Allyl isothiocyanate

Also written AITC, mustard oil, allyl-isothiocyanate, isothiocyanates

Organosulfur compoundC4H5NSTrigeminal sensationAroma

Quick answer

The volatile sulfur compound behind mustard, horseradish and wasabi — a pungency that goes up the nose rather than into the tongue, and that does not exist until the plant is damaged.

What it does

The sensation is sharp, fast, and felt in the nasal passages and behind the eyes rather than on the tongue. It arrives within seconds, peaks almost immediately, and is gone in under a minute, which is exactly the opposite of the slow build and long decay of chilli. Because the compound is volatile, most of the effect is delivered through the back of the nose rather than by contact with the tongue at all — which is why holding the mouth closed intensifies it and why a mouthful of air disperses it. It carries its own aroma alongside the sting: pungent, green, faintly cabbage-like.

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How it works

The physiological account, where one is established.

Allyl isothiocyanate irritates sensory nerve endings through a different receptor system from the one capsaicin acts on — the population responsive to reactive and irritant chemicals rather than to heat. That is the physiological reason the sensations feel categorically different rather than merely differing in strength, and the reason tolerance to chilli confers none to mustard.

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How it behaves when cooked

Four properties of the molecule, and what they mean at the stove.

Volatility: high
Fat solubility: moderate
Water solubility: moderate
Heat stability: low

This is the most fragile pungency in the cupboard and every rule about these three plants follows from that. It must be generated before it can be tasted: the intact seed or root holds a stable precursor and an enzyme in separate compartments, and only crushing, grating or chewing brings them together. It then begins to disappear immediately, so grated horseradish and wasabi are made to order and are noticeably weaker within half an hour. Heat destroys it outright, which is why mustard stirred into a hot sauce loses its bite within a minute and must be added off the heat, why cooked mustard tastes of the seed rather than the sting, and why no amount of horseradish will survive a bake. Acid slows the reaction and stabilises what has formed, which is the working principle behind vinegar in prepared mustard, and cold water at the mixing stage gives a hotter result than hot water, which inactivates the enzyme before it has finished.

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Not present until the plant is damaged

Sinigrin + Myrosinase

The intact tissue stores the glucosinolate sinigrin apart from the enzyme myrosinase. Physical damage brings the two into contact in the presence of water, and the enzyme cleaves the glucosinolate to release allyl isothiocyanate. This is a defensive mechanism of the plant, not a flavour the plant possesses, and it is why an undamaged mustard seed or a whole horseradish root smells of almost nothing.

Spices that carry it

Recorded on the compound rather than on each spice, so the two cannot disagree.

HorseradishPrincipal component
Mustard seedPrincipal component
Brown and black mustard yield allyl isothiocyanate; yellow mustard yields a related but non-volatile compound instead, which is why yellow mustard stings the tongue rather than the nose and why the two are not interchangeable where the sensation is the point.
WasabiPrincipal component
True wasabi carries a broader mixture of isothiocyanates than horseradish, which is why its pungency reads as sweeter and less purely brutal — and why the horseradish-and-colouring sold as wasabi is a recognisably different sensation rather than merely a cheaper one.

Not to be confused with

Compounds a reader is likely to take for this one, and why they differ.

Allicin
Both are sulfur compounds generated by enzyme action on a stored precursor when the plant is damaged, and the parallel is genuine. They are structurally unrelated and taste nothing alike.

Safety

What the concern is. The dose, the population and the evidence level live on the spice profiles, which is where a reader with that question actually arrives.

The concentrated compound is a strong irritant to skin, eyes and airways, and grating fresh horseradish or wasabi in a confined space is unpleasant for that reason. At the exposures produced by eating, the effect is transient irritation rather than harm.