In the past we’ve talked about the skunking of beer, which is an off flavor produced by beer gone bad via light waves, but there are many other flavors and aromas that can indicate when a beer has gone bad, one of which is diacetyl, or as mists say, die-uh-cee-til.
Diacetyl is a chemical that presents a buttery flavor or aroma and if you’ve ever made microwave popcorn, then you’ve ingested it, because it’s what’s manufacturer’s use to give it that “real butter flavor.”
Diacetyl is also known as a culprit behind popcorn lung, which isn’t as fun as it sounds. Popcorn lung, or bronchitis obliterans in fancy science man terms, first gained widespread notoriety in the early 2000s when a number of workers at a popcorn factory developed this condition due to the airborne inhalation of diacetyl.
Diacetyl production is a perfectly normal part of beer fermentation and it’s produced by normal yeast metabolism. However, you don’t need to worry about the diacetyl produced through beer fermentation, because the amount you’re consuming won’t approach harmful levels.
If brewed correctly, the buttery diacetyl flavors will mostly be cleaned up by the yeast and the presence of a little bit diacetyl is considered normal in some beers. However, diacetyl is hard to control and as a beer ages the diacetyl flavors can spin out of control, become unpredictable and of course ruin the beer.
A variety of factors can cause a diacetyl bomb, which is when a beer goes bad and you get a mouthful of butter.
First, high levels diacetyl can be produced if you’re unknowingly using mutant yeast and not in the fun X-men weay. Mutant yeast are yeast colonies that lose their ability to metabolize oxygen properly and they won’t be able to clean up diacetyl appropriately.
A second culprit behind diacetyl is a bacteria known as pediococcus, which can make a beer both sour or buttery. Brewers routinely test their beer for this bacteria, but test results can take up to a week.
A third culprit is unsanitary conditions, and not just on the part of the brewer. Sanitation is important at all stages of a beer’s life, from the word go to the tap lines that are used to deliver beer into your glass.
So, if you’re ever having a beer and you notice a strong buttery or butterscotch flavor, now you know that diacetyl is the villain. If you’re at a pub and they’re not cleaning their tap lines every two weeks, you need to alert the brewer, talk to the bar manager, and consider going somewhere else.