PhotoEver wonder about those packs of "pre-washed" and "tripled-washed" salads? What does it mean to say a pack of spinach, for example, has been triple-washed.

Not much according to engineers at the University of California, Riverside. They discovered that small peaks and valleys in baby spinach leaves could be a key reason why there have been numerous bacterial outbreaks involving leafy green vegetables.

The reason is that the varied topography of the spinach leaf means that nearly 15 percent of the leaf surface may not receive adequate exposure to the bleach disinfectant in which it's rinsed. 

The disinfectant is put into the rinse water, not rubbed into the leaf's surface, the researchers in the Bourns College of Engineering noted.

As a result, as the leaves move through the processing facility after being rinsed the bacteria may continue to live, grow, spread, and contaminate other leaves and surfaces within the facility.

Following rinsing under the low bleach condition, upwards of 90 percent of adhered bacteria were observed to remain attached to and survive on the leaf surface.

Bacteria protected

"In a sense the leaf is protecting the bacteria and allowing it to spread," said Nichola M. Kinsinger, a post-doctoral researcher working with Sharon Walker, a professor of chemical and environmental engineering. "It was surprising to discover how the leaf surface formed micro-environments that reduce the bleach concentration and in this case the very disinfection processes intended to clean, remove, and prevent contamination was found to be the potential pathway to amplifying foodborne outbreaks."

Kinsinger will present her research on August 19 at the 250th American Chemical Society National Meeting & Exposition.

The Center for Disease Control estimates that 1 in 6 Americans become ill and 3,000 die annually from foodborne diseases. Additionally, this causes and an estimated over $75 billion per year loss for the food industry.

Past research has found about 20 percent of single food commodity outbreaks from 2003 to 2008 were attributed to leafy green produce. Contamination of such minimally processed and ready-to-eat produce is of concern since it is frequently consumed uncooked or raw.

One of these outbreaks, involving spinach, occurred in California in 2006. In all, 199 people in 26 states were infected with the outbreak strain of E. coli. Three died.

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