One of the most common off flavors in beer is diacetyl. Diacetyl smells and taste like butter and it reminds many of movie theater popcorn. That’s not a coincidence, diacetyl is added to oil to make movie popcorn “butter flavoring.” The chemical name of diacetyl is 2,3-butanedione. Tasty, eh? Diacetyl can even taste like butterscotch in high concentrations. Most people who can taste it can also pick up the aroma, just swirl your glass and take a short sniff.
You know that buttery taste in some Chardonnays? Yep, that’s diacetyl. It occurs naturally during all alcoholic beverage fermentations and is produced before yeast start cranking out ethanol. As the yeast finish up fermentation, one of the last things on their checklist is to scrub the diacetyl from the beer. Brewers call this time diacetyl rest. The temperature of the beer is raised slightly for a few days after fermentation and the yeast convert the diacetyl to a tasteless substance. Actually, there are 2 sources of diacetyl. The other is lactic acid bacteria commonly found in dirty beer lines. That’s way worse than that fake buttered popcorn.
Like all beer off flavors, everyone has a different sensitivity level to diacetyl. While some folks can’t even taste it, others perceive it easily. To complicate things, it’s OK to have diacetyl in some beer. Diacetyl isn’t always a bad thing. While it’s considered a flaw in most beers, it’s not uncommon to find it in low levels in traditional British ales, like English Pale Ales, and some Pilsners, like Pilsner Urquell.
To prevent diacetyl, brewers should fully complete fermentation then give the beer time for a diacetyl rest. As for the bacteria-induced diacetyl, bar managers need to regularly clean their beer lines, keg couplers and faucets. As with any off flavor, let your bartender or server know if you get a bad beer. They should pour you a fresh one and look into the source of the problem.
Here’s to delicious beer! Cheers.