Experts investigate why onions suddenly causing more foodborne illnesses

In recent years, onions—once celebrated for their natural antimicrobial properties—have become an unexpected source of foodborne illness outbreaks.

Certain foods are more likely to cause digestive trouble than others. Cucumbers have been linked to Salmonella, peaches to Listeria, and eating a salad can feel like taking a gamble. Romaine lettuce, tomatoes, and sprouts are considered particularly high-risk when it comes to foodborne illnesses, Caliber.Az reports, citing foreign media.

As Scott Faber, a food safety expert at the Environmental Working Group, bluntly put it: “Don’t eat sprouts.” In contrast, onions have always been viewed more favourably. They’re believed to have natural properties that help prevent foodborne illnesses, and they go through a curing process that acts as an extra safeguard. According to a CDC study, onions were responsible for 161 illnesses between 1998 and 2013, whereas leafy greens were linked to over 7,000 cases. Onions were never seen as a major threat, as Susan Mayne, the former head of food safety at the FDA, told me. However, that perception has changed.

Last month, McDonald's briefly halted sales of its Quarter Pounders in several states after at least 90 customers fell ill with E. coli. The CDC announced the likely culprit: slivered onions. This marks the fourth time since 2020 that onions have been implicated in a multistate foodborne outbreak, sickening at least 2,337 people. In comparison, leafy greens were responsible for eight outbreaks during the same period, affecting 844 people. Suddenly, onions are emerging as an unexpected problem, with the cause of these recurring outbreaks still unknown. 

The investigation into the source of the McDonald’s outbreak is still underway, but it likely began in the field, where many foodborne illnesses start. Often, the issue stems from contaminated water used for irrigation, or something as simple as a nearby animal relieving itself near crops. Once the produce is harvested, any further processing—such as slicing onions into prepackaged slivers—can provide bacteria with ample opportunities to spread. This is why the FDA considers most precut raw vegetables to be high-risk. (As with other foods, cooking onions to 165 degrees Fahrenheit effectively kills pathogens.) 

However, the fact that onions have been contaminated with E. coli and Salmonella is surprising, given their long-held reputation for having antimicrobial properties that help protect against bacteria. Hippocrates even recommended onions as a remedy, advising their use as suppositories to cleanse the body, and during the French and Indian War, onions were applied to wounds. Modern science has also confirmed the onion’s antimicrobial benefits. In various laboratory studies, onion juice and dehydrated onions have been shown to inhibit the growth of both E. coli and Salmonella. In fact, a 2004 study found that E. coli in soil died off more quickly around onion plants than around carrot plants, a result the researchers attributed to “the presence of high concentrations of antimicrobial phenolic compounds in onions.” 

In addition to these natural defenses, onions also benefit from their papery skin, which research suggests may act as a barrier, protecting the interior of the onion from surface bacteria. The processing of onions also adds a layer of protection: After harvesting, onions are dried for weeks to extend their shelf life. This curing process should, in theory, kill most bacteria. Stuart Reitz, an onion expert at Oregon State University, who has purposely sprayed onions with E. coli-laced water, found that the curing process significantly reduced the bacterial load—likely due to ultraviolet radiation from the sun and the fact that drier surfaces are less conducive to bacterial growth, according to Reitz. 

Clearly, onions are not immune to contamination. Experts I spoke with proposed some plausible explanations. Linda Harris, a professor of food safety at UC Davis, suggested that bacteria could potentially bypass an onion’s protective skin by entering through the green tops and traveling down into the inner layers. While onions are known to have antimicrobial properties, Michael Doyle, a food microbiologist at the University of Georgia, noted that this might not always be enough to prevent an E. coli infection from taking hold. “Not all onions are created equal,” Doyle said. 

By Naila Huseynova