Bagged Salads Could Be a Breeding Ground for Salmonella


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Ready-to-eat bagged salads could be an ideal growing environment for salmonella, a new British study suggests.

Researchers at the University of Leicester found that the moist environment in bags of salad, combined with leaking nutrients from the chopped leaves, provides the perfect setting for the bacteria to grow.

Bagged salads are usually moist in order to keep them crisp and fresh, and they have also been conveniently chopped into individual leaves.  

Credit: U.S. FDA/Flickr

“Salad leaves are cut during harvesting and we found that even microlitres of the juices (less than 1/200th of a teaspoon) which leach from the cut ends of the leaves enabled salmonella to grow in water, even when it was refrigerated,” Dr. Primrose Freestone, who led the study by University of Leicester microbiologists, said. “These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.”

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Credit: Rusty Clark/Flickr, CC BY 2.0

Once the bags were opened, the bacteria on the leaves grew much faster than when it was kept sealed in a cold refrigerator. After first contamination, 100 salmonella bacteria could increase to 100,000 within just five days, the study found. As a result, the researchers advised to eat bagged salad as soon as possible after opening it.

“That’s more than an infectious dose,” Dr Freestone said.

The study, published in the journal Applied and Environmental Microbiology, only tested how bacteria grew on salad leaves after they were damaged and how the bacteria stuck to the surface of the plastic bag. They did not test store-bought salads for the bacteria.

Image Courtesy: University of Leicester
Credit: University of Leicester

Leafy greens are an important part of a healthy diet, the researchers stressed. However, they were also the second most common source of foodborne illness, including salmonella outbreaks. According to the Centers for Disease Control and Prevention, every year salmonella is linked to nearly one million foodborne illnesses in the United States, causing about 19,000 hospitalizations and 380 deaths.

“Consumers seem to be more preoccupied with nutritional facts, but they should not forget that foodborne pathogens can be deadly,” said Dr. Kimon Andreas Karatzas, assistant professor of Food Microbiology at the University of Reading, who was not involved in the study. “Avoiding fresh produce is not a solution, but if possible it would be preferable to buy uncut fresh produce over chopped, and to always wash it before you eat – even the ones that are already washed. Furthermore, keeping these foods in the refrigerator is important.”

Although avoiding fresh produce is not the solution, researchers urged for consumers to be careful and hope to find better protocol when bagging salads.

Related: Good Bacteria Can Limit E. Coli, Salmonella Infections