Malic acid is present in almost all fruits and berry plants. Its aroma is associated with green (unripe) apples and a biting taste.
Formerly sporadic, because poorly known and poorly monitored, malolactic fermentation (MLF) is now carried out in a targeted manner; for us, always in barrels and without any external addition of bacteria.
In the vineyard
In the vine, malic acid is formed in the tissue containing the chlorophyll of the berry and participates in the enzymatic reactions that transport energy throughout the vine. The concentration of malic acid in the berries is at its highest just before the colour change and then decreases until the harvest; one more tool to follow the maturity of the grapes. The amount of malic acid at harvest time will depend on the vintage, the growing area, the microclimate, the grape variety and the balance between technological maturity and phenolic maturity chosen by the winegrower. If all the malic acid is consumed before the harvest, the grapes are too ripe or senescent. The wines produced would be flat and insufficiently stable from a microbial point of view, so acidification would have to be carried out, i.e. adding "foreign" acid during vinification.
The mechanisms of malolactic fermentation (MLF) have only been understood since the 1960s and have since been used specifically in wine production.
In the cellar
During the MLF winemaking process, malic acid is reduced again. The action of indigenous lactic acid bacteria transforms the rather aggressive malic acid into lactic acid, which is more pleasant to taste.
For red wines, this conversion is beneficial; it is the disappearance of the green taste, due to the malic acid, which gives way to something softer, mellower and a little sweet. The texture also evolves to give a more voluptuous and round wine. It is said that malolactic fermentation "melts" the tannins. By erasing the taste of the malic acid, the wine matures. The degradation of the acid must nevertheless be controlled so as not to generate turbidity.
One of the few whites for which MLF is systematically used is chardonnay, hence its characteristic buttery aromas. The soft, milky lactic acid contributes to a creamier sensation in the wine. For white wines such as Chenin Blanc and Riesling, and for most rosé wines, MLF produces unpleasant buttery aromas on tasting; the wines lose their fruitiness and liveliness. To avoid this, they are sulphited just after alcoholic fermentation.
Therefore, malolactic fermentation is important for the final taste of the wine as well as for its evolution over time. It is part of all the processes necessary for the elaboration of most great red wines. And as always, it's all about balance!