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Scoville scale

The Scoville scale is a measurement of the spicy heat (or piquance) of a chili pepper. The number of Scoville heat units (SHU) indicates the amount of capsaicin present. Capsaicin is a chemical compound that stimulates chemoreceptor nerve endings in the skin, especially the mucous membranes.

The scale is named after its creator, American pharmacist Wilbur Scoville. His method, devised in 1912, is known as the Scoville Organoleptic Test. The modern commonplace method for quantitative analysis uses high-performance liquid chromatography, making it possible to directly measure capsaicinoid content.

Scoville organoleptic test

In Scoville's method, an alcohol extract of the capsaicin oil from a measured amount of dried pepper is added incrementally to a solution of sugar in water until the "heat" is just detectable by a panel of (usually five) tasters; the degree of dilution gives its measure on the Scoville scale. Thus a sweet pepper or a bell pepper, containing no capsaicin at all, has a Scoville rating of zero, meaning no heat detectable. The hottest chilis, such as habaneros and nagas, have a rating of 200,000 or more, indicating that their extract must be diluted over 200,000 times before the capsaicin presence is undetectable. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity. Tasters taste only one sample per session.

High-performance liquid chromatography

Spice heat is usually measured by a method that uses high performance liquid chromatography (HPLC). This identifies and measures the concentration of heat-producing chemicals. The measurements are used in a mathematical formula that weights them according to their relative capacity to produce a sensation of heat. This method yields results, not in Scoville units, but in American Spice Trade Association (ASTA) pungency units. A measurement of one part capsaicin per million corresponds to about 15 Scoville units, and the published method says that ASTA pungency units can be multiplied by 15 and reported as Scoville units. This conversion is approximate, and spice experts Donna R. Tainter and Anthony T. Grenis say that there is consensus that it gives results about 20–40% lower than the actual Scoville method would have given. Results vary widely, up to 50%, between laboratories.

List of Scoville ratings

Scoville heat units Examples
15,000,000–16,000,000 Pure capsaicin
8,600,000–9,100,000 Various capsaicinoids (e.g., homocapsaicin, homodihydrocapsaicin, nordihydrocapsaicin)
5,000,000–5,300,000 Law enforcement grade pepper spray, FN 303 irritant ammunition
855,000–1,463,700 Naga Viper pepper, Infinity Chilli, Bhut Jolokia chili pepper, Trinidad Scorpion Butch T pepper
350,000–580,000 Red Savina habaneron
100,000–350,000 Habanero chili, Scotch bonnet (pepper), Datil pepper, Rocoto, Madame Jeanette, Peruvian White Habanero, Jamaican hot pepper
50,000–100,000 Byadgi chilli, Bird's eye chili, Malagueta pepper, Chiltepin pepper, Piri piri (African bird's eye), Pequin pepper
30,000–50,000 Guntur chilli, Cayenne pepper, Ají pepper, Tabasco pepper, Cumari pepper (Capsicum Chinese), Chipotle
10,000–23,000 Serrano pepper, Peter pepper, Aleppo pepper
3,500–8,000 Espelette pepper, Jalapeño pepper, Guajillo pepper, New Mexican varieties of Anaheim pepper, Hungarian wax pepper, Tabasco sauce
1,000–2,500 Anaheim pepper, Poblano pepper, Rocotillo pepper, Peppadew
100–900 Pimento, Peperoncini, Banana pepper
0 No significant heat, Bell pepper, Cubanelle, Aji dulce

The chilis with the highest rating on the Scoville scale exceed one million Scoville units, and include specimens of naga jolokia or bhut jolokia and its cultivars, the "Dorset naga" and the "Ghost chili," neither of which has official cultivar status.

Numerical results for any specimen vary depending on its cultivation conditions and the uncertainty of the laboratory methods used to assess the capsaicinoid content. Pungency values for any pepper are variable, due to expected variation within a species—easily by a factor of 10 or more—depending on seed lineage, climate (humidity is a big factor for the Bhut Jolokia; the Dorset Naga and the original Naga have quite different ratings), and even soil (this is especially true of habaneros). The inaccuracies described in the measurement methods above also contribute to the imprecision of these values. When interpreting Scoville ratings, this should be kept in mind.

The Scoville scale may be extrapolated to express the pungency of substances that are even hotter than pure capsaicin. One such substance is resiniferatoxin, an alkaloid present in the sap of some species of euphorbia plants (spurges). Since it is 1000 times as hot as capsaicin, it would have a Scoville scale rating of 16 billion.

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