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.

Alcoholic fermentations may become problematic at several points. Sometimes they are slow to start, sometimes they become sluggish, and sometimes they become completely stuck.

Fermentation is Slow to Start:

When yeast is inoculated it undergoes a lag phase where yeast cells are adapting to their new environment. It might seem like nothing is happening (no sugar drop, no bubbles), but the yeast are likely still alive and preparing for growth and active fermentation. If fermentation is slow to start, several factors may be extending the lag phase:

Juice may be too cold

Lag phase length is significantly influenced by temperature. To shorten it, ensure that the fermentation temperature is warm enough but within the suggested temperature range for the selected yeast.

There may be a lack of nutrients

At the start of fermentation, yeast have a high demand for vitamins and minerals and a moderate demand for nitrogen. Grapes naturally contain some of these nutrients, but they may be deficient and/or quickly consumed by native microflora and supplementation is necessary.

Rehydration nutrients can supply vitamins and minerals, and fermentation nutrients added at inoculation can supply necessary nitrogen to get fermentation started. If proper pre-fermentation nutrient additions were not made or the juice/must was otherwise deficient, an addition of a fermentation nutrient (FERMAID™ or STIMULA™) can help invigorate fermentation.

Juice/wine might be too clear

If the juice is over-clarified (<50 NTU), yeast may struggle to stay in suspension during the early phases of fermentation. In this case, increase the turbidity by adding RESKUE™ and stir the fermentation. It is also worth noting that proper use of rehydration nutrients will increase the turbidity of the juice and may help avoid this problem.

Yeast may not have been acclimatized

Rehydrated yeast must have time to acclimatize to juice/must conditions (temperature, pH, etc.). Improper acclimatization can shock the yeast, increasing the lag phase or killing the cells. In this case, a reinoculation might be necessary. To avoid this problem, use the “Saccharomyces Yeast Rehydration” protocol which details proper steps for acclimatization.

There may not be enough yeast

If you did not inoculate or you did not inoculate at the recommended rate, it is possible that there is not a sufficient population of yeast. In this case, a reinoculation might be necessary

Fermentation Becomes Sluggish or Stuck:

The most common fermentation problems are stuck and sluggish fermentations:

  • A fermentation is considered stuck if the sugar has not dropped for >48 hours.
  • A fermentation is sluggish and may become stuck if it is approaching ~1 °Brix and slows to <0.25 °Brix per day

Many times, stuck and sluggish fermentations will require a restart, but not always. Consider the following before conducting a restart:

Temperature may be problematic

Each yeast strain has an optimal temperature range and anything outside of this range can cause yeast stress. 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 reinoculation will be necessary. If the temperature is too low, try slowly warming the wine.

To help avoid this problem, try adjusting the temperature of the wine to 20-22°C (68-72°F), especially toward the end of fermentation.

Alcohol may be too high

Certain yeast strains can tolerate more alcohol than others. If you have exceeded the tolerance level of your selected strain, you may need to reinoculate with a strain that has a higher tolerance.

Wine might need a detoxification

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

There might be unwanted microbial populations

Lactic acid bacteria (LAB) can produce volatile acidity (VA) and inhibit yeast activity. Volatile acidity >0.6 g/L can be challenging for yeast (especially in high alcohol conditions), and VA >0.8 g/L can be inhibitory.

To control LAB, use DELVOZYME if malolactic fermentation (MLF) isn’t complete but is desired, or BACTILESS™ if MLF is complete or not desired. In any condition, uncontrolled LAB should be controlled before trying to restart a stuck fermentation.


If a restart is necessary, use the protocol linked below. Please note, fermentations stuck when the sugar level is >3 °Brix and <11.5% (v/v) alcohol are relatively easy to restart if there are no other compounding factors. Fermentations with less sugar and more alcohol become increasingly difficult to restart.