A study published this month in Applied and Environmental Microbiology, conducted at the University of Leicester, shows that juices from damaged salad leaves “massively” stimulate Salmonella growth. In fact, that lab study shows that those juices increase Salmonella growth 2,400 fold over a control group. The leached juices also increased the bacteria’s ability to form a strong, wash-resistant attachment to salad leaves and they enhance the pathogen’s virulence, increasing its ability to cause illness.
This is all bad news for bagged salad lovers. Between 2000 and 2007 in the United States, there were 38 Salmonella outbreaks associated with leafy greens that sickened 1,409 people. Leafy greens and Salmonella as a produce-pathogen pair is the second most common risk for outbreaks. And that doesn’t count the recalls of bagged salad products for possible pathogen contamination.
For instance, in 2012, Dole salads were recalled for possible Salmonella contamination. Trader Joe’s Kale and Edamame Salad was recalled in May 2016 for possible Salmonella contamination. Dole bagged spinach was recalled in October 2015 for Salmonella concerns. And in 2015, Taylor Farms Food Service recalled spinach for possible Salmonella and E. coli. This research did not look for Salmonella in these products; rather it examined how Salmonella grows on salad leaves when they are damaged.
Lead author of the study, Dr. Primrose Freestone, said, “Salad leaves are cut during harvesting and we found that even microliters 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. 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.
“This strongly emphasizes the need for salad leaf growers to maintain high food safety standards as even a few Salmonella cells in a salad bag at the time of purchase could be become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.”
This study also reminds consumers to eat a bagged salad as soon as possible after it is opened. Once opened, bacteria present on the leaves grew much faster, even when the salad was stored at refrigerator temperatures.
To help protect yourself, always buy bagged salads with a use by date as far into the future as possible. Look at the bag before you buy it to make sure there are as few damaged leaves as possible. Store it immediately in the refrigerator, and eat the product as soon as possible, preferably within a day of purchase.