The Scoville scale is the measurement of spicy heat of chili peppers or other spicy foods. Specifically it’s a measurement of capsaicin concentration reported in Scoville heat units (SHU). Capsaicin is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact. Capsaicin, and several related compounds, are called capsaicinoids and are produced by chili peppers as a deterrent.
The scale is named after its creator, American pharmacist Wilbur Scoville. His method, devised in 1912, is known as the Scoville Organoleptic Test. Until 1912, there was no widely used method for measuring the heat of a pepper.
Wilbur’s method of measurement (the Scoville Organoleptic Test) is imprecise, due to the use of human testers. Capsaicin is the compound in chile peppers that makes them irritatingly hot. A measured amount of capsaicin oil is extracted from a pepper using an alcohol solution. Five experienced human testers are gathered and sugar water is added to shot glasses of the capsaicin until three out of five panelists can no longer perceive any heat. The Scoville heat unit (SHU) rating is then assigned based on the quantity of dilution
Now I’m sure you can see the problems with using this method. A taster’s palate and their number of mouth heat receptors varies greatly among people. You may be able to handle some of the hottest peppers out there, and perhaps you know someone that thinks jalapeños are deadly hot. You probably have less mouth heat receptors than your friend, making it not hurt quite as much. There are people out there without any heat receptors at all, for whom ghost chiles are as mild as a bell pepper.
Another weakness is sensory fatigue: the palate is quickly desensitized to capsaicins after tasting a few samples within a short time period making it difficult to compare multiple samples of the same pepper. Results vary widely between laboratories that use this testing method.
Understanding that this testing method probably isn’t the best, Dr. Paul Bosland, professor of horticulture at New Mexico State University, helped devise a system called high performance liquid chromatography (HPLC), allowing scientists to accurately determine how many parts per million of heat-causing alkaloids are present in a given chile pepper.
Researchers and people in the industry now use this more reliable HPLC. The fruit are dried and ground, then water is filtered through the remains like coffee to extract the capsaicinoids, which are placed in a machine to measure their exact quantities.
Armed with this knowledge, I would say use the Scoville scale as a guideline. Results for any pepper can vary depending on its cultivation conditions and the laboratory methods used test it’s heat. Plus, heat values for any pepper can vary depending on seed lineage, climate and soil (this is especially true of habaneros).