School districts soon will be able to opt out of a common ammonia-treated ground beef filler that critics have dubbed “pink slime,” Philly.com reports. 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…