Taking Control of Malolactic Fermentation with your Wine

Most wine, under the right conditions, naturally goes through the chain of reactions called malolactic fermentation.

Today, many home and commercial winemakers employ malolactic fermentation, though they also can prevent it if they choose.

What Is ML?

A malolactic (ML for short) fermentation is so called because it involves a microorganism (bacteria) and a chain of chemical reactions similar in principle to yeast fermentation. In the primary fermentation of a wine, yeast turns sugar into ethanol and carbon dioxide gas. In a malolactic fermentation a class of bacteria, usually of the Lactobacillus or Leuconostoc species, turns the malic acid naturally present in grapes and other fruits into lactic acid and carbon dioxide gas.

The bacteria live on the grapes, on winery equipment, and in the air. They also can be purchased as a pure culture and introduced by the winemaker before, during, or after the primary yeast fermentation.

The lactic acid bacteria effect three major changes in juices, musts, and wines: stabilization, deacidification, and the addition of flavor and aroma compounds. Each winemaker should evaluate his program and decide whether these effects will make the wine better.


Because lactic acid bacteria is widespread in the winemaking environment and malic acid is naturally present in juices, musts, and wines, there is always the possibility that a malolactic fermentation will occur. This is wonderful if you are encouraging it but potentially disastrous if you aren’t. Many winemakers actively encourage malolactic fermentation at an early stage because if the malic acid isn’t all used up by the time the wine is bottled, then the malolactic fermentation can happen after the wine is bottled. If it does, the bottled wine will be spritzy and prone to unpleasant
off-flavors and aromas.

Since the lactic acid bacteria aren’t the only microorganisms that can use malic acid as a food source or a substrate for other necessities, ML fermentation can be an insurance policy. If you encourage the disappearance of malic acid in a controlled manner, there’s no chance that any spoilage bacteria can use it later. This is why the malolactic fermentation is said to “stabilize” young wines.


Before the development of appropriate soil management, viticultural operations, and modern rootstocks, winemakers often had to deal with unripe, poorly balanced grapes that were too low in sugar and too high in acid. For vintners dwelling in the colder northern regions of Europe, it was difficult to get their grapes to ripen enough to make a wine that wasn’t too tart. Enter the malolactic fermentation that, even today, is a boon for those who must make wine with fruit that is less than perfect.

The lactic acid bacteria take up malic acid as a carbon source and spit out lactic acid in the same ratio. Lactic acid is a “less acidic” acid per unit than is malic acid. Once all the malic acid in the wine has been consumed and turned into lactic acid, the wine essentially has been de-acidified to a moderate degree. Some say that malic acid is a harsher acid on the palate and that ML fermentation smooths out the rough edges.

Even if your grapes are perfectly ripe, you can use the ML fermentation to stabilize, de-acidify, or make flavor changes. Be careful, however, because malic acid decreases with ripeness, so any grapes more than 22° Brix may need additional acid in the form of tartaric acid (TA) to make up for the acid lost due to the ML fermentation. Red wines typically should have total acidity in the 4.5 to 7.5 grams per 100 mL region and whites should be around five to eight grams per 100 mL. It’s a good idea to keep an eye on the TA before, during, and after any malolactic fermentation to make sure the process hasn’t de-acidified your wine too much. A normal ML fermentation will not drop the TA too much below these levels if the rest of the acids are in their appropriate balances.

Aroma Changes

ML fermentation can cause a significant and noticeable difference in the flavor profile of wine. As a byproduct of the ML fermentation, many lactic acid bacteria generate a compound called diacetyl, which shows up in wine as a buttery aroma.

This buttery note adds complexity to dry white and red wine varietals that also see a little oak aging, namely Chardonnay, Sauvignon Blanc, Cabernet Sauvignon, Merlot, and Pinot Noir. Vintners employ ML in many other winemaking schemes, excluding their white wines only when fruitiness, low alcohol, or high acidity might make both the buttery aroma and the acidity-lowering effects unwelcome.

Aside from the diacetyl alone, the lactic acid bacteria produce other compounds that have not been shown to consistently contribute any significant aroma/flavor changes to wine, though some think they add to the overall complexity. It is up to you to determine whether or not these flavor and aroma changes fit into your personal winemaking style.

Taking Charge of ML

The following advice covers how to encourage malolactic fermentation. To discourage any lactic acid bacteria from carrying out an ML fermentation, simply do the opposite. The ML fermentation can last for several days or be protracted over a few weeks, depending on the strain of bacteria, the amount of malic acid present, temperature and pH of the wine, and other factors.

Buying a commercial strain of lactic acid bacteria through a reliable winemaking supply source is best. Follow the directions for inoculation included with the bacteria. You can monitor ML fermentation — watch for the disappearance of malic acid (once it is gone, the ML fermentation is over) with a chromatographic solvent kit, available through home winemaking stores, catalogs, and Web sites. Or you can simply wait for any visible (or audible) evidence of carbon dioxide gas to disappear.

ML in Primary or Secondary

• To ensure that the bacteria will have enough malic acid to use as energy, do not use grapes that are too ripe (more than 24° to 25° Brix). Check the TA of your wines during all stages of their early development, adjusting with tartaric acid if the levels get below your liking.

• Keep the sulfur dioxide in your  wine low (as low as you can safely allow, keeping below 20 ppm free). Lactic acid bacteria are very sensitive to SO2 as an antimicrobial agent.

• Keep the pH between 3.2 and 4. Lactic acid bacteria can’t survive in a very low pH environment.

• Keep the temperature of your juices, musts, or wine warmer than 55° F. If the ambient temperature is not warm enough, wrap an electric blanket around the fermenter.

• Don’t filter your juice or wine at any point before inoculating with the lactic acid bacteria. The bacteria are nutritionally fastidious and do poorly on material that has been stripped of vitamins and nutrients.

• Add yeast hulls (yeast ghosts) or a vitamin mix for yeast to give the bacteria the vitamins they need.

Issue: October 1999