Some like it hot --really hot!

©Robert Sekuler 2004

For a very long time people have used psychophysical methods to characterize and quantify experiences. For example, more than 2000 years ago Hipparchus, a Greek astronomer and mathematician (190 - 120 BC), 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 chemist working for the Parke Davis drug company, was interested in the potential health benefits of chile peppers. For years people believed that eating peppers stimulated digestion, etc. Scoville's work on peppers was 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 answers, Scoville had 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 stimulate the sense organs. For example, one can talk about the organoleptic properties of things as wonderful as Morbier cheese or Cape Cod brand potato chips, or as dreadful as SPAM luncheon meat.

Chile peppers belong to the genus Capsicum, a large group of plants that include non-hot members such as bell peppers and pimentos. The genus gets its name from the fact that with some imagination the plants' fruit do sort of look like boxes --and "caps" is Latin for box.

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, all chile peppers are not created equal in pungency. The heat generated by eating a habenero pepper is far, far greater than the heat produced by a jalapeno. Why do different kinds of chile peppers produce different pungencies (heat levels)? 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 will curtail your respiration

Different varieties of Capsicum vary in the amount of Capsaicinoids they contain, and also in their relative amounts of various Capsaicinoids. Capsaicinoids are somewhat soluble in water, but are very soluble in fats, oils and alcohol. BTW: this is why rinsing your mouth with water will do very little good in conquering any undesirable, lingering mouth burn from a chile pepper: the burning ingredients resists solution in water. Whole milk --not skim-- is way better.

Scoville's original test consisted of soaking a specific amount of a chile in alcohol (ethanol) for a specific amount of time. Capsaicinoids are alcohol soluble. Then he 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.

Suppose, the heat generated by some 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 could be detected when 40,000-50,000 units of water had been added to 1 unit of pepper extract. Habaneros, 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.

Selected pepper varieties and their Scoville values
Scoville Units Type of pepper
0 Bell Pepper
Sweet Italian
100-500 Pepperoncini
500-1,000 New Mexican
1,000-1,500 Espanola
1,000-2,000 Ancho
1,000-2,500 Cascabel
1,500-2,500 Rocotillo
2,500-5,000 Jalapeno
5,000-10,000 Hungarian Wax
5,000-20,000 Serrano
15,000-30,000 de Arbol
30,000-50,000 Cayenne
50,000-100,000 Chiltepin
100,000-200,000 Jamaican
100,000-350,000 Habanero
Scotch Bonnet
575,000-600,000 Red Savina
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. Adaptation of the tongue limited any taster 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, then ground. After the capsaicinoids are extracted, the extract analyzed. HPLC breaks down a mixture of compounds (solutes) into separate components, producing 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 will show 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 the ASTA (American Spice Trade Association) scale. But the Scoville scale is so popular that on most packages of chile peppers the ASTA units are translated into good old Scoville units. 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 the old psychophysical approach, 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, the Mo Hotta, Mo Betta company (Savannah, Georgia, USA) runs a great website for hot sauce fanatics. In addition to recipes and sales information, the site provides Scoville unit equivalents for the hot pepper sauces it sells. And many of those sauces have evocative names such as "Mad Dog Inferno" (~90K Scoville units), "Widow - No Survivors" (~84K Scoville units), and "Blair's 2 a.m. reserve" (~700K Scoville units). To put these frightening values into perspective, ordinary Tabasco©  sauce is merely 2K 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.

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.