Why Some Chili Peppers Are Hotter Than Others: The Plant Science of Capsaicin
Every chili pepper's heat comes down to a single family of compounds: capsaicinoids, the most abundant of which is capsaicin. Understanding where and why a pepper plant produces it explains a lot about why heat levels vary so much, not just between different pepper species but between two pods picked off the very same plant.
What capsaicin is and where it's made
Capsaicin is produced in specialized glands on the placenta of the pepper — the pale, spongy tissue that seeds are attached to, more commonly called the pith. It's synthesized as the fruit develops and tends to build up further as the pepper ripens, which is part of why a fully ripe red jalapeño is often noticeably hotter than a green one picked earlier from the same plant. The seeds themselves contain very little capsaicin directly, but because they sit right against the pith, they pick up plenty of it by contact — which is why removing both the pith and seeds together, rather than just the seeds, does the most to tame a pepper's heat.
A likely evolutionary purpose
From the plant's perspective, capsaicin appears to be a defense mechanism, and a fairly targeted one. Mammals have receptors that respond strongly to capsaicin, which is why chewing a hot pepper produces such an intense reaction in humans and other mammals. Birds, by contrast, largely lack a comparable sensitivity and can eat even superhot peppers without much apparent reaction. Because birds swallow seeds whole and disperse them over a wide area through their droppings, while mammals tend to chew seeds and destroy them in the process, a pepper that mammals find unpleasant but birds happily eat has a clear advantage in spreading its seeds successfully. This selective effect on different animal groups is one of the more compelling explanations for why the trait evolved in the first place.
Genetics: the biggest single factor
Different pepper species and varieties carry different versions of the genes controlling capsaicinoid production, which is the fundamental reason a bell pepper (which has lost the ability to produce capsaicin altogether through a genetic mutation) sits at 0 SHU while a Carolina Reaper, bred specifically for extreme heat, can exceed 1,700,000. Within a single variety, individual plants still carry some genetic variation, which is why growers chasing record heat levels grow out many plants from the same seed stock and selectively save seed only from the hottest individuals across several generations — slowly shifting the population's average heat upward over time.
Environmental stress raises capsaicin production
Beyond genetics, how a pepper plant is grown measurably affects its heat. Plants grown with restricted water, in nutrient-poor soil, or in hotter conditions tend to produce more capsaicin than the same variety grown with abundant water and mild conditions — a pattern consistent with capsaicin functioning as a stress response as well as a defense mechanism. This is part of why the same variety of pepper can taste noticeably different depending on where and how it was grown, even before any difference in ripeness or seed genetics comes into play.
Why heat varies pod to pod on the same plant
Even accounting for genetics and growing conditions, individual pods on the same plant can vary in heat, largely due to differences in ripeness, position on the plant (fruit that develops in more direct sun tends to run hotter), and the pepper's own developmental variation. This is exactly why Scoville ratings are published as ranges rather than fixed values, and why a home cook working from a recipe or a heat calculator should treat any specific SHU number as a reasonable estimate to build from and adjust to taste, rather than an exact promise of how a particular pepper will perform.