When shredded mustard is exposed to the air for a long time, oxidation will occur, and the phenolic substances within the shredded mustard will be secreted in large quantities, thus forming black pigments on the surface of the shredded mustard and making the shredded mustard black.

Mustard is a cruciferous, annual herbaceous plant of the genus Brassica, cultivated throughout China. Leaves salted for food; seeds and whole grass for medicinal use, seeds ground powder called mustard, for seasoning; pressed oil called mustard oil; this species is an excellent honey plant. It is rarely cultivated in Europe and the United States, and originated in Asia. Li Shizhen wrote "Materia Medica" recorded the medical value of medical mustard.