What Makes Chillies Hot?

Your mouth contains receptors called TRPV1, whose normal job is to signal temperatures above about 42 degrees Celsius, along with certain kinds of chemical damage. When they fire, the brain interprets it as burning.

Capsaicin binds directly to those receptors and activates them at ordinary temperature. The signal reaching your brain is identical to the one from a hot drink, so you sweat, your face flushes and your eyes water. It is a genuine sensation caused by a false alarm.

Capsaicin is fat soluble and barely dissolves in water. Drinking water spreads it around your mouth without removing much of it, which is why the relief lasts about two seconds.

What works is something that dissolves or binds it. Milk is the classic answer for two reasons: it contains fat, and it contains casein, a protein that helps lift capsaicin away from the receptors. Yogurt, ice cream, and oily or starchy foods such as rice and bread also help.

The Scoville scale was devised in 1912 and originally measured how much sugar water was needed to dilute a chilli extract until a panel of tasters could no longer detect heat. It was subjective and inconsistent between panels.

Modern measurement uses liquid chromatography to quantify capsaicin directly, then converts to Scoville units for familiarity. Rough figures:

• •Bell pepper, zero
• •Jalapeno, roughly 2,500 to 8,000
• •Cayenne, around 30,000 to 50,000
• •Habanero, around 100,000 to 350,000
• •The hottest cultivated varieties, over 2 million

Capsaicin is a defence, and a strikingly targeted one. Mammals have TRPV1 receptors that respond to it, so they find chillies painful and avoid eating the fruit. Birds do not respond to capsaicin at all.

That distinction is useful to the plant. Mammal teeth destroy seeds, while birds swallow them whole and disperse them intact over long distances. The chilli evolved a chemical that deters exactly the wrong seed carriers and ignores the right ones.

Repeated exposure genuinely reduces sensitivity. Frequent capsaicin contact causes the nerve endings to become less responsive over time, which is why people who eat spicy food regularly can tolerate far more than they once could.

This effect is used medically. Capsaicin creams and high strength patches are prescribed for certain kinds of nerve pain, working by deliberately exhausting the pain signalling in the treated area.

Most people believe the seeds are the hot part, and it is close but not quite right. Capsaicin is produced in the glands of the white pith, the spongy membrane the seeds attach to.

The seeds pick up heat only by contact with that membrane. This is why removing the pith reduces heat far more effectively than removing the seeds alone, and why a chilli can be used for flavour with much of its heat taken out. Handle hot varieties with gloves, since capsaicin transfers readily to skin and then to eyes long after washing.

Other pungent foods use different molecules and sometimes different receptors. Black pepper owes its bite to piperine, which acts on the same heat receptor as capsaicin, though more weakly. Mustard, horseradish and wasabi work differently. Their sharpness comes from compounds called isothiocyanates, which form when the plant tissue is crushed and activate a separate receptor called TRPA1. These compounds evaporate easily, which is why wasabi rushes up your nose and fades quickly, while chilli heat stays in the mouth and lingers, because capsaicin barely evaporates at all.

Mint plays the same trick in reverse. Menthol activates TRPM8, a receptor that normally responds to cool temperatures, so mint feels cold even at room temperature. The discovery of these temperature sensors earned the 2021 Nobel Prize in Physiology or Medicine, shared by David Julius and Ardem Patapoutian. Julius's team used capsaicin as a chemical key, searching for the gene that made cells respond to chilli, and so identified TRPV1. Food chemistry, in other words, helped scientists uncover how the body senses temperature and pain.

For much of the twentieth century, doctors widely believed that stomach ulcers were caused by stress, acid and rich or spicy food, and patients were told to eat bland diets. Two Australian researchers, Barry Marshall and Robin Warren, challenged this in the early 1980s by showing that a bacterium, Helicobacter pylori, was present in most ulcer patients. Marshall famously drank a culture of the bacteria and developed stomach inflammation himself. Their work, rewarded with the Nobel Prize in 2005, showed that infection and regular use of painkillers such as aspirin and ibuprofen cause most ulcers.

Chillies are therefore not a cause of ulcers, though they can still make some people uncomfortable. Capsaicin can worsen heartburn or irritate an already sensitive gut, and some people with digestive conditions find that spicy meals aggravate their symptoms. The familiar burning sensation the next day has a simple explanation too. Heat-sensing TRPV1 receptors are found along the digestive tract, not just in the mouth, and capsaicin that survives digestion can trigger them again at the far end. The feeling is unpleasant, but like the burn in the mouth it is a signal rather than damage.

Chillies are native to the Americas, and people in what is now Mexico were growing them thousands of years ago. When Columbus reached the Caribbean in 1492 he was looking for a sea route to the spice lands of Asia, and he called the fiery fruit he found there a kind of pepper, a name that has caused confusion ever since. Chillies and black pepper are unrelated plants. Black pepper is the dried berry of a climbing vine from southern India, while chillies belong to the same family as tomatoes and potatoes.

What happened next was astonishingly fast. Portuguese and Spanish traders carried chilli seeds along their routes to Africa, India and Southeast Asia, and the plants grew easily in warm climates, so farmers could produce their own heat instead of buying expensive imported pepper. Within a century or two, chillies had become central to cooking in places as far apart as India, Thailand, Korea and Sichuan in China. Dishes that now seem ancient and inseparable from their national cuisines, from vindaloo to the red kimchi familiar today, depend on an ingredient that arrived only a few hundred years ago.

Since capsaicin triggers a pain signal, it is fair to ask why so many people seek it out. The psychologist Paul Rozin, who studied chilli eating in Mexico and the United States, proposed an idea he called benign masochism. The burn registers as danger, but the mind knows that no real damage is being done, so the experience becomes a safe thrill, in the same family as a roller coaster ride, a frightening film or a very hot bath. Keen chilli eaters often enjoy pushing right to the edge of what they can stand, which fits this explanation well.

A liking for chilli is also learned. Young children in cultures where chillies are common usually dislike the burn at first and come to enjoy it gradually, helped by watching adults and by meals where small amounts are added to familiar foods. Other mammals rarely make that shift. Attempts to train laboratory rats to prefer chilli flavoured food have largely failed, which suggests the human taste for the burn depends on social learning and on our ability to enjoy a sensation precisely because we know it is harmless. The body may also release its own pain-relieving chemicals during the burn, though how much this contributes is debated.

Chillies are hot because capsaicin activates the receptor that detects burning heat, water cannot wash it away because it is fat soluble, and the plant evolved it to deter mammals while leaving birds unaffected.