How many times have you heard someone say “This Chard is a butter bomb!”? If you’ve wondered what makes certain wines produce such unctuous buttery qualities, it’s malolactic fermentation, “MLF” in winemaker lingo, a secondary fermentation process where malic acid is converted into lactic acid. Lactic acid is also commonly found in milk. So when the malic acid is converted to lactic acid, the wine obtains a creamier, sometimes even “buttery” quality. MLF is responsible for other things aside from producing buttery flavors: it helps de-acidify wine, adds flavor and complexity, and changes the microbial environment in a wine. MLF typically happens shortly after the end of primary fermentation (when a grape’s sugar is converted into alcohol by yeast) when Lactic acid bacteria break down the grape’s malic acid (similar in flavor as tart green apple) and convert it into lactic acid (the buttery flavor), releasing CO2 in the process.
So is malolactic fermentation a good thing for wines? In short, MLF lowers the acidity, smoothening the rough edges of harsh wine. It can add body, smooth tannin in red wines, and add interesting flavor notes such as honey, vanilla, nuts. It’s also why wines which undergo MLF pair beautifully with foods that are enhanced by butter, think crab, lobster, and light pasta. But, it is possible to have too much of a good thing, so winemakers must be careful not to allow too much MLF to avoid developing “rancid butter” or “burnt caramel” notes that can dominate a wine and prevent the other natural fruit and mineral notes from shining through. And some wines taste better with more green, fruity, and acidic qualities (i.e. a crisp sauvignon blanc).
In general, MLF tends to enhance a wine’s body and flavor; by reducing acidity, the wine tastes softer and less crisp, may be less fruity in character but slightly more complex. Malolactic fermentation is often done intentionally by winemakers to avoid it happening in the bottle without their control. Since ML bacteria grow much more slowly than yeast and do not produce nearly as much CO2, it’s a lot less noticeable that malolactic fermentation is taking place. This can fool novice winemakers into bottling wine before ML is complete, which can spoil wine if MLF begins again in the bottle. Since wine microbes grow better in low acid conditions, MLF can make a wine more prone to spoiling, so wines with less MLF that are more highly acidic tend to be better suited for aging, over a similar wine with low acid. However, winemakers can add tartaric acid to a wine when MLF is complete to help compensate for these effects on stability and ageability.
Wines that are most famous for undergoing MLF are full-bodied dry white wines (like Chardonnay) to medium to full bodied dry reds (like Cabernet). Most reds, sparkling wines and about 20% of whites benefit from MLF. In general, since the overarching effect of MLF is the lowering of acidity, it’s best for wines that start out a bit too tart rather than in wines that are already low in acid. So the next time you smell or taste “butter” in your Chardonnay, rest assured, no one dropped a pat in your glass, voila: it’s Malolactic Fermentation.