San Francisco chefs who feel they played a role in popularizing the Impossible Burger are steaming mad that Bay Area-based Impossible Foods cut off their supplies for weeks while apparently fulfilling a contract with Burger King.

The meat-free, "bleeding" Impossible Burger has taken off nationwide after its local launch in 2016, and last month began appearing in Burger King's Impossible Whopper in several American cities last month. As part of the launch in the Bay Area, Redwood City-based Impossible Foods had to supply meatless meat patties to 111 Burger King locations in San Francisco, Oakland, San Jose, and all across the region by the second week in June, and that production crunch appears to have stressed the young company to the point that it had to stop supplying the small businesses in San Francisco where the Impossible Burger got its start — and where many a vegan and vegetarian now rely on it for their burger fix.

The burger meat is made with soy protein, potato protein, coconut oil, sunflower oil, and heme — a bioengineered, plant-based substance using the same molecule found in hemoglobin that gives the Impossible Burger some of its meat-like look and taste, and makes it "bleed" like real beef.

Local chefs tell the Chronicle that the trouble began around late April, when shipments from Impossible Foods started being less regular. Then by June, the shipments ceased completely, and local restaurants that feature the burger on their menus had to tell customers they were out of luck — in some cases replacing the burger with another vegan option. Hi Tops in the Castro, Violet’s in the Richmond, Outer Orbit in Bernal Heights all went without Impossible Burgers for over a month, and just last week the company sent out a letter to customers informing them that they could begin ordering the burgers again — only now they have to take five-pound blocks of the plant-based meat, instead of the pre-formed patties that were sold before.

The bitterness, however, remains.

"[The burgers] disappeared off our shelves and then showed up at Burger King across the street,” says Christian Gainsley, owner of Outer Orbit, speaking to the Chronicle. "It felt like a betrayal. The success of Impossible Burger was borne on the backs of the restaurateurs who took it on and made a good product out of it."

Danny Stoller, who just opened the Detroit-style pizza place Square Pie Guys this month, tells the Chronicle that he put a vegan burger on his menu made with Impossible's competitor product, Beyond Meat, instead, because he couldn't get the Impossible people to call him back. And Stoller says that not getting the pre-formed patties would be a problem anyway, because of the costly extra labor involved.

The Chronicle also wonders aloud if Impossible Foods is doing the patty-making for Burger King (likely), but the company declined to confirm this.

Impossible Foods' success is nothing to sniff at, and hopefully they'll get their supply chain moving more smoothly in the coming months and years. They're now on the hook to supply 7,300 Burger Kings with their product by the end of the year, and they already reportedly supply 10,000 restaurants with Impossible Burger meat, including locations of White Castle and Little Caesar's.

The meatless burger craze has gone international, though Impossible Burgers aren't going to get shipped to Europe for now. As Vox reports, the key "bleeding" ingredient in the Impossible Burger, heme, is derived through genetic modification, which is banned in the EU.

Sidebar: To promote the launch of the so-named "Rebel Whopper" in Sweden, Burger King rolled out the ad below in which assorted Swedes express complete disbelief that the burgers are meat-free. One woman even calls it a "bloody mind-fuck." The chain is offering what it calls the "50/50 Challenge," letting customers do blind taste tests to see if they can tell whether their burger is beef or not-beef. It remains unclear wherefrom the non-meat product for the Rebel Whopper is being sourced.

Related: The 25 Best Burgers In San Francisco