Bagged salad named salmonella risk

A study carried out by the University of Leicester and published in Applied and Environmental Microbiology has found that bagged salad can facilitate the growth of food poisoning bacteria such as Salmonella.

The research warned that food poisoning bacteria thrived even when salad is kept in the fridge and advised people to eat bagged salad on the day of purchase.

The news comes after 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.

The study specifically looked at how bags facilitated the growth of bacteria and found that they provided a moist environment for sugars, proteins and minerals to leak from the cut leaves into the water in the bag.

It found an initial contamination of 100 Salmonella bacteria would increase to 100,000 within five days.

Researcher Dr Primrose Freestone commented: “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 emphasises 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.

"It also serves as a reminder to consume a bagged salad as soon as possible after it is opened. We found that once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge.

“This research did not look for evidence of salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged."

Dr Jeri Barak, from the Food Research Institute at the University of Wisconsin-Madison, added: "It would be fair to conclude that if Salmonella is present in salads, it might grow to infectious doses.

"The rates of produce that have been found to be contaminated are between zero to three per cent. Consumers should treat bagged salads as temperature-sensitive food products, like milk and ice-cream."