An estimated 70 percent of the ground beef supply contains these lean bits of meat derived from muscle and connective tissue. The industry calls the trimmings Lean Finely Textured Beef.

The U.S. Department of Agriculture said it believes beef trimmings known officially as "Lean Finely Textured Beef" are safe to eat. Nonetheless, it announced that owing to "customer demand" it will give schools the chance to opt out of it in the next school year.

Wal-Mart has become the latest food retailer to announce that it's making changes after listening to customer concerns about lean finely textured beef, known by detractors as "pink slime."

Thousands of people are adding their name to petitions urging the government stop buying beef trimmings. But food safety officials say the trimmings are still safe to eat.