We don't use much black mustard in Europe, with brown cultivars dominating the market. But black mustard is important as a spice in Indian cooking, both in seed and oil form.
A field of black mustard
Black mustard flowers
Black mustard seeds
Black mustard is endemic in the Southern Mediterranean region and as it has been cultivated for millennia there are numerous cultivars. Botanically different, but of equal culinary use, are Romanian brown mustard or b. juncea from Eastern Europe and Indian brown mustard, b. integrifolia, a fertile hybrid of b. nigra and b. campestris from India and Central Asia. Of all three species, the latter is most commonly sold in the West. Despite having been introduced alongside white mustard during the Middle Ages under Charlemagne's Capitulare de Villis, black mustard is rarely planted in Europe these days, with brown mustard the dominant quality on the European market. The reason is that brown mustard, unlike its black counterpart, can be harvested by machines which make production cheap in countries where labour is relatively expensive.
Seed grains. These are globular, dark brown and about 1mm in diameter. The seeds of the brown mustard species are larger and somewhat less dark.
Brassicaceae (cabbage family).
The dried seed do not have any fragrance, but exhibit a pungent taste after being chewed for a while. Roasted seeds (more grey in colour) have a rich, nutty odour.
Genus name brassica is Latin for "cabbage", which belongs to the same genus. Species nigra is from the Latin nigr- or niger "black".
The German senf is a loan from Latin sinapi and from the Old English senep (mustard paste was introduced to central and North Europe by the Romans). The same word is also found in Greek (sinapi, also napy), but its ultimate origin is uncertain but probably Egypt. "Mustard", similar words in Roman languages and the German mostrich for mustard paste are all derived from Latin mustum, "must". Although mustard paste is today predominantly prepared with vinegar and wine, the Romans used must "young wine". The Sanskrit names krishnaka and krishnasarshapa derive from the adjective krishna "black". English, French and Spanish common names indicate the modern country of origin ("Indian mustard", moutarde de l'Inde, mostaza de Indias), although in France it is attributed a Chinese origin, moutarde de Chine.
Only a small part of the world's harvest of black and brown mustard seeds is used for the production of mustard paste. This is because mustard paste made from black mustard is too pungent for Western tastes, despite the fact that this pungency is unstable and decreases after some time. Chemically, the pungent principle of black mustard seeds, allyl isothiocyanate, is unstable in water. For this reason, white mustard is usually preferred in making mustard pastes.
Black mustard is more important as a spice and oil plant, especially in India. Indian mustard oil is essential to achieve the authentic flavours of several Indian regional cuisines, especially those of Bengal, Kashmir, Maharashtra and Goa.
In both West Bengal and Bangladesh, mustard oil usage is widespread. It is the preferred cooking medium in these territories and contributes a characteristic flavour to Bengali cooking which is particularly noticeable as strong spices are used with moderation in this cuisine. Mustard oil produced in Bengal contains enough isothiocyanates to have a pungent mustard flavour and it is often used as a flavouring, e.g. by drizzling the oil over boiled vegetables before serving. Such oil is difficult to obtain outside of India and chefs in the West generally substitute it by mustard paste (preferably Dijon type) or by mustard powder.
Because of its erucic acid and isothiocyanates, mustard oil is not a legal foodstuff in most Western countries (including the EU and the US) and may not be sold as cooking oil. Nevertheless, Indian food shops often sell mustard oil labelled "for external use only". In India, it is common to heat mustard oil initially up to the smoking point and let it then cool to cooking temperature before use. This heating procedure is believed to be useful for detoxification as well as for taste improvement.
Black or brown mustard seeds are also used as a direct spice. Their pungency is completely destroyed by cooking and therefore the ground seeds should be added as late as possible if pungency is desired. In India, black mustard seeds are commonly toasted, or fried in a little oil, until they acquire a grey hue (the pan must be covered with a lid as the seeds will pop and disperse themselves all over the kitchen if left open). This frying procedure changes the character of black mustard seeds completely, so that they are no longer pungent but display an interesting and unique nutty taste. This flavour is loved in South India, where mustard seeds are often fried in butter fat to give perfumed butter or tadka.
Black mustard seeds are also a component of the Bengali spice mixture panch phoron and the South Indian mix sambaar podi.