By Dan Mouer

To ML or Not to ML??

As May comes around I find myself making tasting notes on the new wines in bulk storage, trying to decide whether or not they have gone through malolactic fermentation (ML) or not. If they haven’t I decide whether or not to force the issue by inoculating the wines with a culture of ML bacteria such as that packaged by Wyeast. For many years I was completely mystified about ML, which is traditionally called “secondary fermentation.”

I began making wine twenty years ago using traditional Old World grapes (Vitis vinifera) grown in Central Virginia. Typically, grapes from Eastern North America are not harvested fully ripe and are, therefore, a bit high in total titratable acidity (TA). After the first (alcoholic) fermentation would be completed in my wines—sometimes long after—a slow, quiet secondary fermentation would usually commence unless I kept the sulfite levels fairly high. I was afraid that “secondary fermentation” meant bacterial “spoilage,” wbich is precisely how many California winemakers viewed it (and many still do!). However, when this secondary fermentation ended, usually well into the Spring sometime, I found the wines not to be spoiled at all. Rather, they had transformed themselves from somewhat harsh, green wines to rounder, more supple ones, complete with new nuances of complexity in their flavors and aromas.

Malolactic fermentation is a completely normal part of the transformation of grapes to wine. Certain strains of bacteria, which live on the grapes in the vineyard, slowly attack malic acid and transform it to lactic acid. Malic is the acid that gives green apples their sharp flavor. Lactic is the much milder acid that lends tanginess to yogurt and buttermilk. Wines that undergo ML come out with reduced TA and higher pH. In California, where wines ripen easily and are usually harvested with low acidity, ML is dangerous. Very low acid wines are subject to “flabby” flavor, at best, and to microbial infection and oxidation, at worst. For that reason, California winemakers tend to keep sufite levels up and temperatures down to discourage the action of ML bacteria. Mass-market wines from hot regions like the Central Valley are sterile filtered to remove ML bacteria to prevent secondary fermentation in the bottle.

These days it is easy to get excellent wine grapes, juice, and concentrates from all over the world just by stopping by your local retailer. It’s a challenge to the hobby winemaker to decide whether or not a given batch will profit from, or be hurt by, malolactic fermentation. Sterile musts and concentrates in wine kits do not contain any ML bacteria. If, however, you ferment in the same cellar where you have fermented wine pressed from fresh grapes, or if you have inoculated another batch with a cultured strain, then ML bacteria will probably be growing wild in your environment and can easily find its way into the wine. If you don’t want a wine to undergo secondary fermentation, then maintain about 50 ppm of free SO₂ and try to keep temperatures well below 70 degrees. If you do want ML, inoculate your wines using a good commercially cultured strain following the manufacturer’s instructions.

With the except of wines with very low acidity, malolactic fermentation is essential for getting the best wine from your juice. Most wines will benefit from ML. Get used to thinking of your need to manage two distinctive fermentations in your wine, and you will be well rewarded.