A former high-ranking BPI executive -- who the company said was fired and is now disgruntled -- disputed BPI's claim that its raw material, the beef trim, is the same as any other scraps used to make ground beef.
"Pink slime," said Kit Foshee, "comes from cuts or fat that is most-highly susceptible to contamination during [the] slaughter process. Removing hide ... that's exactly where the fat is harvested from ... when they centrifuge, they're going to concentrate harmful bacteria."
And it was those extra pathogens that led BPI to use ammonia gas to kill the bacteria.
Zirnstein's former colleague at the USDA, Carl Custer, also a retired microbiologist, said the claim that "pink slime" is as nutritious as ground beef is wrong.
"Microbiologically safe and nutritionally complete are two different issues," Custer said. "It may be pink [but], nutritionally, it is not equivalent to whole-muscle tissue."
Custer said the ammonia gas does kill E. coli and salmonella if done properly, but much of the protein in lean finely textured beef is different than protein in pure ground beef.
"It would be sort of the equivalent to something like Jell-O or gelatin" said Custer. "Gelatin is connective tissue. It's been boiled down, but it is a protein. It's just not a complete protein. Add sugar to it and other things and it's delicious. And you do get some nutritional value. It's not as nutritional as whole muscle meat."
"Pink slime" does provide nutrition, but not as much as ground beef, according to Richard Ludescher, a nutritionist at Rutgers University in New Jersey who, at the request of ABC News, reviewed data from a study on lean finely textured beef from Iowa State University.
Ludescher said that because lean finely textured beef has five times the collagen level as standard ground beef it "will have a lower nutritional value than beef muscle."
Collagen is a protein, he said, that is higher in non-essential amino acids and lower in essential amino acids than meat from an animal's muscle.
"Addition of LFTB would thus lower the nutritional quality of ground beef," Ludescher said.
He added that even though it is not as nutritious as ground beef, Americans eat much more protein than we need so eating lean finely textured beef would not impact the average American's diet.
"The effect is certainly inconsequential," he said.
While BPI attempts to make its case to a public clearly concerned about what is in their ground beef, Bettina Siegel, a Houston mother of two who launched the change.org petition to ban the product from school lunches, continues her fight to "just label it."
BPI's vice-president and the wife of the owner responded in her press conference Monday: "What should we label it? It's 100 percent beef. What do you want us to label it?