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There are, however, also mustards made from a mixture of black and white mustard seeds, and some very pungent specialty mustards contain pure black mustard seeds. In order to keep the pungency stable, these mustard pastes usually contain more acidic components than the milder mustards based on white mustard seeds. See white mustard for more information on mustard pastes. See also negro pepper for a comparative review on pungent spices.
Black mustard is more important as a spice and oil plant, especially in India (see also sesame about vegetable oils in general). Indian mustard oil is really essential for the authentic flavours of several Indian regional cuisines, particularly those of Bengal, Kashmir, Maharashtra and neighbouring Goa (see tamarind for the famous Goan pork curry vindaloo which also uses mustard oil).
In both the Indian union state West Bengal and in Bangladesh, mustard oil is very widespread. It is the preferred cooking medium and contributes a characteristic flavour which is particularly noticeable as intensive spices are used with moderation in Bengali cooking (see also nigella). Mustard oil produced in Bengal often contains enough isothiocynates to have a pungent mustard flavour and is often used as a flavouring, e.g., by dribbling the oil over boiled vegetables before serving. Such oil is difficult to obtain outside of India, and people in the West will have to substitute it by mustard paste (preferably of dijon type, see white mustard) or mustard powder (of the Colman type, see also white mustard); I wonder whether freshly grated horseradish might also work.
However, because of the erucic acid and maybe also the isothiocyanates, mustard oil is not a legal foodstuff in most western countries, including the EU and the USA, and it must not be sold as cooking oil. Nevertheless, Indian food shops often sell mustard oil, but to circumvent these paternalistic laws, their mustard oil is labelled “For external use only”. There is no need to take that remark seriously, although mustard oil does have cosmetic use in India (e.g. as hair balm). In India, it is common to heat mustard oil initially to high temperature, up to the smoking point, and let it then cool down to regular cooking temperature, or even to room temperature, before the cooking proceeds. Although I don't know for sure, this heating procedure might be useful for detoxification (or, maybe, it just improves the taste, as does the toasting of dry spices); in any case, it's a good idea to follow that praxis.
Black (or brown) mustard seeds are also much used as a spice directly. Their pungency is completely destroyed by cooking, and therefore the ground seeds should be added as late as possible if some pungency is desired. In India, black mustard seeds are commonly toasted, or fried in a little oil, until they acquire a grayish hue (cover the pan 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: They are no longer pungent, but display an interesting nutty taste hardly comparable to anything else on the spice shelf. This flavour is particularly loved in South India, where mustard seeds are often fried in butter fat to give perfumed butter (tadka, see ajwain).
Black mustard seeds are also a component in the Bengali spice mixture
panch phoron (see nigella)
and the South Indian composition sambaar podi (see
cumin).