Fresh produce such as green salad leaves are part of a healthy diet. Bagged, washed salads are seen as a healthy convenience food, and their consumption in the United States and European Union has increased considerably in recent years.
Salad leaves, both lettuces and spinach, because of their high water content are highly perishable and subject to rapid spoilage by microbes both endogenous and exogenous. To minimize the growth of these bacteria requires rapid processing and special packing. Nonetheless, epidemiological profiling now ranks salads as the second most common source of foodborne illness outbreaks.
In a recent research paper published in the November 18th 2016 issure of Applies and Environmental Microbiology found that traces of juices released from salad leaves as they became damaged through crushing in transport, storage and time significantly increased Salmonella enterica salad leaf colonization by a factor of 2-24 over the controls depending on conditions and media.
Until now very little was known about what happens to salad and the Salmonella bacteria within the actual salad bag. The scientists showed that the juices released from the cut-ends of the salad leaves enabled the Salmonella to grow in water even when it was refrigerated. Salad juice exposure also helped the Salmonella cells to attach to the salad leaves so strongly that washing the salad could not remove them. This study shows that exposure to even traces of salad leaf juice may contribute to the persistence of Salmonella on salad leaves as well as priming it for establishing an infection in the consumer.
This study demonstrates the need for producers and packagers to avoid all bacteria contamination in the processing of bagged salads. Also, the salads must be consumed when they are freshest before they are soggy and it may be safer to buy the salad in plastic clamshells where the lettuce can’t be crushed. Read the article:
Salad leaf juices enhance Salmonella growth, fresh produce colonisation and virulence; Giannis Koukkidis, Richard Haigh, Natalie Allcock, Suzanne Jordan and Primrose Freestone; Appl. Environ. Microbiol. doi:10.1128/AEM.02416-16.