Why Are Some Jalapeños Hot and Others Not? The Science of Heat

If you've ever eaten two jalapeños from the same grocery store bag and found one mild and the other blazing, you're not imagining things. Jalapeños can range from 2,500 to 8,000 Scoville Heat Units (SHU), meaning the hottest jalapeño can be more than three times spicier than the mildest. This variation comes down to a combination of genetics, growing conditions, and ripeness — all of which influence how much capsaicin the pepper produces.

Understanding what makes a jalapeño hotter or milder isn't just a scientific curiosity. It's practical knowledge that helps you cook more consistently, grow spicier peppers, and pick the right pepper at the store.

How Capsaicin Is Produced

Capsaicin is synthesized in the pepper's placenta — the white, spongy tissue that runs along the inside of the pepper and holds the seeds. This is important: the seeds themselves don't produce capsaicin. They may taste hot because they sit in direct contact with the placenta, absorbing capsaicin oil, but they're not the source.

The production of capsaicin is controlled by the Pun1 gene (pungency gene 1). This gene encodes an enzyme called capsaicin synthase, which catalyzes the final step in capsaicin production. All hot peppers have an active version of this gene, while bell peppers have a non-functional version — which is exactly why they produce zero heat.

In jalapeños, the Pun1 gene is always active, but how much capsaicin gets produced depends heavily on environmental factors during growth.

Growing Conditions That Affect Heat

Water Stress

This is the single biggest factor that growers can control. When a jalapeño plant experiences moderate drought stress — periods of reduced watering — it produces significantly more capsaicin. The plant concentrates its defensive compounds (including capsaicin) when it perceives environmental threat.

A study from New Mexico State University found that mildly drought-stressed pepper plants produced up to 30% more capsaicin than well-watered controls. However, severe drought stress reduces yield and can kill plants, so the key is balance. If you're growing your own peppers, reducing watering slightly once fruits begin to set can boost heat.

Temperature and Sunlight

Higher daytime temperatures (85–95°F / 29–35°C) with warm nights increase capsaicin production. Peppers grown in hot climates like the American Southwest or their native Mexico tend to be hotter than those grown in cooler regions.

Direct sunlight also matters. The UV radiation from full sun exposure triggers the plant to produce more capsaicinoids as a protective response. Jalapeños grown in partial shade will typically be milder.

Soil Nutrients

Soil sulfur content has been linked to capsaicin levels. Sulfur is a component of the amino acids that serve as capsaicin precursors. Soils naturally rich in sulfur, or those amended with sulfur-containing fertilizers, tend to produce hotter peppers. Excessive nitrogen, on the other hand, can actually dilute capsaicin concentration by promoting rapid, lush growth.

The Role of Genetics

Even under identical growing conditions, different jalapeño cultivars will produce different heat levels. Plant breeders have developed varieties specifically for heat or mildness:

• TAM Jalapeño: Bred at Texas A&M University to be consistently mild (1,000–1,500 SHU). Widely used in commercial salsa production where predictable heat is needed.
• Early Jalapeño: A standard variety, typically 2,500–5,000 SHU.
• Jalapeño M: A hotter cultivar that reliably reaches 4,500–8,000 SHU.
• Mucho Nacho: A large-fruited variety with moderate, consistent heat.

When you buy jalapeños at a grocery store, you usually don't know which cultivar you're getting, which contributes to the unpredictability.

How Ripeness Affects Heat

Jalapeños go through a clear ripening progression: green to dark green to red. As they ripen, two things happen to heat level:

1. Capsaicin continues to accumulate — a fully ripe red jalapeño has had more time to produce capsaicin than one picked young and green
2. Sugar content increases — which can create the perception of slightly less sharpness, even as actual SHU increases

The result is that red jalapeños are generally hotter than green ones, but the increased sweetness and flavor complexity can make the heat feel more rounded and less harsh.

The Seeds and Placenta Myth

Let's settle this definitively: removing the seeds does not remove most of the heat. The capsaicin is produced by and concentrated in the placenta (the white membrane). Seeds contribute very little heat on their own.

If you want a milder jalapeño, you should remove both the seeds and the white membrane. This can reduce the perceived heat by 50–80%. Leaving the seeds in while removing the placenta will give you a significantly milder result than doing the opposite.

How to Predict Heat at the Store

While you can't know the exact SHU of a jalapeño without lab testing, some visual and tactile clues help:

• White stretch marks (corking): These tiny white lines on the skin indicate stress during growth, which correlates with higher capsaicin. More corking generally means more heat.
• Size: Smaller jalapeños tend to be hotter. The capsaicin is more concentrated in a smaller pepper.
• Color: Darker green and any red coloring suggest more maturity and potentially more heat.
• Firmness: A firm, dense pepper has usually grown more slowly (potentially with some stress), which can mean more capsaicin.
• Pointed tip: Some experienced cooks report that jalapeños with pointed tips tend to be hotter than round-tipped ones, though this is anecdotal.

Testing and Measuring Heat

The Scoville scale was originally based on a taste test (the Scoville Organoleptic Test), where a panel of tasters determined how much sugar water was needed to neutralize the heat. Modern testing uses High-Performance Liquid Chromatography (HPLC), which precisely measures capsaicin concentration and converts it to SHU.

For home cooks, the simplest "test" is to cut a tiny sliver from the placenta area and touch it to your tongue. This will tell you quickly where that specific pepper falls on the mild-to-hot spectrum.

Frequently Asked Questions

Do jalapeños get hotter when cooked?

No — cooking doesn't increase capsaicin. However, it can break down cell walls and distribute capsaicin more evenly throughout a dish, which may make the heat feel more intense.

Are jalapeños from Mexico hotter than American-grown ones?

Often yes, due to the hotter, drier growing conditions and different cultivar selections. But this isn't a universal rule — well-stressed jalapeños grown anywhere can reach high heat levels.

Can you make a jalapeño plant produce hotter peppers?

Yes. Reduce watering slightly after fruit set, ensure full sun exposure, use sulfur-rich soil amendments, and allow peppers to mature longer on the plant. For a deeper dive, see our guide to soil, water, and sunlight.

Why do jalapeños from the same plant have different heat levels?

Even on a single plant, individual peppers may receive different amounts of sun exposure, ripen at different rates, or experience localized stress differently. The variation is natural.