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 University of Leicester in the United Kingdom.
Salad juices increased the growth of Salmonella bacteria by 110 percent over normal levels, researchers found.
Salad greens pose a particular infection risk because they are usually minimally processed after harvesting and consumed raw.
Salad greens linked to food poisoning outbreaks
Researchers are paying more attention to salad produce contamination after 100 people in the United States contracted Salmonella infections from bean sprouts in 2014. According to the Centers for Disease Control and Prevention, Salmonella causes 1.4 million cases of foodborne illness and 400 deaths annually in the United States.
The study does not indicate an increased food poisoning risk from eating leafy salads, but it does provide a better understanding of the factors contributing to food poisoning risks. And it highlights the need for continued good practice in the production and preparation of bagged salad greens.
Salmonella multiplied after five days in fridge
The researchers measured the growth of Salmonella enterica, the strain commonly found in foodborne outbreaks in recent years. They crushed several salad leaf types – including 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 greens, 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 the fact that exposure to the juices released from the salad leaves appeared to enhance the Salmonella’s capacity to establish an infection in the consumers.
Leafy salads carry a 3 per cent risk for food poisoning due to pathogens such as Salmonella. 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.
You shouldn’t avoid eating salad greens, but the researchers recommend, if possible, buying fresh uncut lettuce over chopped.
Food safety advice for salad greens
Salmonella strains typically don’t grow below 7 degrees Celsius; the accepted absolute minimum growth temperature is 5 degrees Celsius (41 degrees F). Check the temperature of your fridge. To keep foods safe, make sure your fridge is set at 4°C (40°F) or colder.
Salad greens and vegetables should be washed thoroughly before consumption. Bagged greens that are prewashed before packaging, however, do not need to be washed again. Rewashing bagged greens could introduce bacteria from your hands, cutting board or the sink. Plus, once bacteria that cause foodbourne illness contaminate greens, it’s difficult to remove them with water. If the bag isn’t labeled “prewashed,” wash your greens thoroughly.
Source: Journal of Applied and Environmental Microbiology, online November 18, 2016.
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