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Contrary to what Harry Potter fans might think, most beer experts agree that butter and beer shouldn’t mix. A buttery smell or flavor is one of the first “off flavors” aspiring beer judges learn to identify, and it’s the result of a pesky little organic compound known as diacetyl.

Diacetyl (pronounced “die-ASS-it-uhl”) can create an aroma and flavor in beer that’s a dead ringer for movie theater popcorn butter. Or, if you’ve ever eaten a popcorn-flavored Jelly Belly, imagine that in liquid form. Pretty gross, right?

While diacetyl is generally a sign of fermentation problems or beer served through dirty draft lines, at low levels, it can actually be an acceptable, pleasant component of certain beer styles. Here’s how to tell the tasty from the terrible.

First, some background: Diacetyl is an organic compound made by yeast during fermentation, so it’s present in every beer at some level. When they’re chomping on sugars and turning into alcohol, yeast produce an amino acid called valine. A middle step in valine production is a compound called acetolactate, most of which stays stuck inside the yeast cell. Some rogue acetolactate leaks out, however, and can oxidize into diacetyl.

But, in a fascinating enzymatic reaction, the yeast themselves clean up the diacetyl they’ve made. (If only children did the same with their food messes.) This occurs during the latter stages of fermentation when diacetyl is converted to 2,3-butanediol, an essentially flavorless organic compound that most people can’t detect. As a result, most beers you’ll encounter have no perceptible trace of diacetyl whatsoever; diacetyl’s flavor threshold is about 0.1 parts per million in light beer.

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So it’s quite off-putting to have a bartender slide you a pint that smells or tastes like Land O’ Lakes. Diacetyl has another tell, which is a slickness or oiliness to the texture of the beer. If you smell butter and feel a slick coating on your tongue, you might have higher amounts of diacetyl in your glass than desirable.

“If you’re getting that diacetyl character out of a beer that you’re drinking out of a bottle or can, I’d certainly reach out to the brewery, because the brewery will want to know that from a quality-assurance perspective,” says Dennis Mitchell, a Beer Judge Certification Program grand master judge and member of the American Homebrewers Association governing committee. “If you’re at a beer bar, you might even ask when the [draft] lines were last cleaned. If they can’t tell you, that might be indicative of a problem.”

In draft beer lines that haven’t been properly cleaned, bacteria can build up and produce all sorts of unintended flavors, from butter to vinegar and beyond. But don’t become too paranoid, dear beer drinkers, because tasting high levels of diacetyl in a professionally made beer is a fairly rare experience.

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Sometimes, in fact, low levels of diacetyl are actually in your glass on purpose and add to the overall character of the beer.

“I think a lot of people get this negative connotation pretty early on that diacetyl is an off flavor and a bad thing across the board,” says Brian Reed, a trade brewer with MillerCoors who recently achieved Master Cicerone certification, the highest level of the Cicerone Certification Program. “In reality, there are a few styles in which diacetyl is desirable and without which it’d be an incomplete beer. And those are some of my favorite beer styles.”

Certain European beer styles, in particular some English ales and Czech lagers, count low levels of diacetyl as one of their calling cards. Ever had a Pilsner Urquell or one of Samuel Smiths’ fruit ales? Both contain pleasant levels of diacetyl as part of their character.

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(This next paragraph is for beer-science nerds only. Not interested? Skip ahead.) That has to do with the ways specific yeast strains ferment beer. Many English ale strains are highly flocculent, a funny little word that means they like to clump together and fall out of solution at the latter end of fermentation. English ale yeast cells usually glob together quickly and drift to the bottom of the tank so they don’t have the surface area to reduce diacetyl completely. As for Czech lagers, their characteristic diacetyl comes from the lack of a diacetyl rest, or a period of time when brewers raise the fermentation temperature of the almost-done-beer from lager temperatures (50-55 degrees F) to 65-68 degrees. Higher temperatures encourage the yeast to go Pac-Man style and eat up diacetyl; without that diacetyl rest, a bit remains in the beer.

Even though diacetyl is a part of both English ales and Czech lagers, Reed says the average drinker probably doesn’t notice it.

“Unless it’s a really aggressive ‘Hey, this stout tastes like espresso’ or ‘This beer is brewed with strawberries,’ in terms of specific flavor compounds, you may not be able to pick those up unless someone points it out for you,” he says.

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There’s also a small percentage of people who are totally unable to perceive diacetyl; it’s thought to be a genetic quirk. Author Randy Mosher in his seminal book Tasting Beer estimates that about 10 percent of the population is unable to perceive diacetyl or DMS, another “off flavor” compound.

And that’s really quite fine, since diacetyl is harmless in beer, aside from its sometimes unpleasant flavor. You’ll find low levels of diacetyl in some chardonnay wines, and for years it was added to microwave popcorn and other foods. Diacetyl did come under fire in the early 2000s, when a Centers for Disease Control investigation into factories where microwave popcorn is made “documented a relationship between cumulative exposure to diacetyl vapor over time and having abnormal lung function,” a disease that came to be known as popcorn lung. The CDC couldn’t pinpoint precisely which of the many compounds and chemicals floating around the factories were most responsible for lung damage, but findings point to diacetyl as “one agent that can cause flavorings-related lung disease.”

For those of us who didn’t work in a microwave popcorn factory during the late 1990s, diacetyl is just a normal, small part of the beer we drink. Sometimes it should be in our beer; sometimes it shouldn’t. The key is moderation: You’re sniffing and tasting for a small hint of butteriness in certain styles, never a glass full of Country Crock.

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