The University of Illinois Urbana-Champaign says new research examines factors that affect E. coli contamination on romaine lettuce, green leaf lettuce, spinach, kale and collards.
“We are seeing a lot of outbreaks on lettuce, but not so much on kale and other brassica vegetables. We wanted to learn more about the susceptibility of different leafy greens,” Mengyi Dong, a postdoctoral research associate at Duke University who conducted the research as a doctoral student, said a news report from UI’s College of Agricultural, Consumer and Environmental Sciences.
Researchers infected whole leaves from each type of leafy green with E. coli O157:H7 and monitored what happened after storage at 39 degrees, 68 degrees and 98.6 degrees Fahrenheit.
According to the university, researchers discovered temperature and leaf surface properties, such as roughness and natural wax coatings, impacted susceptibility.
“At room temperature or higher, E. coli grows very fast on lettuce, but if lettuce is refrigerated at 4° C (39° F), we see a sharp decline in the E. coli population,” Dong said. “However, for waxy greens like kale and collard, we get the opposite results. On these vegetables, E. coli grows slower under warmer temperatures, but if it is already present, it can survive longer under refrigeration.”
The research showed kale and collards to be less susceptible to E. coli contamination than lettuce.
The university said researchers also inoculated cut leaves with E. coli to compare the whole leaf to cut surfaces.
“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 said.
The university said the research team learned juice emitted by spinach, kale and collards exhibits antimicrobial properties that protect against E. coli.
