dropped, and you have not used malic acid-degrading yeast or inoculated with ML bacteria, you may have a lactic acid bacteria (LAB) infection. A strong LAB population can produce VA and inhibit yeast that are already weak at the end of alcoholic fermentation. VA >0.6 g/L can be challenging for yeast, especially in high alcohol situations, and anything greater than 0.8 g/L can be inhibitory. Unwanted LAB activity can be prevented by Lysozyme if malolactic fermentation (MLF) isn't complete but is desired, or BACTILESS™ if MLF is complete or a MLF is not desired. In any condition, uncontrolled LAB should be controlled before attempting to restart a stuck fermentation.


If a full restart is necessary, it is essential to choose the correct protocol for a successful outcome. If a fermentation sticks and the sugar level is >3 °Brix and alcohol is <11.5% (v/v) it is relatively easy to restart as long as there are no other compounding factors. However, fermentations are more difficult to restart when the alcohol is higher and the sugar is lower. We have two protocols on our website:

Restart a Stuck Fermentation using UVAFERM 43 RESTART™

  • This is our preferred method to restart sluggish and stuck fermentations. This protocol and yeast strain were specifically developed in conjunction with Inter-Rhône to make restarts easier under a wide variety of restart conditions. Other yeast strains (even ones within our portfolio, including UVAFERM 43™) should not be attempted with this one-step acclimatization protocol.

Traditional Method for Restarting a Stuck Fermentation

  • If you’d like to use other yeasts within our portfolio like FERMIVIN® CHAMPION and UVAFERM 43, this method works when sugar is still high (>3 Brix) and alcohol (<11.5% v/v) is still low.