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Troubleshooting Guide for Sluggish Malolactic Fermentations

Last Updated: 5/2022

Applies to: Winemakers dealing with sluggish malolactic fermentation. This article covers potential factors and solutions to consider before restarting MLF.

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 an MLF complete successfully. Use the guide below to evaluate the potential causes and solutions.

Step 1: Determine if wine conditions are antagonistic to bacteria

Malolactic fermentation (MLF) is affected by wine conditions. When MLF becomes stuck or sluggish, the first step is to evaluate whether wine conditions are problematic:


Temperature may be too low

Low temperature is the most common reason for slow and stuck MLF. Cellar temperatures are often significantly lower than the optimal range for ML bacteria. Try warming the tank or barrels to 18-20°C (64-68°F).

SO2 may be too high

Even if little or no SO2 has been added, it may still be present during MLF. SO2 can come from several sources including yeast, old barrels, and/or erroneous cellar additions.

If total SO2 (TSO2) has exceeded the tolerance of the bacteria strain, the wine should be reinoculated with a strain that has a higher TSO2 tolerance (click here for restart instructions). TSO2 may also be lowered via blending.

Alcohol may be too high

If the wine alcohol level is higher than the tolerance of the bacteria, restart may be necessary using a strain with a higher alcohol tolerance. Click here for restart instructions.

pH may be too low

If the wine pH is lower than the tolerance of the bacteria, a restart will be necessary using a pH-compatible strain. Click here for restart instructions.

Malic and Lactic Acid may be too high

ML bacteria struggle with the malolactic conversion when lactic acid is too high (>3.0 g/L). When starting malic acid is too high (>7.0 g/L), lactic acid produced during MLF can become inhibitory. Lactic acid >1.5 g/L will slow MLF and levels >3 g/L can inhibit MLF. In some cases, a complete MLF may not be possible without blending or other corrective actions.

Step 2:If wine conditions are not antagonistic, consider the following

The bacteria may be struggling to stay in suspension

If the wine has low turbidity, bacteria may struggle to stay in suspension. Try stirring tanks or barrels more frequently.

The wine might be lacking nutrients

Malolactic bacteria have specific nutrient needs, but there are no easy/cost-effective analyses that can help determine deficiencies in wine. ML bacteria require organic acids (malic, citric, pyruvic), organic nitrogen (amino acids, peptides), vitamins (B group), trace minerals (Mn, Mg, K, Na), and low levels of sugar (fructose, glucose). If any nutrients are deficient, bacteria may not consume malic acid.

Malolactic nutrients can compensate for deficiencies. As it is difficult to determine whether a wine is deficient, we recommend using them preventatively or intervening with a nutrient add within 7 days of inoculation if MLF has not started (and temperature is not problematic). Adding a malolactic nutrient is not as critical if bacteria is co-inoculated with yeast.

The wine might need a detox

If you have already added ML nutrients, the bacteria should have what it needs. Sometimes toxins can be present that inhibit 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.

There may not be enough bacteria

If you did not inoculate with Oenococcus oeni, there may not be enough healthy bacteria to complete MLF. In order for MLF to begin, there must be 1 million cells per mL (and native strains may not be able to achieve this population). Consider inoculating with a commercial strain.

It might just need 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 ≤0.1 g/L (some say ≤0.2 g/L).

Step 3:Restart

If the sluggish malolactic fermentation cannot be fixed by changing wine conditions or by trying any of the other solutions presented, click here for restart instructions.


Learn More

To avoid stuck and sluggish malolactic fermentations, consider using the Malolactic Fermentation Feasibility Evaluation. The scorecard rates how antagonistic several wine conditions are to ML bacteria and returns a total point value representing the predicted difficulty of MLF. Based on this value, Scott Laboratories recommends certain steps be taken to maximize success.