🍷 Diacetyl

Diacetyl is a substance with a buttery smell. It arises in wine during malolactic fermentation.

Malolactic fermentation is carried out by bacteria. They turn sharp malic acid into soft lactic acid and, along the way, create by-products, among them diacetyl. It gives notes of butter, cream and nuts. In white wines such as Chardonnay this aroma is considered pleasant if faint. If there is too much diacetyl, the smell seems heavy and unpleasant.

The same diacetyl gives butter and other dairy products their smell, which is why it is called a buttery aroma in wine. It is one of wine's secondary aromas, that is, those that come from fermentation and ageing rather than from the grape. Winemakers can control the amount of diacetyl: stop malolactic fermentation, change the temperature or add oxygen. In light wines where freshness matters, diacetyl is avoided. In rich ones it rounds the taste. For a taster it is one of the smells that show how a wine was made.