Why Are Chilli Peppers Hot (and How Do You Cool the Burn)?

Chilli peppers are hot because they contain capsaicin, a molecule that switches on the same nerve receptor your mouth uses to detect painful heat, so your brain feels a burn even though nothing is actually burning. Water won't cool it because capsaicin doesn't dissolve in water. Milk, yogurt and other fatty or protein-rich foods work much better because they pull capsaicin away from those receptors.

What makes chilli peppers hot?

The heat comes from a family of compounds called capsaicinoids. The two main ones, capsaicin and dihydrocapsaicin, together make up the large majority of the heat in most chillies. Pure capsaicin is a waxy, colourless solid with almost no smell, yet a single drop of a solution diluted to a few parts per million is still noticeably hot.

Chillies make capsaicin as a defence, and the target audience is fascinating. Mammals like rodents chew seeds and destroy them, and capsaicin discourages them. Birds swallow chillies whole and spread the seeds intact, and birds don't feel the burn at all because their version of the heat receptor isn't sensitive to capsaicin. There's also evidence that capsaicin helps protect the fruit from fungi. Humans are the odd mammals who decided the pain was worth it.

Why does capsaicin feel like burning?

Your mouth, eyes and skin are lined with sensory nerve endings that carry a receptor called TRPV1. Its everyday job is to detect dangerous heat: it opens when tissue reaches about 43°C (109°F), lets ions flood into the nerve, and fires a "hot, ouch" signal to your brain.

Capsaicin fits neatly into TRPV1 and opens it at body temperature. The nerve fires exactly as it would for scalding heat, and your brain has no way to tell the difference. That's why chillies make you sweat, flush, tear up and reach for a drink. Your body is responding to a fire that isn't there. The scientist who identified TRPV1 by studying capsaicin, David Julius, shared the 2021 Nobel Prize for the work.

This also explains a couple of everyday quirks:

• Hot food feels spicier than cold food. Real heat and capsaicin both push on TRPV1, so a steaming curry feels more fiery than the same curry eaten cold.
• It's not a taste. Chilli heat travels through pain and temperature nerves, not taste cells. That's why you can feel it on your lips, in your eyes, or on your fingers, which have no taste buds at all. Scientists call this kind of sensation chemesthesis.

Why do some people love the burn?

Repeated exposure to capsaicin makes TRPV1 nerves less responsive, so regular chilli eaters genuinely feel less pain from the same pepper. The body's response to the alarm, including a rush of adrenaline and endorphins, may add to the appeal. Psychologists describe the enjoyment as a kind of "benign masochism": the thrill of a scary sensation you know is safe, a bit like a roller coaster.

Chef Fizz says: A chilli is a smoke alarm with a sense of humour. Lots of noise, no actual fire.

Which part of the chilli is hottest?

Most people blame the seeds, but capsaicin is actually made in the placenta, the pale, spongy ribs inside the pepper where the seeds attach. The seeds pick up heat by sitting against those ribs. The flesh near the tip, furthest from the stem and ribs, is usually the mildest part.

That gives you a simple way to control heat:

• Keep the ribs and seeds for maximum fire.
• Scrape out the ribs and seeds with a teaspoon to cut most of the heat but keep the flavour of the flesh.
• Use whole, uncut chillies in a stew and pull them out before serving for a gentler warmth.

Heat also varies from pepper to pepper, even on the same plant. Stress from heat and dry weather can make chillies hotter. Always taste a tiny piece before tipping a whole one into a dish.

How is chilli heat measured?

The Scoville scale dates back to 1912, when pharmacist Wilbur Scoville dissolved chilli extract in sugar water and asked tasters how far it had to be diluted before they couldn't feel the heat. Today labs measure capsaicinoids directly with chromatography and convert the result to Scoville Heat Units (SHU).

The jump between the rows is huge. A habanero can be 40 times hotter than a jalapeño, and a superhot can be hundreds of times hotter. Those top-end peppers can cause genuine distress, including vomiting and, in rare cases, severe headaches, so they're not for children or casual dares.

Why doesn't water cool the burn?

Capsaicin is an oily molecule. It dissolves well in fat, oil and alcohol, but hardly at all in water. Gulping water mostly sloshes capsaicin around your mouth and spreads it to new nerve endings. The cold brings brief relief, then the burn comes back.

What actually helps:

• Milk. In taste studies comparing drinks, milk came out on top. Interestingly, skim and whole milk performed similarly, which suggests casein, the main milk protein, matters at least as much as fat. Casein seems to bind capsaicin and help carry it away from the receptors.
• Yogurt, raita, sour cream, lassi. Same casein-plus-fat combination, which is why so many spicy cuisines serve dairy alongside.
• Something sweet. Sugary drinks did better than water in the same studies. Sugar may help by distracting the brain with a pleasant taste.
• Starch. Rice, bread and tortillas act like a mop, physically absorbing and wiping away capsaicin.

Beer is mostly water with only around 5% alcohol, nowhere near enough to dissolve much capsaicin, and its fizz can add its own prickle. A cold lager feels nice but isn't a real fix.

The same chemistry matters on your hands. Rinsing chilli-coated fingers in plain water barely works. Rub them with a little cooking oil first, then wash with dish soap, which breaks up oily residues. Better still, wear gloves when cutting hot chillies, and never touch your eyes or contact lenses afterwards.

Want to master heat on purpose? The Cooking Scientist app has short lessons on chilli heat, rescuing a too-spicy dish and how chilli heat differs from black pepper, each with a quick mini game.

Is black pepper heat the same as chilli heat?

Not quite, though they overlap. Several kitchen ingredients trigger pain nerves, but they use slightly different receptors and feel different as a result.

• Chilli (capsaicin) works mainly on TRPV1. It builds slowly, lingers and spreads across the whole mouth.
• Black pepper (piperine) hits TRPV1 and its cousin TRPA1. It's sharper and quicker, then fades sooner.
• Ginger (gingerols, shogaols) gives a warm, rounded heat that builds as dried or cooked ginger develops shogaols.
• Mustard, wasabi and horseradish (allyl isothiocyanate) trigger TRPA1. These compounds are volatile, so they rush up your nose in a sharp, sinus-clearing blast and vanish within seconds.

Knowing the difference helps when building heat. Capsaicin is stable to heat and dissolves into fat, so frying chillies in oil spreads their heat through a whole dish. That's also why toasting and blooming spices in oil gives such a deep, even warmth. Wasabi and mustard, by contrast, fade quickly once mixed or heated, so they're usually added at the end.

How do you fix a dish that's too spicy?

There's no way to remove capsaicin once it's in, but you can dilute and soften it:

• Make more of everything else. Add more of the unspiced base, such as broth, tomatoes, beans or vegetables, to lower the capsaicin per bite.
• Add dairy or coconut milk. Stir in cream, yogurt or coconut milk. If adding yogurt to a hot curry, temper it first with a little hot sauce and add it off the heat so it doesn't split. See why cream curdles in sauce.
• Add fat and sweetness. Peanut butter, a spoon of sugar or honey, or a little butter can round off harsh heat.
• Serve with cooling sides. Plain rice, flatbread, cucumber and raita give every mouthful a break.

Try it at home

The rib-versus-tip test proves where the heat lives. You'll need one jalapeño, a knife and gloves.

1. Wearing gloves, cut the jalapeño in half lengthways.
2. Slice a tiny sliver, about the size of a grain of rice, from the flesh at the very tip, away from any rib.
3. Slice an equally tiny sliver from a pale rib near the stem.
4. Taste the tip sliver first. Wait until your mouth is completely back to normal.
5. Then taste the rib sliver, with a glass of milk nearby.

The tip will taste green, grassy and mild. The rib will be noticeably hotter, sometimes several times so. For a bonus test, sip water after one bite and milk after another, and notice which brings the burn down faster. Wash your hands well afterwards and keep them away from your eyes.

Common mistakes

• Drinking water to put out the fire. Capsaicin doesn't dissolve in water, so it just spreads. Reach for milk, yogurt or a spoonful of rice instead.
• Cutting hot chillies bare-handed. Capsaicin soaks into skin oils and can burn for hours, especially if you then touch your eyes. Wear gloves, or wash with oil and dish soap.
• Assuming all chillies of one type are equally hot. Two jalapeños can differ several-fold. Taste a sliver before adding the whole pepper.
• Removing only the seeds. The ribs hold most of the heat. Scrape out both to really tame a pepper.
• Frying dried chillies over high heat with no ventilation. Capsaicin gets carried into the air with the smoke and can make the whole kitchen cough. Use moderate heat and turn on the extractor fan.
• Adding cold yogurt straight to a boiling curry to cool it. The acid and heat can make it curdle. Temper the yogurt first and stir it in off the heat.