Researchers at the University of Leicester found that juices from damaged leaves in bags of spinach and mixed salad increased Salmonella counts by 2400X compared to a control.
Even tiny amounts of juice — less than 1/200th of a teaspoon — enabled Salmonella growth. Juices also helped Salmonella stick to the bag and grow inside the leaf where it could not be washed off.
Dr. Primrose Freestone of the Department of Infection, Immunity and Inflammation explained:
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.”
That does not mean Salmonella is always in your salad — but if the bag contained even a few cells of Salmonella, there could be thousands by the time the salad leaves hit your plate.
Researchers also found that bacteria grew much faster as soon after the bag was opened, even in the refrigerator, which is a good reminder to eat a bagged salad as soon as possible after it is opened.
There risk of food poisoning is low, but outbreaks have occurred. While researchers only investigated Salmonella, it is likely that other bacteria would also grow well in a moist bag of salad.
In January, Dole Fresh Vegetables Inc. recalled all salad mixes processed in Springfield, Ohio due to Listeria contamination. The outbreak infected 19 people in nine states. All were hospitalized, and one person died.
Source: “Juices from damaged salad leaves massively stimulate Salmonella growth and salad leaf colonisation, study shows” — University of Leicester (November 2016)