Scoville scale

The Scoville scale is a measure of the "hotness," or more correctly, piquancy, of a chili pepper. These fruits of the Capsicum genus contain capsaicin, a chemical compound which stimulates thermoreceptor nerve endings in the skin, especially the mucus membranes. The number of Scoville heat units (SHU) indicates the amount of capsaicin present. Many hot sauces use their Scoville rating in advertising as a selling point. The scale is named after its creator, American chemist Wilbur Scoville.

Scoville Organoleptic Test
Scoville's original method for testing hotness was called the Scoville Organoleptic Test, which he developed in 1912. As originally devised, a solution of the pepper extract is diluted in sugar water until the "heat" is no longer detectable to a panel of (usually five) tasters; the degree of dilution gives its measure on the Scoville scale. Thus a sweet pepper or a bell pepper, containing no capsaicin at all, has a Scoville rating of zero, meaning no heat detectable even undiluted. Conversely, the hottest chiles, such as habaneros, have a rating of 300,000 or more, indicating that their extract has to be diluted 300,000-fold before the capsaicin present is undetectable. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity.

High performance liquid chromatography (the "Gillett Method")
Spice heat is now usually measured by a method using high performance liquid chromatography (HPLC, also known as high pressure liquid chromatography). This identifies and measures the heat-producing chemicals. They are then used in a mathematical formula in which they are weighted according to their relative capacity to produce a sensation of heat. This method yields results, not in Scoville units, but in "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. This conversion is approximate, and Tainter and Grenis say that there is consensus that it gives results about 20-40% lower than the actual Scoville method would have given.

List of Scoville ratings
Pungency values for any pepper, stated in "Scoville units," are imprecise, due to expected variation within a species&mdash;easily by a factor of 10 or more&mdash;depending on seed lineage, climate 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. When interpreting Scoville ratings, this should be kept in mind.