Malolactic Fermentation: Softening Acidity in Red and White Wines

What Is Malolactic Fermentation?

Malolactic fermentation (commonly abbreviated MLF) is a biological process in which bacteria convert malic acid in wine into lactic acid and carbon dioxide. Despite its name, MLF is not technically a fermentation in the traditional sugar-to-alcohol sense. It is a bacterial conversion that happens after (or sometimes concurrently with) the primary alcoholic fermentation.

The chemical equation is straightforward: one molecule of L-malic acid is converted into one molecule of L-lactic acid plus one molecule of carbon dioxide. The practical effect on the wine is significant. Malic acid is sharp and aggressive, often described as the bite of a green apple. Lactic acid is smooth and mild, associated with the creaminess of yogurt or milk. This conversion softens the wine's acidity, lowers its perceived sharpness, and adds complexity to both the flavor and mouthfeel.

MLF is a standard practice for nearly all red wines worldwide and for certain styles of white wine, most famously oaked Chardonnay. Understanding when and how to manage MLF gives home winemakers a powerful tool for controlling the acidity, texture, and flavor profile of their wines.

The Bacteria Behind MLF

The primary bacterium responsible for MLF in wine is Oenococcus oeni (formerly known as Leuconostoc oenos). This species is uniquely adapted to the harsh environment of wine: high alcohol, low pH, and high sulfite levels that would kill most bacteria.

Other lactic acid bacteria, including various species of Lactobacillus and Pediococcus, can also perform MLF. However, these species are less desirable because they may produce off-flavors, biogenic amines, and other unwanted byproducts. This is why controlled inoculation with a commercial Oenococcus oeni culture is strongly preferred over relying on wild bacteria to complete MLF spontaneously.

What MLF Does to Wine

The effects of MLF extend well beyond simple acid reduction:

Acidity reduction: Total titratable acidity decreases as the stronger malic acid is replaced by the weaker lactic acid. The pH rises by approximately 0.1-0.3 units
Flavor changes: MLF produces diacetyl, the compound responsible for buttery aromas and flavors. At low levels, diacetyl adds complexity. At high levels, it can taste like artificial butter or movie popcorn
Mouthfeel enhancement: The conversion to lactic acid creates a rounder, creamier, more voluptuous texture on the palate
Microbial stability: Once all the malic acid is consumed, a major nutrient source for spoilage bacteria is eliminated, making the wine more biologically stable
Aroma complexity: MLF produces a range of secondary metabolites that contribute to aromatic complexity, including esters, aldehydes, and various flavor-active compounds

When to Use Malolactic Fermentation

Almost Always: Red Wines

Virtually all red wines benefit from MLF. Red grapes tend to have high levels of malic acid, and the sharp, angular character of malic acid conflicts with the tannin structure and body that define good red wine. MLF softens this harshness, integrates the tannins more smoothly, and creates the round, supple mouthfeel that red wine drinkers expect.

If you are making red wine from grapes, plan for MLF as a standard part of your process.

Sometimes: White Wines

MLF in white wine is a stylistic choice, not a necessity. The decision depends on the wine style you are pursuing:

Use MLF for:

Full-bodied, oaked Chardonnay: MLF is essential for the classic rich, buttery style. The combination of MLF diacetyl and oak vanillin creates the archetypal California or Burgundian Chardonnay character
White wines with excessively high acidity: If your grape must has a TA above 9 g/L or a pH below 3.1, MLF can bring the acidity into a more balanced range
Blended whites that need a rounder component to soften the blend

Avoid MLF for:

Crisp, aromatic whites (Riesling, Sauvignon Blanc, Gewurztraminer): MLF eliminates the bright acidity that defines these styles and can mask delicate aromatic compounds
Light, refreshing whites where freshness and acidity are the primary appeal
Fruit wines: Most fruit wines rely on their natural acidity for balance and do not benefit from MLF

Rarely: Rose Wines

Most rose wines are made without MLF to preserve their bright, fresh character. However, rose wines with uncomfortably high acidity may benefit from partial MLF, where the conversion is stopped before completion to achieve a target acidity level.

How to Inoculate for MLF

Choosing a Culture

Commercial MLF cultures are available in freeze-dried and liquid forms. The most popular strains include:

CH16 (Oenococcus oeni): A versatile, reliable strain that works well across a wide range of conditions. Good for beginners
VP41: Known for producing minimal diacetyl, preferred for wines where a buttery character is not desired
CH35: Produces moderate diacetyl, good for balanced buttery character in Chardonnay
Alpha (MBR): Extremely robust, can tolerate high alcohol and low pH. The best choice for challenging conditions

For home winemakers, freeze-dried direct-addition cultures are the most convenient. A single packet typically treats 66 gallons, but many suppliers sell smaller packets sized for 5-6 gallon batches.

Timing the Inoculation

The optimal time to inoculate for MLF is immediately after primary fermentation is complete or during the last stages of primary fermentation when the sugar level has dropped below 5 Brix (specific gravity below 1.020).

Co-inoculation (adding MLF bacteria at the same time as yeast) is gaining popularity in commercial wineries but requires careful management. For home winemakers, the traditional approach of sequential inoculation (yeast first, bacteria after) is more reliable.

Critical rule: Do not add sulfite (potassium metabisulfite) to the wine before MLF inoculation. Even a small addition of 10-15 ppm SO2 can kill or inhibit the bacteria. If you have already added sulfite, wait at least 24-48 hours before inoculating, and consider a higher inoculation rate.

Preparation and Addition

Warm the wine: MLF bacteria prefer temperatures between 65-75F (18-24C). If the wine is cold, warm it gradually before inoculating
Rehydrate the culture: Follow the manufacturer's instructions exactly. Most freeze-dried cultures are rehydrated in small amount of water at 77F (25C) for 15-20 minutes
Add to wine: Pour the rehydrated culture directly into the wine and stir gently to distribute
Maintain temperature: Keep the wine at 68-72F (20-22C) throughout MLF. Temperature drops below 60F (16C) will slow or stall the process
Do not add sulfite: No sulfite additions until MLF is confirmed complete

Monitoring MLF Progress

MLF does not produce visible signs like the bubbling of alcoholic fermentation. You may notice a slight haze or a faint increase in CO2 activity, but these are unreliable indicators.

The only reliable way to track MLF is through chromatography or enzymatic testing:

Paper chromatography: An inexpensive test ($10-15 for a kit) that visually shows the relative levels of malic and lactic acid in the wine. You spot wine samples on chromatography paper, develop the paper in a solvent, and compare the resulting spots. When the malic acid spot disappears and a strong lactic acid spot appears, MLF is complete. Test every 1-2 weeks during MLF
Enzymatic malic acid test: More precise than chromatography. These kits measure the exact concentration of malic acid remaining. MLF is complete when malic acid drops below 0.1 g/L

MLF Timeline

Under optimal conditions (warm temperature, healthy bacterial population, adequate nutrients), MLF typically takes 4-8 weeks to complete. However, it can take much longer if conditions are suboptimal:

Low temperature: MLF can take 3-6 months at cellar temperature (55-60F)
High alcohol (above 14%): Stresses the bacteria and slows conversion
Low pH (below 3.2): Inhibits bacterial growth
High SO2: Even residual sulfite from pre-fermentation additions can delay MLF

Managing MLF Challenges

Stuck MLF

If MLF stalls before completion, possible causes include:

Temperature too low: Warm the wine to 68-72F
Nutrient deficiency: Add a malolactic nutrient supplement (available from winemaking suppliers). These contain amino acids and vitamins that support bacterial growth
SO2 inhibition: Free SO2 above 10 ppm inhibits Oenococcus oeni. If SO2 was added prematurely, time will reduce the free SO2 level as it binds to other compounds in the wine
Bacteriophage: Viral infection of the bacteria. Rare but possible. The only solution is re-inoculation with a fresh culture

If MLF remains stuck after addressing these factors, re-inoculate with a fresh, robust culture like Alpha (MBR) and ensure optimal conditions.

Excessive Diacetyl

If the wine develops an overpowering buttery or butterscotch aroma, the diacetyl level is too high. This typically occurs when MLF is slow or incomplete. Ironically, the best remedy is to encourage MLF to complete. The same bacteria that produce diacetyl will reabsorb and metabolize it in the later stages of MLF, a process called diacetyl reduction. Maintaining warm temperatures (68-72F) after MLF completion for an additional 1-2 weeks allows the bacteria to reduce diacetyl to acceptable levels.

Unwanted MLF

If MLF begins spontaneously in a wine where you do not want it (such as a crisp Sauvignon Blanc), you need to stop it:

Add sulfite: Add 50 ppm SO2 (potassium metabisulfite) immediately
Cool the wine: Drop the temperature below 55F (13C)
Consider sterile filtration: A 0.45 micron filter removes bacteria and prevents MLF from restarting

To prevent unwanted MLF in future batches, maintain adequate SO2 levels (25-35 ppm free SO2) and keep the wine cool after primary fermentation.

MLF and Wine Chemistry

pH Considerations

MLF raises the wine's pH. For wines that already have a relatively high pH (above 3.6), this increase can push the pH into a range where the wine becomes microbiologically unstable and vulnerable to spoilage. In these cases, consider making an acid addition before or after MLF to bring the pH back into a safe range.

Conversely, wines with very low pH (below 3.1) may be too acidic for MLF bacteria to thrive. In this case, a small acid reduction before inoculation can make the environment more hospitable for the bacteria.

SO2 Management After MLF

Once MLF is confirmed complete by testing, immediately add sulfite to protect the wine. A standard post-MLF addition is 50 ppm SO2 (approximately 1/4 teaspoon potassium metabisulfite per 5-6 gallons). This addition kills any remaining bacteria and prevents them from producing unwanted byproducts during aging.

From this point forward, maintain standard SO2 levels (25-35 ppm free SO2) throughout aging and prior to bottling.

Frequently Asked Questions

How do I know when malolactic fermentation is complete?

The only reliable method is testing. Paper chromatography is the most accessible test for home winemakers. When the malic acid spot disappears from the chromatography paper and only a lactic acid spot remains, MLF is complete. Do not rely on taste, smell, or the cessation of CO2 activity as indicators, as these are unreliable.

Can MLF happen on its own without adding bacteria?

Yes. Wild Oenococcus oeni and other lactic acid bacteria are naturally present on grape skins and in winery environments. Spontaneous MLF can occur if conditions are favorable (warm temperature, low SO2, available malic acid). However, spontaneous MLF is unpredictable and carries a higher risk of off-flavors from undesirable bacterial strains. Controlled inoculation with a commercial culture is strongly recommended.

Does MLF reduce the alcohol content of wine?

No. MLF converts malic acid to lactic acid. It does not involve sugar or alcohol. The alcohol content remains unchanged.

Can I do MLF in a carboy with an airlock?

Yes, this is the standard approach for home winemakers. Use an airlock because MLF produces a small amount of CO2. The airlock allows this gas to escape while preventing oxygen from entering. Do not seal the vessel airtight during MLF.

Why does my wine taste buttery after MLF?

The buttery flavor comes from diacetyl, a byproduct of MLF. Some diacetyl production is normal and desirable, adding complexity to styles like oaked Chardonnay. If the buttery flavor is excessive, maintain warm temperatures (68-72F) for 1-2 weeks after MLF completion. The bacteria will metabolize the excess diacetyl during this period. If you prefer no buttery character at all, choose a low-diacetyl MLF strain like VP41.