The key element in each Impossible Burger is soy leghemoglobin, which is found at the root of soybean plants but is created in Impossible Foods' labs. The FDA is unsure that these are safe for humans to eat, according to May 2015 documents secured through a FOIA request by ETC Group. The company rejected the agency's suggestion that the core protein in soy leghemoglobin would threaten consumers and stated it "is substantially similar to proteins consumed daily by the global population, in the form of meat and other vegetables."

Not that the FDA explicitly stated Impossible Burger's ingredient was dangerous, or forbade Impossible Foods from selling its meat substitute. But the company still wants the agency to confirm that its product is safe to eat, and plans to petition the FDA again, according to The New York Times.