The Scoville scale is an empirical measurement dependent on the capsaicin sensitivity of testers and so is not a precise or accurate method to measure capsaicinoid concentration, however, capsaicin concentration can very roughly be estimated as ~18 µg/gram/SHU.
In Scoville's method, an exact weight of dried pepper is dissolved in alcohol to extract the heat components (capsinoids), then diluted in a solution of sugar water. Increasing concentrations of the extracted capsinoids are given to a panel of five trained tasters, until a majority (at least three) can 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 sensitivity to pungency; the human palate is quickly desensitised to capsaicins after tasting a few samples within a short time period. Results vary widely, ± 50%, between laboratories.
Spice heat is usually measured by a method that uses high performance liquid chromatography (HPLC). This identifies and measures the concentration of heat-producing chemicals. The measurements are used in a mathematical formula that weighs them 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.
Scoville units are a measure for capsaicin content per unit of dry mass. 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.
Since Scoville ratings are defined per unit of dry mass, comparison of ratings between products having different water content can be misleading. Typical fresh chili peppers have a water content around 90 percent, whereas, for example, Tabasco sauce has a water content of 95 percent. For law-enforcement-grade pepper spray, values from 500 thousand up to 5 million SHU have been mentioned, but the actual strength of the spray depends on the dilution, which could be a factor of 10.
Numerical results for any specimen vary depending on its cultivation conditions and the uncertainty of the laboratory methods used to assess the capsaicinoid content. Pungency values for any pepper are variable, owing 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 Bhut Jolokia; 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.
^ abcdTainter, Donna R.; Anthony T. Grenis (2001). Spices and Seasonings. Wiley-IEEE. p. 30. ISBN0-471-35575-5. "Interlab variation [for the original Scoville scale] could be as high as +/−50%. However, labs that run these procedures could generate reasonably repeatable results."
^"Chemical hazards in law enforcement". The Police Policy Studies Council. Retrieved 2009-02-09. "Most law enforcement sprays have a pungency of 500,000 to 2 million SHU. One brand has sprays with 5.3 million SHU."
^"The Truth About Defensive Spray Heat". Sabre red. "Sabre Red = 10% OC @ 2,000,000 Scoville Heat Units. Thus, 90% of the formulation dilutes the 2,000,000 SHUs creating a Scoville Content of 200,000."
^ ab"Tezpur/Naga Jolokia – The Hottest Chile?" (PDF). The Chile Pepper Institute Newsletter11 (2) (The Chile Pepper Institute, New Mexico State University). 2000. p. 5. "...the Red Savina Habanero whose Scoville rating is around 555,000 Scoville Heat Units (SHU), the 'Naga Jolokia' possesses 855,000 SHU."
^"Burning Questions" (PDF). The Chile Pepper Institute Newsletter13 (3) (The Chile Pepper Institute, New Mexico State University). 2002. p. 7. "...'Red Savina' habanero...came in at a whopping 577,000 Scoville Heat Units."
^ abLillywhite, Jay M.; Simonsen, Jennifer E.; Uchanski, Mark E. (2013). "Spicy Pepper Consumption and Preferences in the United States.". HortTechnology23 (6): 868–876. "Any pepper type with ≥ 1 SHU could be considered spicy. However, for this study, paprika (0–300 SHU), New Mexico long green or red chile (300–500 SHU), and poblano/ancho (≈1369 SHU) types were included as mild spicy peppers (Table 1)."