When your unopened salad package starts collecting a clear liquid slime at the bottom of the bag, you might want to throw it out.
New stomach-turning research from the University of Leicester in England finds that this “salad juice” can help dangerous bacteria like salmonella cling to salad greens, potentially increasing consumers’ risk for food poisoning.
Microbiology researchers and study authors Primrose Freestone and Giannis Koukkidis explain in the video below that when juice from damaged leaves starts forming in a closed salad bag, it can encourage the salmonella bacterium to grow and stick to the greens and the interior of the plastic bag, even when stored in the refrigerator. This appears to make the salmonella more virulent compared to bags of greens without salad juice, potentially increasing the risk of an infection.
Of course, the salad juices don’t generate dangerous pathogens ― they just help spread ones that may already exist through transfer of bacteria during the salad picking or packaging process.
Every year in the U.S., salmonella is estimated to cause about one million cases of food poisoning, 19,000 hospitalizations and 380 deaths, according to the U.S. Centers for Disease Control and Prevention. Symptoms include diarrhea, fever and abdominal cramping and can appear any time from 12 to 72 hours after eating the contaminated food. In 2016, salmonella outbreaks in the U.S. have so far been linked to alfalfa sprouts, pistachios and egg shells.
Eat salad like a researcher
While this research may make it seem like pre-packaged salads pose a scary risk, the researchers themselves were quick to say they still eat bagged salads. But they make sure to look for packages that have appropriate use-by dates and crisp-looking leaves. They stay away from salads that have mushy, slimy-looking greens, or bags with accumulated salad juice at the bottom. And they make sure to eat the greens within one day of purchase.
“Our project does not indicate any increased risk to eating leafy salads, but it does provide a better understanding of the factors contributing to food poisoning risks,” said Freestone.
If you feel like it, you can wash greens that have already been pre-washed by manufacturers just before eating, but Freestone says this doesn’t have much of an effect on the salmonella bacteria that may already be attached or internalized by the leaves.
Her study was published in the journal Applied and Environmental Microbiology.