Scoville scale

The Scoville scale is a measurement of the pungency (spicy heat) of chili peppers—such as the jalapeño, the bhut jolokia, and the world's current hottest pepper, the Carolina Reaper—or other spicy foods as reported in Scoville heat units (SHU), a function of capsaicin concentration. Capsaicin is one of many related chemicals, collectively called capsaicinoids. The scale is named after its creator, American pharmacist Wilbur Scoville. His method, devised in 1912, is known as the Scoville Organoleptic Test.

Unlike methods based on high-performance liquid chromatography, the Scoville scale is a subjective measurement dependent on the capsaicin sensitivity of testers and so is not a precise or accurate method to measure capsaicinoid concentration.

In Scoville's method, 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 capsinoids 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 their number of mouth heat receptors, which varies greatly among people. Another weakness is sensory fatigue: the palate is quickly desensitised to capsaicins after tasting a few samples within a short time period. Results vary widely (up to ± 50%) between laboratories.

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 percent, whereas Tabasco sauce has a water content of 95 percent. 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, which could vary by a factor of 10.

...
Wikipedia