Malolactic fermentation: practical Q&A for winemakers

This technical guide presents a few questions we have been asked about malolactic fermentation and the use of our Anchor and Maloferm bacteria.

Download this technical resource listing many questions you may have had with their detailed answers.

WHY PAY FOR BACTERIA IF MALOLACTIC FERMENTATION (MLF) HAPPENS SPONTANEOUSLY?

Spontaneous MLF means losing control over timing, microbial population and fermentation outcome.
Without inoculation, the development of indigenous bacteria can lead to slow or unpredictable fermentations, with an increased risk of spoilage (by bacteria or by Brettanomyces).
In practice, this may result in deviations such as volatile acidity, mousy off-flavours or bitterness (e.g. acrolein), as well as textural defects linked to compounds like mannitol. It can also delay cellar operations and create
inconsistencies between batches.

KEY MESSAGE: Inoculation with selected wine bacteria avoids contamination and/or spoilage.

HOW TO AVOID SPONTANEOUS MLF IF YOU PREFER CONTROLLED FERMENTATION?

pH adjustment to pH 3.5 or lower (in combination with moderate SO₂: less than 50 ppm).
Co-inoculation with a robust Oenococcus oeni / Lactiplantibacillus plantarum / blend of O. oeni and L. plantarum for the early dominance of a selected strain.
Inoculate at the recommended dosage of 1 g/hL and avoid propagation to ensure the efficacy of the inoculated culture.

KEY MESSAGE: Co-inoculation with selected commercial cultures ensures a controlled fermentation.

WHY SHOULD YOU CARE ABOUT USING SPECIFIC WINE LACTIC ACID BACTERIA (LAB)?

The selection and use of specific wine LAB serve two main purposes: it allows for a secure, complete and timely MLF, especially under difficult conditions, as well as being a biological tool, similar to the use of a selected yeast strain that can enhance a specific sensory profile of the final wine, be it increased complexity, aroma or varietal characters.
Different bacteria strains have different metabolic capabilities and can ferment under a range of pH, alcohol, sulphur concentrations and temperature conditions.
Propagating bacteria creates a dual risk: you reduce the selected bacteria population size, which makes the bacteria less efficient and increases the risk of delayed, sluggish or stuck malolactic fermentation. Similarly, the impact on the sensory profile is reduced/diminished.

KEY MESSAGE: A selected commercial bacteria is a biological tool to ensure a successful fermentation and enhance the sensory profile of the wine.

WHEN IS THE BEST TIME TO INOCULATE BACTERIA?

The decision on when to inoculate depends on both practical and wine style considerations. In general, coinoculation of yeast and bacteria (0-24 hours after yeast) can be a tool to facilitate MLF in more difficult wine conditions or cultivars and result in a more fruit-driven, fresher wine style (less diacetyl results in less masking of fruit aromas). Sequential inoculation allows for the
completion of AF, after which the bacteria are inoculated, with wine styles being more complex, buttery and structured.

KEY MESSAGE: Decide on your final wine style and practical winemaking to select when to inoculate.

WHAT IS THE BEST WAY TO ADD MY BACTERIA?

Liquid and frozen cultures can be more challenging in terms of transport, storage and shelf-life. Freeze-dried cultures offer greater stability and are easier to handle in the winery.

They can be inoculated:

directly from the sachet
after rehydration (10 minutes in chlorine-free water, 1:20 ratio) to improve dispersion

KEY MESSAGE: Always follow supplier recommendations regarding usage instructions.

WHAT ARE SOME OF THE MAIN FACTORS
INFLUENCING THE SUCCESS OF MLF?

Some of the main juice/wine parameters include the following (tolerance is strain-dependent):

pH: ideal > 3.4 ; more difficult < 3.1 ; pH levels >3.7 increase the risk of spontaneous MLF and the presence of spoilage bacteria, e.g. Pediococcus.
Fermentation temperature: ideal 18-22 °C; more challenging < 14 °C and > 29 °C.
Alcohol: ideal < 13%; more challenging >15%.
Sulphur: ideal conditions are free SO₂ < 5 ppm and total SO₂ < 30 ppm; MLF becomes more challenging above 12 ppm free and 50–60 ppm total. The molecular SO₂ fraction increases at lower pH and higher temperature and/or alcohol, with lethal levels around 0.3–0.5 mg/L.
Malic acid concentration: ideal 2-4 g/L; more challenging < 1 g/L and > 5 g/L.
Yeast selection & fermentation quality: A malolactic-friendly yeast with low to moderate nitrogen requirements supports a smooth alcoholic fermentation. This reduces risks for malolactic fermentation, influenced by the yeast through malic acid consumption, nutrient modification, and production of ethanol, sulphur, and inhibitory peptides or fatty acids.
Bacterial nutrition: nutrient deficiencies can limit malic acid consumption; targeted solutions such as Maxaferm ML Force support bacterial activity and improve MLF reliability.

KEY MESSAGE: Measure your juice/wine parameters and ensure a successful alcoholic fermentation.

CAN THE LAB SELECTION INFLUENCE THE AROMA / FLAVOUR OF THE FINISHED
WINE?

Findings have shown that the genetic diversity within wine LAB is very rich and that it can have implications for the sensory profile:

Decreased acidity/increased softness due to malic acid degradation.
An increase/decrease in buttery/creamy notes linked to the bacteria citric acid metabolism and timing of inoculation (less diacetyl production during coinoculation).
Decrease in green or vegetative aromas by metabolising aldehydes, leading to a more refined sensory profile.
Increased fruity, floral or spicy aromas based on bacteria-specific esterase and/or glycosidase activity.
During ageing, enhanced oak aromas due to glycosidase activity and interaction with oak.
The production of other compounds (polysaccharides, etc.) that can influence the astringency, volume and mouthfeel perception.

KEY MESSAGE: The selection of specific LAB strains can be a tool to tailor wine flavour, aroma, and ageing potential.

WHAT TO DO IN THE CASE OF A SLOW OR
STUCK MLF?

A stuck malolactic fermentation is difficult to resolve, disrupts cellar operations and delays downstream processing. Several factors may be involved, including inhibitory residues (e.g. pesticides, chitosan, lysozyme), unsuitable bacteria use or timing of inoculation.

The first step is to evaluate the main wine conditions and their possible solutions:

If the alcohol level is higher than the tolerance of the bacteria, a restart may be necessary (using a strain with a higher alcohol tolerance).
Low temperature is the most common reason for slow and stuck MLF. Cellar temperatures are often significantly lower than the optimal range for bacteria. Try warming the tank or barrels to 18-22 °C.
If the wine pH is lower than the tolerance of the bacteria, a restart will be necessary using a pH-compatible strain.
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.
Lactic acid > 1.5 g/L can slow MLF and levels > 3 g/L can inhibit MLF. If initial malic acid > 7.0 g/L, an inhibitory amount of lactic acid may be produced from the malolactic conversion and a complete MLF may not be possible without blending or other corrective actions.

If the delayed MLF is not linked to one of the conditions mentioned before, consider the following corrective actions:

Try stirring tanks or barrels more frequently to keep bacteria in suspension.
If nutrients are deficient, bacteria may not consume malic acid. Malolactic nutrients (e.g. Maxaferm ML Force) can compensate for deficiencies and support MLF completion.
Sometimes toxins can be present that inhibit MLF. Extraferm® D’tox, a yeast cell wall for treating stuck fermentations, can be extremely beneficial for detoxification.
The start of MLF requires at least 10⁶ cells/mL. If you did not inoculate, the indigenous bacteria population may not reach this required biomass, and you may need to inoculate with a commercial bacteria strain.
Under certain conditions, MLF may take weeks to months to complete. Measure the malic acid, which should be below 0.3 g/L for a complete fermentation.

If all the mentioned factors have been evaluated, measured and/or adjusted for and the fermentation is still stuck, the next step is to inoculate/restart the fermentation. This step requires three solutions:

The use of specific yeast cell walls. Extraferm^® D’tox will help to detoxify the wine by trapping the main inhibitors, mainly pesticide residues and medium-chain unsaturated fatty acids.
Inoculation with a robust O. oeni bacteria. Addition of a robust bacteria like Anchor^® Solo Select or Maloferm^® Plus at a 2 – 2.5 g/hL dosage is recommended.
Addition of bacteria nutrient. Maxaferm^® ML Force is rich in bioavailable peptides, minerals, and vitamins and also contains yeast cell walls, which adsorb fatty acids that inhibit bacteria.