Oenococcus oeni: The Wine’s Silent Architect
Oenococcus oeni is a specialized lactic acid bacterium (LAB). This microorganism is uniquely adapted to the chemically hostile environment of wine, becoming its primary habitat after alcoholic fermentation. It possesses exceptional tolerance for high concentrations of ethanol (often exceeding 10% ABV) and highly acidic conditions, sometimes thriving at a pH below 3.0. This resilience allows O. oeni to dominate the bacterial community and drive malolactic fermentation.
The Process of Malolactic Fermentation
The core function of Oenococcus oeni is the chemical conversion of L-malic acid into L-lactic acid and carbon dioxide. Malic acid, a dicarboxylic acid naturally present in grape must, has a sharp, tart flavor reminiscent of green apples. The bacteria use an enzyme to remove a carboxyl group from the malic acid molecule, releasing carbon dioxide. This process is technically a decarboxylation, as the bacteria break down an organic acid rather than metabolizing sugar for energy.
This conversion yields L-lactic acid, a monoprotic acid perceived as significantly softer and creamier on the palate. The change from diprotic malic acid to monoprotic lactic acid results in measurable deacidification, typically reducing total acidity by 1 to 3 grams per liter. This chemical shift also causes a slight increase in the wine’s pH (usually 0.1 to 0.3 units), contributing to a smoother profile. This decrease in sharpness is sought after in nearly all red wines and certain white wines, such as Chardonnay.
Flavor and Texture Contributions
The most noticeable sensory change caused by Oenococcus oeni activity is the introduction of new aromas and modification of the wine’s texture. Beyond the primary acid conversion, the bacteria metabolize other minor compounds, notably citric acid. The most important secondary product is diacetyl, a diketone compound responsible for the distinct buttery or butterscotch notes in wine.
Winemakers carefully manage the concentration of diacetyl, as it determines the final aromatic impact. At low concentrations (1 to 4 milligrams per liter), it contributes a pleasant, subtle complexity often described as hazelnut or caramel. If the concentration exceeds approximately 5 milligrams per liter, the overwhelming buttery flavor can be considered a fault. The overall mouthfeel also changes; the reduction in total acidity and the release of cell wall components contribute to a rounder, fuller, and richer texture on the mid-palate.
Environmental Controls in Winemaking
Winemakers actively manage the activity of Oenococcus oeni to achieve their desired wine style. The bacteria’s growth is heavily influenced by four environmental factors: low pH, high ethanol concentration, temperature, and the presence of sulfur dioxide (\(text{SO}_2\)). Although O. oeni is uniquely tolerant, its growth can be inhibited if the wine’s pH drops below 3.0 or the alcohol content rises above 14% by volume.
Encouraging Malolactic Fermentation
To encourage MLF, winemakers often inoculate the wine with commercial freeze-dried cultures of known strains. This provides a competitive advantage over wild bacteria that could introduce off-flavors. They may also warm the cellar environment, as the bacteria are generally more active at temperatures between 68 and 72 degrees Fahrenheit.
Preventing Malolactic Fermentation
Conversely, to prevent MLF, a winemaker may add sulfur dioxide, as the free \(text{SO}_2\) binds to and inhibits the bacteria’s growth. Filtration through a fine membrane physically removes the bacteria from the wine. Alternatively, the wine is kept cold to slow or halt metabolic activity, ensuring a fresh, fruit-forward style.