What Is Diacetyl?

The molecule responsible for butter flavor is also behind Popcorn Lung, a deadly lung disease.

After about a year of working at the Glister-Mary Lee microwave popcorn factory in Jasper, MO, Eric Peoples started having breathing problems his doctors couldn’t explain. Eventually, he was diagnosed with bronchiolitis obliterans and informed he would likely need a double lung transplant in the future. A suspiciously high number of his colleagues at the same factory received the same diagnosis. Lawsuits followed and, because he had the worst symptoms, Peoples’s case was heard earliest. In 2004, he and his wife were awarded a total of $20 million in the first so-called “popcorn lung” case.

The culprit of Peoples’s ailment turned out to be diacetyl, the exact chemical that's used to make buttery popcorn so delicious. It’s found naturally in dairy products like sour cream, buttermilk and, yes, butter, and before the Peoples case, it was a common ingredient in artificial butter flavoring. (If you enjoyed “buttered” popcorn at a movie theater or out of the microwave before the mid-2000s, you almost definitely know what diacetyl tastes like.) It’s safe to eat, but it can cause permanent damage to the bronchioles—the narrowest parts of the branching airways in the lungs—if you inhale it. Like if, say, you’re a microwave-popcorn-factory employee working over the giant tank of flavorings.

Dozens of popcorn factory employees (and even one consumer—a Colorado man who ate two bags of popcorn every day for 10 years) won millions of dollars in lawsuits over the chemical, and diacetyl became molecule non grata in the processed-food world. Orville Redenbacher’s declined an interview for this story, but a spokesperson said that it stopped using diacetyl in its products in 2007, along with all other popcorn products produced by companies owned by its corporate parent, Conagra Brands, which include ACT II and Jiffy Pop. Diacetyl is still entirely legal (and safe!) to use as a flavoring, but it would be rolled up in “natural flavorings” or “artificial flavorings” on an ingredients list: You’d never see it named on a label. (I reached out to Jelly Belly, whose buttered popcorn-flavored jelly beans may use diacetyl—or may have used it in the past—but the company also declined an interview.)

Givaudan, the world’s largest maker of flavorings and fragrances, also stopped using diacetyl in the US in 2007. “Although it has been designated as generally recognized as safe (GRAS) for use in food by the FDA, we made the decision based on the ongoing threat of litigation and the related public perception,” says Stephen Collins, head of communications for Givaudan Taste & Wellbeing North America.

But if you’re a wine drinker, diacetyl has a very different reputation. “In wine, there’s a sharp acid called malic acid that tastes like green apples,” says Maggie Campbell, a wine and spirits expert who is president and head distiller of Privateer Rum, a member of the boards of the American Craft Spirits Assocation and the Wine & Spirits Education Trust, and a frequent speaker at drinks conferences about things like yeast physiology. “If you want to soften that, you can do a bacterial fermentation called malolactic fermentation that turns malic acid into lactic acid, and that also produces diacetyl.” One of the more common white wines to use malolactic fermentation is Chardonnay, which means that when a wine geek calls a Chardonnay “buttery,” it’s literally true.

In addition to being a huge fermentation nerd, Campbell has been technically trained as a taster to pick out individual flavor molecules, and she notes that besides Chardonnay, nearly all red wines undergo malolactic fermentation, and the subtle hint of movie-theater-popcorn that produces is actually useful for sommelier-level wine analysis. “If I’m tasting a red wine and notice a note of diacetyl, that can help me figure out the balance of malic and lactic acid, which helps me figure out how ripe the grapes were and the climate where they were grown,” Campbell says.

Buttery Chardonnay is a good thing, but buttery lager, less so. “Diacetyl in general is hated in beer,” Campbell says, explaining that it’s produced there by yeast fermenting very quickly or with insufficient nutrients. Sometimes, makers of slower-fermenting beers like lagers even include a “diacetyl rest” period during production that lets the yeast break down any diacetyl that might be around and get rid of any lingering buttery flavor. But there are a handful of beer styles where a little bit of butteriness is a good thing. Take British-style stouts (or Irish-style ones, like Guinness): A bit of diacetyl contributes butterscotch notes and a slightly oily mouthfeel.

Will drinking too much Chardonnay give you lung disease? No. Campbell hadn’t even heard of “popcorn lung” before our interview, and isn’t aware of any safety regulations about diacetyl for beer, wine, or spirits producers. Fermentation produces it at low concentrations, and even then, the diacetyl is unlikely to get into the air where it can be inhaled.

“Popcorn lung” associated with actual popcorn has pretty much disappeared in recent years, but diacetyl has reared its ugly head as a flavoring agent in the world of nicotine vapes. A 2015 study at Harvard University found diacetyl in more than 75 percent of the flavored e-cigarettes and refills tested. And late last year, a Canadian doctor published a paper on a 17-year-old with “popcorn lung” apparently caused by vaping. The FDA banned flavored vape products at the beginning of the year, but that was done to keep teens from getting hooked; the announcement makes no reference to diacetyl.

So the next time you catch a whiff of that old movie theater popcorn flavor, it's a good bet that what you're sniffing is diacetyl, whether you've got your nose buried in a Chardonnay, a glass of badly fermented beer, or a bag of butter-flavored anything, even if you can't find it listed on the ingredients. But you've been warned: delicious as it is, you may not want to breathe that sweet, sweet scent in too deeply.