Mustard seeds are the small seeds of the various mustard plants. The seeds are about 2 mm in diameter, and may be colored from yellowish white to black. They are important spices in many regional cuisines because of the amazing taste. The seeds can come from three different plants: black mustard (B. nigra), brown Indian mustard (B. juncea), and white or yellow mustard (B. hirta/Sinapis alba). It is often whimsically referred to as "eye of newt.
In the Indian subcontinent they are often used whole, and are quickly fried in oil until they pop to impart a flavor to the oil.
The French have used mustard seeds as a spice since 800 AD, and it was amongst spices taken by the Spanish on explorations throughout the 1400s. Pope John XXII was particularly fond of mustard, and created a new position in the Vatican, 'grand moutardier du pape', or 'mustard maker to the pope'.
Mustard oil can be extracted from the seeds. It is formed from mustard oil glucosides (glucosinolates) immediately after crushing. Different kinds of mustard give different mustard oils: black mustard gives a highly pungent mustard oil due to its content of sinigrin, while white mustard gives a much less pungent mustard oil due to its content of sinalbin. The seeds, particularly the yellow ones, can also be ground into flour, and mixed to a thick paste with a little water to make the condiment mustard. The ground mustard powder is usually mixed with ordinary flour to reduce the strength of the resulting condiment.
When initially mixed the sauce is mild in flavor, but it develops in time. Strong mustard has a very powerful (and painful) effect on the nasal membranes if eaten carelessly.
The whole seeds can be soaked in liquid before grinding to create whole grain mustard.
It is possible to buy prepared mustard in many places.