ALBANY, N.Y. >> School districts soon will be able to opt out of a common ammonia-treated ground beef filler critics have dubbed "pink slime."

Amid a growing social media storm over so-called "lean finely textured beef," the U.S. Department of Agriculture was set to announce Thursday that starting in the fall schools involved in the national school lunch program will have the option of avoiding the product.

Under the change, schools will be able to choose between 95 percent lean beef patties made with the product or less lean bulk ground beef without it. The change won’t kick in immediately because of existing contracts, according to a USDA official with knowledge of the decision.

Though the term "pink slime" has been used pejoratively for at least several years, it wasn’t until last week that social media suddenly exploded with worry and an online petition seeking its ouster from schools lit up, quickly garnering hundreds of thousands of supporters.

The low-cost ingredient is made from fatty bits of meat left over from other cuts. The bits are heated to about 100 F and spun to remove most of the fat. The lean mix then is compressed into blocks for use in ground meat. The product, made by South Dakota-based Beef Products Inc., also is exposed to "a puff of ammonium hydroxide gas" to kill bacteria, such as E. coli and salmonella.

The USDA source, who spoke on condition of anonymity because the official announcement was not made yet, said that the agency believes the ammonia treatment is safe, but that it wanted to be transparent and that school districts wanted choices.

"School districts have made requests and school districts want, basically, choice," the official said Wednesday evening. "And we respect that, they’re our customers."

The USDA buys about a fifth of the food served in schools nationwide.

There are no precise numbers on how prevalent the product is, and it does not have to be labeled as an ingredient. Past estimates have ranged as high as 70 percent; one industry official estimates it is in at least half of the ground meat and burgers in the United States.

The product has been on the market for years, and federal regulators say it meets standards for food safety. But advocates for wholesome food have denounced the process as a potentially unsafe and unappetizing example of industrialized food production.

The phrase "pink slime," coined by a federal microbiologist, has appeared in the media at least since a critical 2009 New York Times report. Celebrity chef Jamie Oliver has railed against it, and it made headlines after McDonald’s and other major chains last year discontinued their use of ammonia-treated beef.

But "pink slime" outrage appeared to reach new heights last week amid reports by The Daily and ABC News. The Daily piece dealt with the USDA’s purchase of meat that included "pink slime" for school lunches.

The story touched a nerve with Houston resident Bettina Siegel, whose blog "The Lunch Tray" focuses on kids’ food. On March 6, she started an online petition on Change.org asking Agriculture Secretary Tom Vilsack to "put an immediate end to the use of ‘pink slime’ in our children’s school food."

"When I put it up, I had this moment of embarrassment," she said, "What if only 10 people sign this?"

No problem there. Supporters signed on fast. By Thursday morning, the electronic petition had more than 225,000 signatures. Organizers of Change.org said the explosive growth is rare among the roughly 10,000 petitions started there every month.

Meanwhile, Google searches for "pink slime" spiked dramatically. It has become the food version of Joseph Kony, the rogue African warlord virtually unknown in the United States until this month, when an online video campaign against him caught fire.

But why is "pink slime" striking a chord now?

Issues can to go from a simmer to an explosion when content with broad interest — such as like food safety — is picked up and disseminated by widely connected people, said Marc A. Smith, director of the Social Media Research Foundation. These people act like "broadcast hubs," dispersing the information to different communities.

"What’s happening is that the channels whereby this flood can go down this hill have expanded," Smith said "The more there are things like Twitter, the easier it is for these powder kegs to explode."

In this case, Siegel thinks the added element of children’s school lunches could have set off this round.

"That’s what upset me. This idea that children are passively sitting in a lunch room eating what the government sees fit to feed them and McDonald’s has chosen not to use it, but the government is still feeding it to them," she said. "That really got my ire."

The USDA this year is contracted to buy 111.5 million pounds of ground beef for the National School Lunch Program. About 7 million pounds of that is from Beef Products Inc., though the pink product in question never accounts for more than 15 percent of a single serving of ground beef.

Beef Product Inc. stresses that its product is 100 percent lean beef and is approved by a series of industry experts. The company’s new website, pinkslimeisamyth.com, rebuts some common criticisms of the product ("Myth 4: Boneless lean beef trimmings are produced from inedible meat").

The National Meat Association also has joined the fight, disputing claims that the product is made from "scraps destined for pet food" and other claims. The industry group also said that ammonium hydroxide is used in baked goods, puddings and other processed foods.

Association CEO Barry Carpenter, who has visited BPI plants and watched the process, said critics don’t seem to have the facts.

"It’s one of those things. It’s the aesthetics of it that just gets people’s attention," Carpenter said. "And in this case, it’s not even legitimate aesthetics of it. It’s a perception of what it is."