Certain foods are more likely than others to wreak havoc on your stomach. Cucumbers have carried Salmonella, peaches have been contaminated with Listeria, and eating a salad feels a bit like Russian roulette. Romaine lettuce, tomatoes, and sprouts are all considered high risk for foodborne illnesses. (Scott Faber, a food-safety expert at the Environmental Working Group, put it to me bluntly: “Don’t eat sprouts.”)
By comparison, onions have an almost-divine air. They are blessed with natural properties that are thought to prevent foodborne illnesses, and on top of that, they undergo a curing process that acts as a fail-safe. According to one analysis by the CDC, onions sickened 161 people from 1998 to 2013, whereas leafy greens sickened more than 7,000. Onions haven’t been thought of as a “significant hazard,” Susan Mayne, the former head of food safety at the FDA, told me.
Not anymore. Late last month, McDonald’s briefly stopped selling its Quarter Pounders in certain states after at least 90 people who ate them fell sick with E. coli. Last Wednesday, the CDC announced the likely culprit: slivered onions. This is the fourth time onions have caused a multistate foodborne outbreak since 2020, in total sickening at least 2,337 people, according to available data. In that same time span, leafy greens have caused eight multistate outbreaks that have affected 844 people. All of a sudden, the United States seems to have an onion problem—and no one knows for sure what is causing it.
The investigation into the cause of the McDonald’s outbreak is still ongoing, but the problem likely started where many foodborne illnesses begin: in the field. The culprit, in many instances, is contaminated water used to irrigate crops. An outbreak can also start with something as simple as a nearby critter relieving itself near your veggies. Any additional processing, such as when onions are cut into prepackaged slivers, can give bacteria lots of opportunities to spread. That’s why the FDA considers most precut raw vegetables to be high risk. (As with other foods, cooking onions to 165 degrees Fahrenheit kills pathogens.)
But the fact that onions appear to get contaminated with E. coli and Salmonella at all is striking. Onions have long been thought to have antimicrobial properties that can help them fight off bacteria. Hippocrates once recommended that onions be used as suppositories to clean the body, and onions were placed on wounds during the French and Indian War. Medical knowledge has thankfully advanced since then, but the onion’s antimicrobial properties have been documented by modern science as well. In various lab experiments, researchers have found that onion juice and dehydrated onions inhibit the growth of E. coli and Salmonella. And in 2004, researchers found that E. coli in soil died off faster when surrounded by onion plants than when surrounded by carrot plants, a result the authors said might be due to “the presence of high concentrations of antimicrobial phenolic compounds in onions.”
Onions have another powerful weapon in their food-safety arsenal: their papery skin, which research suggests may act as a barrier protecting the insides of an onion from surface bacteria. The way that onions are processed should add an additional layer of protection: To extend their shelf life, onions are left to dry, sometimes for weeks, after they are harvested. This curing process should, in theory, kill most bacteria. Stuart Reitz, an onion expert at Oregon State University who has intentionally sprayed onions with E. coli–laced water, has found that the curing process kills off a significant amount of the bacteria—likely because of ultraviolet radiation from the sun and because drier surfaces are less conducive to bacteria growing, Reitz told me.
But clearly, onions are not contamination proof. Onion experts I spoke with floated some plausible theories. Linda Harris, a professor of food safety at UC Davis, posited that bacteria could hypothetically bypass an onion’s protective skin by entering through the green tops of the onion and then traveling down into the layers of the onion itself. And although onions might have antimicrobial properties, that might not always be enough to prevent an E. coli infection from taking hold, Michael Doyle, a food microbiologist at the University of Georgia, told me; when it comes to antimicrobial activity, he said, “not all onions are created equal.” And the McDonald’s onions could have become infected simply by way of probability. One of Reitz’s recent studies on the effect of curing found that 2 percent of onions sprayed with E. coli still had detectable levels of the bacteria after being cured.
Still, none of this explains why onions seem to be causing more foodborne illnesses now. Harris told me that she and a colleague have “spent a lot of time trying to figure out how these outbreaks happen, and I will tell you: We don’t have an answer.” Unfortunately, we may never understand the cause of the onion’s heel turn. In many cases, regulators are unable to figure out exactly what causes a foodborne outbreak. They failed to find a definitive cause in the three other recent onion outbreaks, and perhaps the same will be true of the McDonald’s debacle.
The entire situation demonstrates the maddening inscrutability of foodborne illness. The reality is that although these outbreaks are rare, they can be dangerous. One person died after eating a contaminated Quarter Pounder, and a 15-year-old had to undergo dialysis to stave off kidney failure. Yet for all of the technology and science that goes into food safety—the genome sequencing of foodborne pathogens, blockchain technology that traces crops from farms to store shelves—we continue to be stuck with more questions than answers. America has less of an onion problem than an everything problem.