Spinach contaminated with a dangerous strain of E. coli has made 114 people sick so far, according to the Centers for Disease Control and Prevention. Produce consumers have been advised to stay away from all fresh spinach—even a careful washing won't get rid of the bacteria. Wait, does washing fruits and vegetables do any good at all?
Probably. Food safety experts say that a thorough rinsing can reduce the number of microorganisms on fresh produce by about 90 percent. (Commercial "vegetable wash" products don't seem to do much better.) The water won't clean off the remainder of the bacteria, which are either nestled in grooves on the vegetable's surface or bound to it by interacting electric charges. If the bacteria have been present long enough, they can start to form a "biofilm"—a slimy matrix that makes them even tougher to wash off.
The cleaning of vegetables is measured in "log reductions," which correspond to successive 90-percent decreases in the number of bacteria present. When you wash a tomato in the sink, for example, you've performed about one log reduction in microorganisms. By comparison, low-acid canned goods frequently undergo a 12-log reduction of deadly C. botulinum.
A single-log reduction can help reduce the risk of illness from certain kinds of bacteria, especially if they start out in very low numbers. But it won't help very much with the contaminated spinach. The strain of E. coli in question can make people ill even in very tiny numbers—as few as 10 or 20 cells will do the trick. If your leaf of spinach started with fewer than 100 E. coli bacteria, you could get it down to safe levels with a single washing. But it's likely to have many, many more. A single speck of manure might contain a million E. coli cells. (You'd need at least a five-log reduction to take that many cells down to a safe number.)
Washing has more of an effect in cleaning off "spoilage bacteria." These microbes generally won't make you sick, but they'll reduce the shelf life of your veggies. A thorough washing and drying can make a head of lettuce last a little longer before it rots. It might also rinse off some pesticide residue. There's disagreement over the health risks posed by this residue. The British Food Standards Agency endured fierce criticism when in concluded in 2002 that fruits and vegetables need not be washed to remove pesticide. (The agency did concede that food should be washed on "hygiene grounds.")
Under some conditions, washing can be counterproductive. A pressure gradient may form if there's a big difference in temperature between the fruit or vegetable and the water being used to wash it. Cold water can be pulled inside warmer produce—along with potentially harmful bacteria. Water baths can also lead to cross-contamination among pieces of produce. Farmers and packagers often use chlorinated water to reduce this risk.
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Explainer thanks Vivian Chi-Hua Wu of the University of Maine, Luke LaBorde of Pennsylvania State University, and Suresh Pillai of Texas A&M University.