New Study: Some Bacteria Grow Worst on These Leafy Greens

Updated: Mar. 05, 2024

Romaine, spinach and kale were among the varieties food scientists investigated, finding some greens contain anti-microbial properties from nature.

While E. coli is sometimes associated with ground beef, the lettuce in your salad is also susceptible to contamination with this dangerous pathogen. In fact, over the past ten years, outbreaks of foodborne illness involving vegetables were predominantly linked to leafy greens.

With that in mind, researchers at the University of Illinois Urbana-Champaign wanted to observe the effects of various susceptible greens under different temperature conditions. The aim was to simulate the journey of a contaminated product to a person’s home and assess which types of greens required careful handling, particularly regarding immediate refrigeration, and which might be safe at room temperature. 

“We are observing numerous outbreaks related to lettuce, but not as many involving kale and other brassica vegetables. We aimed to delve deeper into the susceptibility of different leafy greens,” explained Mengyi Dong, the lead author of the study, who conducted the research as a doctoral student in the school’s Department of Food Science and Human Nutrition.

The researchers selected five types of greens frequently implicated in outbreaks: Romaine lettuce, green-leaf lettuce, baby spinach, kale, and collard greens. These were sprayed with E. coli O157:H7, the strain most commonly causing illnesses. They then assessed how the bacteria grew at three different temperatures: Refrigerator temperature (39°F), room temperature (68°F), and on a very warm day (98.6°F).

Their findings revealed differences in bacterial growth on the leafy greens. Spinach, lettuce, and romaine were more susceptible, especially to warmer temperatures, whereas kale and collard greens showed some resistance. Even under refrigeration, damaged greens exhibited greater pathogen growth. “Whole leaves and freshly cut leaves present different situations. When the leaf is cut, it releases vegetable juice, which contains nutrients that stimulate bacterial growth,” Dong explained, particularly concerning delicate leaves.

Surprisingly, kale and collards displayed some resistance to bacterial growth at higher temperatures. Additionally, damage to their cellular walls was less concerning as the juice inside was shown to have the ability to kill or control the pathogen.

In conclusion, the researchers noted, “E. coli’s survival on leaves depends on produce-specific traits and temperature.” This emphasizes the importance of proper handling from harvesting to consumption. While washing certain types of lettuce may provide some benefit, it may not always be sufficient to prevent illness. Purchasing undamaged greens that have been handled correctly and are not subject to recall is crucial. One intriguing suggestion was to explore the potential use of kale and collard juices as a natural microbial-reducing spray or coating for susceptible greens in the future.

Overall, the researchers emphasized that, despite past issues, the U.S. food supply is generally safe when common-sense techniques are followed. “The USDA imposes high standards for food production,” they assured. By adhering to proper buying, storage, and preparation guidelines for lettuce, most of the time, eating greens is healthy and perfectly safe. 

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How to buy the safest greens

The key to taking the researchers’ findings and implementing them in your kitchen is to start with the best greens. If romaine, other types of lettuce, and spinach are on your menu you should always pay attention to any local recalls, throw out any affected foods, and clean any surface they touched. As the study determined washing and proper storage will not minimize the risk if your lettuce or greens are contaminated. 

Next, follow the Centers for Disease Control’s (CDC) guidelines for buying, storing, and serving leafy greens:

  • Don’t buy greens that are damaged or expired. You can see through the package for a reason. As mentioned above, bruised greens are more likely to cause bacteria to spread and all of it can’t be washed off.
  • Refrigerate your leafy greens at or below 40°F, especially pre-cut, immediately, and transport and store them away from raw meats.
  • Don’t wash anything labeled ready to eat, triple washed, or no washing necessary. 
  • Wash greens under running water. Never use bleach or soap. Be sure to your own hands and food surfaces before washing. 
  • Use running water. The CDC cautions that if microbes are present soaking could allow the pathogens to spread.
  • Vinegar, lemon juice, or fancy produce washes are unnecessary.
  • Use separate cutting boards for vegetables and meats. 
  • Refrigerate your food promptly. All cut or cooked foods, including salads, need to be refrigerated within two hours or one hour if the temperature is over 90°F.