There's a new reason for public school students to avoid the cafeteria meatloaf.

A Monday report has revealed that an ammonia-treated meat byproduct has long been used in beef products at U.S. schools. This pink-hued substance, which the U.S.D.A calls lean meat trimmings, has historically been an ingredient in dog food and cooking oils. But when sterilized with ammonium hydroxide, it can be blended with ground beef and eaten by humans.

The mysterious filler came into existence 11 years ago, when a company called Beef Products Inc. (BPI) sought an edge over the competition. They wanted to capitalize on certain byproducts -- fatty trimmings and connective tissues -- that weren't fit for consumption.

These byproducts were notoriously prone to salmonella and E. coli, but in 2001 BPI commissioned a study and found that an ammonia process could kill those pathogens. Their method was approved by the U.S.D.A., and the lean beef trimmings produced by the company became popular with fast food chains and public school cafeterias.

Then, in 2009, a New York Times investigation found records indicating that E. coli and salmonella pathogens have been found dozens of times in BPI meat, challenging claims by the company and the U.S.D.A. about the effectiveness of the treatment.  Until those findings were shared with top officials, BPI facilities had been fully approved and exempt from product recalls.

Though subject to stricter regulations after the New York Times story, the ammonia-treated trimmings were still widely used as burger-filler in cafeterias and fast food joints nationwide. So in April of 2011, celebrity chef Jamie Oliver began a television campaign to alert consumers to the existence of pink slime in their burgers. And in January of this year, McDonalds released a statement saying that they had stopped using the stuff in August.

For a number of years prior to 2011, to assist with supply, McDonald's USA used some lean beef trimmings treated with ammonia in our burgers, they explained. We were among other food retailers who used this safe product. At the beginning of last year, we made a decision to stop using this ingredient. It has been out of the McDonald's USA supply chain since last August.  We wanted to be consistent with our global beef supply chain and we're always evolving our practices. Both Burger King and Taco Bell have also cut the filler out of their menus.

Now, concerned parents and educators are asking: if fast food chains have refused to use this ammonia-treated byproduct, why is it still showing on lunch trays at public school cafeterias?

The Daily revealed that unlike McDonalds, the U.S.D.A. has never stopped buying the filler and is ordering seven million pounds of it for the National School Lunch Program, which provides free or discounted healthy lunches to eligible children at public schools.

Years ago, Food Safety Inspection Service veterans Carl Custer and Gerald Zirnstein were among the first to inspect the product. Zirstein coined the term pink slime. Custer labeled it high risk, telling The Daily that he and colleagues had objected to it because it used connective tissues instead of muscle... my objection was that it was not meat. But his findings were ignored -- possibly because an undersecretary of the USDA had strong ties to the beef industry.

To this day, the pink slime is considered officially safe by the government. And the compound ammonium hydroxide is not unique to beef; it's included in many U.S. products to kill pathogens. And because the U.S.D.A. considers the ammonia process a part of production procedure rather than an ingredient, it is not listed on food labels.

As the government has not yet responded to concerns about using the filler in public school lunches, the fate of this mysterious meat is in the hands of the public. I have a 2-year-old son, said Zirnstein to The Daily. And you better believe I don't want him eating pink slime when he starts going to school.