The USDA has made an amazing discovery. Water that has been used several times to wash produce is dirtier than water that has been used just once. I wonder how much it cost to figure that out.
Processors of “fresh-cut”, washed produce can economize by recycling water. The water is chlorinated, of course, to take care of those pathogens that might be lurking in the lettuce (or spinach). You know which ones - E. coli O157:H7 and Salmonella. After all, put enough chlorine in the wash water and the microbes will be killed. Most of them will, anyway.
The catch is that organic material - soil, bits of produce, fecal matter and whatever else is washed off the produce - inactivates the chlorine. And the more times the water is recycled through the produce washing processing line, the more organic material it collects and the less active chlorine is left to kill the microbes.
This isn’t rocket science - it’s simple arithmetic. Nor is it particularly new. A 1981 research paper described how turbid and organic-laden water reduced the ability of chlorine to kill bacteria. It's nice of USDA to go to the trouble of reconfirming that for us.
Packaged “fresh-cut, washed” produce is neither “fresh-cut” nor particularly well washed. Is it any wonder that spinach and lettuce have been associated with outbreak after outbreak of food borne disease for the last ten years and more?
Personally, I’m sticking with buying intact heads of lettuce and washing the individual lettuce leaves under running tap water just before making my salad.