Yum, chillis—such a delicious burn. But why do spicy foods physically feel hot? Let’s find out.
Chilli peppers contain a chemical called capsaicin, which is what gives them their spice. Different peppers have different amounts, which is why some chillis like the jalapeño are a bit spicy, and others like the bird’s eye chilli are seriously over-the-top spicy.
What’s most fascinating about chilli peppers is that they don’t just taste hot, but that your body reacts as if they are hot. Your mouth contains a loads of different receptors. There are, of course, tastebuds that respond to flavor, but there are also pain receptors that react to mechanical stimulation like pinching and cutting, and yet others that react to temperature. Through a total accident of biology, capsaicin binds to these pain receptors, stimulating them, and tricking your body into thinking that there’s something hot in your mouth and that it’s physically painful. In other words, your mouth feels like its on fire, even though it’s not.
This is why your body reacts to the “heat” of chilli peppers as if you are actually experiencing real heat—your nervous system is telling it that it is. You start to sweat and blood rushes to the surface of your skin, giving you that red flush. It’s the same cooling symptoms you get if you go for a jog on a hot day.
The receptors that respond to capsaicin aren’t just in your mouth, they’re located all over your body, particularly in sensitive areas like your eyes, nose, and genitals. If you’ve ever rubbed your eyes (or picked your nose, don’t judge me) after preparing some chillis, you’ll have felt a pretty extreme burning sensation. This is why compounds that contain huge amounts of capsaicin are used as pepper spray—a little in the eyes, nose, or lungs is enough to leave people in absolute agony.
How hot a chilli feels, or rather how much capsaicin is in it, is measured using the Scoville scale, developed by Wilbur Scoville. To test each chilli, it is dried and then a precise amount is dissolved in a small amount of alcohol to extract the capsaicin. The capsaicin-alcohol mix is then diluted with sugar water. A panel of five testers (who are expert chilli-eaters) are given increasingly more diluted versions until at least three of them can no longer detect any heat of it. How much the capsaicin-alcohol mix had to be diluted determines the Scoville Heat Unit (SHU) rating.
The mildest chillis, such as the bell pepper and pimento pepper, have SHUs of between 0 and 100. Slightly spicier chillis like the jalapeño have SHUs between 1,000 and 10,000. Things like cayenne pepper and tabasco have an SHU somewhere between 10,000 and 100,000, while truly hot chillis like the habanero and Scotch bonnet fall around 100,000 to 350,000. The hottest chillis, like the Carolina reaper, can have SHUs of over 3,000,000.
Chilli peppers aren’t the only foods that confuse your body. Menthol has the opposite effect, tricking your tongue into thinking it’s cold. Menthol is found in mint oils like peppermint or eucalyptus, and is responsible for the cool, fresh feeling you get when you suck on a Fisherman’s Friend lozenge or brush your teeth. It’s also the active ingredient in pain relief cooling gels.