The Scoville scale is a measurement of the pungency (spicy heat) of chili peppers, or other spicy foods, as reported in Scoville heat units (SHU), a function of capsaicin concentration. Capsaicin is one of many related pungent compounds found in chili peppers, collectively called capsaicinoids. The scale is named after its creator, American pharmacist Wilbur Scoville whose 1912 method is known as the Scoville organoleptic test.
In modern times, high-performance liquid chromatography is used to determine the pungency. The older method is a subjective measurement dependent on the capsaicin sensitivity of testers, and so is not a precise or accurate method to measure capsaicinoid concentration.
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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 varies 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.
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Pungency units
Since the 1980s, spice heat has been measured by high-performance liquid chromatography (HPLC) which measures the concentration of heat-producing chemicals, such as capsaicin. HPLC results enable use of a mathematical formula that weights them according to their relative capacity to produce perceived heat ("pungency"). This method yields results in American Spice Trade Association (ASTA) "pungency units" which are then multiplied by 15 to produce a Scoville score defined as one part capsaicin per million in a unit of dried pepper mass.
Scoville ratings
Considerations
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 1 million SHU have been mentioned, 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--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.
Capsicum peppers
Capsicum chili peppers are commonly used to add pungency in cuisines worldwide. Subjective sensory feedback from people experienced with tasting hot chili peppers indicates five possible categories related to perceiving chili heat and the corresponding Scoville units: 1) development of heat sensation over time, 2) duration, 3) location (lips, mouth, throat, stomach), 4) sensation of heat, and 5) intensity (specifically related to Scoville units). 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).
Capsaicinoids
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.
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