Why are chili peppers hot?
Their capsaicin triggers the body’s heat sensors, so the brain reads it as “hot” and “painful.”
43°C
Take one bite of a blazing curry or mapo tofu, and sweat beads on your forehead as your mouth seems to catch fire. In English we call spicy food “hot,” and Japanese speaks of a heat “fierce enough to breathe fire.” But put a thermometer in your mouth and you will find the temperature has barely changed.
So where does that burning sensation come from? Follow the trail, and it turns out our bodies are being neatly fooled by the chili pepper.
Spiciness isn’t a taste
Sweet, salty, sour, bitter and umami: these are the five basic tastes detected by the taste buds on your tongue, and spiciness is not among them. Spiciness is less a taste than a sensation, closer to pain or heat.
The compound that makes chili peppers hot is capsaicin. At the ends of the nerves in your mouth sits a sensor called TRPV1. Its real job is to respond to heat above about 43°C (109°F) and send the brain a signal that says “Hot! Danger!” Capsaicin, however, latches onto this sensor and flips the switch even when there is no heat. The brain takes the incoming signal at face value and concludes that your mouth is burning. Sweating and turning red in the face are also thought to be the body’s attempts to shed heat.
The sensor was identified in 1997 by a research team led by the American physiologist David Julius. For this and other work, Julius received the Nobel Prize in Physiology or Medicine in 2021. An everyday food, the chili pepper, became the key to unlocking how humans sense heat and pain.
The heat is in the pith, not the seeds
When cooking with chilies, people often say that removing the seeds tones down the heat. But it is not the seeds themselves that make capsaicin. It is especially concentrated in the white ribs that hold the seeds inside the pod, a part botanists call the placenta. Seeds taste hot because capsaicin rubs off onto them from the placenta right next to them.
The strength of a pepper’s heat is expressed in Scoville units, a scale devised in 1912 by the American pharmacist Wilbur Scoville. The method diluted a chili extract with sugar water and counted how many times it had to be diluted before people could no longer detect any heat. Today, it is more common to measure the amount of the compound by machine and convert it, and pure capsaicin is said to reach a staggering value of around 16 million.
Birds don’t mind a bit
Curiously, birds barely feel the heat of chili peppers at all. A 2002 study of chicken TRPV1 revealed that the birds’ sensor responds to heat but hardly responds to capsaicin.
A leading theory holds that this is tied to the chili pepper’s survival strategy. Mammals such as rodents chew the fruit and crush the seeds along with it, but birds swallow the seeds whole, carry them far away and drop them in their droppings. The heat drives off the mammals alone, while letting the birds, those skilled seed-sowers, eat their fill. In 2001, an experiment using wild chili peppers reported results supporting this idea.
Why water won’t put out the fire
Gulping water after eating something spicy doesn’t bring much relief. That is because capsaicin dissolves readily in oil but hardly at all in water. Rinsing your mouth with water does little to wash away the compound clinging to your sensors.
When you want to cool the burn, milk or yogurt is said to work well. The fat and proteins in dairy are thought to wrap around the capsaicin and pull it away. Serving lassi alongside Indian food turns out to be a rather sensible pairing.
Still, why do people go out of their way to seek a heat they know will hurt? One theory suggests it is a thrill like riding a roller coaster—enjoying a “fear” we know is safe—but no clear answer has emerged yet.