How hot is a Thai bird's eye chili? Lab data, and why first picks burn most
Kasetsart researchers measured heat in 10 Thai chili cultivars by HPLC: most 50,000-80,000 SHU, one only 20,250, and every cultivar hottest at its first harvest.
- Nine of ten Thai cultivars measured were highly pungent, from about 50,000 SHU up, with two above 80,000; the tenth reached only 20,250.
- Every cultivar was hottest at its first harvest and got milder in later pickings, so two chilies never mean exactly the same heat.
- Heat sits mostly in the white pith inside the fruit; the seeds do not make capsaicin themselves.
- SHU from HPLC uses a factor of 15 or 16 depending on the source, and labs can differ widely. Read it as a range, not a fixed number.
Most Thai bird’s eye chilies (phrik khi nu, พริกขี้หนู) sit at roughly 50,000-80,000 SHU, according to Kasetsart University and Department of Agriculture researchers who measured the heat compounds in ten cultivars by HPLC. The real spread is much wider than that: the mildest cultivar reached only 20,250 SHU, and every cultivar was hotter at its first harvest than at later ones.
That is the science behind why “two chilies” from the same stall is not the same heat every day. This follows how hot to order, which covers the words to use and what to do when it is too hot. Here we look at how heat is measured.
What makes a chili hot
Heat comes from capsaicinoids. The two main ones are capsaicin and dihydrocapsaicin, which in most hot chilies make up about 69% and 22% of the total. They bind to a receptor called TRPV1 on nerve endings, the same one that senses heat, so the brain reads it as burning.
The compounds are concentrated in the white pith inside the fruit (the placenta). The seeds do not make capsaicin themselves; they carry what rubs off from the pith. Capsaicin also barely dissolves in water, which is why plain water does little for the burn.
How SHU is measured
The Scoville Heat Unit (SHU) comes from Wilbur Scoville’s 1912 method: dilute a chili extract in sugar water and have five tasters try it until at least three can no longer feel heat. The dilution level is the SHU. Because it depends on human tongues, results can vary by up to ±50% between labs.
Today labs use HPLC (high-performance liquid chromatography) to separate and weigh capsaicinoids directly, then multiply to get SHU. One source uses a factor of 15, another 16 (pure capsaicin = 16,000,000 SHU). The Kasetsart study used 15.
The study: ten Thai cultivars
Published in the Kasetsart Journal (Natural Science) in 2008, the study grew ten cultivars at the Kanchanaburi Horticultural Research Center from January to September 2004: six Capsicum annuum and four Capsicum frutescens. Fully ripe fruit was picked three times, 30 days apart. Stalks and seeds were removed, and the flesh with its pith was dried at 50°C, ground, extracted and measured by HPLC.
All figures are milligrams per gram of dry weight. Fresh chilies are mostly water, so the per-gram figure for a fresh chili is lower.
| Cultivar | Species | Total capsaicinoids (mg/g) | SHU | Class |
|---|---|---|---|---|
| Huarua | C. annuum | 5.81 | 87,135 | Very highly pungent |
| K07 (Kanchanaburi 07) | C. frutescens | 5.39 | 80,805 | Very highly pungent |
| Huayseeton SK1 | C. annuum | 5.21 | 78,075 | Highly pungent |
| Pijit 007 | C. annuum | 4.50 | 67,425 | Highly pungent |
| Jinda | C. annuum | 3.42 | 51,255 | Highly pungent |
| K05 (Kanchanaburi 05) | C. frutescens | 1.35 | 20,250 | Lowest in the set |
The ranges do not quite agree, even within the paper. The abstract puts the highly pungent group at 45,000-80,000 SHU, while the body text says 50,000-80,000. Wikipedia gives bird’s eye chilies 50,000-100,000 SHU in its text, but up to 250,000 in the infobox on the same page. Fair to say most Thai bird’s eye chilies fall somewhere from fifty thousand to the high eighty thousands, with no single number right for every chili.
Worked example: from mg/g to SHU
- Huarua has 5.81 mg/g of capsaicinoids.
- 1 mg/g = 1,000 ppm, so 5.81 mg/g = 5,810 ppm.
- Multiply by 15, the study’s factor: 5,810 × 15 = 87,150 SHU (the paper’s table shows 87,135 because it used unrounded values).
- K05: 1.35 mg/g = 1,350 ppm × 15 = 20,250 SHU.
- Compare: 87,135 ÷ 20,250 ≈ 4.3 times hotter.
With a factor of 16 instead, Huarua would come to 92,960 SHU, about 7% higher from the formula alone.
Species does not tell you everything
K05 is a C. frutescens, a species often described as very hot, yet it was the mildest in the set. The authors note that K05 has thick flesh and a small placenta, while the other frutescens cultivars had thin flesh and a large placenta. That fits earlier work finding thin-walled fruit tends to be hotter. Their conclusion: heat depends on cultivar and fruit shape more than on species.
Why the first pick is hottest
Every cultivar had its highest capsaicinoid content at the first harvest (April for C. annuum, June for C. frutescens) and less in later ones. K07’s capsaicin fell from 4.85 to 3.71 to 2.72 mg/g across the three pickings, a drop of about 44%. The average capsaicin across all cultivars fell from 3.34 mg/g at the first harvest to 2.62 mg/g at the second.
The authors link this to plant age. Early on, a plant carries few fruit, so they compete little for nutrients. As the plant ages and sets more fruit, competition rises and each fruit makes less capsaicinoid.
Two chilies of the same size can differ in heat several times over, depending on cultivar, which picking they came from and how they were grown. Counting chilies helps, but it is not a precise measure.
Using this when you order
- At a new place, or early in the chili season, order one level milder than usual and add heat from the chili fish sauce on the table.
- Ask for chili on the side (kho phrik yaek, ขอพริกแยก) at made-to-order, som tam and yam stalls, and add a little at a time.
- Cooking at home, to keep the chili flavour with less heat, split the chili and scrape out the white pith, not just the seeds. Wash your hands before touching your eyes.
Common mistakes
- Treating an SHU figure as exact. It depends on cultivar, harvest, method and conversion factor. Always read it as a range.
- Comparing dry-weight figures with fresh chilies. This study measured dried chili; fresh chili per gram is much lower.
- Removing only the seeds. The heat is mainly in the white pith.
- Assuming smaller is always hotter. Among short chilies, the hottest and mildest cultivars differed about fourfold.
Summary
Most Thai bird’s eye chilies measure about 50,000-80,000 SHU, but one cultivar came in at 20,250 and two topped 80,000. The first harvest is hottest and heat fades in later pickings. Heat lives in the white pith, not the seeds. These numbers explain why “two chilies” is still a gamble, and why chili on the side is the most reliable way to control heat.
Sources
FAQ
What is an SHU?
A Scoville Heat Unit. Originally a chili extract was diluted in sugar water until tasters stopped feeling heat. Today labs measure capsaicinoids by HPLC and convert the result to SHU.
Does removing the seeds make a chili milder?
Only if you scrape out the white pith too. The heat is concentrated in the pith; the seeds do not produce capsaicin, they just pick it up from the pith.
Is a bird's eye chili always hotter than a long chili?
Generally yes. In the study, long chilies of 9-15 cm ranged from very mild to medium-hot, and short chilies of 2-7 cm from medium to very hot. But among the short ones, heat still varied about fourfold.
Ingredients and heat vary by kitchen and recipe. If you have a food allergy, ask before you order, every time.