Malolactic fermentation
A secondary fermentation converting malic acid to softer lactic acid.
Malolactic conversion, often called malolactic fermentation or MLF, is a winemaking step where sharp-tasting malic acid—naturally found in grape must—gets turned into milder lactic acid. This usually happens as a secondary fermentation right after the main one ends, though it can also occur at the same time. Most red wines go through this process, and it’s common for certain white grapes like Chardonnay, where it can add a buttery flavor from diacetyl, a byproduct of the reaction.
The work is done by lactic acid bacteria, including *Oenococcus oeni* and various *Lactobacillus* and *Pediococcus* species. Chemically, it’s a decarboxylation, meaning carbon dioxide is released. These bacteria convert L-malic acid, one of the two main grape acids, into L+ lactic acid. This can happen naturally, but in commercial winemaking, winemakers usually add a starter culture of desirable bacteria, typically *O. oeni*, to prevent unwanted strains from creating off-flavors. When a fruity or floral white wine—like Riesling or Gewürztraminer—is the goal, winemakers actively stop MLF to keep a tart, acidic profile.
The process gives wine a rounder, fuller mouthfeel. Malic acid tastes like green apples, while lactic acid is richer and more buttery. Grapes from cool climates often have high acidity, much of it from malic acid. MLF generally boosts body and flavor persistence, making wines feel softer on the palate. Many winemakers also believe it helps fruit and oak character blend better if the conversion happens while the wine ages in barrel.
A wine undergoing MLF looks cloudy from the bacteria and may smell like buttered popcorn, thanks to diacetyl. If MLF starts in the bottle, it’s usually considered a fault, since the wine appears to still be fermenting due to CO₂ production. However, early Vinho Verde producers saw this slight fizz as a feature, though they had to use opaque bottles to hide the cloudiness and sediment from in-bottle MLF. Today, most Vinho Verde makers finish MLF before bottling and add carbonation artificially.
Malolactic fermentation is probably as old as wine itself, but understanding its benefits and controlling it are recent. For centuries, winemakers noticed a mysterious activity in barrels during warm spring months after harvest. Like primary fermentation, it released CO₂ and changed the wine, not always for the better.
- Ta reduction
- 1 to 3 g/L
- Ph increase
- 0.3 units
- Ethyl lactate after mlf
- as high as 110 mg/L
Lore & Background
Malolactic fermentation is possibly as old as the history of wine, but scientific understanding of its positive benefits and control is a relatively recent development. For many centuries, winemakers noticed an activity in their wines stored in barrel during warm spring months following harvest, which released carbon dioxide gas and profoundly changed the wine. It was described as a second fermentation in 1837 by the German enologist Freiherr von Babo, who encouraged winemakers to respond by racking, adding sulfur dioxide, and further racking and sulfuring. In 1866, Louis Pasteur isolated the first bacteria from wine and determined that all bacteria in wine were a cause of spoilage, though he did not link the acid reduction to consumption of malic acid. In 1891, Swiss enologist Hermann Müller theorized that bacteria may be the cause, and with peers explained his theory of biological deacidification in 1913 as caused by Bacterium gracile. In the 1930s, French enologist Jean Ribéreau-Gayon published papers on the benefits of this bacterial transformation, and during the 1950s advances in enzymatic analysis allowed better understanding of the chemical processes. Émile Peynaud furthered understanding, and soon cultured stock of beneficial lactic acid bacteria became available.
Reader's Guide
The primary role of malolactic fermentation is to deacidify wine, converting the harsher diprotic malic acid to the softer monoprotic lactic acid. This reduces titratable acidity by 1 to 3 g/L and increases pH by 0.3 units. Malolactic fermentation can also affect sensory aspects, making the mouthfeel smoother and adding potential complexity in flavor and aroma. Most red wines worldwide, many sparkling wines, and nearly 20% of white wines go through MLF. The process can aid in making wine microbiologically stable by consuming leftover nutrients that spoilage microbes could use, but the rise in pH can also make wine slightly unstable, sometimes requiring later addition of tartaric acid. Lactic acid bacteria convert malic acid into lactic acid as an indirect means of creating energy via chemiosmosis, using the pH gradient between inside the cell and the wine to produce ATP. The decarboxylation of malate into L-lactic acid releases carbon dioxide and consumes a proton, generating the pH gradient. Sensory changes include softer acidity, reduced perceived sourness, and altered mouthfeel related to pH increase and production of polyols such as erythritol and glycerol, as well as ethyl lactate. The influence on aroma is complex and difficult to predict with different strains of Oenococcus oeni.
Did You Know?
- Malolactic fermentation is actually a decarboxylation, not a true fermentation, and releases carbon dioxide.
- The buttery flavor in some Chardonnay comes from diacetyl, a byproduct of malolactic fermentation.
- Malic acid is associated with the taste of green apples, while lactic acid is richer and more buttery tasting.
- In-bottle malolactic fermentation is usually considered a wine fault, but was once a distinguishing trait for early Vinho Verde.
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