Back in 2016, the original Impossible Burger debuted as a veggie burger that could almost pass as beef. Its meaty secret was a molecule called heme, which contains iron and is largely responsible for the flavors we associate with cooked flesh. But, according to Impossible CEO Pat Brown, it requires a protein to bind it. The original Impossible Burger used wheat protein, which worked, but had some drawbacks. First, it meant the Impossible Burger wasn’t gluten-free, but it also put some limitations on the meat’s form factor. The wheat worked for burger patties that stayed in a relatively static shape, but it couldn’t crumble or take on other shapes—like meatballs—without losing its integrity. The solution was a switch to soy.