A recent study by researchers at the University of Leicester found that traces of salad green juices released when the leaves become damaged can significantly encourage the growth of Salmonella enterica.
The researchers reported that they concentrated on Salmonella because it is “an aggressive pathogen that has been implicated in salad-associated infections.” Their work was posted online Nov. 18 by the journal Applied and Environmental Microbiology.
“We found that juices released from the cut-ends of the salad leaves enabled the Salmonella to grow in water, even when it was refrigerated — this was a surprise as Salmonella has a temperature preference of 37 C,” according to the study.
The research team consisted of Giannis Koukkidis, Richard Haigh, Natalie Allcock, Suzanne Jordan and Primrose Freestone, who are all affiliated with the University of Leicester in Leicester, England.
Other findings from their research were that salad leaf juices enhanced attachment of the pathogen to the plastic bag and that over a five-day period, which researchers noted was the typical storage time for bagged salad, even traces of the juice in salad bag fluids increased Salmonella growth in water by up to 280 fold over control cultures.
“Collectively, this study shows that exposure to salad leaf juice may contribute to the persistence of Salmonella on salad leaves, and strongly emphasizes the importance of ensuring the microbiological safety of fresh produce,” the researchers wrote.
Previously, little had been understood about how Salmonella bacteria behave within the confines of a salad bag, despite the increasing risk of foodborne illness from Salmonella enterica and other pathogens, they added.
“Salad juice exposure also helped the Salmonella cells to attach to the salad leaves so strongly that washing could not remove them. Collectively, 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,” said Freestone, a senior lecturer in clinical microbiology and an author of the study.
She noted that there have been several illness outbreaks linked to salad greens and involving Salmonella, Listeria and E. coli.
For example, bean sprouts contaminated with Salmonella infected more than 100 people in the U.S. in 2014, at least 50 people in Australia developed salmonellosis in February of this year after eating bagged salad and, in July, 161 people were sickened and two died in the U.K. after eating mixed salad.
According to 2013 data from the European Food Safety Authority, salad greens are the second most common source of foodborne illness in the European Union.
Freestone stressed that the research “does not indicate any increased risk to eating leafy salads” but that it does point out the continuing need for vigilant food safety practices in the production and preparation of salad greens.
“With regards to eating leafy salads, which are a nutritious part of the diet, they should be stored, prepared and used according to the guidance on the pack — including refrigeration and use-by instructions,” she said. “Avoid bags of salad with mushed-up leaves, avoid any bags or salad containers that look swollen, store in the fridge and use the salad as quickly as possible after purchase to minimize the growth of any pathogens that might be present.”
Additional research involving pathogens that may inhabit bagged salads is being planned by University of Queensland researchers in partnership with Horticulture Innovation Australia Limited.
They intend to spend $800,000 over two years to see whether probiotics, in the form of lactic acid bacteria, can be introduced to such products to help reduce the possibility of outbreaks caused by Salmonella and Listeria.
Professor Mark Turner from the university’s School of Agriculture and Food Sciences said that more than 300 people in Australia were sickened this year in outbreaks linked to bagged salads and sprouts.
“Our project aims to commercialize new strains of bacteria that already naturally occur on veggies,” he said. “These can assist salad producers improve the food safety and health benefits of their fresh and fresh-cut salad products, adding value to them and protecting the health of consumers.”
Other University of Queensland researchers working with Turner include Paul Dennis, Bhesh Bhandari, Nidhi Bansal, Sangeeta Prakash and Van Ho.
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