Leafy green vegetables are important sources of dietary fiber, vitamins (vitamin K, C, carotenoids A and vitamin B, folate), minerals and natural antioxidants. However, they have been repeatedly associated with foodborne outbreaks. In particular, lettuce has frequently been involved in foodborne illness outbreaks in the United States.
A new study from the University of Illinois at Urbana-Champaign (USA) has examined the factors that affect E. coli contamination in five different leafy greens: romaine lettuce, green leaf lettuce and spinach, cabbage kale and cabbages. Escherichia coli, known as E. coli, produces Shiga toxin, which is the main human pathogen associated with green leafy sprouts.
"We're seeing a lot of outbreaks in lettuce, but not as much in kale and other brassicas. We wanted to learn more about the susceptibility of different leafy greens," says lead author Mengyi Dong, a postdoctoral research associate at Duke University ( USA).
The researchers infected whole leaves of each of the five vegetables with E. coli and observed what happened after storing them at 4°C, 20°C and 37°C. Overall, they found that susceptibility was determined by a combination of temperature and leaf surface properties, such as roughness and natural wax coating.
At room temperature or above, the E. coli bacteria grows very quickly on lettuce, but if the lettuce is refrigerated at 4°C, they saw a sharp decrease in the population of this microorganism. However, with waxy vegetables like kale and collards, they got opposite results. In these vegetables, E. coli grows more slowly under warmer temperatures, but if it is already present, it can survive longer in the refrigerator.
Still, kale and collards are generally less susceptible to E. coli contamination than lettuce. Additionally, these vegetables are usually cooked (which kills or inactivates this bacteria), while lettuce is consumed raw. Rinsing the lettuce helps, but it does not eliminate all the bacteria due to its strong adhesion to the leaf.
The researchers also inoculated cut leaves with E. coli to compare the intact surface of a whole leaf with the damaged surface of a cut leaf. Whole leaves and freshly cut leaves present different situations. When the leaf is cut, it releases plant juice, which contains nutrients that stimulate bacterial growth. However, researchers found that juice from spinach, kale, and collard greens actually exhibited antimicrobial properties that protect against E. coli.
To further explore these findings, they isolated juice from kale and cabbage and applied the liquid to lettuce leaves, discovering that it can be used as a natural antimicrobial agent. Potential applications could include antimicrobial sprays or coatings to control contamination by foodborne pathogens, both in the pre- and post-harvest stages, the researchers said.
"We can't completely avoid pathogens in food. Vegetables are grown in soil, not in a sterile environment, and will be exposed to bacteria," says co-author Pratik Banerjee, FSHN associate professor and Illinois Extension specialist.
"It is a complex problem to solve, but we can adopt best practices in the food industry and the food supply chain. There is a lot of interest from the research community and federal agencies to address these issues, and the USDA imposes high standards for food production, so overall the food supply in the United States is quite secure," he adds.
Banerjee and Dong emphasize that they do not want to discourage people from eating fresh fruits and vegetables; They are part of a healthy diet. Just remember to follow food safety guidelines, such as washing your lettuce thoroughly, storing it in the refrigerator, and paying attention to any food safety recalls in your area.
