Mustard seeds consist of 36 percent nutty vegetable oil and essential oils. The essential oils contain the mustard oil glycosides, which are responsible for the pungency and flavour.
Mustard seeds in themselves are odourless and only develop their flavour when they come into contact with liquid. This is due to the enzyme myrosinase, which can only be activated by liquid. Through the activation of this enzyme, the mustard oil glycosides are transformed into various substances.