To find out more about the differences among the many breeds of chiles, I spoke to the source: the Chile Pepper Institute at New Mexico State University, where more than 150 varieties of chiles are grown, including at least 20 that were created at the university by selective breeding. (That’s a fraction of the thousands of chile varieties in existence around the world.)
Dr. Paul Bosland is the cofounder and director of the institute, which is dedicated to researching and educating the world about the potent fruit. He’s published more than 100 papers on various aspects of chiles, their growing, and their heat characteristics.
“There are at least 200 compounds contributing to the flavor of chiles,” Bosland told me. “In terms of the heat, , and they all have a different effect.” Capsaicin is the most common, first to be discovered, and hottest of the capsaicinoid family, but every chile contains a somewhat different mix of capsaicin, dihydrocapsaicin, nordihydrocapsaicin, homodihydrocapsaicin, nornordihydrocapsaicin, and quite a few others. They all fit into TRPV1 receptors in slightly different ways, and the particular mix of capsaicinoids in any given chile is responsible for the specific way we experience its heat.
To characterize the ways in which different chiles burn differently—not just the amount, but the nuances—he has developed a classification system to describe a heat profile, which rates a chile pepper on five different axes. Is the heat instant, or is there a delay before it hits? How long is the delay? Second, how long does the heat linger?
“The third one: is it sharp or flat? Is it like pins pricking you, or a paintbrush painting your tongue?” Then, where in the mouth do you feel the heat? The tip of the tongue, the midpalate, the lips, the throat? And fifth is the heat level, expressed in Scoville heat units.
Though there are exceptions, chiles from the Americas tend to burn slow and long, while Asian varieties, like the tiny bird chiles popular in Thai cooking, give a sharp, immediate, powerful heat that vanishes almost as quickly as it came.
“We were trying to export chile powder to Asia, and they said we didn’t have good quality,” says Bosland. “We didn’t know what they meant. We couldn’t figure it out. Finally we tried sending them a powder that has the sharp heat. Now they’re using millions of pounds of it.”
Bosland characterizes jalapeños’ heat as rapid onset, slow dissipation, and with a flat rather than sharp feeling. Habaneros and ghost chiles both have a flat, back-of-the-throat heat that comes on slowly and lingers painfully.
The most powerful Scoville heat yet discovered is not from a chile at all. A cactus-like mountain succulent from North Africa, the resin spurge, produces a natural chemical called resiniferatoxin (RTX), which is about 1,000 times more potent than capsaicin. Marco Tizzano, a sensory scientist at Philadelphia’s Monell Center who also trained as a cook in his native Italy, administers it to laboratory mice. One dose permanently destroys their chemesthetic sense. Undissuaded, Tizzano once tried RTX himself. “With a friend, we put one drop of a very dilute solution”—he pointed to the tip of his tongue—“and then immediately rinsed it off. At the point where we put it, I couldn’t feel anything for a week.”
Nowadays, chile heat is measured not by mouth, but by machine. A specimen of chile is dried, ground, and its capsaicin extracted with alcohol, then piped through a chromatography column that separates it into its various chemical components, allowing the capsaicinoids that are present to be precisely quantified in parts per million.