What Makes Chillies Hot?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The heat comes from a molecule called capsaicin, which binds to the receptor your body uses to detect burning heat. Nothing is actually burning, and your mouth is not being damaged. Your nervous system is simply being given false information, and it responds exactly as it would to genuine heat.
At the receptor level
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.
Why water does not help
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
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
Why plants make it
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.
Building tolerance
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.
Where the heat is concentrated
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.
The takeaway
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.