Why Are Peppers Spicy? Capsaicin and the 43 °C Sensor

Why are peppers spicy?

Because of capsaicin. It is the main heat compound in chiles, and it causes a burning pain by switching on the nerve cells that report harmful stimuli to your brain¹. In 1997 David Julius's lab found the sensor it switches on, an ion channel later named TRPV1, and it turned out to be a heat sensor too².

Your mouth has no separate sensor for chile, so it borrows the one for scalding heat. That is why a pepper you eat on a snowy porch still makes you flush. The Nobel committee gave Julius the 2021 prize in Physiology or Medicine for this work, shared with Ardem Patapoutian for touch².

Here is the whole chain, from the moment the pod touches your tongue to the moment you reach for a drink.

The last box only lights up when you take in a lot. For a normal plate of chili, the chain stops at your brain reading heat, and the feeling fades as the capsaicin leaves your mouth.

Where in the pepper does the heat come from?

The pale tissue in the middle, not the red flesh. A 2024 review puts about 89 % of a chile's capsaicin in the placenta, the spongy ridge that holds the seeds, and only 5–6 % in the outer wall³.

1Placenta: about 89 % of the capsaicin³2Red wall: 5–6 %³3Seeds, attached to the placenta4Stem cap

1. Placenta: about 89 % of the capsaicin³
2. Red wall: 5–6 %³
3. Seeds, attached to the placenta
4. Stem cap

Seeds taste hot because they sit against the placenta, which Usman's team names as the tissue where capsaicinoids occur⁴. Guinness makes the same point in its Pepper X announcement: the heat sits in the tissue that holds the seeds, not in the seeds themselves⁵.

So if you want the flavor of a jalapeño with less bite, run a teaspoon down the inside of each half and scrape out the pale ridge with the seeds. Most of the heat goes with it. Wear gloves while you do it, because that ridge is exactly the part that ends up on your fingers. The pepper seeds page takes the seed question further.

What are capsaicinoids?

They are the family of heat compounds, and capsaicin is only the best known one. Capsaicin and dihydrocapsaicin together make up over 90 % of the capsaicinoids in a fruit⁴, and a Danish study of 15 cultivars found capsaicin alone ranging from 31 % to 71 % of the total⁶.

The mix changes from pepper to pepper. One HPLC study measured all five in dried chiles from northern Mexico, and the mix chart on this page shows how far apart three of them sit⁷.

In that study, dihydrocapsaicin outweighed capsaicin in every variety tested⁷. The guajillo stands out: more than a third of its heat compounds are homodihydrocapsaicin. You can taste a chile de árbol and a guajillo side by side and feel two different burns, even before you think about how much heat each one has.

Does the mix change the Scoville number?

Labs add the capsaicinoids up and convert the total into SHU, so the number tracks how much of the family a pod holds. Pure capsaicin sits at the top of the scale at 16 million SHU⁴. The capsaicin page covers how each compound gets counted.

Does capsaicin actually burn you?

Not the way a stove does. Capsaicin raises no temperature. It opens the same channel that painful heat opens, and in cells studied in the lab that channel opens for heat at about 43 °C⁸. It also responds to acid, so even mildly acidic conditions can switch it on at room temperature⁹.

That does not make capsaicin harmless on your skin. The poison center calls it intensely irritating to skin, eyes and the gut, and describes a cook who got pain and blistering after handling hot chiles¹⁰. Gloves and the pepper burn on hands page handle that side.

TED-Ed made this short animation on the science of spiciness, so you can watch the nerve channel and the burn explained in motion after reading the steps in words.

Play video: The science of spiciness - Rose Eveleth 3:54

The science of spiciness - Rose Eveleth

From TED-Ed

VideoPlays from YouTube when you press play.Watch on YouTube

1 source

Why does the same pepper feel milder the tenth time?

Part of the answer sits in the channel itself. In lab studies, once capsaicin has switched TRPV1 on, the channel goes into a long refractory state, a stretch where it stops answering⁸.

That finding comes from nerve cells in a dish, so treat it as a clue to why your fifth bite of a hot curry hurts less than your first, not as proof of how tolerance builds over months. The spice tolerance page looks at what repeated eating does.

Why do peppers make capsaicin at all?

To pick who eats the fruit. Tewksbury and Nabhan found that capsaicin discourages the animals that destroy chile seeds without putting off the birds that spread them¹². Birds simply do not feel it: their TRPV1 still senses heat, but it ignores capsaicin¹³.

The plant gets a burn that works on the animals it wants to keep away. You, as a mammal, sit on the wrong side of that deal.

What calms the burn, and when do you need help?

Milk, most of the time. In the test, 72 adults swallowed 30 mL of spicy tomato juice and then drank 40 mL of one of seven drinks. Every drink lowered the burn, and whole milk, skim milk and cherry Kool-Aid cut it the most¹⁴.

Skim milk did as well as whole, so the authors point to the protein in milk more than the fat. The sweet drink did well too, and they name sugar as one possible reason without testing it¹⁴. The mouth burn page compares the options in more detail.

What you do first depends on where the capsaicin landed. The poison center gives a different first move for each place¹⁰.

1. 1

   In your mouth or stomach

   Take small sips of milk or water. The poison center also suggested ice cream or an antacid for one teenager with a stomach ache after a hot pepper.¹⁰

2. 2

   On your skin

   Get away from the source first, then try warm water flushes, vegetable oil, or soaks in vinegar diluted with water. A liquid antacid spread on the skin may help.¹⁰

3. 3

   In your eyes

   Rinse with plenty of warm water for 15 minutes. Cool compresses afterwards may ease the pain.¹⁰

If the problem is a pot of chili rather than your tongue, the fixes live with the cooking techniques for taming heat in a dish.

What goes wrong when you try to cool it down?

Most of the trouble comes from moving the capsaicin somewhere new, not from the first bite. These are the mistakes the sources describe, with what they suggest instead.

• Rubbing your eyes after you cut chiles. The poison center tells you not to touch your face or eyes after you prepare hot peppers, and to wear rubber, latex or nitrile gloves while you work¹⁰.
• Leaning over the pan. Breathing in capsaicin can set off an asthma attack or breathing trouble in people prone to it¹⁰, so open a window when you dry-toast or fry very hot chiles.
• Gulping water and expecting a cure. Water did lower the burn in the drink test, just less than milk¹⁴. France's food safety agency goes further and tells you to skip water and fizzy drinks and reach for dairy or a fatty food¹⁵.
• Waiting out a bad reaction. ANSES reports that large amounts can raise blood pressure, cause palpitations or loss of consciousness in some people, and that children and people with heart or gut conditions react more strongly¹⁵.

Which burn question do you have next?

Capsaicin itself has more chemistry than this page can hold, from its four cousins to how labs count it. If you wonder whether the burn does you any good, the health studies sort what research shows from what it does not.

If you got juice in your eyes, the pepper in your eyes page walks you through the rinse.