More reasons to prefer whole heads of lettuce over pre-cut salad leaves and mixes: the cut and broken leaves in packed salad mixes raise the risk of salmonella contamination by as much as 2400.

A new study published in Applied and Environmental Microbiology by researchers at Leicester University is highlighting an overlooked aspect of food safety in salad mixes. Packed salad offer many advantages to the consumer: they are convenient and easy to use, there is a huge selection to choose from, and they help raise the consumption of healthy leafy greens. But most consumers are unaware of the fact that such pre-cut salads are more sensitive to contamination, and possess a much larger risk for potential microbial outbreaks.

The researchers explained that juices oozing from the damaged leaves promote salmonella growth, and also increase its virulence (its ability to thrive and cause disease in the unlucky consumer). Even tiny amounts of liquids were enough for the salmonella to grow. The growing salmonella could thrive also in refrigeration (at below 4 degrees), whereas until now it was believed that salmonella cannot grow below 5.2 degrees. The researchers found that the salmonella somehow used the juices to attach itself so strongly to the leaves and even to the bag, that no washing could remove it.

So what can we do in order to enjoy the benefits of leafy greens, and keep ourselves safe and healthy?

  • Always prefer whole heads of salads over pre-cut leaves. Cutting and preparation should be done as close to meal time as possible.
  • Avoid bags with liquids in them – it shows the produce is not fresh.
  • When choosing between whole heads of salads, prefer hydroponically grown salads, which are cleaner and safer to consume to begin with.