Open the menu at Momofuku Nishi or a greasy spoon called Food Factory in Fairbanks, Alaska, and there it is: the Impossible Burger, served with fries and special sauce for $18 at David Chang’s joint, or if you’re in the Alaskan Interior, a side of cottage cheese and pineapple for a $1 upcharge. Stanford biochemist Pat Brown’s “bleeding” vegan patty started winning over carnivores four years ago — Chang was the first high-profile convert, and since then, the burger has surfaced on almost 2,000 menus, including Food Factory’s last month. It’s been miniaturized into White Castle sliders, fancified into tartare, and just this past weekend, transformed by Questlove into the world’s first Impossible Cheesesteak. (Of course, Philly’s rigid culinary traditions demand that the sandwiches are made “wit Wiz.”)

The food start-up’s success is undeniable — and a noticeable affront to Big Meat — but Impossible Foods’ hasty rise has also made the $400 million Silicon Valley darling an unexpected set of enemies: consumer groups and environmentalists who exposed FDA letters questioning the safety of the burger’s “magic ingredient,” then started demanding more proof that Impossible’s alt-meat is the harmless, vegan product it claims.

If this fight sounds like a fantasy dreamed up by food-policy nerds (there has to be at least a couple of people like that, right?), the actual debate really boils down to what Impossible Burgers are, and what they aren’t. Almost everyone agrees that the vegan patty is a game changer that looks, smells, and even sizzles like real beef. Compared to traditional hamburger patties, the Impossible Burger also requires 95 percent less land and 74 percent less water to produce. And some of America’s smartest food writers have declared it “very impressive” and “remarkably like beef,” despite the fact that it’s a combination of wheat, potato protein, coconut oil, and a protein called soy leghemoglobin, or SLH, that delivers the so-called “magic” blood — heme, a ferrous molecule abundant in animal muscle.

Brown, the biochemist, has given an entire TED Talk on the substance, detailing how it gives meat its strangely alluring metallic taste, and to create a plant burger packed with this much heme, Impossible must manufacture the SLH in a lab using yeast. Therefore, what the Impossible Burger isn’t is organic. It’s also a creation that’s never been found before in the human food supply.

Given their novel situation, Impossible asked the FDA to review the burger’s safety, despite being under no obligation to do so. That was in 2015, and it went badly: The agency responded with a letter saying that Impossible’s data “do not establish the safety of SLH for consumption, nor do they point to a general recognition of safety.” Impossible Foods took a mulligan, sending 1,000-plus pages of new test data that the company argued better demonstrate SLH’s safety. The FDA gave itself until the end of April to reply, but just extended that deadline by another 90 days.

That alarmed Friends of the Earth, ETC Group, and other staunch anti-GMOers. Impossible spokesperson Rachel Konrad says their heme is “identical to the heme humans have been consuming for hundreds of thousands of years in meat,” but these critics argue that simply having the same 3-D structure isn’t good enough to prove safety. “This is a protein produced with genetic engineering,” says Friends of the Earth senior food and technology campaigner Dana Perls — therefore, it’s “a new food ingredient.”

Michael Hansen, a senior staff scientist with Consumers Union who’s on the ETC Group board, argues that it’s “sort of outrageous” Impossible even took the burger to market without the FDA’s all clear. “A few months after the FDA’s letter said they hadn’t demonstrated it was safe, they withdrew their notification,” he says. “Then they turned right around and started selling the burger at Momofuku.” He’s concerned because the heme they manufacture has 46 “unknown” proteins attached (that are also entering our food supply).

Impossible Foods’ Konrad counters that this is a silly argument, since we eat “hundreds of thousands of proteins” every day that are technically “unknown,” and they’d quit selling Impossible Burgers in a nanosecond if even “a shred of doubt” about their safety arose.

Critics argue that the FDA letter does raise doubts. They also criticize how the vegan start-up tried remedying them in 2016 — with animal testing that wasn’t revealed until last August, when Brown admitted that the ordeal had been an “agonizing dilemma.” During said tests, researchers fed rats immense quantities of heme for 28 days. That may sound like a long time, but for new food additives like SLH, some researchers suggest that it would have been more prudent to perform a long-term study lasting two years. (Impossible Foods, again, disagrees, saying that there’s no consensus around two-year studies.)

The spat has turned onetime allies, like the Sustainable Foods Summit and PETA, into some of Impossible’s most hostile audiences. Opponents also take issue with Impossible’s marketing tactics — while it doesn’t hide that SLH is a GMO, the company is very strategic about where it discloses that information. For instance, the home page describes heme as a “magic molecule” made “by fermentation” (like beer!); inside the burger is a mix of “the right ingredients from the plant kingdom.” Dig halfway down the FAQ page, and you’ll find the GMO process explained in detail. But per Internet Archive, that edit didn’t occur until between April and May of 2017, almost ten months after the burger debuted at Momofuku Nishi.

There is, of course, no consensus that genetically modified ingredients are necessarily bad for you, but years of marketing have convinced many consumers that they aren’t exactly great for you, either, so it’s not surprising that Impossible was quiet about their inclusion, and that the company’s restaurant partners would also omit any mention of GMOs.

Some of those partner restaurants even seem confused about what, exactly, they are selling when they sell Impossible Burgers. Take Bareburger, a popular organic burger chain where, according to the marketing material, “everything going into your body is coming from nature.” The chain also supports GMO Free NY’s food-labeling campaign. It was Impossible’s first multi-restaurant partner, and several of its 48 locations bragged about their new menu item using hashtags like “#organic” and “#nongmo”:

Confused, I visited a Brooklyn location this week to see how the Impossible Burger gets explained to customers in real life. I ordered one, cooked medium, with a side of fries, but told the server that I wanted the sales pitch — was “bleeding” fake meat Bareburger’s next step in sustainable, entirely organic burgers? “No, actually,” the worker replied, clearly well-trained on the burger’s specs, “to make it, they have to use genetically engineered ingredients.” I pointed out that wasn’t exactly in keeping with the Bareburger ethos. He half-shrugged, and added: “If you want a veggie burger that’s organic, you have to order one of the others.”

When I got in touch with Bareburger’s corporate office to ask about this discrepancy, they replied that “sustainability does not always take the form we think it should,” adding that just because something’s “certified organic or local” doesn’t necessarily make it “the most sustainable version of that food.” By contrast, there are “both good and bad GMOs,” and the Impossible Burger is “by far one of the good ones.” The chain says, regardless, it’s “extremely tasty, which is the real reason it’s on our menu!”

Bareburger isn’t the only restaurant where Impossible Burgers probably confuse regular customers: Hopdoddy, a burger chain in six states, added the nonorganic item to a long list of sandwiches “made with fresh, all-natural ingredients.” At California mini-chain Mendocino Farms, the Impossible Burger joined other items “made with fresh organic” ingredients. Impossible Burgers are also on the menu at South Beach’s Full Bloom Gourmet Vegan Cuisine, a place that specializes in “fresh organic ingredients.” Meanwhile, the Chicago chain M Burger straight-up calls Impossible’s patty “100% free of hormones, antibiotics, and artificial ingredients.”

Individually, these may seem like minor infractions or misunderstandings, but taken as a whole, the errors make Impossible look sloppy, like maybe it’s hustling to stay ahead in one of Silicon Valley’s hottest industries. For its part, Impossible argues that it’s “never stated or implied that the product is organic,” and restaurants ought to be aware that nonorganic “is the default classification of all products.” Impossible also blames anti-GMO crusaders and a sensationalist press for these attacks. The company says that it openly discusses genetic modification with anyone who’ll listen, disclosed the animal testing voluntarily, sought the FDA’s approval when it wasn’t required, and that a panel of independent safety experts has given the green light to the veggie burgers — twice. Also, as both Impossible and Bareburger point out, studies have repeatedly shown that genetic modification is safe in the abstract. The problem is what humans do with it.

The response is predictable, and it’s also somewhat at odds with the way the Impossible Foods are advertised: They sell “meat on a mission,” they make a “better planet,” they “do the impossible,” they are, after all, trying to create a world with lots of tasty burgers and no slaughtered cows. But this is also just future-focused sales babble that is firmly grounded in today’s standard Silicon Valley aspirational disruption stance, where it can be tough to slow down a decent idea once it gets rolling.