How Oenococcus oeni Bacteria Convert Tart Malic Acid into Buttery Diacetyl During Winemaking

When a winemaker inoculates a barrel of Chardonnay with Oenococcus oeni bacteria, they are initiating a microscopic transformation that fundamentally alters how the wine feels in the mouth. This secondary process, known as malolactic fermentation (MLF), does not rely on yeast or produce alcohol. Instead, it targets malic acid—the sharp, tart compound responsible for the bite in a green apple. Over the course of several weeks, the bacteria consume this diprotic acid and excrete lactic acid, the softer, richer compound found in milk and cultured dairy products.[3]

The result is a profound textural shift. The harsh edges of the wine soften, and the mid-palate takes on a creamy, almost oily weight that defines many classic white wines. During the late 20th century, this buttery profile became the hallmark of highly sought-after California Chardonnays, though the style has seen shifting consumer preferences in recent years. Yet, beyond the trends, the biological mechanism remains a critical tool for managing a wine's structure and longevity.[3]

The sensory change is driven by a measurable chemical shift. As the bacteria convert the "harsher" diprotic malic acid into the softer monoprotic lactic acid, the wine loses a significant portion of its titratable acidity—typically dropping by 1 to 3 grams per liter. Simultaneously, the pH of the wine increases by roughly 0.3 units. Because lactic acid only has one acidic proton to donate compared to malic acid's two, the wine becomes measurably less sour.[1][2]

For grapes grown in cooler climates, this deacidification is often essential. Vines in cooler regions naturally retain higher levels of malic acid at harvest, and without bacterial intervention, the resulting wine can be aggressively tart. By encouraging malolactic conversion, winemakers can round out the mouthfeel. As wine writer Rémy Charest notes in SevenFifty Daily, the process "takes the edge off a wine's acidity by converting the sharper malic acid into the softer lactic acid... which rounds out the mouthfeel and makes a wine more accessible."[1][3]

But the bacteria do not stop at malic acid. Oenococcus oeni also metabolizes the small amounts of citric acid naturally present in the grape must. One of the intermediary byproducts of this citric acid degradation is a volatile compound called diacetyl. Diacetyl is the exact chemical responsible for the aroma of melted butter and toasted popcorn. When a Chardonnay smells heavily of butter, it is not because of the oak barrel itself, but because the malolactic conversion has left behind a high concentration of diacetyl.[3][4]

The maximum concentration of this buttery compound typically peaks just as the malic acid is fully exhausted. Winemakers can manipulate the intensity of this butter note by controlling the environment in the cellar. The final concentration of diacetyl is highly sensitive to oxygen exposure, redox potential, and the addition of sulfur dioxide (SO2).[4]

Diacetyl binds strongly with SO2; if a winemaker adds sulfur immediately after the fermentation completes, the buttery aroma is masked. However, this reaction is reversible. If the free SO2 levels drop during the wine's time in the bottle, the diacetyl is released, and the buttery aroma can suddenly re-emerge months or years later. Furthermore, the temperature of the fermentation plays a critical role: studies from Oregon State University demonstrate that conducting MLF at 15 °C versus 21 °C produces significantly different aromatic profiles and mouthfeel perceptions.[2][4]

While malolactic fermentation is famous for its role in white wines, it is actually far more common in reds. Winemakers routinely block the process in crisp whites like Riesling or Sauvignon Blanc to preserve their zesty, refreshing bite, usually by chilling the tanks and adding sulfites. In contrast, almost all red wines undergo full malolactic conversion.[3]

In a Cabernet Sauvignon or a Pinot Noir, the reduction in acidity and the slight bump in pH make the tannins feel rounder and more approachable. Without this bacterial intervention, many red wines would remain harsh, astringent, and microbiologically unstable in the bottle. By removing the malic acid, the winemaker eliminates a potential food source for spoilage organisms later on.[2][3]

The timing of the inoculation also dictates the final style. Before the mid-1900s, malolactic fermentation was a spontaneous, unpredictable event that occurred when the cellar warmed up in the spring. Today, commercial cultures of Oenococcus oeni, which became widely available in the 1980s, allow winemakers to trigger the process precisely. Some choose to co-inoculate the bacteria alongside the yeast during the primary alcoholic fermentation, while others wait until the yeast has finished its work. This decision, combined with the specific strain of bacteria used, gives the winemaker extraordinary control over the wine's final texture, stability, and flavor profile, turning a sharp, acidic grape into a seamless, velvety pour.[2]