MAP is best defense against E. coli in shredded lettuce, says USDA

Scientists at the USDA have found that modified atmosphere packaging (MAP) provides consumers with the best protection against the food safety threat of E. coli in shredded lettuce.

Writing in the Journal of Food Protection, scientists at the USDA Agricultural Research Service sought to determine the impact of different packaging options and storage temperatures on the development of E. coli in fresh cut lettuce.

The scientists looked specifically at E. coli O157:H7 – a strain of the bacterium that has caused food poisoning in leafy greens. And under the microscope were three packaging types; MAP conditions in gas-permeable film with N2, near-ambient conditions in a gas-permeable film with microperforations, and high CO2 and low O2 conditions in a gas-impermeable film.

Research results

They found that MAP conditions did not promote increased virulence of E. coli while the other two pack types, with ambient air conditions, were associated with higher expression of virulence factors. And this held true at 4oC as well as 15oC.

One of the study authors, Manan Sharma, said: “The implication for the industry is that current packaging conditions (MAP) do not promote increased virulence of E. coli O157:H7.”

But the research microbiologist warned that once packs of lettuce are opened consumers need to be aware that potential danger levels are higher.

“Opened packages of lettuce that are temperature-abused (stored at elevated temperature at 15oC) may allow bacteria to become potentially more virulent than under the commercial MAP conditions. Bags that are not completely consumed should be stored at appropriate temperatures in household refrigerators so as to minimize temperature abuse and also potential increased virulence of E. coli O157:H7 on contaminated lettuce.”

Increase in outbreaks

Leafy greens, such as shredded lettuce, have been flagged up as a problem area in food safety. An investigation by the Centers for Disease Control and Prevention (CDC) in 2008 found that, between 1996 and 2006, food borne illness outbreaks linked to leafy greens increased 39 per cent.

The government agency said part of this growth may be explained by a 9 per cent increase in consumption of leafy greens. But that does not explain the full increase. It recommended that federal agencies study the supply chain from farm to fork to determine how to reduce risk.

Source: Journal of Food Protection