The Scoville scale is the 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. The scale is named after its creator, American pharmacist Wilbur Scoville. His method, devised in 1912, is known as the Scoville Organoleptic Test.
The Scoville scale is an empirical measurement dependent on the capsaicin sensitivity of testers and so is not a precise or accurate method to measure capsaicinoid concentration, however, capsaicin concentration can very roughly be estimated as ~18µg/gram/SHU.
Scoville organoleptic test Edit
In Wilbur Scoville's method, an exact weight of dried pepper is dissolved in alcohol to extract the heat components (capsinoids), then diluted in a solution of sugar water. Increasing concentrations of the extracted capsinoids are given to a panel of five trained tasters until a majority of three can 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 sensitivity to pungency; the human palate is quickly desensitized to capsaicins after tasting a few samples within a short time period. Results vary widely, ± 50%, between laboratories.
High-performance liquid chromatography Edit
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 weighs 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.
Scoville units are a measure of capsaicin content per unit of dry mass. 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.
Since Scoville ratings are defined per unit of dry mass, comparison of ratings between products having different water content can be misleading. Typical fresh chili peppers have a water content around 90 percent, whereas, for example, Tabasco sauce has a water content of 95 percent. For law-enforcement-grade pepper spray, values from 500 thousand up to 5 million SHU have been mentioned, but the actual strength of the spray depends on the dilution, which could be a factor of 10.
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 cultivar, the "Ghost chili", which does not have 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, 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.