Let's hope you didn't eat a hamburger before clicking on this story.

A former U.S. Department of Agriculture scientist has come forward with a startling tale of how a substance known as "pink slime" has been embedded in about 70 percent of ground beef sold in the U.S. -- a topic ABC News investigated for a segment Wednesday night.

"Pink slime" is largely made up of connective tissue that used to be reserved only for dog foods. It was not classified as "meat" because it was largely seen as unfit for human consumption. It also contains ammonia, which is used to kill off bacteria so people who eat it do not get sick.

But in the early 90s, former undersecretary of agriculture Joann Smith decided that it was meat, allowing it to enter the human food chain. When she left her post in 1993, she immediately took a job with Beef Products, Inc. on their board of directors.

The meat industry now refers to it as "lean finely textured beef," but in a government memo USDA scientist Gerald Zirnstein coined the term "pink slime," which now appears to have stuck. Zirnstein and fellow former USDA scientist Carl Custer told ABC News that it has become so prolific, "pink slime" can now be found in approximately 70 percent of U.S. ground beef.

"Pink slime" was in the news last month after several major fast food chains announced they would no longer use it as filler in their hamburgers. Despite those restaurants' plans, the U.S. government has continued to purchase "pink slime" for use in school lunches, according to a report out this week by the News Corp.-run iPad newspaper The Daily.

This video is from ABC News, broadcast Wednesday, March 7, 2012.

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