🍷 Diacetyl

Diacetyl is a compound with a buttery or butterscotch smell that forms in wine during malolactic fermentation.

Malolactic fermentation is carried out mainly by the bacterium Oenococcus oeni. It converts L-malic acid into L-lactic acid, which reduces the acidity of the wine, and it also shapes aroma and flavour. Diacetyl is one of the major compounds these bacteria produce; it is formed through the metabolism of citric acid. Above its sensory threshold, about 0.2 mg/L in Chardonnay, it gives the wine a buttery or nutty character. At high concentrations the wine tastes overtly buttery, which is considered undesirable, and strong oak or very fruity character can mask the buttery note.

Winemakers can influence the amount. According to the AWRI, the diacetyl concentration is higher with fewer bacteria added, a longer malolactic fermentation, a lower temperature (18 °C rather than 25 °C) and more oxygen (anaerobic conditions give less), and it is lower after long contact with yeast lees. Sulfur dioxide binds diacetyl, so adding it can cut the concentration. Diacetyl is also formed in dairy products fermented by lactic acid bacteria.

Air makes wine buttery. In a Chardonnay trial, malolactic fermentation under semi-aerobic conditions pushed diacetyl to about 13 mg/L, against roughly 2 mg/L without oxygen. Sulphur dioxide works the other way: 80 mg/L bound enough diacetyl to cut the measured level from about 20 to 5 mg/L, though the binding is reversible. Cabernet Sauvignon hides the note better than Chardonnay, since its sensory threshold is about 2.8 mg/L, against about 0.9 for Pinot Noir.