Malolactic Fermentation: What It Is and Why It Matters

Malolactic fermentation (MLF) is a bacterial process that converts malic acid into lactic acid. Malic acid tastes like "crisp, green apple," while lactic acid is described as "soft, creamy," and the conversion softens a wine's taste and reduces its total acidity. Lactic acid bacteria carry out this secondary fermentation, primarily from the genera Oenococcus, Lactobacillus, Pediococcus, and Leuconostoc. Beyond altering acidity and texture, MLF contributes to microbial stability by reducing malic acid, a nutrient source for spoilage organisms (Westgarth Wines). Most red wines undergo complete malolactic fermentation to lessen acidity and prevent it from occurring in the bottle. For white wines, the decision to use MLF varies. Lighter aromatic wines like Riesling generally do not use it. Fuller white wines such as barrel-fermented Chardonnay commonly do, sometimes partially. According to AWRI, this process can also introduce "buttery" or "butterscotch" characters due to diacetyl production by bacteria. At low levels (1-4 mg/L) diacetyl can add complexity. At high levels (>5 mg/L) it may be considered objectionable (AWRI: wine faults and taints).

What is malolactic fermentation?

Malolactic fermentation (MLF) is a bacterial process where specific strains of bacteria convert malic acid into lactic acid. It typically occurs during or after the primary alcoholic fermentation. Lactic acid bacteria from the genera Oenococcus, Lactobacillus, Pediococcus, and Leuconostoc primarily carry out this conversion. Malic acid, found in grapes, is described as "crisp, green apple," while the resulting lactic acid is "soft, creamy". The process reduces the total acidity of the wine. Malic acid has two acid radicals (-COOH), whereas lactic acid has only one. This bacterial action modifies the wine's taste profile. It also enhances microbial stability by removing malic acid, a potential food source for spoilage organisms (Westgarth Wines). Winemakers can initiate this fermentation intentionally by inoculating the wine with desired bacteria. It can also occur spontaneously if uncultivated lactic acid bacteria are present. Understanding such processes is key to fine wine investment strategies.

How does malolactic fermentation change wine's taste and stability?

Malolactic fermentation significantly alters a wine's sensory profile by converting sharp malic acid into milder lactic acid. That can improve the taste of wines with high malic acid levels. This reduction in acidity can make the wine feel softer and less sour on the palate. Lactic acid is an acid found in dairy products. Beyond taste, MLF contributes to microbial stability by reducing the availability of malic acid, a nutrient source for spoilage organisms (Westgarth Wines). The process also influences the wine's texture, often imparting a "buttery" or "butterscotch" character due to the production of diacetyl. At low concentrations, between 1 and 4 mg/L, diacetyl can add complexity to a wine. At higher levels, exceeding 5 mg/L, its aroma may be considered objectionable (AWRI: wine faults and taints). According to AWRI, the formation of diacetyl primarily occurs from the catabolism of citric acid by bacteria during MLF. It generally happens after all malic acid has been converted (AWRI: wine faults and taints). You can learn more about how condition impacts value in our guide to wine ullage levels explained.

Which wines typically undergo malolactic fermentation?

Most red wines undergo complete malolactic fermentation to both lessen their acidity and eliminate the risk of the process occurring in the bottle. For white wines, the application of MLF varies depending on the desired style. Lighter, aromatic white wines, such as Riesling, generally do not go through the process, in order to preserve their crisp character. In contrast, fuller-bodied white wines, like barrel-fermented Chardonnay, are more commonly put through MLF, sometimes partially, such as less than 50%. Regional climate can also influence the necessity and prominence of the process. It is usually more accepted in colder regions of production. In warmer wine regions, where higher pH is common, it may be less necessary and potentially damaging to the final product. The decision to use MLF is a key winemaking choice. It shapes the final character and aging potential of the wine, as you can explore with the style split in white Burgundy we score.

What are the sensory characteristics influenced by malolactic fermentation?

MLF can introduce or enhance several sensory characteristics in wine, most notably a "buttery" or "butterscotch" aroma from diacetyl. Bacteria typically produce diacetyl during MLF from the catabolism of citric acid. At levels of 1 to 4 mg/L, it contributes positively to wine complexity (AWRI: wine faults and taints). However, if diacetyl concentrations exceed 5 mg/L, the aroma can become objectionable, potentially rendering the wine defective (AWRI: wine faults and taints). The process also reduces the perception of "crisp, green apple" notes associated with malic acid. It replaces them with the "soft, creamy" sensation of lactic acid. This change in acidity can make the wine feel rounder and less tart on the palate. The Court of Master Sommeliers' deductive tasting format includes "Cream" and "Butter" as non-fruit descriptors that can arise from winemaking. That indicates their relevance in sensory evaluation (Court of Master Sommeliers: deductive tasting format).

Here is a summary of the sensory impacts of the process:

Characteristic	Malic Acid (Before MLF)	Lactic Acid (After MLF)	Diacetyl (MLF Byproduct)
Taste	"Crisp, green apple"	"Soft, creamy"	"Buttery" or "butterscotch" (AWRI: wine faults and taints)
Acidity	Higher, sharper	Lower, milder	N/A
Texture	Leaner, more tart (implied by "crisp")	Rounder, softer (implied by "creamy")	Adds richness, creaminess (implied by "buttery")
Threshold (positive)	N/A	N/A	1-4 mg/L for complexity (AWRI: wine faults and taints)
Threshold (objectionable)	N/A	N/A	>5 mg/L (AWRI: wine faults and taints)

How is malolactic fermentation managed and controlled?

Winemakers manage malolactic fermentation in one of two ways. They either inoculate the wine intentionally with cultivated strains of lactic acid bacteria, or they allow it to occur naturally if uncultivated bacteria are present. The formation of diacetyl, a key byproduct, is dependent on factors such as the bacterial strain, the oxygen (O2) tension of the wine, citric acid concentration, and temperature, according to AWRI: wine faults and taints. An increase in O2 concentration favors the oxidation of α-acetolactate to yield diacetyl (AWRI: wine faults and taints). Winemakers must also monitor pH during MLF. It should not rise above a pH of 3.55 for white wines or 3.80 for red wines. The pH can be reduced by approximately 0.1 units per 1 gram/liter of tartaric acid addition. To prevent or stop MLF, sulfur dioxide (SO2) can be added immediately after alcoholic fermentation. For white wines, maintaining available sulfur dioxide at 30 mg/L until bottling can inhibit bacterial activity. For red wines, a maintenance level of about 20 mg/L is recommended after MLF. Without sufficient SO2, wines can readily suffer bacterial spoilage. For those interested in the broader context of wine production, our producer atlas by region offers detailed insights.

What are the potential faults and risks associated with malolactic fermentation?

While malolactic fermentation is often beneficial, it carries risks, including the potential for spoilage and undesirable off-flavors if not properly managed. One significant risk is the production of diacetyl at high concentrations. Diacetyl contributes "buttery" notes at low levels (1-4 mg/L). Above 5 mg/L it can become objectionable and make the wine defective (AWRI: wine faults and taints). Another potential fault is "mousiness," an off-flavor reminiscent of caged mice or cracker biscuit. It is generally perceived late on the palate and can render a wine undrinkable (AWRI: wine faults and taints). Most strains of lactic acid bacteria, particularly heterofermentative species like Lactobacillus hilgardii, Lactobacillus plantarum, Lactobacillus brevis, and Oenococcus oeni, are capable of producing this taint (AWRI: wine faults and taints). Mousy taint is more likely to occur in wines with low concentrations of sulfur dioxide (SO2) and low acidity, according to AWRI: wine faults and taints. The AWRI reports an increased incidence of mousy wines when winemakers moved to lower SO2 regimes. Those winemakers also experimented with extended lees aging, high pH, minimal SO2, oxidative aging, and minimal clarification or filtration in white wines (AWRI: wine faults and taints). There is no satisfactory method to remove mousy off-flavor once it develops (AWRI: wine faults and taints). Proper wine storage is crucial to prevent such issues from developing post-bottling.

Additionally, if sorbic acid is present during MLF, lactic acid bacteria can metabolize it to sorbic alcohol. That alcohol then rearranges and reacts with ethanol to form 2-ethoxyhexa-3,5-diene, causing a "geranium" off-odor (AWRI: wine faults and taints). This "geranium" off-odor has an extremely potent detection threshold of less than 1 ng/L. It is practically impossible to remove, persisting even after carbon treatment, distillation, or significant dilution (AWRI: wine faults and taints). Consequently, it is imperative to maintain wine conditions that strongly inhibit lactic acid bacteria growth and to avoid using sorbic acid to treat red wines (AWRI: wine faults and taints). Monitoring wine quality is a continuous process, from fermentation to bottling, and includes various laboratory tests. For a broader view of market trends, consult our live fine wine market index. Understanding these winemaking choices is essential for anyone looking to buy wine at auction.

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