Scoville Unit

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 active components found in chili peppers, 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.

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.

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 their number of mouth heat receptors, which varies greatly 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 more precisely measured by a method that uses high-performance liquid chromatography (HPLC). This method measures the concentration of heat-producing chemicals. The measurements uses a mathematical formula that weights them according to their relative capacity to produce perceived heat (pungency). This method yields results, not in Scoville units, but in American Spice Trade Association (ASTA) pungency units. ASTA pungency units are then multiplied by 16 in order to be reported as Scoville units, as according to the published method, a measurement of one part capsaicin per million corresponds to about 16 Scoville units. Scoville units are a measure for capsaicin content per unit of dry mass.

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