You probably choose some grocery items for convenience, hoping to save time when meal prepping. Things like prewashed bagged greens—lettuce or spinach—might seem like a smart choice for maintaining a healthier diet. But, hold on…
Food safety experts suggest that prewashed bagged greens aren’t as safe as you might think. In fact, they can harbor bacteria and contamination risks that are often overlooked.
“While consumers look for convenience and safety, the truth is that prewashed bagged greens often rank among the most hazardous items in grocery stores due to contamination risks all along the supply chain,” explains Darin Detwiler, a food policy professor at Northeastern University. He admits to steering clear of bagged greens because of the potential for dangerous E. coli contamination. “Looking at previous outbreaks, bagged salads have often been a key culprit in foodborne illnesses, with some incidents leading to severe health complications like hospitalizations, kidney failure, or even death,” he adds.
You’re not imagining it — outbreaks are common.
Several elements in the supply chain can taint the greens with pathogens—E. coli, listeria, or salmonella—that might lead to illness. “Bagged greens are washed and processed in bulk, which can effectively spread pathogens around,” says Olivia Roszkowski, a chef-instructor specializing in plant-based culinary arts and food safety at the Institute of Culinary Education.
Interestingly, leafy green outbreaks occur more frequently than one would think. To combat this, the FDA introduced a Leafy Greens STEC Action Plan back in 2020, updating it in 2023 to tackle different strains of E. coli associated with leafy greens. Detwiler explains that Shiga toxin-producing E. coli O157:H7 is a particularly dangerous strain known for its ability to cause severe illness even with minimal exposure. “This risk isn’t just speculative—it’s been evidenced repeatedly through various outbreaks,” he states.
Despite ongoing efforts to minimize these outbreaks, leafy greens are responsible for “up to 9.2% of all known foodborne illnesses” in the U.S., per a report in the Journal of Foodborne Illness. It is estimated that over 2.3 million cases of illness each year can be traced back to contaminated leafy greens.
How does the contamination happen?
On the farm
Contamination can take place at several points before bagged salad greens even reach your fridge. “In the fields, crops can get contaminated from sources like tainted irrigation water, wildlife, soil, or even air,” says Kimberly Baker, who directs food systems and safety programs at Clemson University. The risk of contamination rises when farming areas are near livestock operations or feedlots, adds Roszkowski.
California and Arizona are among the top producers of leafy greens in the U.S. and are often adjacent to large cattle operations, according to Detwiler. He notes that E. coli, salmonella, and listeria can infiltrate these crops through contaminated water sources linked to nearby cattle farms. “Manure runoff can seep into irrigation canals, introducing harmful bacteria like E. coli O157:H7,” Detwiler explains. When it comes to irrigation, using untreated water from rivers or lakes can also elevate contamination risks.
Risks continue to multiply during harvesting, when human contact, contaminated equipment, or water may transfer bacteria onto the lettuce, Baker warns.
In the processing plant
Once the greens hit processing plants, they encounter more chances for pathogen exposure. Typically, bagged greens come from multiple farms, complicating tracing back any contamination sources. “In processing facilities, greens are mixed, washed, and packaged together, meaning just one tainted leaf can affect thousands of bags across various states,” Detwiler points out.
Lettuce is often cleaned in large vats, which may contradict assumptions about safety, according to Rosemary Trout, a culinary arts and food science professor at Drexel University. Surprisingly, plant foods are frequently linked to foodborne illness more than animal products.
During transportation
Keeping the greens chilled is vital to inhibiting bacterial growth. Roszkowski notes that failing to maintain consistent cold temperatures throughout harvesting, processing, and transport can allow bacteria to thrive. Refrigeration, while helpful, doesn’t kill bacteria. As Detwiler says, “It slows their growth but doesn’t eliminate the risk of illness from contaminated greens.”
Is washing prewashed salad greens worth it?
If you’re contemplating rinsing your prewashed greens for extra safety, rethink that. Washing them at home often doesn’t improve safety since you’re unlikely to eliminate pathogens, according to Detwiler. In fact, rinsing can actually introduce new pathogens that are on your counter or sink. Baker agrees, emphasizing that prewashed greens should not be washed again at home to avoid introducing contamination.
Bacteria like E. coli or salmonella can form biofilms, keeping them firmly attached to the greens. Once they cling on, they don’t simply wash off. “The only effective way to eliminate these harmful pathogens is through cooking, as heat kills them. Unfortunately, lettuce is generally consumed raw, which complicates efforts to reduce foodborne illness risks,” Baker adds.
Tips for purchasing bagged greens safely
If you still prefer the convenience of bagged greens, consider taking these precautions:
Check for recalls
Before buying any salad greens, check for recalls. Detwiler and Roszkowski recommend staying informed via FDA and CDC websites. If your greens are implicated in an outbreak, dispose of them immediately.
Examine expiration dates
Experts encourage looking at expiration dates prior to purchase. Trout suggests only buying what you can consume in a day or two.
Assess freshness
Avoid bags that appear wet or overly moist, as these environments can be breeding grounds for bacteria. Baker advises inspecting the package for fresh-looking lettuce, without wilting or discoloration. Steer clear of bags containing slimy leaves, Roszkowski adds.
Minimize temperature fluctuations
Once you bring greens home, prevent letting them sit in a hot car or on the counter. “Drastic temperature changes—a hot car or outdoor heat—can create condensation that worsens spoilage and encourages bacteria growth,” Roszkowski notes.
Store properly
Keep your greens refrigerated at temperatures of 40 degrees Fahrenheit or lower, Detwiler and Trout advise.
Opt for whole heads of lettuce
Buying whole heads of lettuce or bunches of spinach may require a bit more prep, but it can help reduce contamination risks. Detwiler suggests rinsing under cold water—not soaking—to remove soil and any surface bacteria before consuming.
