Mustard oils

The heat that goes up your nose: mustard, horseradish and wasabi

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Mustard, horseradish and wasabi get their heat from isothiocyanates, the mustard oils. They form only when the plant's cells are damaged, and they trigger TRPA1, the receptor for irritant chemicals.

What it is¶

"Mustard oils" are not the pressed oil sold for cooking, but a family of volatile, pungent compounds called isothiocyanates. The most important in the kitchen is allyl isothiocyanate (AITC), the source of the eye-watering heat of brown and black mustard, horseradish and wasabi. Related isothiocyanates give the pepperiness of rocket, radish, watercress and nasturtium, all members or near relatives of the cabbage family, Brassicaceae.

A two-part chemical defence¶

The plant does not store mustard oil ready-made. It keeps two separate ingredients in different compartments of its cells:

a glucosinolate, a sulphur-containing sugar compound that is not pungent. In black and brown mustard, horseradish and wasabi the main one is sinigrin.
an enzyme, myrosinase, which can split it.

When an insect chews the leaf — or a cook grinds a seed or grates a root — the compartments break, the two meet in the presence of water, and myrosinase cuts the glucosinolate apart to release the isothiocyanate. The plant's defence is, in effect, a bomb that assembles itself on impact.

🔬 The TRPA1 receptor

Allyl isothiocyanate activates TRPA1 (transient receptor potential ankyrin 1), an ion channel on pain-sensing nerves that responds to a wide range of reactive, irritating chemicals, including the compounds in tear gas and wood smoke. AITC reacts directly with the channel protein, switching it on. TRPA1 was identified as the receptor for mustard oil and wasabi in 2004 by researchers including David Julius's and Ardem Patapoutian's groups, and is sometimes called the "wasabi receptor".

Unlike capsaicin, AITC is highly volatile. It evaporates at room temperature and travels up the back of the throat into the nose, which is why mustard and wasabi produce a short, sharp rush through the sinuses rather than a lingering burn on the tongue. It also fades fast: the sensation peaks in seconds and is usually gone within a minute.

Brown, black and white mustard¶

The species of mustard decides the mustard oil.

Mustard	Glucosinolate	Product	Character
Black (Brassica nigra)	Sinigrin	Allyl isothiocyanate	Fiercest; volatile, nose-clearing
Brown (Brassica juncea)	Mostly sinigrin	Allyl isothiocyanate	Hot; the basis of Dijon and English mustards
White or yellow (Sinapis alba)	Sinalbin	4-hydroxybenzyl isothiocyanate	Milder, not volatile, heat stays on the tongue

American yellow mustard is made mostly from white mustard, which is why it is mild. English mustard powder combines brown and white seed, usually with wheat flour and turmeric for colour.

The chemistry in the kitchen¶

Because the heat is made by an enzyme, the conditions under which mustard is mixed decide how hot it gets.

Water starts the reaction. Dry mustard powder is not hot. Mixed with water it develops full heat in about 10 to 15 minutes.
Cold water gives the hottest result. Myrosinase is damaged by heat; mixing mustard powder with boiling water produces a weaker, more bitter paste.
Acid slows the enzyme. Adding vinegar, wine or verjuice at the start gives a milder, more stable mustard. Adding it after the heat has developed "fixes" a hotter one. Horseradish follows the same rule: grate, wait a couple of minutes for the heat to build, then stir in vinegar.
Cooking drives it off. AITC is volatile and breaks down with heat, so mustard stirred into a sauce at the start mellows markedly. Add it at the end for bite.
Whole seeds fried in oil do not burn. In Indian tempering, mustard seeds popped in hot oil develop a nutty, mild flavour: the oil is dry and hot enough to disable myrosinase before it can work.

💡 Fresh wasabi

Real wasabi root is grated very finely, traditionally on sharkskin, just before serving. Its heat and sweetness peak within a few minutes and fade noticeably within about a quarter of an hour. Most "wasabi" sold outside Japan is horseradish with mustard and green colouring — pleasant, but a different thing.

Mustard oil, the pressed cooking oil used in Bengal and Punjab, contains some AITC, which gives it its pungency; it is traditionally heated to smoking and cooled before use to soften the sharpness.

Safety¶