The Scoville scale is a measure of the hotness of a chile pepper (hot pepper). These fruits of the Capsicum genus contain capsaicin, a chemical compound which stimulates heat-receptor nerve endings in the tongue, and 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 Scoville Scale is named after Wilbur Scoville, who developed the Scoville Organoleptic Test 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, 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. 15 Scoville units is equivalent to one part capsaicin per million. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity.
An alternative method for quantitative analysis uses high performance liquid chromatography, making it possible to directly measure capsaicinoid content. Spice heat is now usually measured by this method. This identifies and measures the concentration of 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 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. 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.
Pungency values for any pepper, stated in Scoville units, are imprecise, due 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 Naga as 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. When interpreting Scoville ratings, this should be kept in mind.