Washing fresh fruits and vegetables — even with chlorine disinfectants — may not be enough to rid them of certain bacteria that cause food poisoning, say researchers, who found that irradiating the food is the most successful means of killing microbes.

E. coli, salmonella and listeria can all cause illnesses, but destroying the bugs can be problematic, said microbiologist Brendan Niemira, lead investigator of a study looking at ways to make fresh produce safer for consumers.

Eliminating bacteria is difficult if they have made their way inside the leaves of lettuce, spinach and other vegetables and fruit, where surface treatments cannot reach them, said Niemira, who presented his findings Thursday at the American Chemical Society annual meeting in New Orleans.

Destroying E. coli and other food-borne pathogens can be even more difficult if the bacteria are lurking within biofilms — tightly knit communities that coat fruits and vegetables and protect the microbes from harm, he said.

Even humans harbour biofilm: that sticky substance that coats our teeth and gums is one example, said Niemira, lead scientist of the Produce Safety Research Project for the U.S. Department of Agriculture's eastern region.

In fact, up to 100 or more different types of bacteria can live together within a biofilm, which protects them from the environment. Niemira's group wanted to know if they could use irradiation to destroy disease-causing bacteria that had taken up residence inside vegetable leaves or within biofilms.

"The answer is: Yes, you can," he said. "Based on my studies, the basic punch line is if you have E. coli inside a lettuce leaf where chlorine can't kill it, irradiation will."

Irradiation to bump off microbes in food — including meat, eggs and produce — is for the most part not allowed in Canada (imported spices are one exception). The U.S. permits irradiation of some foods, including certain meat products, but the FDA is reviewing whether to add fresh produce to that list.

Electron beams destroy pathogens, insects

The process exposes food to a source of electron beams that inactivates parasites and destroys pathogens and insects, depending on the dose. Using this technique on fresh and fresh-cut fruits and vegetables could provide a reliable way to reduce cases of food-borne illnesses reported each year in North America, Niemira said.

Concern over pathogens increased following two outbreaks of E. coli disease in the United States in 2006 due to contamination of U.S.-grown spinach and lettuce. The tainted produce sickened about 275 people in all, killing three.

Fresh fruits and vegetables carry the risk of contamination because most are grown in open fields with exposure to microbes from soil, irrigation water, manure, wildlife or other sources.

To conduct the study, Niemira's lab cut leaves of romaine lettuce and baby spinach into pieces and submerged them in a mixture containing E. coli. The bacteria were pushed inside the leaves with a vacuum perfusion process and the leaves then were treated with either a three-minute water wash, a three-minute chemical treatment or irradiation.

The researchers found washing with plain water was ineffective at reducing E. coli levels in either spinach or lettuce. The chemical treatment did not cause a significant drop in bacteria in spinach leaves, and gave less than a 90 per cent reduction in the romaine lettuce.

Ionizing radiation cut E. coli by 99.9%

However, ionizing radiation significantly cut the E. coli population in both spinach and lettuce leaves, with reductions of about 99.9 per cent. Irradiation was also able to kill salmonella and E. coli within biofilms.

The use of irradiation isn't without controversy. A number of consumer activist organizations have long lobbied against its use in foods, arguing the process damages food quality, destroys vitamins and enzymes and can create toxins.

Dr. Keith Warriner, a food microbiologist at the University of Guelph, said long-term studies of food irradiation show it poses no risk to human health. But some people associate it with radiation from nuclear bombs, he said from Guelph, Ont.

"The reality of the situation is irradiation is perfectly safe. It's just got a bit of a bad press through the years."

Warriner advises consumers to keep most produce refrigerated, especially green leafy vegetables, and wash them thoroughly under running water before eating.