There are three types of mustard seeds, white, brown and black. They are the small round seeds of the mustard plant which are dried to create the spice. The whole seeds lack aroma but as you crush and cook with them their punchy, nutty flavour is quickly released.
Black ones have the strongest taste, brown (or reddish-brown) are slightly less pungent than black, but hotter than the sweeter tasting white ones (which are a sandy yellow colour). Brown and black mustard seeds are favoured in Indian cooking and go particularly well with fish, whereas European cooking tends to use more white seeds, for such things as marinades and pickles.
IN THE KITCHEN
Many southern Indian dishes fry mustard seeds whole in hot oil at the beginning of cooking, a method known as 'tempering' or 'tarka', which brings out a sweeter side of their taste.
To do this, heat a little sunflower or vegetable oil or ghee in a heavy-based pan until very hot, add the seeds and fry for about 10 seconds. They will soon start to jump and pop in the pan, so keep them moving by gently shaking the pan, then remove from the heat before they have a chance to burn and become bitter tasting.
In some parts of India, especially Bengal, the seeds are crushed to make a hot paste.
Mustard seeds will keep for up to a year if kept airtight in a dry place.