Bagged salad can fuel the growth of food-poisoning bugs like Salmonella and make them more dangerous, a study says.
University of Leicester scientists said the moist environment combined with nutrients leaching out of chopped leaves created the perfect breeding ground for bacteria.
The researchers said they were shocked at the way the bacteria thrived, even in the fridge.
They advised people to eat bagged salad on the day they bought it.
Despite their wholesome nature, fresh green leaves and salad foods are often involved in food poisoning.
An outbreak that affected more than 2,000 people across Europe in 2011 was traced back to bean sprouts and this year, officials in England traced an outbreak that killed two people back to bags of rocket leaves.
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.