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Malolactic Fermentation: Complete Guide

Updated: February 2026 | Reading Time: 12 minutes

If you've ever savored the buttery smoothness of a well-aged Chardonnay or the soft, rounded character of a fine Burgundy, you've experienced the magic of malolactic fermentation. This secondary fermentation is one of the most transformative processes in winemaking, converting sharp, green-apple acids into softer, creamier lactic acids that fundamentally change a wine's character.

Unlike primary fermentation—which most winemakers understand as the conversion of sugar to alcohol—malolactic fermentation (often called "MLF") is optional, controllable, and surprisingly nuanced. Understanding when to encourage it, when to prevent it, and how to manage it puts a powerful tool in your winemaking toolkit.

What Is Malolactic Fermentation?

Malolactic fermentation is a biological process where specific bacteria convert malic acid (the sharp, green apple acid found in grapes) into lactic acid (the softer, milk-derived acid found in dairy products). This is not a fermentation in the traditional sense—no alcohol is produced. Instead, it's a deacidification process that fundamentally changes the wine's chemistry and mouthfeel.

The bacteria responsible for MLF are primarily Oenococcus oeni, though other lactic acid bacteria can also perform this conversion. These bacteria are naturally present on grapes and in wineries, though they're often added intentionally as a bacterial culture for more predictable results.

Why Winemakers Use Malolactic Fermentation

Malolactic fermentation accomplishes several important goals:

Smoother Mouthfeel

Lactic acid is far less harsh than malic acid. A wine that tastes sharp and tart before MLF often becomes soft and round afterward. This "buttery" quality is particularly prized in Chardonnay and other full-bodied white wines.

Biological Stability

Once malolactic fermentation is complete, there's no malic acid left for unwanted bacteria to convert into acetic acid (vinegar). This makes the wine more stable during aging.

Complex Flavor Development

MLF produces several flavor compounds beyond lactic acid, including diacetyl (which contributes buttery, creamy notes), acetaldehyde (which adds nutty complexity), and various other compounds that enhance the wine's aromatic profile.

pH Adjustment

By converting malic acid to lactic acid, MLF naturally raises the wine's pH slightly (makes it less acidic), which can improve balance in high-acid wines.

💡 Did You Know?

The "buttery" descriptor in wine is directly linked to diacetyl production during malolactic fermentation. Winemakers can control how much diacetyl is produced by managing the timing and conditions of MLF.

🔬 Why This Works: The Biochemistry

Malic acid (C₄H₆O₅) is a dicarboxylic acid—it's a larger molecule with two acid groups. Lactic acid (C₃H₆O₃) is a smaller hydroxycarboxylic acid with just one acid group. The conversion process removes one carbon dioxide molecule through decarboxylation:

Malic Acid → Lactic Acid + CO₂

This biochemical reaction is catalyzed by the enzyme malolactic enzyme (MLE), which Oenococcus oeni produces. The reaction releases energy that the bacteria use for growth.

The key difference: malic acid tastes like sharp green apples, while lactic acid tastes like the creaminess of milk. Even though both are technically "acids," our taste perception is dramatically different. This is why MLF can transform a wine from harsh to smooth without actually reducing acidity (the pH changes minimally).

Which Wines Benefit from Malolactic Fermentation?

Wines That Typically Undergo MLF

Wines That Typically Skip MLF

The general principle: wines made for aging and complexity typically benefit from MLF, while wines made for immediate freshness typically avoid it.

How to Conduct Malolactic Fermentation

Timing: Sequential vs. Concurrent

You have two main approaches:

Sequential (Consecutive) MLF: This is the traditional approach. You complete primary fermentation completely (until fully dry), then rack the wine and initiate MLF. This gives you more control and allows you to evaluate the wine before committing to MLF.

Concurrent (Simultaneous) MLF: Here, you add the MLF bacteria at the same time as or shortly after adding wine yeast. This approach can produce more integrated flavors but offers less control—you can't easily stop MLF once it starts.

Step-by-Step Guide

Step 1: Ensure Proper Conditions

MLF bacteria prefer:

Step 2: Prepare Your Culture

While you can rely on wild bacteria, using a commercial MLF culture gives more predictable results. Most come in freeze-dried form and need to be rehydrated according to package directions, typically in a small amount of wine or water at room temperature.

Step 3: Inoculate

Add the prepared culture to your wine. If doing sequential MLF, add after primary fermentation is complete and the wine has been racked to a clean container. If doing concurrent MLF, add 24-48 hours after yeast inoculation.

Step 4: Monitor Progress

MLF takes time—typically 2-6 weeks, sometimes longer. You can track progress using:

Step 5: Stop MLF When Complete

Once MLF is complete (malic acid below detection), you can stop it by adding sulfites (about 50 ppm total SO₂). This stabilizes the wine and prevents any unwanted bacterial activity.

Preventing Malolactic Fermentation

Sometimes you don't want MLF—especially for crisp white wines where you want to preserve freshness. Here's how to prevent it:

Troubleshooting MLF Problems

MLF Won't Start

Possible causes: Temperature too low, pH too low, sulfite levels too high, alcohol too high

Solutions: Warm the wine, check pH, wait for sulfites to dissipate, choose a more alcohol-tolerant strain

MLF Takes Too Long

Possible causes: Temperature too low, nutrient deficiency, weak bacteria

Solutions: Increase temperature slightly, add nutrients, consider adding a fresh culture

Unwanted MLF (In Wines You Wanted Crisp)

Solutions: If MLF starts when you don't want it, you can stop it by adding sulfites immediately and storing the wine cold

The Art of MLF Management

Experienced winemakers use MLF as a flavor tool, not just a technical process. Here's how they think about it:

Partial vs. Complete MLF

You don't have to convert all the malic acid. Partial MLF—where perhaps half the malic acid is converted—can give you some of the smoothness without the full buttery character. This is a technique some winemakers use to fine-tune their white wines.

MLF and Oak

MLF pairs beautifully with oak aging. The soft, round character that MLF creates integrates beautifully with oak tannins and vanillin, creating wines of exceptional complexity. Many premium Chardonnays undergo MLF in oak barrels.

MLF and Aging

Wines that have undergone MLF often age differently than those that haven't. The biological stability that MLF provides can allow for longer bottle aging, during which additional complexity develops.

Conclusion

Malolactic fermentation is one of the winemaker's most powerful tools for shaping wine character. Whether you want the buttery richness of a classic California Chardonnay or the crisp freshness of a Sauvignon Blanc, understanding MLF gives you control over your wine's fundamental personality.

Remember: MLF is optional and controllable. Use it when you want softness, complexity, and biological stability. Avoid it when you want freshness, crispness, and vibrant fruit character. The choice is yours—and that's the beauty of understanding this transformative process.

Ready to learn about extraction techniques? Check out our article on Punch-Downs and Pump-Overs: Extraction Techniques for red winemaking methods.