Malolactic fermentation is a winemaking process by which certain bacteria convert sharp malic (apple) acid, found naturally in wine, to less sour lactic (milk) acid. Since malic acid remaining in red wines can make their tannins taste harsher, this softening is desired for almost all red wines. It also can be encouraged in white wines like Chardonnay, which are not particularly aromatic. One by-product of the process is diacetyl, which has a buttery aroma, often associated with highly regarded Chardonnays.
The yeasts that initially turn grape juice into wine create an atmosphere of carbon dioxide (CO2); and, since malolactic bacteria need oxygen to survive, their “secondary” fermentation usually does not occur until after the grape juice has become wine and the CO2 has dissipated.
In addition, the wine shouldn’t be too cold (kept above 20C/ 68F), and its protective level of Sulfur Dioxide (SO2) can’t be very high. This is no problem for red wines, somewhat protected by their tannins. But white wines are kept cooler to preserve grape aromas and need more help from SO2 to repel oxidizing bacteria.
How Does Malolactic Fermentation Apply to Traditional Wine Making Techniques?
Malolactic fermentation has happened naturally for centuries, long before winemakers understood the process, so it’s a very traditional winemaking technique. The required bacteria are usually present in the winery, but they can be purchased, just as some winemakers purchase commercial yeasts instead of relying on yeast in the winery’s atmosphere. It’s a very natural way to make a wine taste softer and easier to drink.
WSET addresses this process differently at different Levels. In Level 2 we wait until our Chardonnay week, so we can show its effects by pouring, side by side, bottles with and without “ML.” In Level 3 we teach it during winemaking week, and return to it in several subsequent weeks, such as white Burgundy week, again using illustrative examples. In Level 4, which has six separate units, it’s part of our vinification unit; our unit on light wines (e.g., again, White Burgundy); and even our sparkling wine unit, as some Champagnes go through the process.