Could high-power ultrasound protect produce from pathogens?

A litany of food scares--and rules for organic produce--have pushed the industry to seek new solutions for food safety.

Scientific American
4 min read

Perfectly sanitized dimpled spinach leaves or tender greens like baby lettuce has been high on the wish list of the $3.1 billion bagged-salad industry since its inception. The race to develop better wash systems for cleaning took off in earnest in 2006, after the high-profile E. coli O157:H7 outbreak linked to bagged spinach killed five people and sickened more than 200, leaving the leafy green industry with a black eye and an ego-bruising $350 million price tag in recalls and lost sales.

Advances to date in cleaning salad greens have mostly centered on chlorine-based washes and plenty of testing throughout the supply chain. But for organic salad producers, such as Earthbound Farm, a wash additive may not be an option because it has not been approved for organic use. So the company teamed up with the Institute for Food Safety and Health (IFSH) at the Illinois Institute of Technology to look for solutions outside the bag. One of the most promising: high-power ultrasound.

High-power ultrasound looks to be one of the most promising advances in cleaning salad greens. Courtesy of Earthbound Farm

When applied to leafy greens, high-power ultrasound creates millions of tiny bubbles along a leaf's surface. As they burst at a rate of a thousand times a second, they provide high-energy shock waves that can get into the leaf's nooks and crannies to dislodge pathogens, which are then whisked away in the sanitized wash. (Earthbound is looking at citrus and peracetic acid-based sanitizers, both sanctioned for use with organic products.)

"Mostly we're after E. coli O157:H7; norovirus that causes winter vomiting, and we'll continue working with salmonella and Listeria as well," says IFSH director, Robert Brackett.

Will Daniels, senior vice president of operations and organic integrity at Earthbound, says they hope to move the equipment out of the lab and apply it to their process within the next few months. "That's assuming the pilot studies between now and then are successful and show we don't end up with pureed lettuce at the end of the line. That would be a deal breaker," Daniels adds.

It's not the first time high-power ultrasound has been used as a sanitizer. The wine industry has used it (PDF) to clean oak barrels since 2006. Employing ultrasound, however, does not guarantee sterile produce, and Earthbound says they will not put forward such a claim.

But early results about its effectiveness in eliminating pathogens are promising--and come at a particularly key time. The U.S. Department of Agriculture's Microbiological Data Program is on the federal chopping block thanks to budget cuts, though it currently tests more than 15,000 samples of fruits and vegetables a year from 11 different produce points across the U.S.

Others in the industry are exploring other sanitization techniques that include ultraviolet light, cold plasma and high pressure to eliminate pathogens on produce. The developers at Earthbound declined to specify the cost of the new ultrasound procedure compared with traditional methods but it will undoubtedly be more expensive--it may add as much as $200,000 to the cost of a sanitation line, according to Cavitus, a company working on the technology.

Organizations like the Center for Produce Safety at the University of California at Davis, have been dispensing research grants to study produce safety.

"If you think about a 50-acre field of spinach, it's grown outside and is likely to get contaminated somehow--either through dust-borne E. coli, overflying geese, pigs that get loose. That's the functional reality of things that are grown outside," says Bill Marler, a Seattle-based attorney who specializes in food safety cases.

Even if a small portion of a commercial field is infected by a pathogen, the way produce is harvested, bundled together, and washed means cross-contamination can escalate a problem, unless it is detected early.

Robert Whitaker, chief science and technology officer for the Produce Marketing Association says there has been an emphasis on preventive controls throughout points in the industry's supply chain--from harvest, cooling, processing, storage, and distribution centers, all the way to the consumer's home.

"One of those points are washes," Whitaker explains. "You want to keep the wash water from being a source of contamination. If pathogens are killed in the water, they don't move through the water. It's controlling the quality of the wash water. That's what's going on here."

Daniels says if it works in practice, the technique will not be kept proprietary for Earthbound's use. All producers will be able to undertake a leaf zap to keep the bugs away from the consumer.

Follow Scientific American on Twitter @SciAm and @SciamBlogs. Visit ScientificAmerican.com for the latest in science, health and technology news.
© 2012 ScientificAmerican.com. All rights reserved.