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Safer Salad In The Bag

By Doris Stanley Lowe
February 10, 1999

Irradiating bagged salads after washing the vegetables in a chlorine solution reduces harmful and non-harmful microorganisms without affecting quality, scientists with USDA’s Agricultural Research Service report. That’s good news for health-conscious consumers who are eating more salads. Sales of packaged lettuce alone were more than $1.2 billion in the United States in 1997.

From a food safety perspective, salads are considered to be among the safest foods. However, some segments of the population often exclude salads and other uncooked fruits and vegetables from their diets. These foods have the potential to be a food safety risk for the young, old, pregnant or immunocompromised, because of the presence of human pathogens. These groups can’t risk exposure to microorganisms that, for the general population, may not be a major concern.

Although the food industry uses chlorine to control microbes on fresh-cut lettuce, chlorine doesn’t eliminate all the organisms that can be present, like Shigella and E. coli O157:H7.

In lab experiments at the U.S. Citrus and Subtropical Products Research Laboratory, Winter Haven, FL, ARS scientists found that irradiation significantly reduced the microbial and yeast populations on cut iceberg lettuce. Eight days after zapping chlorine-washed lettuce with only 0.2 kilogray (kGy) of irradiation, microbial counts were 290 colony-forming units (CFU) and 60 CFU of yeast. Control samples showed microbial counts of 220,000 CFU and 1,400 CFU of yeast. (The U.S. Food and Drug Administration has approved up to 1 kGy of irradiation for fresh produce.)

Irradiated lettuce had about the same shelf life as untreated samples. Manufacturers of salads for retail sales claim normal shelf life of 14 to 16 days from packaging date.

Scientists also irradiated chlorine-washed, shredded carrots in modified-atmosphere packaging. Nine days after irradiation, on the expiration date, microbial count was 1,300, compared with 87,000 for nonirradiated, chlorinated control samples.

This combination treatment could help fresh-cut salads to be included in diets of people who otherwise can’t enjoy them because of a potential microbiological health risk.

Scientific contact: Robert D. Hagenmaier, ARS Citrus and Subtropical Products Research Laboratory, Winter Haven, FL; phone (941) 293-4133, X123, fax (941) 299-8678, email