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News Release

May 18, 2006

U of G Food Scientist Finds Effective Way to Clean Produce

Buying prewashed lettuce can save you time, but it can also make you sick, as close to two dozen U.S. consumers discovered last year. Now University of Guelph food scientists have found a more effective way of cleaning vegetables that can dramatically reduce the risk of contamination.

“Pathogens can actually get into the internal tissue of the lettuce,” said Prof. Keith Warriner of the Department of Food Science. “You can wash it for as long as you like, but you’re not going to remove all the pathogens because they can hide in cut edges and the pores of the lettuce leaves.”

When lettuce is harvested for bagged salads it’s kept cool in containers of water and then it’s washed again at the processing plant, he said. “If the water is contaminated, which it sometimes is, bacteria will be passed onto the lettuce, and simple washing can’t remove them.”

This could be what happened in October 2005 when 23 people in three states became sick from eating lettuce contaminated with E. coli O157:H7, said Warriner. Most people aren’t aware that, next to ground beef, fresh produce is the most common culprit in food-borne illness, he said.

At least 19 food-borne illness outbreaks have been linked to leafy greens since 1995, resulting in two deaths and 425 people becoming seriously ill, according to the U.S. Food and Drug Administration.

To find a way to eliminate pathogens in vegetables, Warriner, along with researcher Christina Hajdok, decided to apply the same method used to decontaminate food cartons. Like fresh produce, the surface of carton packaging material is full of crevices that can provide protective sites for microbes. Milk, juice and soup cartons are sterilized by being sprayed with hydrogen peroxide at the same time they are illuminated with UV light. The UV light converts the hydrogen peroxide into antimicrobial free radicals that penetrate into the packaging material to inactivate microbes.

To test this method on produce, Warriner artificially contaminated tomatoes, cauliflower, iceberg lettuce, romaine lettuce, Spanish onions and broccoli with Salmonella. After “cleaning” the vegetables using the hydrogen peroxide/UV method, “we managed to achieve 99.999-per-cent inactivation of the Salmonella,” he said.

This inactivation is huge in food-safety terms. “The good thing about hydrogen peroxide and UV is that they make free radicals that can penetrate right into the subsurface of vegetables so we can ensure the pathogens in the lettuce leaf can be inactivated, something that washing cannot do,” said Warriner.

You wouldn’t actually be consuming any hydrogen peroxide by eating vegetables that have been cleaned by this method, said Warriner. Plants contain enzymes called catalase that degrade hydrogen peroxide into water. “These free radicals are so short-lived that within seconds they do their job and are converted to water as the by product.”

Warriner has determined the optimal levels of hydrogen peroxide and exposure time. Next, he will test his decontamination method on produce contaminated with E. coli O157:H7 and viruses to show the true potential of the system. This new way of cleaning produce will not only make food safer to consume, but it should also extend the shelf life of products because vegetables are often spoiled by microbial action, said Warriner.