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 — and why a chart of "typical" Scoville ratings is always describing an average, never a guarantee.

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 alone, does the most to tame a pepper's heat, and why "deseeding" a pepper without also scraping out the pale membrane behind the seeds barely tames it at all.

Why capsaicin feels hot at all

Capsaicin doesn't damage tissue the way an actual burn does — it works by binding to a specific receptor (called TRPV1) that sits on nerve endings and normally responds to real heat, upward of roughly 109°F (43°C) in human tissue. Capsaicin activates that same receptor directly, at any temperature, which is why the brain interprets a bite of hot pepper as genuine heat even though nothing has actually gotten hotter. It's the same receptor, doing the same job it would do if your tongue had actually touched something hot — capsaicin is just tricking it into firing. That's also why a hot pepper's "burn" spreads and lingers the way a real burn does, and why drinking ice water gives only brief relief: the receptor is still bound and firing regardless of the water's temperature. Fat and dairy work better for the same reason cold water doesn't — capsaicin is fat-soluble, not water-soluble, so milk or oil-based food actually helps carry it away from the receptor rather than just numbing the area temporarily.

A likely evolutionary purpose

From the plant's perspective, capsaicin appears to be a defense mechanism, and a fairly targeted one. Mammals have TRPV1 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, have a version of the same receptor that doesn't respond to capsaicin the same way, 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, and it's also why capsaicin shows up as an ingredient in some topical products marketed for minor muscle or joint discomfort — it's the same receptor-binding mechanism at work on skin, a factual point about how the compound behaves, not a recommendation for treating any specific condition.

The on/off switch behind "hot" and "not hot"

Whether a pepper plant produces capsaicin at all comes down to a specific gene locus, generally referred to in pepper genetics as the "pungency" locus, that controls an enzyme needed to build capsaicinoid molecules from their precursors. When that gene is disabled, as it is in bell peppers, the plant still makes all the other compounds that give a pepper its color, sweetness, and aroma — it simply can't complete the final step that would make any of it taste hot, no matter how the plant is grown or how ripe the fruit gets. That's a fundamentally different situation from a "mild" hot pepper like a poblano or Anaheim, which still has a working version of the pathway and is capable of producing real heat, just less of it than a jalapeño or serrano under the same conditions. It's also why no amount of stress, ripening, or growing technique will ever coax measurable heat out of a bell pepper — the pathway that produces capsaicin isn't just turned down, it's absent.

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, is commonly reported in the range of roughly 1.4 to 2.2 million SHU. 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. This is also why two packets of "Carolina Reaper" seed from different growers can produce plants with noticeably different heat ceilings; the name describes a lineage that's still being actively selected, not a fixed genetic constant.

Breeding for extreme heat also isn't just a matter of crossing two hot peppers and hoping. Serious breeders typically grow out large numbers of plants from a promising cross, test or taste-screen the hottest-looking candidates, and save seed only from the individuals that measure highest — repeating that selection over several generations before a new "record" variety is considered stable enough to sell as seed. It's a slow, statistical process much closer to selective livestock or crop breeding than to a single lucky mutation, and it's part of why genuinely new superhot varieties don't appear especially often, despite how much attention the category gets, and why a "new" record-chasing variety is usually still years of quiet selection work by the time it reaches a seed catalog.

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, and it's also why a home-grown pepper from a stressed, under-watered plant can occasionally out-heat a store-bought pepper of the same variety grown under easier commercial conditions. It's worth being clear that this isn't a recommendation to stress a plant deliberately for more heat — a badly stressed plant also produces fewer, smaller fruit overall, so a grower chasing maximum total harvest and a grower chasing maximum heat per pod are often making different, sometimes opposing, choices about how much water and fertilizer to hold back.

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.

What this means when you're cooking from a chart

None of this makes a reference chart useless — it just means the number is a planning tool, not a lab certificate for the specific pod in front of you. The pepper reference table lists the commonly cited range for each variety alongside its midpoint, rather than a single false-precise figure, for exactly this reason: a habanero's real heat can land anywhere across a genuinely wide band even when every plant is the same variety, grown by the same person, in the same season — the biology described above, not measurement error alone, is why that band exists in the first place. When a recipe or a blend calculator asks for a pepper's SHU, feeding it the midpoint of that range is a reasonable default — just one worth adjusting toward the low or high end if you know your specific peppers ran milder or hotter than usual that season — grown under more stress or picked further into ripeness, lean toward the top of the range; grown easy and picked early, lean toward the bottom.