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Pre-Washed Spinach May Not Be As Clean As You Think

While the rinse used on spinach may be effective at cleaning dirt and debris from the leaves, it’s not necessarily effective at killing pathogens, thanks to the topography of the spinach leaves.

Researchers at the University of California-Riverside tested different wash conditions on spinach contaminated with E. coli and measured the rates at which the pathogen sticks to, and detaches from, the vegetable.

They also did some computational modeling to simulate how the shape of spinach leaves influences the ability of the rinse to actually disinfect.


University of California-Riverside post-doctoral researcher Nichola Kinsinger studied the topography of spinach leaves.

The food industry adds chlorine at 50 to 200 parts per million into rinse water, which does the trick to clean away dirt and debris.

But the UCR team found that nooks and crannies on leaves keep out the disinfectant, “leaving live bacteria present on the leaves and the possibility of an infection from those,” says Sharon Walker, a professor of chemical and environmental engineering whose laboratory conducted the study.

“What we’ve learned is that these concentrations have their limitations,” she adds.

While the researchers can’t comment on industry practices for other leafy greens, such as lettuce or kale, they do believe that the leaf topography and how it allows pathogens to persist should be considered for other types of leaves as well.

In the past, Walker’s research group has studied the mechanisms of how bacteria stick to different surfaces, applying their findings to water treatment and making antimicrobial surfaces. This was their first foray specifically into food surfaces.

They started with spinach in part because of the 2006 E. coli outbreak linked to spinach from California that sickened 199 people in 26 states. Three of them died.

Members were able to transition to studying food when post-doctoral researcher Nichola Kinsinger joined the group with a fellowship through the USDA National Institute for Food and Agriculture.

Kinsinger leaves the university at the end of the month for a policy fellowship with the American Association for the Advancement of Science, but Walker’s group will continue to work on processes for improving spinach safety.

“There are lots of things on the table,” she says. “We can think about the chemistry of what’s in the rinse water, we can think about the physics and the flow dynamics of the rinse water, and we can think about ways to disinfect that are not chemically based.”

Kinsinger presented the research on Aug. 19, 2015, at the American Chemical Society National Meeting & Exposition. She is also writing a paper, co-authored by Walker and Madeline Luth, an undergraduate student, about the research findings, which will soon be submitted for publication.

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  • Mike_Mychajlonka_PhD

    This seems like an excellent starting point. Leafy greens need a sanitation step that works, especially after the washing of produce has been recommended to consumers as an intervention. When the paper has been published, I hope Food Safety News will provide a link to it.

  • Carl Custer

    Cilantro is even better at hanging on to bacteria. What is needed is better preharvest control of pathogens in food animal production. Those facilities spread pathogens into the environment via wind, runoff, insects, birds, and other feral animals. See Berry et al in the January Applied Environmental Microbiology for one recent example.

    • Capt. Obvious at your service

      I don’t know about that. Hepatitis A isn’t typically coming from your dreaded “food animal production”, Carl. Aren’t you just blowing smoke up our skirts with your blanket accusation? You usually make reasonable comments. What’s gone wrong with you?

      • Carl Custer

        Excellent point but the article was about “mechanisms of how bacteria stick to different surfaces,”

    • LP

      It is widely reported and accepted by many that removal of riparian zones does not deliver any benefit with regard to pathogen contamination….something local DNRs have been telling us for years. Carl, the animals to blame here are the bipedal ones. Hygiene, sanitation and proper handling have been and will remain the primary cause for good or bad outcomes in food production.

  • Angelina Popovich

    I wash my produce in a vinegar, salt and water solution, then rinse. Especially effective on mushrooms. Have never had any associated illnesses.

    • C Condit

      Will you share the quantities of each for us?