Chiles get their heat from a group of chemical compounds called capsaicinoids, the best known being capsaicin.
Most of the capsaicin in a chile is concentrated in the inner whitish pith (also called ribs), with progressively smaller amounts in the seeds and the flesh. In lab tests we conducted on jalapeños, we found the pith contained 100 times the amount of capsaicin in the flesh, while the seeds had 15 times as much.
Forget anything you’ve heard about appearance being an indicator of chile heat. Small chiles, for example, have no more heat than larger ones. “Corking” (thin white striations) on the skin also has no correlation to a chile’s kick. According to Denise Coon of the Chile Pepper Institute at New Mexico State University, capsaicin production is mainly tied to the environment. Chiles grown in high, arid deserts undergo stress—and stressed chiles produce more capsaicin than those grown in wetter, cooler lowland climates. The upshot: The same variety of chile can have widely varying heat levels, depending on how much it was stressed. And unfortunately, there’s no way to look at a chile and determine whether it has led a stressed or a charmed life.