Should you keep your lettuce at room temperature or in the fridge? If you do it wrong, you may get infected with E. coli!

Compiled by: Gong Zixin

Green leafy vegetables are an important source of dietary fiber, vitamins, minerals, and natural antioxidants, however, they also harbor harmful pathogens and are often associated with outbreaks of foodborne illness. (In the decade from 2011 to 2020, there were 132 foodborne outbreaks related to edible vegetables in the United States. Among them, outbreaks involving green leafy vegetables accounted for 68.9% of all agricultural product-related outbreaks.)

Recently, a new study from the University of Illinois at Urbana-Champaign investigated the attachment, survival and growth potential of Escherichia coli on five common green leafy vegetables (lettuce, spinach, kale, etc.).

The researchers infected whole leaves of five types of vegetables with E. coli O157:H7 and observed what happened after storage at 4°C, 20°C and 37°C to understand the susceptibility of different leafy greens. Overall, they found that the susceptibility of green vegetables to E. coli was determined by a combination of temperature and leaf surface properties, such as roughness and natural wax coating.

"E. coli grows very quickly on lettuce at room temperature or higher, but if the lettuce is kept in a refrigerated environment at 4 degrees Celsius, you see a sharp drop in the number of E. coli," said lead author Mengyi Dong, a doctoral student in the Department of Food Science and Human Nutrition at the University of Illinois.

However, the opposite was true for waxy greens like kale, where E. coli grew more slowly at warmer temperatures but, if already present, survived longer under refrigeration.

Even so, kale overall is less susceptible to E. coli contamination than romaine lettuce. In addition, these vegetables are often cooked, which can kill or inactivate E. coli, while romaine lettuce is often eaten raw. Rinsing romaine lettuce does help, but it doesn't remove all bacteria, the researchers said.

The researchers also inoculated cut leaves with E. coli O157:H7 to compare intact surfaces of whole leaves with damaged surfaces of cut leaves.

The results showed that whole leaves and freshly cut leaves showed different results. When leaves are cut, vegetable juices are released, which contain nutrients that stimulate bacterial growth. However, the researchers found that the juices of spinach and kale actually showed antibacterial properties that protected against E. coli.

To further explore these findings, the researchers isolated the juice (lysate) from kale and applied the liquid to lettuce leaves, finding that it could be used as a natural antimicrobial agent. Potential applications could include antimicrobial sprays or coatings to control foodborne pathogen contamination during pre-harvest and post-harvest stages, the researchers noted.

"This is a complex problem that is difficult to solve, and we can't completely avoid pathogens in our food because vegetables grow in soil, not in a sterile environment, and they are exposed to bacteria," said co-author Pratik Banerjee, associate professor of food science and human nutrition at the University of Illinois. "But we can adopt best practices in the food industry and in the food supply chain."

The researchers stressed that eating fresh fruits and vegetables is part of a healthy diet, but food safety guidelines should be followed. If you like to eat lettuce raw, be sure to wash it thoroughly and refrigerate it in the refrigerator.

References:
https://www.sciencedirect.com/science/article/abs/pii/S0740002023002198?via%3Dihub