The Impossible Burger is a plant-based patty made by Silicon Valley startup Impossible Foods, which has backing from Bill Gates.

Some advocates have said the burger is unsafe because of a key ingredient called heme, or soy leghemoglobin.

The FDA initially said it couldn't deem heme safe for consumption without further research. But on Wednesday, the FDA classified soy leghemoglobin as a color additive, clearing the way for Impossible Foods to sell its "bleeding" burgers in stores.

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The magic ingredient that makes Silicon Valley's favorite veggie burger "bleed" has officially been approved by federal regulators.

On Wednesday, the Food and Drug Administration (FDA) cleared the way for a protein called soy leghemoglobin to appear in food items sold in grocery stores. When soy leghemoglobin is cooked, it produces heme, the iron-rich compound that gives the "bleeding" veggie burger made by Impossible Foods its signature blood-red color.

Read more: This Silicon Valley startup behind the veggie burger that 'bleeds' just fixed its biggest problem

In July 2018, the FDA determined that soy leghemoglobin was safe to eat, or in official parlance, "generally recognized as safe" (GRAS). But until now, Impossible Foods had still been waiting to hear whether the ingredient would be recognized as a color additive, which requires a separate approval process.

The FDA's latest decision is a big win for the company, which now plans to sell its burgers in select grocery stores starting in September.

Impossible Foods battled with the FDA over its star ingredient

Impossible Foods has received funding from Bill Gates, inked deals with major fast-food chains like White Castle, and even made its burgers available on Air New Zealand flights. But the company hit a snag last summer after voluntarily sending data about its burger to the FDA; the agency said the paperwork wasn't sufficient to "establish the safety" of soy leghemoglobin.

In the meantime, everyone from journalists to environmental activists glommed onto the controversy, taking issue not only with soy leghemoglobin in the Impossible Burger but also the fact that it was made using genetically engineered ingredients, or GMOs.

Impossible Foods went back to the FDA with more research about soy leghemoglobin's safety. In July 2018, the agency reversed its original statement and concluded that heme — and the Impossible Burger — were safe to consume.

"We are the farthest thing from surprised," Pat Brown, Impossible Foods' CEO, told Business Insider last year.

Heme, the essential nutrient you've never heard of

Although today's veggie burgers can be described by a handful of delicious-sounding adjectives, "meaty" wasn't really one of them until Impossible Foods came along. The Impossible Burger's meat-like flavor is largely attributed to heme, an essential nutrient in many proteins that is found in just about every living thing on Earth.

In our bodies, heme is found in a molecule in our blood called hemoglobin. Heme is also found in smaller concentrations in some plants, including soy, which is where the Impossible Burger's heme comes from. Impossible Foods genetically modifies yeast in order to turn the organisms into tiny heme factories that churn out enough of the ingredient to give the burgers their meaty flavor.

Brown has said that the FDA's green light simply highlights what he and the rest of the Impossible Foods team already knew — that the burger is safe to eat.

"We would have been kicking ourself in the foot if we hadn't already done the research and proven that this was safe," Brown said last year.