Crushed leaves in bagged lettuces at the supermarket may leak juice that fosters the right environment for Salmonella growth, according to a new study from the UK.
Salad juices increased the growth of Salmonella bacteria by 110 percent over normal levels, researchers found.
"Salad leaves pose a particular infection risk because they are usually minimally processed after harvesting and consumed raw," said senior study author Primrose Freestone, a clinical microbiology lecturer at the University of Leicester.
Researchers are paying more attention to salad produce contamination after 100 people in the United States contracted Salmonella infections from bean sprouts in 2014. Salmonella causes 1.4 million cases of foodborne illness and 400 deaths annually in the United States, according to the Centers for Disease Control and Prevention.
"Our project does not indicate any increased risk for eating leafy salads, but it does provide a better understanding of the factors contributing to food poisoning risks," Freestone told Reuters Health by email. "It also highlights the need for continued good practice in salad leaf production and preparation."
Freestone and colleagues measured growth of Salmonella enterica, the strain commonly found in foodborne outbreaks in recent years. They crushed several salad leaf types - such as spinach, red chard and red romaine lettuce - to obtain leaf juice.
During a five-day refrigeration period, which is typical storage time for bagged salad, 100 Salmonella bacteria multiplied to more than 100,000. Salad leaf juice also enhanced the bacteria's ability to attach to the sides of the plastic bags and containers, as well as to the leaves themselves.
"Most concerning was that we found exposure to the juices released from the salad leaves appeared to enhance the Salmonella's capacity to establish an infection in the consumer," Freestone said. "Salad leaves are an important part of a healthy diet but have been associated in recent years with a growing risk of food poisoning."
Leafy salads carry a 3 percent risk for food poisoning due to pathogens such as Salmonella, the study authors write in the Journal of Applied and Environmental Microbiology. The European Food Safety Authority has classified leafy green salads as one of the top sources of foodborne infections, with salmonellosis accounting for more than 30 percent of outbreaks.
"Consumers seem to be more preoccupied with nutritional facts, but they should not forget that foodborne pathogens can be deadly," said Kimon Karatzas, an assistant professor of food microbiology at the University of Reading in the UK who was not involved with the study. "Avoiding fresh produce is not a solution, but if possible, consumers should buy fresh uncut produce over chopped."
Future studies should investigate how Salmonella survives in different kinds of fresh produce, Karatzas told Reuters Health by email. His research team is developing disinfectants that eliminate microorganisms from fresh produce.
"The fact that bacteria growth is enhanced by the presence of nutrients from a food is not very surprising," said Martin Adams, a food microbiology professor at the University of Surrey in the UK who was not involved with the study. "What did concern me was that the particular strain of Salmonella was able to grow at 4 degrees Celsius (39.2 degrees F), or refrigeration temperature."
Salmonella strains typically don't grow below 7 degrees Celsius, Adams said, and the accepted absolute minimum growth temperature is 5 degrees Celsius (41 degrees F).
"It is very important that salad vegetables are washed thoroughly before consumption, which is good advice that goes back many years," Adams told Reuters Health by email. "Although prepared bagged salads have already been washed, another washing before use would give an added level of reassurance."