BOSTON — Andy Gomez, a ninth grader at Brighton High School, was not sure why hamburgers and meatballs had disappeared from the cafeteria, but he was not happy about it. “Today I just ate peanut butter and jelly,” he said. “I don’t like the chicken patty.”

The absence of ground beef at lunch last week — at Brighton High and 43 other public schools here — could be explained by a peek into the freezer, where 21 boxes of ground beef products sat, cordoned off from the rest of the meat by a clinical-looking cover of white paper reading “Do not use.”

This is the frozen mass at the center of growing public concern, stoked by news coverage and social media outrage, over so-called pink slime, the low-cost blend of ammonia-treated bits of cow. It turns out that it constitutes some of the ground beef distributed by the U.S. Department of Agriculture through its school lunch program, and that it can be found in at least some grocery store beef, though many chains have said they will not sell beef that contains it.

This year, McDonald’s and other fast-food restaurants also said they would stop using the substance, a filler formally known as lean finely textured beef, in their meat products. And on March 15, as an outcry resulted in hundreds of thousands of people signing online petitions, the Agriculture Department announced that next year it would offer schools a ground beef option that does not contain pink slime.

But for some school districts — with administrators fielding phone calls from concerned parents and fretting about past food scares — next fall is not soon enough. The Boston school district, among others, has taken the step of purging all ground beef from its menus. Other districts, like the New York City schools, have begun phasing out ground beef containing the additive from their lunchrooms.

Michael Peck, the director of food and nutrition services for the Boston schools, said the district had decided to hold and isolate its entire inventory of ground beef, leaving over 70,000 pounds of beef — worth about $500,000, Peck estimated — confined to a warehouse until the district knows more about what is in it.

“It’s another example of the alteration of our food supply,” said Peck, who is concerned about the use of ammonia hydroxide gas to kill bacteria in the product. “Have we created another unknown safety risk?”

BOSTON — Andy Gomez, a ninth-grader at Brighton High School, was not sure why hamburgers and meatballs had disappeared from the cafeteria, but he was not happy about it. “Today I just ate peanut butter and jelly,” he said. “I don’t like the chicken patty.”

The absence of ground beef at lunch last week — at Brighton High and 43 other public schools here — could be explained by a peek into the freezer, where 21 boxes of ground beef products sat, cordoned off from the rest of the meat by a clinical-looking cover of white paper reading “Do not use.”

This is the frozen mass at the center of growing public concern, stoked by news coverage and social media outrage, over so-called pink slime, the low-cost blend of ammonia-treated bits of cow. It turns out that it constitutes some of the ground beef distributed by the U.S. Department of Agriculture through its school lunch program, and that it can be found in at least some grocery store beef, though many chains have said they will not sell beef that contains it.

This year, McDonald’s and other fast-food restaurants also said they would stop using the substance, a filler formally known as lean finely textured beef, in their meat products. And on March 15, as an outcry resulted in hundreds of thousands of people signing online petitions, the Agriculture Department announced that next year it would offer schools a ground beef option that does not contain pink slime.

But for some school districts — with administrators fielding phone calls from concerned parents and fretting about past food scares — next fall is not soon enough. The Boston school district, among others, has taken the step of purging all ground beef from its menus.

Michael Peck, the director of food and nutrition services for the Boston schools, said the district had decided to hold and isolate its entire inventory of ground beef, leaving over 70,000 pounds of beef — worth about $500,000, Peck estimated — confined to a warehouse until the district knows more about what is in it.