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The Scoville scale measures the hotness or piquancy of a chili pepper, as defined by the amount of capsaicin it contains. Capsaicin is a chemical compound which stimulates chemoreceptor nerve endings in the skin, especially the mucous membranes. The number of Scoville heat units (SHU) indicates the amount of capsaicin present.

The scale is named after its creator, American chemist Wilbur Scoville, who developed a test for rating the pungency of chili peppers. His method, which he devised in 1912,[1] is known as the Scoville Organoleptic Test. An alternative method for quantitative analysis uses high-performance liquid chromatography, making it possible to directly measure capsaicinoid content.

Some hot sauces use their Scoville rating in advertising as a selling point. Chili peppers, fruits of the Capsicum genus, contain a great deal of capsaicin.

Scoville Organoleptic Test

In Scoville's method, a solution of the pepper extract is diluted in sugar syrup 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 chilis, such as habaneros, have a rating of 200,000 or more, indicating that their extract has to be diluted 200,000 times before the capsaicin presence is undetectable. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity.

High-performance liquid chromatography

Spice heat is now usually measured by a method using high performance liquid chromatography (HPLC). 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.

List of Scoville ratings

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.

Below is a list of Chile peppers and their Scoville Heat Units. Due to variations in growing conditions, soil and weather, peppers tend to vary between the lower and upper levels listed, but can go beyond them.