Refrigerating may prevent lettuce from contamination, not the saags, study finds

New Delhi: Refrigerating may help prevent lettuce from E. coli contamination, but not kale and collards - types of saag - a new research has found.

Seeing outbreaks on lettuce but not on kale and other brassica vegetables like broccoli, cabbage, cauliflower, collard greens and others, researchers analysed the tendencies of leafy greens to get infected with E. coli, or Escherichia coli, a bacterium species found in lower intestines of humans. While generally harmless, some of its strains are occasionally responsible for food contamination and can cause food poisoning.

The researchers at the University of Illinois at Urbana-Champaign, US, infected whole leaves from five leafy greens - romaine lettuce, green-leaf lettuce, spinach, kale, and collards - with E. coli (serotype O157:H7) and observed what happened after storing them under these temperatures - 4 degrees Celsius (refrigeration), 20 degrees Celsius (room temperature) and 37 degrees Celsius (normal body temperature).

Overall, a combination of both temperature and leaf surface properties such as roughness and the natural wax coating were found to govern susceptibility to bacterial contamination, the researchers found.

"At room temperature or higher, E. coli grows very fast on lettuce, but if lettuce is refrigerated at 4 degrees Celsius, 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," said Mengyi Dong, who conducted the research as a doctoral student in the Department of Food Science and Human Nutrition (FSHN), at the University of Illinois.

Even so, kale and collard are overall less susceptible to E. coli contamination than lettuce and while rinsing lettuce does help, it doesn't remove all the bacteria because of their tight attachment to the leaf, said Dong, lead author of the study published in the journal 'Food Microbiology'.

Further, the researchers inoculated cut leaves with the E. coli strain to compare the intact surface of a whole leaf to the damaged surface of a cut leaf, which they said presented different situations.

"When the leaf is cut, it releases vegetable juice, which contains nutrients that stimulate bacterial growth," said Dong.

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

Dong and colleagues emphasised that they didn't want to discourage people from eating fresh fruits and vegetables. They said eating them was part of a healthy diet but that pathogens in food cannot be completely avoided, as they are grown in soil and not in a sterile environment.

They urged people to follow food safety guidelines, wash the lettuce thoroughly and store it in the refrigerator.