Mustard seed comes from three large shrubs, Brassica juncea, (brown mustard), Brassica nigra (black mustard) and Brassica hirta (white mustard). All three produce bright yellow flowers that die off to leave small round seeds. The brown mustard seed is more pungent than the white and is used predominantly in Indian cooking.
Mustard seeds are small, matt, hard, spherical, and either brown, white or black. When heated, they taste bitter, nutty, hot and aromatic. They are a key ingredient in some vegetable dishes and in pickles. In Bengal, they are often ground to make sauces for fish. Cooks in southern India fry a small quantity with other seasonings, such as cumin and curry leaves, before eating them take care when you do this: the seeds pop in the hot oil and fly about with a life of their own. The spiciness of mustard seeds, no matter how pungent, does not linger, and they impart a rich, earthy taste to any dish.