When animal or seed milks are heated in an uncovered pot, a skin of coagulated protein forms on the surface. The skin forms because proteins unfolded by the heat concentrate at the surface, get tangled up with each other, and then lose their moisture to the dry room air. As they dry, they get even more tightly tangled, and form a thin but solid protein sheet, entrapping oil droplets and developing a fibrous, chewy texture.
Such skins are usually an annoyance, but some cultures make a virtue of them and turn them into a dish. The Indians do this with cow’s milk, and for several centuries the Chinese have been using soymilk to make dou fu pi, the Japanese yuba, which they layer together to form a variety of sweet and savory products, some of them shaped into flowers, fish, birds, even pigs’ heads. The skins are also meltingly delicious when eaten just as they’re taken from the milk. At some Japanese restaurants, a small pot of soy milk is heated at the table so diners can remove and eat the skins as they form, then add a pinch of salts to the remaining liquid and coagulate it into soft tofu.