Combine irradiation with a chlorine wash to make lettuce safe.

Bagged salads are one of the most popular items in the fresh produce section of supermarkets today. Sales of packaged lettuce in the United States were more than $1.2 billion in 1997. Salads are considered by some to be among the safest foods. However, some segments of our population often exclude salads and other uncooked fruits and vegetables from their diets. Because of the high levels of microbial agents found on fresh-cut produce, salads are often not recommended for the young, old, pregnant or immunocompromised.

These people can't risk exposure to microorganisms that, for the general population, are normally considered nonpathogenic. Even though commercial food processors use chlorine to control microbial growth on fresh-cut lettuce, the treatment doesn't eliminate all the organisms that can be present, such as Shigella and E. coli O157:H7. Although E. coli is primarily found on meat, it has recently shown up in apple juice, sprouts and lettuce.

Outbreaks of food poisoning from Shigella on iceberg lettuce have occurred in Sweden, England and Wales. USDA/ARS researchers (Citrus and Subtropical Products Research Laboratory, PO Box 1909, Winter Haven, FL 33883) have found a way to reduce these and other pathogenic and nonpathogenic microorganisms. They combine an ionizing irradiation treatment with a chlorine wash.

Although the FDA has approved the use of up to 1 kilogray (kGy) of ionizing irradiation for fresh produce, the ARS scientists use much less. In lab experiments, they 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 kGy of irradiation, microbial counts were 290 colony-forming units (CFUs) and yeast, 60 CFUs. Control samples that had not been irradiated showed microbial counts of 220,000 CFUs and yeast, 1400 CFUs.

Low levels of irradiation were used to minimize changes in the texture or appearance of the lettuce. Irradiated lettuce had about the same shelf life as untreated samples. The normal shelf life claimed by manufacturers for retail sales of salads is between 14 and 16 days from the packaging date.

Investigators also irradiated chlorine-washed, shredded carrots packed in modified atmosphere packaging. Nine days after irradiation on the expiration date, the microbial count was 1300, compared with 87,000 for nonirradiated, chlorinated controls. Texture and appearance were unchanged.

Further information. Robert Hagenmaier; phone: [phone]; fax: [phone].