Is That Spinach Really Clean?

To wash or not to wash—that is the question, as you stand in your kitchen clutching your newly bought bag of spinach. It says “prewashed” or perhaps “ready to eat,” on the package, but should you trust those labels? No, according to the researchers at Bourns College of Engineering at UC Riverside.

Why were a bunch of engineers playing with spinach, you may ask? They were looking at the peaks and valleys within the spinach leaves, and think that this natural design is a main cause of bacterial outbreaks. When produce companies disinfect produce to kill bacteria like Listeria or E. coli, they put chemicals in the rinse water, not directly onto the leaves’ surfaces. Because of the bumpy topography of the spinach, nearly 15 percent of the leaf will not receive enough of the bleach disinfectant. The little hotspots of bacteria stay on the leaves and thrive, multiplying as they travel through the processing facility, and contaminating other leaves and surfaces as they go.

“In a sense the leaf is protecting the bacteria and allowing it to spread,” wrote Nichola M. Kinsinger, Ph.D., a post-doctoral scientist who recently presented this research on salad safety to the American Chemical Society. “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.”

Green, leafy foods have frequently caused outbreaks of disease because they are often eaten uncooked. A 2006 incident, for example, sickened 199 people in 26 states with E. coli from spinach.

So when you’re making your salad, yes, go ahead and wash the leaves again, recommends Kinsinger. Here’s how:

• Keep spinach stored at 40 degrees or below in the fridge.
• Wash your hands with soap and water before you touch the spinach.
• Put the spinach under a stream of cool or cold water. The FDA does not recommend vegetable washes, soap or detergent.
• Rub each leaf really well with your hands to get into all the creases.
• Dry the spinach using a fresh, clean dishtowel or paper towels.