Tofu skin, tofu coat. Li Shizhen said in Compendium of Materia Medica: "The method of tofu began with Liu An, king of Huainan in Han Dynasty. Black beans, soybeans, white beans, mud beans, peas and mung beans are all acceptable. Methods: Soak in water, crush, filter to remove Me, fry, precipitate with salt water or alum juice or vinegar, and then collect. Others are put in vats, and gypsum is collected at last. Generally, bitter, salty, sour and spicy things can be restrained. Those condensed on the surface are exposed and dried, which is called tofu skin.