One of the chief selling points of the Impossible Burger, a much ballyhooed plant-based burger patty, is its resemblance to meat, right down to the taste and beeflike “blood.”

Those qualities, from an ingredient produced by a genetically engineered yeast, have made the burger a darling among high-end restaurants like Momofuku Nishi in New York and Jardinière in San Francisco, and have attracted more than $250 million in investment for the company behind it, Impossible Foods.

Now, its secret sauce — soy leghemoglobin, a substance found in nature in the roots of soybean plants that the company makes in its laboratory — has raised regulatory questions.

Impossible Foods wants the Food and Drug Administration to confirm that the ingredient is safe to eat. But the agency has expressed concern that it has never been consumed by humans and may be an allergen, according to documents obtained under a Freedom of Information request by the ETC Group as well as other environmental and consumer organizations and shared with The New York Times.