Washington Post Food section columnist Robert Wolke, professor emeritus of chemistry at the University of Pittsburgh, says all parts of the leafy green would have to be heated to a temperature of 160 degrees and held at that temperature for at least a couple of minutes. There's no easy way to check for consistent temperature, and people just don't like to cook fresh spinach that long.
Because "a ridiculously low number of bacteria, estimated to be as few as 10, is enough to cause illness," Wolke said in an e-mail, "even cooking can be risky. Best to throw the stuff out -- and wash your hands well if you have handled it."