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.