The Scoville scale is a measurement of the pungency (spiciness or "heat") of chili peppers and other spicy foods, as recorded in Scoville Heat Units (SHU) based on the concentration of capsaicinoids, among which capsaicin is the predominant component. The scale is named after its creator, American pharmacist Wilbur Scoville, whose 1912 method is known as the Scoville organoleptic test. In the 21st century, high-performance liquid chromatography (HPLC) is used to quantify the capsaicinoid content as an indicator of pungency. The most expedient method for estimating SHU is a subjective assessment derived from the capsaicinoid sensitivity by people experienced with eating hot chilis.
Scoville organoleptic test
In the Scoville organoleptic test, an exact weight of dried pepper is dissolved in alcohol to extract the heat components (capsaicinoids), then diluted in a solution of sugar water. Decreasing concentrations of the extracted capsaicinoids are given to a panel of five trained tasters, until a majority (at least three) can no longer detect the heat in a dilution. The heat level is based on this dilution, rated in multiples of 100 SHU.
A weakness of the Scoville organoleptic test is its imprecision due to human subjectivity, depending on the taster's palate and number of mouth heat receptors, which vary widely among people. Another weakness is sensory fatigue; the palate is quickly desensitized to capsaicinoids after tasting a few samples within a short time period. Results vary widely (up to ± 50%) between laboratories.
Since the 1980s, spice heat has been assessed quantitatively by HPLC, which measures the concentration of heat-producing capsaicinoids, typically with capsaicin content as the main measure. As stated in one review: "the most reliable, rapid, and efficient method to identify and quantify capsaicinoids is high-performance liquid chromatography (HPLC); the results of which can be converted to Scoville Heat Units by multiplying the parts-per-million by 16." HPLC results permit the measurement of a substance’s capsaicin capacity to produce perceived heat ("pungency"). This method yields results in American Spice Trade Association (ASTA) "pungency units", which are defined as one part capsaicin per million parts dried pepper mass.
For parts per million (ppm) measurements: "Scoville Heat Units are calculated in parts per million of heat (ppm H) based on sample dry weight according to the following equation from ASTA (1985): ppm H = [Peak area of capsaicin + (0.82)(peak area of dihydrocapsaicin)](ppm standard)(ml acetonitrile) ÷ (Total capsaicin peak area of standard)(g sample). Conversion to Scoville Heat Units can be made by multiplying ppm H by a factor of 15" (modified to a factor of 16 by others).
Another source using subjective assessment stated: "Conventional methods used in determining the level of pungency or capsaicin concentration are using a panel of tasters (Scoville Organoleptic test method). ... Pepper pungency is measured in Scoville Heat Units (SHU). This measurement is the highest dilution of a chile pepper extract at which heat can be detected by a taste panel (Bosland et al., 2007)."
Since Scoville ratings are defined per unit of dry mass, comparison of ratings between products having different water content can be misleading. For example, typical fresh chili peppers have a water content around 90%, whereas Tabasco sauce has a water content of 95%. For law-enforcement-grade pepper spray, values from 500,000 up to 5 million SHU have been reported, but the actual strength of the spray depends on the dilution.
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, owing to expected variation within a species, possibly by a factor of 10 or more, depending on seed lineage, climate and humidity, and soil composition supplying nutrients. The inaccuracies described in the measurement methods also contribute to the imprecision of these values.
Capsicum chili peppers are commonly used to add pungency in cuisines worldwide. The range of pepper heat reflected by a Scoville score is from 100 or less (sweet peppers) to over 3 million (Pepper X) (table below; Scoville scales for individual chili peppers are in the respective linked article).
|Scoville heat units||Peppers|
|800,000 to 3,200,000||Pepper X, Carolina Reaper, Dragon's Breath, Naga Morich|
|350,000 to 800,000||Red Savina pepper, Chocolate habanero|
|100,000 to 350,000||Habanero, Scotch Bonnet|
|10,000 to 100,000||Malagueta pepper, Cayenne pepper|
|1,000 to 10,000||Guajillo pepper, Jalapeño|
|100 to 1,000||Banana pepper, Cubanelle|
|0 to 100||Bell pepper, Pimento|
The class of compounds causing pungency in plants like chili peppers is called capsaicinoids, which display a linear correlation between concentration and Scoville scale, and may vary in content during ripening. Capsaicin is the major capsaicinoid in chili peppers.
|Scoville heat units||Chemical|
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Interlab variation [for the original Scoville scale] could be as high as +/−50%. However, labs that run these procedures could generate reasonably repeatable results.
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Most law enforcement sprays have a pungency of 500,000 to 2 million SHU. One brand has sprays with 5.3 million SHU.
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