By Dominique Fink, MSc Biology
What are the Risks of an Unmanaged Malolactic Fermentation?
Once the alcoholic fermentation is over and done, it is time to think about the malolactic fermentation.
Why perform a malolactic fermentation?
What are the advantages and associated risks of this secondary fermentation if it becomes sluggish or simply stops?
We will see a few solutions in this short article.
What is Malolactic Fermentation?
As its name implies, malolactic fermentation is the conversion of L-Malic acid into L-Lactic acid through the action of bacteria. This transformation may occur naturally after alcoholic fermentation if microbial flora is already present in the tank. Usually, winemakers add a defined amount of lactic bacteria such as Œnococcus oeni to ensure proper homogeneity and efficiency. This secondary fermentation is often used to modulate sensory attributes and helps stabilize red wine. Some whites and rosés also are positively enhanced by this process. Œnococcus oeni is a good choice for this operation as it is relatively resistant to high alcohol levels and low pH. However, it is best to perform this operation at the end of the alcoholic fermentation to avoid the appearance of mouse taint. This fault may occur when Œnococcus oeni degrades sugars instead of L-malic acid. An end of alcoholic fermentation analysis will help determine if the lactic bacteria will be able to complete the malolactic fermentation. For example, an alcohol by volume higher than 14% v/v might affect the fermentation efficiency. Same goes for a pH lower than 3.3.
Why Perform a Malolactic Fermentation?
This secondary fermentation is generally done on red wines and occasionally on some white wines such as Chardonnay. L-Lactic acid is a softer acid than L-Malic acid and therefore, transformation of L-Malic acid into L-Lactic acid positively affects mouthfeel with increased roundness and suppleness. However, some white grapes like Chardonnay benefit from malolactic fermentation as this operation will lead to the characteristic buttery aromas. On other white or rosé wines, winemakers refrain from doing this secondary fermentation as it will lead to a loss of fruity aromas and loss of freshness or acidity. It all depends on the vision of the winemaker.
What are the Risks if a Malolactic Fermentation Becomes Sluggish?
An unmanaged malolactic fermentation may cause several important wine faults. For example, lactic bacteria such as Lactobacillus may degrade tartaric acid. This action causes an increase in levels of acetic and lactic acids. The wine becomes brown and no longer drinkable. Lactic bacteria can also consume glycerol which increases bitterness significantly. Pediococcus-typed bacteria are responsible for the apparition of glycans therefore increasing viscosity. The wine becomes “stringy” and unpalatable. These faults can be avoided by a quick malolactic start and a quick finish.
What are the Reasons Behind a Sluggish Malolactic Fermentation or if it Stops?
Several factors are important to ensure efficiency. Lactic bacteria are sensitive to sulphites. So during and after the alcoholic fermentation, sulphiting should be avoided. Temperature is also a big factor. It should be maintained near 20-22oC for maximum efficiency. Lactic bacteria do not tolerate well high acidity and high alcohol levels. OEnologists may therefore recommend to decrease acidity by wine deacidification based on the results obtained from the alcoholic fermentation testing.
How Do We Measure L-Malic Acid?
The malolactic fermentation is considered to be over when the L-Malic acid value is under 0.2 g/L. It is thus essential to follow its evolution in order to avoid problems. In the cellar, winemakers generally use paper chromatography which is a very useful and cheap tool for monitoring. A quick check of the malolactic fermentation in the cellar 2 to 3 times per week is good practice. However practical, this method is not very sensitive. It is for this reason that professional laboratories use methods like enzymatic analysis or high pressure liquid chromatography. These methods have quantification levels much lower than 0.2 g/L. Analytical testing through a laboratory will remove the guess work and garantee you that the malolactic fermentation is over. Once this secondary fermentation is done, the following step will be racking and sulphiting in order to avoid transformation of citric acid into diacetyl, an action also done by lactic bacteria.
To Conclude…
Malolactic fermentation is an important step in winemaking. It can lead to many faults if left unchecked but can be well managed by precise testing. The OENOSCIENCE laboratory is at your side during all winemaking steps and notably, during malolactic fermentation.
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