Advanced Malolactic Fermentation: Controlling MLF for Complexity

The Science Behind Malolactic Fermentation

Malolactic fermentation (MLF) is not a true fermentation but a bacterial conversion in which Oenococcus oeni and related lactic acid bacteria (LAB) decarboxylate L-malic acid into L-lactic acid and carbon dioxide. This transformation reduces perceived acidity, enhances mouthfeel, and generates a suite of secondary metabolites that contribute to wine complexity. While the basic concept is straightforward, advanced practitioners understand that controlling the timing, bacterial population dynamics, and environmental conditions of MLF allows precise manipulation of wine style.

Malic acid tastes sharp, angular, and green. Lactic acid is softer, rounder, and perceived as creamier on the palate. The conversion typically reduces titratable acidity by 1 to 3 g/L, depending on the initial malic acid concentration. But the impact extends far beyond simple acid reduction.

Beyond Acid Conversion: Secondary Metabolites

During MLF, bacteria produce diacetyl (the buttery compound), acetoin, 2,3-butanediol, various esters, and volatile sulfur compounds. The concentration and balance of these metabolites depend on the bacterial strain, nutrient availability, temperature, pH, and timing of MLF relative to alcoholic fermentation.

Diacetyl production is perhaps the most discussed MLF variable. At low concentrations (1 to 4 mg/L), diacetyl adds desirable buttery, creamy, and toasty notes, particularly valued in barrel-fermented Chardonnay. Above 5 mg/L, it becomes overtly buttery and can mask fruit character. Advanced winemakers manage diacetyl through strain selection, temperature control, and yeast lees contact, as active yeast metabolize diacetyl into the less aromatic acetoin.

Bacterial Strain Selection

Choosing the right malolactic culture is as consequential as yeast selection, yet many winemakers default to whatever their supplier stocks. Advanced practitioners match bacterial strains to wine chemistry and stylistic goals.

Oenococcus oeni Strains

Oenococcus oeni remains the dominant species for winemaking MLF because of its tolerance to low pH (surviving at pH 3.0 and below), high alcohol (up to 15 to 16% ABV), and elevated SO2 levels. Commercial strains vary significantly in their metabolic profiles:

• High-diacetyl producers (such as Lalvin VP41 and Chr. Hansen Viniflora CH16) are suited to Chardonnay, Viognier, and other whites where buttery complexity is desirable
• Low-diacetyl producers (such as Lalvin MBR and Enoferm Alpha) preserve fruit character in reds like Pinot Noir and Gamay where freshness is the priority
• Robust strains tolerant of pH below 3.2 and alcohol above 14.5% (such as Lalvin VP41 and Scott Labs PN4) are essential for challenging wine matrices

Lactobacillus and Pediococcus

Lactobacillus plantarum has emerged as an alternative to Oenococcus for MLF, particularly for early inoculation strategies. It metabolizes malic acid through a direct pathway that produces less diacetyl and fewer off-flavors. Lactobacillus is less tolerant of high alcohol and low pH, making it better suited to co-inoculation during alcoholic fermentation when alcohol levels are still low.

Pediococcus species are generally undesirable in winemaking as they produce elevated levels of diacetyl, biogenic amines, and exopolysaccharides that cause ropiness. Avoid conditions that favor Pediococcus by maintaining appropriate SO2 levels and avoiding prolonged storage at elevated pH without MLF completion.

Timing Strategies for MLF

The timing of MLF initiation profoundly affects wine style, and advanced winemakers use timing as a deliberate tool.

Post-Alcoholic Fermentation Inoculation

The traditional approach inoculates bacteria after alcoholic fermentation is complete. This allows the winemaker to assess the base wine before committing to MLF. The disadvantage is that post-AF conditions are hostile to bacteria: high alcohol, low nutrients (consumed by yeast), and potential SO2 exposure create a challenging environment that can lead to sluggish or stuck MLF.

To optimize post-AF MLF, avoid sulfite additions between AF completion and MLF inoculation. Maintain temperature at 64 to 68 degF (18 to 20 degC). Consider adding a malolactic nutrient preparation containing amino acids, vitamins, and minerals to compensate for the depleted environment.

Co-Inoculation

Co-inoculation involves adding malolactic bacteria 24 to 48 hours after yeast inoculation, while alcoholic fermentation is still in its early stages. This strategy has gained significant traction because it exploits the more favorable chemical environment: lower alcohol, abundant nutrients, and warmer fermentation temperatures.

Co-inoculation typically completes MLF within days of AF completion rather than weeks or months. It reduces the risk of stuck MLF and minimizes the vulnerable window between AF completion and MLF finish when the wine lacks both yeast-derived and SO2 protection against spoilage. Research has shown that co-inoculated wines often show less diacetyl and better fruit preservation than sequentially inoculated wines.

Spontaneous MLF

Allowing native LAB populations to initiate MLF without inoculation produces the most variable and potentially the most complex results. Spontaneous MLF relies on bacteria present on grape skins, in the winery environment, and on barrel surfaces. The risk is inconsistency: spontaneous MLF may take months, produce excessive diacetyl or biogenic amines, or fail to complete entirely.

pH and Acidity Management

Wine pH is the single most critical parameter governing MLF success. Oenococcus oeni functions optimally between pH 3.3 and 3.6. Below pH 3.1, even robust strains struggle, and MLF may stall or fail entirely. Above pH 3.8, the risk of spoilage from Lactobacillus and Pediococcus increases significantly.

Pre-MLF Acid Adjustment

If your wine's pH is below 3.2, consider a modest deacidification with potassium bicarbonate (1 to 2 g/L) before MLF inoculation. Target pH 3.3 for reliable MLF initiation. After MLF completion, you can re-acidify if necessary with tartaric acid to reach your target pH for bottling.

For wines with pH above 3.7, exercise caution. MLF will proceed rapidly, potentially too rapidly for diacetyl management. At high pH, the conversion happens quickly but the wine becomes more vulnerable to microbial instability. Ensure you have a clear plan for SO2 addition immediately upon MLF completion.

Monitoring MLF Progress

Paper chromatography is the traditional home-winemaker method for tracking MLF. It separates malic, lactic, and tartaric acids on chromatography paper, providing a visual confirmation of conversion progress. However, paper chromatography is qualitative, not quantitative.

For precise monitoring, use enzymatic malic acid test kits that provide numerical malic acid concentrations. MLF is considered complete when residual malic acid falls below 0.3 g/L (30 mg/100 mL). Some practitioners set a stricter threshold of 0.1 g/L to ensure stability.

Temperature Control During MLF

Temperature profoundly influences both the rate of MLF and the metabolic byproducts produced.

Optimal Temperature Ranges

The optimal temperature for most Oenococcus oeni strains is 64 to 72 degF (18 to 22 degC). Within this range, MLF proceeds steadily and produces a balanced metabolic profile. Below 59 degF (15 degC), MLF slows dramatically and may stall. Above 77 degF (25 degC), bacteria may produce elevated volatile acidity and biogenic amines.

Cold MLF for Freshness

Some winemakers deliberately conduct MLF at 55 to 60 degF (13 to 16 degC) to slow the process and reduce diacetyl production. This cold MLF approach preserves primary fruit aromas and produces a cleaner, more fruit-forward profile. It requires patience (MLF may take two to three months) and robust bacterial populations, but the results can be exceptional for varieties like Pinot Noir and Gamay where freshness is paramount.

Warm MLF for Richness

Conversely, conducting MLF at the upper end of the optimal range (70 to 72 degF) accelerates the process and tends to produce more diacetyl and richer textural contributions. This approach suits full-bodied whites like Chardonnay and Viognier where creamy, buttery characters are stylistically appropriate.

MLF in Barrel vs. Tank

The vessel in which MLF occurs affects the final wine character significantly.

Barrel MLF

Conducting MLF in barrel integrates the bacterial metabolites with oak-derived compounds simultaneously. Diacetyl interacts synergistically with vanillin and oak lactones, creating a more complex and seamless buttery-toasty character than adding these components separately. Barrel MLF is the standard approach for premium Chardonnay and many red wines.

The barrel's micro-oxygenation also promotes diacetyl stability, as the small oxygen exposure prevents the excessive reduction that can metabolize diacetyl to acetoin in the lees. However, barrel MLF requires monitoring each barrel individually, as MLF progression can vary barrel to barrel.

Tank MLF

Tank MLF provides more uniform conditions and easier monitoring. It is efficient for large volumes and when the winemaker wants to complete MLF quickly before racking to barrel. Tank MLF with subsequent barrel aging separates the bacterial and oak contributions temporally, giving the winemaker more independent control over each element.

Preventing and Correcting MLF Problems

Stuck MLF

A stuck MLF (no malic acid reduction for two weeks or more) can be rescued by:

• Raising temperature to 68 to 72 degF
• Adding a fresh malolactic nutrient supplement
• Re-inoculating with a fresh, high-viability bacterial culture, ideally a different strain than the original
• Ensuring free SO2 is below 10 ppm and molecular SO2 is below 0.5 ppm

Excessive Diacetyl

If diacetyl levels become objectionable, stir the lees (batonnage) to bring active yeast into contact with the diacetyl. Yeast cells metabolize diacetyl into the much less aromatic acetoin and 2,3-butanediol. Two to three weeks of lees contact after MLF completion is standard practice for diacetyl management. This is one reason many winemakers avoid racking immediately after AF completion.

Biogenic Amines

Histamine, tyramine, and putrescine are biogenic amines produced by certain LAB strains during MLF. They can cause headaches and allergic reactions in sensitive individuals. Minimize biogenic amine production by using commercial bacterial cultures with tested low-amine profiles, avoiding prolonged MLF at high temperatures, and maintaining adequate nutrient levels throughout the process.

Partial MLF for Stylistic Control

Not every wine benefits from complete MLF. Partial malolactic fermentation deliberately arrests MLF before all malic acid is consumed, preserving some of the bright, fresh acidity of malic acid while gaining the textural softness of lactic acid conversion.

To arrest MLF at the desired point, add SO2 (40 to 50 ppm free) and chill the wine to below 50 degF (10 degC). Sterile filtration at 0.45 microns removes remaining bacteria. This technique is particularly effective for aromatic whites like Riesling, Viognier, and Chenin Blanc where complete MLF would strip desirable acidity and freshness.

Partial MLF requires precise monitoring, as the arrest must occur at exactly the right malic acid concentration. It is inherently less stable than complete MLF and demands diligent SO2 management to prevent unintended resumption in bottle.

Frequently Asked Questions

Should all red wines go through malolactic fermentation?

Most red wines benefit from complete MLF because the acid softening improves mouthfeel and integrates tannins. However, some light reds like Beaujolais Nouveau or certain Pinot Noir styles may benefit from partial MLF to retain vibrancy. The decision depends on the wine's pH, total acidity, and stylistic goals.

Can I do MLF in a carboy or demijohn?

Yes, MLF can proceed in any vessel. Glass carboys maintain stable temperatures and are easy to monitor. The challenge is temperature control, as carboys are difficult to warm evenly. Place them in a temperature-controlled space at 65 to 70 degF and ensure the vessel is topped up to minimize headspace.

How do I know if MLF has started spontaneously?

Look for fine, persistent CO2 bubbles in the airlock weeks after alcoholic fermentation has finished. The wine may develop a slightly creamy or buttery aroma. Confirm with paper chromatography or an enzymatic malic acid test showing declining malic acid concentration.

Is it safe to add SO2 during MLF?

No. Even modest SO2 additions during active MLF can inhibit or kill the bacteria, causing a stuck MLF. Wait until MLF is confirmed complete (malic acid below 0.3 g/L) before adding any sulfite. After MLF completion, promptly add 25 to 35 ppm free SO2 to protect the wine.

What causes a mousy off-flavor after MLF?

Mousiness is caused by tetrahydropyridines produced by certain Lactobacillus and Brettanomyces species. It is more common in wines with low SO2, high pH, and spontaneous MLF. Once present, mousiness is extremely difficult to remove. Prevention through proper SO2 management, controlled bacterial populations, and maintaining pH below 3.7 is the best strategy.