You likely buy some grocery items for convenience, to save a couple of steps when it’s time to prepare a meal. Prewashed bagged greens, such as lettuce or spinach, seem like a no-brainer if you’re trying to eat nutritiously. But not so fast ...
According to food safety experts, prewashed bagged greens aren’t the safest choice. Bacteria and contamination are more common than you may suspect in this particular grocery item.
“While consumers expect convenience and safety, the reality is that prewashed bagged greens remain one of the riskiest items in the grocery store due to contamination risks throughout the supply chain,” says Darin Detwiler, a professor of food policy at Northeastern University and author of “Food Safety: Past, Present, and Predictions.” He avoids buying bagged greens because of the risk of deadly E. coli contamination. “When you look at past outbreaks, bagged salads have been a leading cause of foodborne illness, with some outbreaks resulting in hospitalizations, kidney failure (hemolytic uremic syndrome), and even death,” Detwiler said.
You’re not imagining it — there are a lot of outbreaks.
Many factors in the supply chain can contaminate the greens with pathogens — such as E. coli, listeria or salmonella — that can leave people sick, hospitalized or worse. “Bagged greens are washed and processed in large batches, which is an effective way to spread pathogens,” said Olivia Roszkowski, chef-instructor of plant-based culinary arts and food safety certification at the Institute of Culinary Education’s New York City campus.
Leafy green outbreaks happen more than you’d expect, and the FDA created a Leafy Greens STEC Action Plan in 2020 to reduce different strains of E. coli in leafy greens and updated the plan in 2023. (For example, shiga toxin-producing Escherichia coli O157:H7 is the pathogenic strain that is often associated with lettuce and leafy green outbreaks, and Detwiler says it “is particularly dangerous because it takes a very small amount to cause severe illness. ... The risk isn’t theoretical — it’s been repeatedly proven in outbreak after outbreak.” )
And despite the efforts to reduce leafy green-related outbreaks, they continue to occur. Leafy greens are attributed to “up to 9.2% of known pathogen-caused foodborne illnesses” in the U.S., according to a report in the Journal of Foodborne Illness published in April 2024. The report estimates over 2.3 million illnesses every year are related to contaminated leafy greens.
How the food supply chain becomes contaminated
On the farm
Contamination can occur at different stages before bagged salad greens reach the refrigerated produce section and eventually your plate.
“In the field, produce can become contaminated with pathogens from sources such as contaminated irrigation water, wild or domesticated animals, soil, or air,” said Kimberly Baker, a food systems and safety program team director at Clemson University. It’s more common to have contaminated greens “when the field is in close proximity to livestock farms or feedlots where animals are being held in densely populated conditions,” Roszkowski explained.
Arizona and California are big producers of leafy greens in the United States and are often near large cattle operations, according to Detwiler. E. coli, salmonella, or listeria can be introduced to the crops through contaminated irrigation water from nearby cattle farms, he said.
“Runoff from manure lagoons can seep into irrigation canals and contaminate fields with deadly bacteria like E. coli O157:H7,” Detwiler continued. Manure lagoons are large pits where livestock waste is stored, per Environmental America. And it’s not only contaminated water that can be a problem. Some farms use unregulated water sources to irrigate the fields. “Some farms use untreated or minimally treated surface water from rivers, lakes or canals, increasing the risk of contamination,” Detwiler said.
When it’s time to harvest, there are added risks for pathogens and bacteria to come in contact with lettuce. “Contamination can occur through human handling, contaminated equipment or water used in post-harvest processes to transport, produce or remove soil,” Baker said.
In the processing plant
There are opportunities for pathogens to come in contact with leafy greens once they reach a processing plant. The greens you get in a bag don’t typically come from one field but from multiple farms, complicating traceability and increasing contamination.
Greens are processed in a centralized area and distributed — which is convenient for all the companies involved in the process— but not ideal from a food safety standpoint. “Greens from different farms are mixed, washed and packaged together, so one contaminated leaf can impact thousands of bags across multiple states,” Detwiler explained.
Lettuce is washed in large vats, added Rosemary Trout, associate professor and program director of Culinary Arts & Food Science at Drexel University. What you may not suspect, according to Trout, is that “plant foods are usually more often associated with foodborne illness than animal products.”
On the journey to store shelves
Keeping the greens cold is necessary to prevent any pathogens from growing and spreading. According to Roszkowski, “Not keeping the greens consistently cold throughout the process of harvesting, processing and transporting can cause bacterial growth to occur.”
However, refrigeration doesn’t eliminate the chance of pathogens; it can help slow them down. “Refrigeration slows bacterial growth but does not kill bacteria, meaning contaminated greens can still cause illness, even if stored properly,” Detwiler said.
Will washing prewashed salad greens help?
If you’re thinking, “Well, I’ll wash my prewashed greens for extra safety measures,” don’t. Washing greens at home doesn’t make them safer because water won’t remove the pathogens, Detwiler said. Rinsing prewashed greens can introduce pathogens that may be lurking on your counter or sink. “Prewashed greens should not be rewashed at home, as doing so can introduce contamination if the sink, utensils, equipment or hands of the preparer are not properly cleaned or sanitized,” Baker said.
Bacteria or pathogens aren’t obvious and can’t be seen. “If greens are contaminated with pathogens, they cannot be detected by sight, smell or taste … and can’t be removed by washing,” Baker explained. According to Detwiler, “Bacteria like E. coli and salmonella can form biofilms that help them stick to the surface of the leaves [and] once attached, these bacteria do not simply rinse off.”
Harmful pathogens need to be cooked to be eliminated, as heat kills them, said Detwiler. A big issue with lettuce is it’s usually eaten raw, not cooked, “making it challenging to reduce the risk of foodborne illness,” Baker said.
Recommendations for buying bagged greens
If you still prefer buying bagged greens for the convenience, there are several precautions to take.
Check for recalls
Before you purchase any salad greens, check if there are any recalls, advised Detwiler and Roszkowski. “Stay updated on recalls via the FDA and CDC websites,” Detwiler said. “If your greens are part of an outbreak, throw them away immediately.”
Check expiration dates
Experts say you should look at the expiration dates before purchasing. Trout advises to only buy what you will eat within a day or two.
Look for freshness
Avoid bags with excess moisture that look wet, as damp environments are a haven for bacteria to thrive, Detwiler said. “Inspect the package to ensure the lettuce looks fresh, without wilting or discoloration, as these signs may indicate improper storage or reduced freshness,” Baker said. You also want to stay away from any bags that have slimy leaves, Roszkowski added.
Minimize temperature changes
Once you’ve purchased greens, don’t let them sit in a hot car or garage or out on the counter. “Avoid drastic changes in temperature when transporting them from the store and when preparing a meal,” Roszkowski said. “This leads to condensation and creates a moist and warm environment that can encourage both spoilage and bacterial growth.”
Store properly
Greens should be kept refrigerated at or below 40 degrees Fahrenheit, say Detwiler and Trout.
Buy heads of lettuce
Purchasing a head of lettuce or a bunch of spinach may have the extra step of rinsing and drying before tossing in a salad bowl or stir-frying, but you could be minimizing the risk of lurking pathogens. “Whole heads of lettuce or spinach have less surface area for contamination compared to pre-cut, mixed bagged greens,” Detwiler said. He suggests rinsing them under cold water — not soaking — to remove dirt, debris, any insects and surface-level bacteria. “Wash them at home before eating.”