According to University of Leicester scientists, the moist environment of bagged salads, combined with nutrients leaching out of chopped leaves, created the perfect breeding ground for bacteria, which can fuel the growth of food-poisoning bugs like Salmonella and make them more dangerous. The researchers said they were shocked at the way the bacteria thrived, even in the fridge, reports the BBC. They advised people to eat bagged salad on the day they bought it.
Studies have shown that salad can carry bacteria, but the research team showed the bag made things much worse.
Bags are often moist to help keep the salad crisp and fresh and the plants have often been chopped into individual leaves for convenience.
The study showed sugars, proteins and minerals escaped from the cut leaves into the water in the bag.
"That's a reasonable amount of nutrients if you're a bacterium," said researcher Dr Primrose Freestone.
The study, published in Applied and Environmental Microbiology, showed that an initial contamination of 100 Salmonella bacteria would increase to 100,000 within five days.
"That's more than an infectious dose," Dr Freestone said.
She said she was also "shocked" that the bugs did so well in the chilled environment in the fridge.
The studies showed that Salmonella did especially well in bags containing spinach, while E. coli loved rocket leaves.
Dr Freestone told the BBC News website: "Juices that naturally leach from the leaves have the potential to increase the growth of any pathogen that might be present and establish them so strongly that washing wouldn't be enough to eradicate them.
"Don't be alarmed, we still eat bagged salad, but don't keep bagged salad any longer than you need to, we normally buy it on the day we eat it.
"Buy the bag with the best sell-by date, avoid lots of mushed leaves and if it's inflated then don't use it."