According to a statement issued late Thursday, the South Korean Agriculture and Forestry Ministry found 6.26 picograms of the toxic substance in one gram of fat, part of a 10.2-ton shipment of U.S. beef which arrived on Dec. 1.

South Korean standards allow no more than 5 picograms per gram of fat. A picogram is equivalent to a trillionth of a gram.

The discovery was the latest bad news for the U.S. cattle industry in South Korea, already dealing with the rejection of three recent shipments of beef for including banned bone fragments, which South Korea fears could potentially harbor mad cow disease.

Seoul barred U.S. beef in December 2003 after the first reported U.S. case of the disease, known as bovine spongiform encephalopathy. Imports recently resumed after a nearly three-year ban, but so far no beef has made it to South Korean food stores or restaurants.

Officials said the beef with the dioxin was in the third of the rejected shipments.

Seoul has asked Washington to explain why the beef contained the dioxin, said a South Korean Agriculture and Forestry Ministry official, who refused to give his name.

Source: CBS News