Green leafy vegetables are important sources of dietary fiber and nutrients, but they can also be dangerous “sources” of harmful pathogens. Lettuce in particular is often “guilty” of outbreaks of food poisoning and serious infections. Now a new study by researchers from the University of Illinois at Urbana-Champaign published in the journal Food Microbiology zooms in on the factors that influence the contamination of five different green leafy vegetables with the potentially highly dangerous E.coli bacteria — specifically researchers focused on romaine lettuce, collard greens, spinach, collard greens and Swiss chard.
“We see a lot of outbreaks of E.coli in lettuce but not as often in kale and other cruciferous vegetables. So we wanted to find out how vulnerable different green leafy vegetables are to the bacterium,” said study lead author Megji Dong, who was a postdoctoral researcher in the Department of Food Science and Nutrition at the University of Illinois (and is now a postdoctoral researcher at the University Duke).
Study at three different temperatures
The researchers infected whole leaves from each of the five vegetables with E. coli (strain O157:H7 which is responsible for serious gastrointestinal infections) and observed what happened after they were stored at 4 degrees Celsius, 20 degrees Celsius and 37 Celsius degrees. As they saw, how susceptible each vegetable was to the bacterium was determined by a combination of ambient temperature and properties of its leaf surface (eg, how rough the surface was).
Lettuce in the fridge, kale and chard out of the fridge
“At room temperature or higher E. coli grows very quickly on lettuce. But if the lettuce is kept at 4 degrees Celsius, i.e. in the refrigerator, a sharp reduction in the E. coli population is observed. For other leafy vegetables such as kale and chard the results are opposite. In these vegetables, E. coli grows more slowly at higher temperatures – however, if the bacterium is already present in the vegetable, it can survive longer if stored in the refrigerator,” the study states.
Washing the lettuce well is not enough
Even so, kale and collard greens are less susceptible to E. coli contamination than lettuce. Furthermore, these vegetables are usually eaten cooked – which kills the bacteria – while lettuce is eaten raw. Washing the lettuce helps, noted Dr. Dong, but it doesn’t remove all the bacteria as they are very well “hooked” on the leaves.
Antimicrobial spinach juice
The researchers also infected cut vegetable leaves with E. coli O157:H7 to compare whether there was a difference between the intact surface of a whole leaf and the damaged surface of cut leaves. “When the leaf is cut, it releases sap that contains nutrients that promote bacterial growth,” Dr. Dong explained. However, the researchers found that the juice of spinach, kale and Swiss chard has antimicrobial properties that protect against E. coli.
In spray or coating form
In order to further investigate this latest finding, the scientists isolated juice from kale and Swiss chard and added it to lettuce leaves – as they saw, this juice could be used as a natural antimicrobial. Potential applications could be an antimicrobial spray or coating applied to vegetables both before and after harvest, according to the researchers.
The existence of pathogens in vegetables is inevitable
“We cannot completely avoid pathogens in food. Vegetables are grown in soil, not in a sterile environment,
and it is next that they will be exposed to bacteria,” said the study’s other lead author Pratik Banerjee, an associate professor in the Department of Food Science and Nutrition at the University of Illinois and added: “This is a complex problem, but we can adopt best practices in the food industry but also in the supply chain”.
Instructions for consumers
Both researchers emphasized that they do not want to discourage the public from consuming fresh fruits and vegetables, which are an integral part of a healthy diet. “Just wash your lettuce very well, refrigerate, and be aware of possible food safety recalls in your country or region,” they concluded.
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