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Ask Paul: Why Do Different Chiles Burn Your Mouth Differently?

It's not just about Scoville units.
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Published Mar. 30, 2022.

A student asked: “Why does the heat hit faster in some chiles or linger longer in others?”

Even non-chile-heads are familiar with the Scoville scale, which measures the fieriness of chile peppers. It goes from zero Scoville Heat Units (SHU), for a bell pepper, to millions for headline-grabbing record-setting varieties like the ghost pepper (1,001,304 SHU) and the Carolina Reaper (1,641,183 SHU).

Between the extremes are all the chiles we know and adore, like the jalapeño, which varies widely but generally clocks in around 5,000 SHU; and the habanero, with an easy 100,000 SHU or more. A Thai chile also runs about 100,000 SHU.

But the sensation of eating a habanero is very different from that of eating a Thai chile. (Feel free to bite into raw peppers as you read if you want to follow along at home.) The habanero seems to take its time, letting you chew and swallow before the heat starts to build at the back of your mouth and throat—and it takes its time leaving as well, with a burn that lingers for minutes on end.

The Thai chile is very different. As soon as it hits your tongue, it unleashes its keen heat, more sharply and more suddenly than the habanero, and further forward on the tongue. And it fades faster, leaving you flushed and ready for another bite.

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The simple linear Scoville scale doesn’t capture these facets of chile heat, which is why Paul Bosland of New Mexico State University’s Chili Pepper Institute developed a “heat profile” to characterize peppers on five separate criteria. One is the Scoville heat level. The others:

  • Development: How soon does the heat start? Is it rapid? Delayed by several seconds?
  • Duration: How long does the heat last?
  • Location: Where does it hurt? On the lips, tip of the tongue, in the throat?
  • Feeling: What is the sensation like? Sharp like a needle or flat and steady like a broad hot surface?

Why is it so varied? Capsaicin is the primary chemical compound responsible for the burn of peppers, and until the 1960s it was believed to be the only one. But in fact, chiles produce quite a few different compounds that are chemically similar to capsaicin—the capsaicinoids, such as homocapsaicin, dihydrocapsaicin, and nordihydrocapsaicin.

These capsaicinoids stimulate our receptors in subtly different ways, evoking different sorts of heat sensations. And as different pepper cultivars contain varying amounts of the various compounds, so they each burn us differently.

Ask Paul Adams, senior science research editor, about culinary ambiguities, terms of art, and useful distinctions: paul@americastestkitchen.com

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