Study At UNC-CH Shows Chlorinated Water Dramatically Cuts Strawberry Contamination

May 21, 1998

CHAPEL HILL - Washing strawberries with chlorinated water significantly cuts levels of bacteria, hepatitis A virus and other viruses that indicate possible contamination by animal or human wastes, according to a new study.

The research, conducted at the University of North Carolina at Chapel Hill, showed that after five minutes' exposure to water containing 10 parts per million of chlorine, between 90 and 99 percent of the disease-causing contaminants had disappeared.

Graduate student Michael J. Casteel and his mentor, Dr. Mark Sobsey, professor of environmental microbiology at the UNC-CH School of Public Health, performed the study. They presented their findings Thursday (May 21) at an American Society for Microbiology meeting in Atlanta.

"Contaminated produce has become an important source of food-borne disease in the United States and worldwide," Sobsey said. "Many produce commodities that could become contaminated with human and animal wastes are eaten raw and unprocessed."

Raspberries, strawberries, lettuce and basil leaves have caused outbreaks of food-borne viral and parasitic diseases such as gastroenteritis and infectious hepatitis, he said. Last spring, for example, contaminated strawberries, distributed through U.S. Department of Agriculture-sponsored school lunch programs, caused an outbreak of infectious hepatitis A that sickened more than 150 children and school workers in Calhoun County, Mich.

In their experiments, the UNC-CH researchers washed strawberries in the chlorine solution and then used a simple but efficient method of recovering any remaining microbes to determine how effective washing had been. They found their method "highly successful" in eliminating most viruses and bacteria.

"Our work is important because it demonstrates for the first time that hepatitis A virus can be inactivated on strawberries by a simple chlorination procedure," Sobsey said. "This will make it possible to reduce levels of the virus and contamination from bacteria like E. coli on strawberries and probably other produce as well and in so doing dramatically cut the risk of infectious enteric diseases from fecally contaminated fruits and vegetables."

The strawberry industry has begun implementing the chlorination procedure the UNC-CH researchers identified as effective, he said. In California at least, the treatment may become standardized and required.

Sobsey recommends that consumers wash fruits and vegetables that will be eaten raw, especially those from developing countries in Central and South America, for up to 10 minutes in a gallon or two of cold water containing a half teaspoon of Clorox bleach. After soaking, the produce should be thoroughly rinsed in cold tap water.

"We believe that this research needs to be expanded to determine chlorination efficiency against other disease-causing organisms like Salmonella bacteria and other viruses and to other types of produce such as tomatoes and apples," he said.

The Clorox Co. and Ramsey-SIAS, an agricultural processing company, supported the UNC-CH research in cooperation with the California Strawberry Commission and the Processed Strawberry Advisory Board of California.

Note: To reach Sobsey, call David Williamson at (919) 962-8596.

University of North Carolina at Chapel Hill

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