Troubleshooting & Restarting Stuck Malolactic Fermentations

Last updated: 6/2021

Applies to: Winemakers dealing with stuck malolactic fermentations. This guide contains factors that affect the success of MLF and a protocol for re-start.

There are many factors that can influence the success of a malolactic fermentation (MLF). Before jumping into a full restart, it is a good idea to assess each of these factors. Sometimes only a small adjustment is needed to help a MLF complete successfully. If MLF continues to struggle after considering the factors listed below, a full MLF restart may be necessary. Our MLF restart protocol can be found at the end of this article.

Factors to Consider

Wine temperature might be too low.

Try warming the tank or barrels; 18-20°C (64-68°F) is optimal. A MLF will progress much slower at cooler temperatures.

Wine might be lacking nutrients.

Try adding an ML-specific nutrient like ML RED BOOST™ or OPTI’MALO BLANC™.

Wine might need a detox.

If you have already done a nutrient addition, the bacteria should have what it needs. Sometimes toxins can be present that impede the success of an MLF. RESKUE™, a specific inactivated yeast for treating stuck fermentations, can be extremely beneficial for detoxification. RESKUE should be added and racked after 48 hours.

Wine chemistry might be challenging or inhibitory.

It is essential that you check your pH, alcohol, and free and total SO2 to see if one of these factors might be inhibiting the fermentation. Winemakers are often surprised by the amount of SO2 in a wine even if they have added little to no SO2. SO2 can come from several sources including yeast during alcoholic fermentation, vineyards, old barrels, or erroneous cellar additions. Total SO2 is just as important to check. SO2 is often bound to acetaldehyde and when bacteria consume acetaldehyde, free SO2 is liberated which can inhibit MLF. If wine chemistry is challenging, make sure you have inoculated with a strain that can handle the challenges.

There may not be enough healthy bacteria.

If you did not inoculate with Oenococcus oeni, there may not be enough healthy bacteria to complete MLF. Consider inoculating with a known strain that is appropriately suited to handle the chemistry of your wine. If you have already inoculated, and the wine has low turbidity, it is possible that the bacteria are struggling to stay in suspension. Try stirring your tanks or barrels more frequently.

You might just need to give it more time.

MLF can be a test of patience taking weeks, or even months, to complete. To determine if MLF is complete, malic acid must be measured — it is nearly impossible to determine completion by sensory analysis — and is considered complete when malic acid is ≤0.1g/L (some say ≤0.2 g/L). If initial malic acid content was high (>7.0 g/L) then lactic acid produced by MLF may be high. Lactic acid levels >3g/L can inhibit ML bacteria.


Prepare the Stuck Wine:

  • step 1 Add 30 g/hL (2.5 lb/1000 gal) of RESKUE™ prior to restarting. Suspend RESKUE in 10 times its weight of warm water 30–37°C (86–98°F). Wait 20 minutes then add to stuck wine
  • step 2 Allow tank to settle for 48 hours then rack off the settled lees.
  • step 3 Adjust temperature of RESKUE-treated wine to 18–22°C (64–72°F)

Malolactic Nutrient Addition:

  • step 4 Add 20 g/hL (1.7 lb/1000 gal) of ML RED BOOST™ to RESKUE-treated wine. When restarting a stuck MLF, ML RED BOOST is used for white, red, and rosé wines.
  • step 5 Mix gently and wait 24 hours before bacteria addition.

Malolactic Bacteria Addition

  • step 6 Add a double dose of LALVIN VP41™ direct inoculation culture (Example: for 1000 gallons, add 3 x 25hL (660 gal) packets).
  • step 7 Check for MLF activity by analyzing L-malic acid degradation every 2–4 days.
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