For ages, people have exploited psychophysical methods in order to characterize and quantify sensory experiences. For example, more than 2000 years ago, Hipparchus, a Greek astronomer and mathematician, used psychophysics to develop what is today known as the scale of stellar magnitude, an upside down scale in which the brightest stars are of the first magnitude, the next brightest of the second magnitude and so forth. 
This scale of stellar magnitude, which is still used today, was based on Hipparchus' own experiences. though it's a long way from stellar magnitude, the heat produced by chile peppers was originally quantified using a similar, simple but ingenious psychophysical procedure.
Wilbur Scoville, a pharmacologist working for Detroit's Parke Davis drug company, was interested in the potential health benefits of chile peppers. For years, people believed that eating peppers stimulated digestion, etc. In fact, as early as 1493, a Spanish physician who had traveled with Columbus to the Americas --where chile peppers were native-- wrote enthusiastically about chile peppers' medicinal value.
Scoville's research efforts were frustated by the difficulty of quantifying any pepper's pungency (heat). He tried various standard chemical assays with little success: the results of those assays failed to square with the perceived heat that people experienced. For the answers that he sought, Scoville was forced to rely on human tongues --his own and those of co-workers. In the process, he developed the Scoville Organoleptic Test (1912), a psychophysical procedure that identifies the dilution threshold --the amount of stimulus that is just detectable. ("Organoleptic" is a term used in the perfume, food, flavor, and beverage industries. It covers the qualities (as taste, color, odor, texture, crispness and mouthfeel) of a substance that stimulates the sense organs. For example, taste scientists can talk about the organoleptic properties of things as wonderful as Brie de Meaux cheese or Cape Cod Potato Chips, or of things less wonderful, such as SPAM© processed luncheon meat.)
Chile peppers belong to the genus Capsicum, a large group of plants that include many non-hot members such as bell peppers and pimentos. Although the origin of the genus' name is in dispute, the Oxford English Dictionary links the name to the Latin word
The fruits of most plants in the genus Capsicum contain organic compounds called Capsaicinoids. These compounds, which are most abundant in the fruit's seeds and ribs, give chile peppers their characteristic pungency, or heat. Clearly, not all chile peppers are created equal when it comes to in pungency. The heat generated by eating a habenero pepper is far greater than the heat produced by a jalapeno. Why do different kinds of chile peppers produce different pungencies (heat levels)? The answer lies in chemicals called Capsaicinoids. Members of the Capsicum family differ how much Capsaicinoid they contain, and also in the particular kinds of Capsaicinoid they have. The most pungent Capsaicinoid is Capsaicin, also known as N-Vanillyl-8-methyl-6-(E)-noneamide. Capsaicin, which is pronounced "cap-say-sin", has the chemical formula C18H27NO3. Swallowing a teaspoonful of pure capsaicin could take your breath away. And I mean this in the literal sense: that teaspoonful would stop your respiration
Capsaicinoids are somewhat soluble in water, but are very soluble in fats, oils and alcohol. Incidenally, this is why rinsing your mouth with water will do very little to conquer the lingering mouth burn you can get from a chile pepper: the burning ingredients resists solution in water. Whole milk --not skim or 1% milk-- is way better. The original version of Scoville's test consisted of soaking a specific amount of a chile in alcohol (ethanol). This allowed some of the chemical to dissolve into the alcohol. After the chile section had soaked for the prescribed time, Scoville took a small volume of the extract, and found out how much sugar water had to be added until the heat was barely perceptible. He started with an extremely dilute solution that would be below threshold, and worked up from there. And when he reached a solution where people could first taste the characteristic chile bite, Scoville worked out how hot the chile must have been.
In his first published work (1912) on the topic, Scoville wrote
The method I have used is as follows: One grain of ground capsicum is macerated over night in 100 cc. of alcohol. After thorough shaking, filtered. This alcoholic solution is then added to sweetened water in definite proportions until a distinct but weak pungency is perceptible on the tongue. By this method, Japan Chillies tested 1 in 20,000 to 1 in 30,000... and Mombassa Chillies 1 in 50,000 to 1 in 100,000.
To understand Scoville's logic, suppose that the heat generated by a really weak pepper became imperceptible when just a little water was added, say 250 units of water to 1 unit of pepper-derived capsaicin-dissolved in ethanol. That would produce a Scoville value of 250. In contrast, Zanzibar chiles had scores of 40,000 to 50,000 units --their pungency was detectable even when 40,000-50,000 units of water had been added to 1 unit of pepper extract. Habanero peppers, by the way, score upwards of 250,000. And pure Capsaicin has a pungency of 16 million Scovilles, which means that to dilute it so that its hotness can just barely be tasted, about 4,000 gallons of sugar water would have to be added to 1g of capsaicin.
| Scoville Units | Type of pepper | 0 | Bell Pepper, Sweet Italian, Pimento |
| 100-500 | Pepperoncini |
| 500-1,000 | New Mexican, Anaheim, Mulato |
| 1,000-1,500 | Espanola Poblano |
| 1,000-2,000 | Ancho, Pasill |
| 1,000-2,500 | Cascabel, Cherry |
| 2,500-5,000 | Jalapeno, Mirasol, Guajillo |
| 5,000-10,000 | Hungarian Wax |
| 5,000-20,000 | Serrano |
| 15,000-30,000 | de Arbol |
| 30,000-50,000 | Cayenne, Tabasco |
| 50,000-100,000 | Chiltepin, Santaka, Thai |
| 100,000-200,000 | Jamaican |
| 100,000-350,000 | Habanero, Scotch Bonnet |
| 575,000-600,000 | Red Savina |
| 5,000,000+ | Law enforcement pepper spray |
| 16,000,000 | Pure Capsaicin |
Until the late 1970's and early 1980's spice companies relied on panels of human tasters to rate peppers' heat content. This was extremely expensive and time-consuming, and sometimes painful. Adaptation of the tongue limited any of the tasters to just six tastes per working day, and the results were not as consistent as one would like.
Today, when chile pepper producers need to check their products consistency, they use High-Performance Liquid Chromatography (HPLC). For this procedure, the pods of chile peppers are dried, and ground. The result is analyzed by HPLC, which breaks down the mixture of compounds (solutes) into separate chemical components. HPLC prouduces a chromatograph in which the amount of each component is plotted against the component's weight in atomic mass units. Because molecules of different capsaisinoids differ in mass, the chromatograph shows peaks of varying height arrayed along the axis that represents atomic mass.
The total heat is quantified, from the HPLC graph by summing the various capsaisinoid-related peaks. The scale employed is one produced by ASTA (American Spice Trade Association) scale. But the old Scoville scale remains so popular that most packages of chile peppers give not only the ASTA units, but also their Scoville unit equivalen. The chromatographic method measures the total heat present, and it also allows the amounts of the individual capsaicinoids to be determined --which a human tongue cannot do. In addition, unlike Scoville's psychophysical approach, which was limited by the tasters' tongues and endurance of pain, a chromatograph makes it possible to assay many samples within a short
If you like/love hot peppers but prefer to take them in liquid form, Tabasco© is the best known hot sauce in the U.S. But this sauce is has nowhere near the maximum pungency that you can put in a bottle. For starters, the sauce greatly waters down the pungency of its namesake ingredient, the tabasco chile pepper. In fact, the Scoville value for tabasco chile peppers is about 10-15 times higher than the Scoville value for their namesake sauce. If you crave a bottled sauce that will really tingle your tongue and nose, the Mo Hotta, Mo Betta company in Savannah, Georgia, USA, is heaven for hot sauce fanatics. In addition to recipes and sales information, its website provides Scoville unit equivalents for the hot pepper sauces it sells. Many of these have evocative and scary names, such as "Mad Dog Inferno" (~90,000 Scoville units), "Widow - No Survivors" (~84,000 Scoville units), and "Blair's 2 a.m. reserve" (~700,000 Scoville units). To put these frightening values into perspective, ordinary Tabasco© sauce weighs in at a puny 2,000 Scoville units! So if a dash of Tabasco sauce on your veggie burger leaves you gasping, "Blair's 2 a.m. reserve" is definitely not for you. Incidentally, none of these hot sauces approaches the 5,000,000+ Scoville unit pepper sprays that police use on protestors and miscreants. That is seriously painful stuff.
Many, many thanks to Robert Cormack, New Mexico Insitute of Mining and Technology, for generously making available some of the information and material for this page.