Lettuce should be refrigerated to minimize E. coli contamination

Attached is E. coli O157:H7 on leaf surfaces, highlighting the susceptibility of leafy greens.


Leafy green veggies are good for us because they have fiber and nutrients. But sometimes, they can have harmful germs in them. Lettuce, primarily, has caused sickness outbreaks in the U.S. A new study from the University of Illinois Urbana-Champaign looks at why E. coli germs get on five kinds of leafy greens: romaine lettuce, green-leaf lettuce, spinach, kale, and collards.

Lead author Mengyi Dong is now a postdoctoral research associate at Duke University. Dong researched as a doctoral student in the Department of Food Science and Human Nutrition (FSHN), part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at the U. of I. said, “We see many outbreaks of lettuce, but not so much on kale and other brassica vegetables. We wanted to learn more about the susceptibility of different leafy greens.”

Researchers tested how E. coli O157:H7 behaves on the whole leaves of five different vegetables. They kept the leaves at three different temperatures: 4°C (39°F), 20°C (68°F), and 37°C (98.6°F). They found that temperature and the surface of the leaves, like how rough they are and if they have a natural wax coating, affect how likely E. coli is to stick to them.

When the temperature is warm, E. coli proliferates on lettuce, but when it’s kept cold at four °C (39°F), the number of E. coli goes down fast. On the other hand, with waxy greens like kale and collard, E. coli grows slower when it’s warm. However, if it’s already there, it can survive longer when refrigerated.

Still, kale and collard are less likely to have E. coli than lettuce. Also, since kale and collard are usually cooked, the heat kills or deactivates E. coli, unlike lettuce, which is eaten raw. Washing lettuce helps a bit but can’t remove all the bacteria because they stick tightly to the leaves.

The researchers also tested what happens when they put E. coli O157:H7 on cut leaves compared to whole leaves. Dong explained, “When a leaf is cut, it releases juice with bacteria-like nutrients. However, they found that spinach, kale, and collard juice fight against E. coli.”

They took juice from kale and collards to learn more and put it on lettuce leaves. They found that it can help kill bacteria. They think this juice could be used to make sprays or coatings that fight against harmful germs in food before and after it’s harvested.

“We can’t completely avoid germs in food. Vegetables grow in soil, not in a sterile place, so they’ll have bacteria on them,” said Pratik Banerjee, an associate professor in FSHN and Illinois Extension specialist.

“It’s a tough problem, but we can use the best food industry and supply chain methods. Many researchers and government agencies are working on this, and the USDA sets high standards for food production. Hence, the food supply in the U.S. is generally safe.” He added

Banerjee and Dong want to clarify that they’re not saying people shouldn’t eat fresh fruits and vegetables; they’re essential for a healthy diet. Just follow food safety rules, wash your lettuce well, keep it in the fridge, and stay updated about any food recalls in your area.

Putting lettuce in the fridge is essential to stop E. coli from getting on it. This shows how important it is to store and handle food properly. Doing these things can make eating leafy greens safer and lower the chance of getting sick from E. coli in food.

Journal reference:

  1. Mengyi Dong, Maxwell J. Holle et al., Fates of attached E. coli o157:h7 on intact leaf surfaces revealed leafy green susceptibility. Food Microbiology. DOI: 10.1016/j.fm.2023.104432.