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Malolactic fermentation (MLF) is stuck or sluggish


A stuck malolactic fermentation (MLF) is a condition where malolactic bacteria (MLB) – good LAB – are no longer able to convert malic acid into lactic acid because of adverse or unfavorable environmental conditions. A sluggish MLF occurs when MLB are struggling to ferment, and it could potentially stop fermenting altogether and become stuck. As there are several factors or causes that can impact MLB activity, stuck MLF is a common occurrence if not managed closely. In fact, you should take extra care during MLF as the required environmental conditions also favor growth of spoilage organisms.

All of the listed corrective actions will benefit from the addition of a fresh MLB culture and nutrients, such as OPTI’MALO PLUS, and, for more difficult conditions, you can first condition the MLB inoculum using commercial apple juice. In addition, for the alcoholic fermentation, choose a yeast strain that favors MLF; some yeast strains are known to inhibit MLB.

Musts from grapes or fresh juice may already contain MLB for MLF to start on its own accord if all conditions are favorable. You should still add a MLB culture to these types of musts to avoid complications.

Concentrates and reconstituted juices are not malolactic-compatible since MLB have been eradicated during the concentration or sterilization procedures. These musts have also been tartrate-stabilized during their production and thus contain a very high proportion of malic acid, which would be converted to lactic acid. The wine would have very little acid, and a pH above 3.8 making it very susceptible to bacterial infections. Therefore, MLF should not be attempted in these types of musts.

Assuming that you have determined the desired style of wine, assessed the environmental conditions, selected an appropriate MLB culture, and confirmed the presence of malic acid by paper chromatography, possible causes of stuck MLF include:




Fermentation temperature is too low or too high
MLB are typically inhibited at temperatures below 18° C (60° F), or as specified by the MLB culture manufacturer. If your fermentation area’s temperature is too low or too high, increase or decrease temperature accordingly to get the must or wine within the recommended range, and very gently stir the MLB culture back into suspension. Do not introduce too much air as most home winemaking MLB strains are anaerobic and therefore sensitive to oxygen.

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Oxygen (air) has inhibited MLB
If you suspect that MLB was overly exposed to air and caused MLF to become stuck, re-inoculate the must or wine with a fresh MLB culture, stir very gently, and ensure that the container is properly topped up and protected from air using a fermentation lock. Avoid any oxidation-promoting operations, such as racking and stirring, during MLF.

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SO2 content is too high
MLB are very sensitive to SO2. For MLF to be successful the SO2 levels should be low enough to allow the bacteria culture to function successfully. Different MLB cultures have different tolerances to SO2 so use a bacteria culture that has a high SO2 tolerance in high SO2 wines.

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pH is too low
MLB can also become inhibited in low-pH musts or wines. The typical minimum pH value is 3.2 although different MLB strains can have slightly different requirements. Lower total acidity (TA) or increase pH, and re-inoculate with a fresh MLB culture, if required.

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Alcohol content is too high
Most MLB strains can tolerate alcohol contents of up to 14.0% alc./vol. If your strain has a lower tolerance or if your wine contains a relatively high level of alcohol, there may not be much you can do except try re-inoculating with a high-alcohol-tolerant MLB strain. If you expect to ferment to a high alcohol level or if you intend to use a weaker MLB strain, inoculate the wine concurrently with the alcoholic fermentation, a technique known as co-inoculation. The MLB will adapt to the higher alcohol levels as they increase and have a much better chance of completing MLF.

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MLB have insufficient nutrients
MLB also need nutrients to properly convert malic acid into lactic acid. Carry out the MLF on the nutrient-rich fine lees. If MLF is stuck even in the presence of fine lees, gently stir the lees back into suspension. Do not stir vigorously; otherwise MLB can become inhibited from excessive exposure to air. Add yeast nutrients, or preferably MLB nutrients, such as OPTI’MALO PLUS, in addition to stirring the lees, or if there are no lees to provide nutrients.

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High levels of inhibitory polyphenols
Red wine with high levels of inhibitory polyphenols can also cause MLF to be inhibited or become sluggish. If you suspect that your wine has an unusually high level of polyphenols, add natural yeast derivative nutrients, such as OptiRED, post fermentation at a rate of 30 g/hL, and then re-inoculate with a fresh MLB culture.

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Lysozyme concentration is too high
If you treated the must or wine with lysozyme, MLB have most probably been inhibited. You can correct this problem by “deactivating” the lysozyme using bentonite and reconditioning the must or wine for a favorable MLF. Prepare a bentonite solution, and add to the must or wine at a rate of 5–20 g/hL. Wait 6–12 hours, rack off the bentonite and then re-inoculate the must or wine with a fresh MLB culture, ensuring that all other MLF environmental conditions are appropriate.

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Low MLB count
All of the above corrective actions will benefit from the addition of a fresh MLB culture and nutrients, such as OPTI’MALO PLUS, and, for more difficult conditions, you can first condition the MLB inoculum using commercial apple juice. In addition, for the alcoholic fermentation, choose a yeast strain that favors MLF; some yeast strains are known to inhibit MLB.

Musts from grapes or fresh juice may already contain MLB for MLF to start on its own accord if all conditions are favorable. You should still add a MLB culture to these types of musts to avoid complications.

Back to Top or Back to Troubleshooting Page

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