Black Mustard Seeds: The Hottest Mustard, and Why It Turns Nutty in Hot Oil

Black mustard seeds are the hottest of the three mustards used as spices, yet true black mustard has mostly vanished from farming, replaced by brown mustard. So the jar labeled “black” may or may not be the real thing. Crush the tiny dark seeds and wet them and they bite hard. Fry them whole in hot oil, a move that’s common in South Indian cooking, and they lose that bite and turn nutty.

At a glance

What black mustard is

Black mustard is a much-branched annual herb, 0.5–1.5 m tall, with a firm taproot. Kew now accepts it as Mutarda nigra, with Brassica nigra as a synonym, so older books, seed catalogs and labels use the Brassica name for the same plant.

Black, brown and white

Three plants give us mustard seed: black mustard, brown mustard (Brassica juncea) and white or yellow mustard (Sinapis alba). Katzer describes black and brown mustard as botanically different but equally useful in the kitchen, with black mustard’s heat slightly stronger than brown’s.³ The rule of thumb: the darker the seed, the spicier.

Why true black mustard is scarce

Black mustard has dropped out of large-scale farming. PROSEA explains that its pods shatter, which makes it a poor fit for large-scale, mechanized growing, and that brown mustard has largely replaced it. Black mustard’s place in international trade is now minimal.² Katzer makes the same point: brown mustard can be harvested by machine and black can’t, and black mustard is hardly planted in Europe anymore.³ Britannica says brown mustard has entirely replaced black in commercial cropping, and that black mustard now shows up mainly as an introduced weed.⁴

We found no survey of what retail “black mustard seed” actually is, so we can’t tell you whether a given jar is true black mustard or dark brown mustard. The botanical name on the label, if there is one, is your only guide.

Where it comes from

Sources disagree on where black mustard comes from:

The seed has been used as a spice and medicine since ancient times in the Middle East, India and Greece.

Flavor and the chemistry of the heat

Finely ground black mustard is a neutral, odorless powder. The heat only shows up once the seed is crushed and wet. The enzyme myrosinase acts on the glucosinolate sinigrin and releases allyl isothiocyanate, a volatile compound.³ PROSEA calls allyl isothiocyanate aggressive and responsible for the pungent taste, and notes that it irritates mucous membranes and skin.² To be clear, myrosinase is the enzyme that makes the hot compound, not the hot compound itself.

Cooking changes everything. Katzer notes that boiling knocks out the enzymes needed to form the pungent taste, and that heating quickly destroys any isothiocyanate already formed. Seeds fried in fat or dry-toasted are no longer pungent but take on a nutty, smoky taste.³

For the practical side of controlling mustard’s heat with water temperature and vinegar, see mustard powder.

How black mustard seeds are used

In South Asian cooking

Katzer describes frying the whole seeds in fat as a typical South Indian technique, and notes that they often turn up in Tamil vegetable curries alongside curry leaves. Black mustard seed is also part of the Bengali spice mix panch phoron, and West Bengal and Bangladesh are, in Katzer’s words, the epicenter of mustard oil use. The seeds also go into pickled raw vegetables, and in Andhra Pradesh a paste of ground seed goes into the mango pickle avakaya.³

Here’s how it usually goes: heat oil until it shimmers, add the seeds, cover the pan loosely, and wait for the popping to slow before adding curry leaves, chilies or whatever’s next. Seeds that aren’t fried long enough stay bitter and sharp. Seeds fried too long burn.

As mustard

Finely ground black mustard seed makes mustard meal, an odorless powder that becomes table mustard when mixed with vinegar. The condiment’s name comes from an early recipe: mixing must, unfermented grape juice (Latin vinum mustum), into the crushed seed.⁵ Katzer credits the Romans with making mustard seed popular in Central and Western Europe and with mixing it with must.

For more ideas, see mustard seed uses.

The mustard seed of the Gospels

In Luke 13:19, the King James Version says the mustard seed “waxed a great tree,” and most modern translations say it “became a tree.”⁶ You’ll often read that this mustard was black mustard and that black mustard grows over ten feet tall. We couldn’t confirm either point. PROSEA gives black mustard’s height as 0.5–1.5 m (about 1.5 to 5 feet),² and Katzer argues that reading it as a tree is hard to defend for an annual herb whose tender branches can’t hold birds.³ Which plant the passage meant is a question for biblical scholars, and we’re not settling it here.

Buying and storage

Look for whole seeds that are small, round, evenly dark and free of dust and debris. True black mustard seed is about 1 mm across.

The hot compound only forms after the seed is crushed and wet, so the smell of dry seed tells you little about its strength. We found no published shelf-life figure for whole black mustard seed. Store it airtight, dry and away from heat. To check an old jar, crush a few seeds with a drop of cool water and sniff after a minute or two.

Nutrition

PROSEA gives this composition for black mustard seed, per 100 g edible portion: water 8 g, protein 29 g, fat 28 g, carbohydrates 19 g, fiber 11 g, and iron 21 mg.² You use seeds by the teaspoon, so they don’t add much to a day’s diet.

Claims that black mustard is a notable source of zinc, magnesium, carotenes, lutein or zeaxanthin couldn’t be traced to a source. We’re not claiming black mustard seed treats or prevents cancer, high cholesterol or any other disease.

Safety

• Allergy. Mustard is one of the 14 allergens food businesses in the UK have to declare.⁷ It’s not one of the nine major food allergens under US law, which are milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat, soybeans and sesame.⁸ US allergen labeling rules apply to those nine, so mustard isn’t covered by them.⁸
• Skin and eyes. Allyl isothiocyanate irritates mucous membranes and skin.² Wash your hands after handling crushed, wet seed or homemade mustard paste.
• Erucic acid (EU). EU law caps erucic acid at 35 g/kg in mustard as a condiment and 50 g/kg in mustard oil, with an exemption for mustard oil produced and eaten locally.⁹
• Folk remedies. PROSEA records mustard plasters and seed teas as traditional remedies.² We don’t recommend them.

Substitutes and comparisons

Brown mustard seed is the closest substitute. Katzer treats the two as equally useful in the kitchen, with black slightly hotter. Yellow mustard seed is milder and doesn’t stand in well for frying. Ratio (Method): Start by swapping brown for black one for one, and taste.

• Black mustard seed substitutes
• Brown mustard seeds and yellow mustard seeds
• Yellow vs. black mustard seeds
• Mustard powder and mustard powder vs. mustard seed
• Panch phoron