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Off-Flavor of the Week: Diacetyl

Commonly described as having an artificial butter flavor.

Dave Carpenter Oct 15, 2014 - 3 min read

Off-Flavor of the Week: Diacetyl Primary Image

Continuing on our trek down the list of Beer Judge Certification Program (BJCP) off flavors, today we come to number four: diacetyl. Commonly described as having an artificial butter flavor (think movie theatre popcorn) that leaves your mouth feeling like an oil slick, diacetyl is actually produced in varying amounts by all yeast strains in all kinds of fermentations.

Bacteria can make diacetyl, too. If you’ve ever tasted a California Chardonnay that reminded you more of butter than of wine, then you’ve experienced firsthand an extreme example of malolactic fermentation, a bacterial process in which malic acid is converted to lactic acid and, you guessed it, diacetyl.

In certain beer styles, particularly those from the British isles, a small amount of diacetyl is acceptable. It should never approach the level of a California Chardonnay butter bomb, but a small amount can complement the fruity esters of indigenous yeasts. On the other hand, any perceptible diacetyl is strictly verboten in German lager brewing, and brewers go to great lengths to be rid of it.

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