There has been recent publicity surrounding the risk of salmonella in salad leaves following the publications of an article by scientists at the University of Leicester in Applied and Environmental Microbiology. This study shows the need for further research to strengthen understanding of the behaviour of salmonella.
Salmonella is a food poisoning bacterium – this means that low levels of the bacteria are usually not sufficient to cause illness and contamination of food alone, unless contamination is very heavy, is generally insufficient to cause illness. Salmonella normally needs to be present in high levels for a healthy person to become ill; of course the infective dose is less for vulnerable groups such as the young, elderly, ill, pregnant women or susceptible individuals.
In order to grow to the levels required to cause illness, salmonella needs the right temperatures and food/nutrients to support multiplication. High protein foods have traditionally been linked with salmonella but this latest research supports the premise that salad leaves could potentially provide the nutrients to support multiplication.
It has widely been recognised that listeria is a harmful bacteria that can grow at the low temperatures found in a refrigerator. Now this study indicates that salmonella can also grow and multiply at low temperatures, something which has not been widely recognised previously.
For pathogens that grow under refrigerated conditions, it is essential to limit shelf life in order to avoid giving them the time they require to grow to harmful levels. Food must always be consumed within the manufacturer’s shelf life but we recommend the sooner the better.
Salad leaves may be contaminated by sources such as animals, soil, water or humans during the growing process. Reputable producers must have rigorous systems in place to prevent and remove contamination and sanitize the product. Salad leaves should be thoroughly washed in clean potable water to reduce levels of potential contamination, especially if they are not supplied ready to eat. The research shows that damaged leaves are a particular concern – bagged salads should be handled and stored carefully in order to minimise damage and reduce risk of contamination.