Chilli heat is a trick played on your heat sensors

Capsaicin, the compound that makes chillies hot, doesn't burn anything. It switches on the same nerve sensor that warns you about real heat.

Bite into a hot chilli and your mouth reports a fire. Nothing is actually burning. You are getting a false alarm from a sensor that is working exactly as it should.

The compound behind it is capsaicin. It is concentrated in the pale pith inside the pepper, where the seeds are attached; the seeds themselves make none. In the nerve endings of your mouth and skin sits a protein called TRPV1, a tiny channel in the surface of the nerve cell. Its usual job is to open when tissue gets hot enough to be harmed, above about 43 °C. When it opens, charged particles flow into the nerve, the nerve fires, and the brain gets the message: this is burning.

Capsaicin binds to TRPV1 and opens it with no heat at all. The brain has no separate line for chilli. It only learns that the burning sensor has gone off, so it reports heat.

Finding the sensor

The sensor was identified in 1997 by the lab of David Julius. His team took genes from pain-sensing nerve cells, put them into cells that normally ignore capsaicin, and searched for the one gene that made those cells react. It coded for TRPV1, and the same channel turned out to respond to painful heat. Julius shared the 2021 Nobel Prize in Physiology or Medicine with Ardem Patapoutian for their discoveries of receptors for temperature and touch.

His lab later found a cousin, TRPM8, which responds to cool temperatures and to menthol. That is why mint feels cold in the same false way that chilli feels hot.

Birds are the telling exception. Their version of TRPV1 doesn't respond to capsaicin, so they can eat the hottest peppers without discomfort. That suits the plant. Seeds that pass through a bird can still sprout, while a mammal's molars crush them, so a common explanation is that the heat steers the fruit toward the animals that spread it.

One practical point follows from the chemistry. Capsaicin barely dissolves in water, so a glass of water mostly moves it around your mouth. Milk may help more: its proteins can draw the capsaicin into an emulsion and carry it away.