Could refrigeration of lettuce reduce E. coli contamination risk?

Leafy green vegetables have been known to harbour harmful pathogens, with some recent outbreaks of foodborne illness reported across the US leading to the development of leafy green safety protocols. Now a study from the University of Illinois Urbana-Champaign examines factors that affect E. coli contamination on five different leafy greens — 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,” said lead author Mengyi Dong, now a postdoctoral research associate at Duke University.

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

“At room temperature or higher, E. coli grows very fast on lettuce, but if lettuce is refrigerated at 4°C, we see a sharp decline in the E. coli population. 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.”

Even so, kale and collard are overall less susceptible to E. coli contamination than lettuce. Furthermore, these vegetables are usually cooked — which kills or inactivates E. coli — while lettuce is consumed raw. Rinsing lettuce does help, Dong said, but doesn’t remove all the bacteria because of their tight attachment to the leaf.

The researchers also inoculated cut leaves with E. coli O157:H7 to compare the intact surface of a whole leaf to the damaged surface of a cut leaf.

“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. However, the researchers found that spinach, kale and collard juice actually exhibited antimicrobial properties that protect against E. coli.

To further explore these findings, they isolated juice (lysate) from kale and collards and applied the liquid to lettuce leaves, finding that it can be used as a natural antimicrobial agent. The potential applications could include antimicrobial spray or coating to control foodborne pathogen contaminations at both pre-harvest 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 they will be exposed to bacteria,” said co-author Pratik Banerjee, Associate Professor in FSHN and Illinois Extension specialist.

The paper, Fates of attached E. coli o157:h7 on intact leaf surfaces revealed leafy green susceptibility, is published in Food Microbiology.

Image credit: iStock.com/zoranm