The produce industry, including Dole, which packaged the affected salads, says that pre-washed salads can be eaten without further washing. So does the U.S. Food and Drug Administration, which oversees the safety of fresh produce. Some food safety experts urge consumers to wash anyway, but caution that it may not eliminate bacteria.
In a society facing an epidemic of obesity, health experts say the risk is considered low compared with the benefits of eating produce.
Bagged lettuce racked up $2.3 billion in retail sales in the past year, according to ACNielsen. Pre-washed, cut produce is credited with boosting consumption of fruits and vegetables by making it easier. It is popular with food service operations for the same ease of preparation, which reduces labor costs, and for the assurance that salads are prepared in facilities with food safety controls.
Food-borne illness linked to fresh fruits and vegetables doubled in the 1990s, according to the federal Centers for Disease Control and Prevention. Packaged spinach and lettuce mixes have been tied to several E. coli 0157:H7 outbreaks in recent years. Earlier this week, the FDA issued a national warning against eating three kinds of Dole salad blends that have been linked to a September outbreak. At least 13 people have been sickened in Minnesota and Wisconsin; four were hospitalized and released.
Investigators are checking E. coli cases in other states to see whether they're related, said Doug Schultz, a spokesman for the Minnesota Department of Health. Investigators have found E. coli bacteria in an unopened bag of the lettuce, he said.
Other packaged salads have been linked in the past few years to outbreaks in restaurants, a nursing home and at a cheerleading camp.
"If you were to eat bagged lettuce, the likelihood of becoming ill is quite low," said Mike Doyle, director of the University of Georgia Center for Food Safety in Griffin. "But that's not to say it doesn't happen."
Once processed lettuce is contaminated, it's difficult to remove bacteria even with chlorinated wash water, Doyle said.
He believes that head lettuce is safer because, if it's contaminated, the bacteria are usually on the outer layers of leaves that are typically removed by consumers or restaurants before washing the interior of the lettuce. Salad processors remove the outer layer and take other safety precautions, according to the International Fresh-cut Produce Association.
Any lettuce might be contaminated by irrigation water or runoff from livestock pastures near growing fields. Processed lettuce may be exposed to other sources of potential contamination, such as rinse water or inadequate hand washing by plant workers.
Bacteria that cause food-borne illness can be destroyed by cooking, but most salads are served raw. That makes the cleanliness of the product even more important.
Dole's Web site mentions the company's stringent food safety controls. Most packaged salads, including Dole's, carry labels that advertise the greenery as pre-washed, washed or ready to eat.
"All salads go through a triple wash process," Eric Schwartz, president of Dole Fresh Vegetables, said in an e-mail. "The first step is a fresh water rinse to remove any field debris, and the second and third step both involve a thorough chlorinated bath. This process makes a bagged salad much cleaner than the average consumer can by just rinsing it under tap water."
But if the lettuce is contaminated with bacteria, that might not help, Doyle said. Researchers at the Center for Food Safety in Griffin have performed experiments with lettuce contaminated with high doses of salmonella or E. coli bacteria, washing it with chlorinated water. It removed only a small amount of the bacteria, Doyle said.
"We know if the organism contaminates the lettuce, it's difficult to wash away," he said. "The key is to keep the fields free of potential contaminants."
Consumers have been advised to check their refrigerators and throw out any bags of salad included in the recall. Because the blends had best-if-used-by dates of Sept. 22 and Sept. 23, the bags should already be off store shelves.
The FDA and the Minnesota Department of Health are looking for the source of the outbreak. The three recalled salad mixes — Classic Romaine, American Blend and Greener Selection — contained three common ingredients: romaine lettuce, carrots and red cabbage.
Dole produced more than 245,000 bags of the salad, which were shipped to distributors nationally. It's not known yet whether the salads wound up in Georgia supermarkets.
Symptoms of e. coli include stomach cramps and diarrhea. More serious cases can cause kidney failure and death, especially those at higher risk of foodborne illness, such as young children, the elderly and those with compromised immune systems.