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.
"F.D.A. believes the arguments presented, individually and collectively, do not establish the safety of soy leghemoglobin for consumption," agency officials wrote in a memo they prepared for a phone conversation with the company on Aug. 3, 2015, "nor do they point to a general recognition of safety."
Impossible Foods can still sell its burger despite the F.D.A. findings, which did not conclude soy leghemoglobin was unsafe. The company plans to resubmit its petition to the agency.
Impossible Foods is finding out what happens when a fast-moving venture capital business runs headlong into the staid world of government regulation.
Investors like Bill Gates and Khosla Ventures have poured money into a variety of so-called alt meat companies. Silicon Valley has noble goals, applying technological solutions to address major issues like climate change, farm animal welfare and food security.
But food is not an app. It is far more heavily regulated by governments and much more heavily freighted with cultural and emotional baggage.
"This rush to market is the Silicon Valley mind-set," said Michael Hansen, a food safety expert who is the senior staff scientist at Consumers Union, an advocacy group. "They think because they're doing something disruptive, the regulations that apply to other companies don't apply to them."
For now, few food start-ups are selling products to consumers. Only Beyond Meat, which uses a traditional pea protein to make its Beyond Burger; Hampton Creek, which makes plant-based sandwich spreads and salad dressings; and Impossible Foods have any notable presence in the market.
Like Impossible Foods, Hampton Creek faced problems with the F.D.A., which challenged its use of the word "mayo" in the name of its vegan spread, Just Mayo. Federal definitions of foods require mayonnaise to contain eggs.
The agency ultimately allowed Hampton Creek to keep the name but required it to use bigger type on the front of the label to say it was egg-free. The label now defines "just" to mean "guided by reason, justice and fairness," instead of suggesting that it was a replica of mayonnaise.
In the case of Impossible Foods, the debate centers on its use of soy leghemoglobin, which the company's engineeered yeast produces and forms an important ingredient behind the business.
The company was started in 2011 by Pat Brown, a chemist at Stanford University. His approach, involving genetics, microbiology and cutting-edge chemistry attracted venture capitalists also eager to find plant-and lab-based replacements for hamburgers and chicken wings.
Impossible Foods sought to woo top chefs with a splashy sales pitch about how the burger mimicked the aroma, attributes and taste of real beef. When soy leghemoglobin breaks down, it releases a protein known as heme, giving it that meatlike texture.
Within three years of its founding, Impossible Foods landed big-name investors like Khosla, Mr. Gates and the Hong Kong billionaire Li Ka-Shing. This month, Temasek Holdings, Singapore's sovereign wealth fund, joined an investment round that added $75 million to the company's coffers.
"I love V.C.s and particularly the ones that invested in us," Mr. Brown said at a TechCrunch conference in May, referring to venture capital firms. "But it's truly astonishing how little diligence they do in terms of the actual science that underlies some tech companies."
The F.D.A.'s approval is not required for new ingredients. Companies can hire consultants to run tests, and they have no obligation to inform the agency of their findings, a process known as self-affirmation.
Impossible Foods adhered to that procedure, concluding in 2014 that soy leghemoglobin was safe. But it went further, seeking the regulator's imprimatur.
"We respect the role the F.D.A. plays in ensuring the safety of our food supply, and we believe the public wants and deserves transparency and access to any information they need to decide for themselves whether any food they might eat is safe and wholesome," Rachel Konrad, a spokeswoman for Impossible Foods, wrote in an email.
The F.D.A., however, wanted the company to show the ingredient was safe specifically for humans. It told Impossible Foods to establish the safety of the more than 40 other proteins that make up part of its soy leghemoglobin. F.D.A. officials said the company's assessment of the potential for the ingredient to be an allergen was deficient.
"This product has been touted as the 'secret sauce' in the Impossible Burger," said Jim Thomas, program director at the ETC Group, the Canadian environmental organization that started the Freedom of Information request. "Now we know that the F.D.A. had questions about it, but it was put on the market anyway."
Ms. Konrad defended the burger, writing it "is entirely safe to eat" and "fully compliant with all F.D.A. regulations." She said the company was "taking extra steps to provide additional data to the F.D.A. beyond what's required."
Impossible Foods, she said, has tested its ingredient on rats fed "well above" the amount of soy leghemoglobin in its burger. Ms. Konrad said the company's expert panel had determined those tests also demonstrated the ingredient was safe, and that the company would thus resubmit its petition for F.D.A. confirmation this month.
Companies have "no requirement" to notify the F.D.A. of a food being determined safe, Megan McSeveney, an agency spokeswoman, said in an email. She added, however, that the F.D.A. could question the basis for any such conclusion and "take appropriate action to protect public health."
Consumer advocates say the experience of Impossible Foods highlights longstanding concerns about the F.D.A.'s oversight of food safety. Congress gave it the responsibility for policing food additives under the Food, Drug and Cosmetic Act of 1938. Twenty years later, it added an exemption to allow a company to sell a product without the agency's review if the additives were deemed safe.
In 2010, the Government Accountability Office raised concerns about the agency's interpretation of its responsibilities under the law. A study by the Pew Charitable Trusts found in 2013 that the F.D.A. was unaware of roughly 1,000 of some 10,000 ingredients used in food because companies had used the self-affirmation process. And in May, the Center for Food Safety and other groups sued the F.D.A. over that process.
"The exemption was meant to cover ingredients that had long been used in the food supply, so that companies didn't have to come in every time they made a new product," said Tom Neltner, chemicals policy director at the Environmental Defense Fund, an advocacy group that is one of the plaintiffs in the lawsuit. "It wasn't meant to allow companies to simply bypass the F.D.A."