Malolactic Fermentation in Winemaking: Process, Implementation, and Effects

Malolactic fermentation (MLF) is a crucial secondary fermentation process in winemaking that significantly influences the wine's flavor, texture, and stability. This article explores the process of malolactic fermentation, the role of bacteria, how to employ them, the sensory effects on wine, and the types of wine that benefit from or should avoid this fermentation.

What is Malolactic Fermentation?

Malolactic fermentation is a secondary fermentation process where malic acid in wine is converted into lactic acid and carbon dioxide by lactic acid bacteria (LAB). The primary bacterium responsible for this conversion is Oenococcus oeni, although other LAB such as Lactobacillus and Pediococcus can also be involved. This process is distinct from primary alcoholic fermentation, which is carried out by yeast converting sugars into alcohol and carbon dioxide.

The Process of Malolactic Fermentation

1. Initiation: After the completion of primary fermentation, winemakers can inoculate the wine with a culture of Oenococcus oeni or allow indigenous bacteria to initiate the process. The bacteria can be added directly to the wine or through a starter culture.

2. Conversion: The bacteria metabolize malic acid, a sharp and tart acid found in grapes, converting it into lactic acid, which is softer and creamier, and releasing carbon dioxide as a byproduct. This conversion reduces the overall acidity of the wine.

3. Monitoring: Winemakers monitor the progress of MLF by measuring the levels of malic and lactic acids using chromatography or enzymatic assays. The process can take a few weeks to several months, depending on factors such as temperature, pH, and bacterial activity.

4. Completion: Once MLF is complete, the wine is stabilized by racking, sulfiting, and sometimes filtering to remove any remaining bacteria and prevent unwanted spoilage.

Employing Malolactic Bacteria

Inoculation Methods

• Direct Addition: Freeze-dried Oenococcus oeni cultures can be added directly to the wine. This method is straightforward and ensures a reliable start to MLF.
• Starter Cultures: A small batch of wine is inoculated with the bacteria and allowed to undergo MLF. This culture is then added to the main batch, promoting a more controlled fermentation.

Optimal Conditions

• Temperature: MLF bacteria thrive in temperatures between 65°F and 75°F (18°C to 24°C). Lower temperatures can slow the process, while higher temperatures can inhibit bacterial activity.
• pH Levels: A pH range of 3.3 to 3.5 is ideal for MLF. Wines with higher acidity (lower pH) may require adjustments to create a more favorable environment for the bacteria.
• Nutrient Availability: Like yeast, MLF bacteria need nutrients such as amino acids, vitamins, and minerals. Nutrient additions may be necessary to ensure successful fermentation.

What the Bacteria Do

Oenococcus oeni and other LAB convert malic acid into lactic acid through decarboxylation. This process not only reduces the acidity of the wine but also contributes to the development of complex flavors and a smoother mouthfeel. The bacteria can also produce secondary metabolites that enhance the wine's aromatic profile.

Sensory Effects on Wine

Positive Effects

• Reduced Acidity: The conversion of sharp malic acid into softer lactic acid results in a smoother, rounder mouthfeel.
• Complex Flavors: MLF can introduce buttery, creamy, and nutty notes, particularly in white wines like Chardonnay.
• Enhanced Aromas: The process can produce diacetyl, a compound that imparts a buttery aroma, adding complexity to the wine.

Potential Drawbacks

• Loss of Freshness: In some wines, especially those meant to showcase bright, fruity acidity, MLF can diminish the fresh, vibrant character.
• Risk of Spoilage: If not properly managed, MLF can lead to off-flavors and spoilage due to unwanted bacterial activity.

Types of Wine and Malolactic Fermentation

Wines that Benefit from MLF

• Chardonnay: MLF is commonly employed to soften acidity and add buttery, creamy notes.
• Red Wines: Most red wines, including Cabernet Sauvignon, Merlot, and Pinot Noir, undergo MLF to achieve a smoother texture and complex flavor profile.
• Viognier: This aromatic white wine can benefit from MLF to add complexity and a fuller body.

Wines that Typically Avoid MLF

• Sauvignon Blanc: Known for its crisp, vibrant acidity, MLF is usually avoided to preserve these characteristics.
• Riesling: The bright acidity and fresh fruit flavors of Riesling are best maintained without MLF.
• Albariño: This light, aromatic white wine is typically produced without MLF to retain its zesty freshness.