Allyl isothiocyanate
Also written AITC, mustard oil, allyl-isothiocyanate, isothiocyanates
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.
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.
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.
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.
- Garlic mustardMajor component
Released from sinigrin in crushed leaf, together with allyl thiocyanate and an epithionitrile: garlic mustard turns its sinigrin into an unusually broad range of products.
What cooking and processing were measured to do to it
Findings from studies in which this compound was followed through heat, fat, water, drying or storage. Each belongs to its study and its conditions.
Simmering and boiling · Converts Allyl isothiocyanate, Diallyl sulfide, Diallyl disulfide, Diallyl trisulfide, Allyl thiocyanate, N,N'-Diallylthiourea
Mustard seed, Horseradish, Wasabi
Allyl isothiocyanate boiled in water for an hour breaks down. The products that could be extracted from the water included diallyl sulfide, disulfide, trisulfide and tetrasulfide, the same family of compounds found in cooked garlic, together with allyl thiocyanate; the major product left in the water was N,N′-diallylthiourea.
Conditions: in water · 100 °C under reflux · 1 h · Pure allyl isothiocyanate in water
The sharpness of brown mustard, horseradish and wasabi is lost to long wet cooking by chemical breakdown as well as by evaporation, and the breakdown products are sulfur compounds of the kind that smell of garlic.
What this does not show: Read in the abstract only, which gives no yields. The pure compound in water for a full hour under reflux, where vapour is returned to the flask; in an open pan much of it would leave as vapour first.
Measured on the pure compound
Acid and alkali · Converts Allyl isothiocyanate, Allylamine, Carbon disulfide, Allyl dithiocarbamate, Diallylthiourea, Allyl thiocyanate
Mustard seed, Horseradish, Wasabi
Held at 80 °C in buffered water, allyl isothiocyanate decomposed at every acidity tested, to different products. In alkaline buffer the major products were allylamine, allyl dithiocarbamate, diallylthiourea and carbon disulfide; at pH 6 and pH 4 carbon disulfide and allylamine were the major ones. It also isomerised in part to allyl thiocyanate.
Conditions: in water · 80 °C · pH Buffers of pH 4, 6 and 8 · Allyl isothiocyanate at 1000 mg/kg
What this does not show: Read in the abstract only, which names the products and not the rates, so it does not show whether acid slows the loss. A buffer model at a single temperature.
Measured on the pure compound
Grinding · Increases Allyl isothiocyanate
Ground mustard seed meal released allyl isothiocyanate when it took up moisture from humid air alone, without being wetted. Both the rate and the amount released rose with relative humidity and with temperature between 5 and 35 °C, and the rate could be altered through the particle size and the fat content of the powder. Between 2 and 17 mg were released per gram of powder within 24 h.
Conditions: in air · 5, 20 and 35 °C · Release followed over 24 h · Meal ground to powders of 5 to 300 µm · Defatted and partly defatted meal in sealed jars at 85 or 100 per cent relative humidity
Water is a reagent in making mustard’s heat, not just a carrier for it, and even damp air supplies enough to start the reaction in ground seed. A mustard powder kept dry is keeping its pungency unspent.
What this does not show: Read in the abstract only. A packaging study on defatted meal, aimed at antimicrobial use, and the abstract does not say in which direction particle size or fat content moved the rate.
Measured on the spice
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.
Questions this page answers
- what is allyl isothiocyanate
- what does allyl isothiocyanate do
- which spices contain allyl isothiocyanate
- what is aitc
- what is mustard oil