Capsaicin

The molecule that makes chillies hot, and why the heat is an illusion

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Capsaicin is the main compound behind the heat of chillies. It triggers the TRPV1 receptor, the same sensor that detects scalding heat, so the mouth reports a burn that is not there.

What it is¶

Capsaicin is a colourless, odourless, waxy compound made by plants of the genus Capsicum — the chillies. Chemically it is an amide: a vanillyl group (the same ring structure found in vanilla's vanillin) joined to a fatty-acid-like tail. It is the best known of a small family called the capsaicinoids; its close relative dihydrocapsaicin is roughly as pungent, and together the two account for most of the heat in most chillies. Minor capsaicinoids such as nordihydrocapsaicin, homocapsaicin and homodihydrocapsaicin make up the rest and differ slightly in where and how long they burn.

Pure capsaicin rates about 16 million Scoville heat units on the-scoville-scale, which is far beyond any chilli: even the hottest varieties, such as the carolina-reaper, contain only a modest percentage of capsaicinoids by dry weight.

Where it is made in the fruit¶

Capsaicinoids are made in glands on the placenta — the pale ribs, or pith, to which the seeds are attached. The seeds themselves contain little capsaicin but sit against the placenta and pick it up, which is why they have a reputation they have not quite earned. Removing ribs and seeds together takes out much of the heat of a fresh chilli; removing only the seeds does less than most cooks expect. In some very hot varieties capsaicinoid glands extend into the flesh, so de-ribbing a habanero or ghost-pepper tames it only a little.

Heat varies within a single variety with growing conditions: stress from heat, drought and bright sun tends to raise capsaicinoid content, which is why the same seed can give a mild plant one year and a fierce one the next (see growing-chillies).

How it works¶

🔬 The TRPV1 receptor

Capsaicin binds to TRPV1 (transient receptor potential vanilloid 1), an ion channel on pain- and heat-sensing nerve endings in the mouth, eyes and skin. TRPV1's normal job is to open at temperatures above roughly 43 °C — the threshold at which heat begins to damage tissue. When capsaicin binds, the channel opens at body temperature, calcium and sodium ions flood in, and the nerve fires exactly as it would for real heat. The brain has no way to tell the difference.

The receptor was cloned in 1997 by David Julius's laboratory at the University of California, San Francisco, using capsaicin as the key to find it. The work contributed to Julius's share of the 2021 Nobel Prize in Physiology or Medicine, awarded jointly with Ardem Patapoutian for discoveries of receptors for temperature and touch — several of which are the subject of pages on this site.

Because the burn is a nerve signal rather than a scald, capsaicin at culinary doses does not damage the lining of the mouth. The body responds as though it were overheating anyway: flushing, sweating (especially on the face and scalp), a running nose and watering eyes are all reflexes triggered by the false alarm. Very high doses from extract-based products are another matter; see extreme-chilli-risks.

Capsaicin is not a taste. It does not act on taste buds, and the tongue's taste receptors are unaffected. It is detected by the trigeminal nerve, the same system that registers the prickle of fizzy drinks, the cool of menthol and the sting of mustard. Food scientists call these sensations chemesthesis.

Why chillies make it¶

The usual explanation is directed deterrence. Mammals chew seeds and destroy them; birds swallow them whole and scatter them, often far from the parent plant. Birds' version of TRPV1 is insensitive to capsaicin, so a bird eats chillies without discomfort while a rodent avoids them. Field studies on wild chillies have also found that capsaicin inhibits a fungus that attacks the fruit. The two ideas are not exclusive, and the deterrence story is generally accepted as the main one.

Chemistry in the kitchen¶

Capsaicin's physical properties explain most kitchen advice about chillies.

Property	What it means for cooks
Fat-soluble, barely water-soluble	Water spreads the burn around; full-fat milk, yoghurt or coconut milk carry it away better
Soluble in alcohol	Strong spirits extract it well; weak beer is mostly water and does little
Stable to heat	Cooking does not destroy it, so long simmering spreads heat through a dish rather than reducing it
Not volatile at room temperature	Whole chillies do not smell hot, but frying or charring them throws capsaicin into the air as a stinging vapour
Clings to skin	It is oily and hard to wash off with water alone, hence gloves (see handling-chillies-safely)

Dairy is often credited to the milk protein casein, which is thought to help lift capsaicin off receptors, but fat content matters too; see fat-and-flavour and cooling-a-dish-thats-too-hot. Sugar and acid take the edge off perceived heat without removing the capsaicin, which is why hot-sauce is so often sweet, sour or both.

Capsaicin also survives drying unchanged, so dried chillies such as chile-de-arbol and powders such as cayenne-pepper keep their heat far longer than their aroma.

History¶

Capsaicin was first extracted in impure form in 1816 by the German chemist Christian Friedrich Bucholz. The English pharmacist John Clough Thresh obtained it in a much purer crystalline form in 1876 and gave it the name capsaicin. Its structure was worked out by E. K. Nelson in 1919, and it was first synthesised in 1930. The quest to measure it produced the-scoville-scale in 1912 and, much later, the laboratory methods that replaced it.

Tolerance and repeated exposure¶

Regular chilli eaters really do feel less burn. Repeated exposure desensitises TRPV1-bearing nerve endings, and the effect persists for a time with continued use. The detail — and why the first bite of a meal can feel milder than the fifth — is covered in spice-tolerance.