Who invented the Scoville scale, and how does it work?

In 1912, Wilbur L. Scoville, a pharmacist, devised the first modern technique for measuring a pepper’s bite. The measure is still used to this day by spice companies and sauce manufacturers everywhere. In fact, if all known peppers were measured using this technique, their scale of pungency would range from 0 Scoville units, for the bell pepper, to 350,000 units, for the Mexican habanero.

Scoville tried first to measure pungency by studying how pepper extract reacted with other chemicals but concluded that none was sensitive enough to offer readings with any degree of precision. He found that the tongue, on the other hand, was far more sensitive, capable of detecting capsaicin (the chemical in hot peppers that is responsible for their heat) dissolved in a solution a million times its volume; no laboratory test could detect such a low concentration.

Of course, this brought sneers from his peers. What could be more subjective than the tongue, they questioned. But Scoville was unfazed: "Physiological tests are tabooed in some quarters, yet when the tongue is sensitive to less than a millionth of a grain, it certainly has an advantage."

Scoville’s method was simple. He soaked each different variety of pepper separately in alcohol overnight. Because capsaicin is soluble in alcohol, the soaking extracted the pungent chemicals from the pod. Then he took a precise measure of the extract and to it added sweetened water in incremental portions until the presence of pungency was barely detectable on his tongue. In the case of Japan chiles, it took sweetened water in volumes between 20,000 to 30,000 times the pepper extract before the pungency was barely discernible. He thus rated the Japan chiles 20,000 to 30,000 Scoville Heat Units. Zanzibar chiles were rated 40,000 to 50,000, and Mombasa chiles 50,000 to 100,000.

Scoville’s name has since become closely associated with the measure of pungency, but the oral test is now being slowly replaced by a modern machine (High Pressure Liquid Chromatograph) that is as sensitive as the human tongue.

One problem with Scoville’s test is that no two tongues ever agreed, so the panelists’ estimates had to be averaged. Another problem was that the number of tests a panelist could do in a day was limited. Because the tongue would temporarily get used to a given level of pungency, it had to be given rest to cool down before resuming the task. In an eight hour period, no more than 6 samples could be run through the panel. The machine has now replaced the human testing panel. Thirty samples can now be tested within 8 hours, and accuracy is not a concern. And because the cost of a pepper is directly related to the pepper’s heat, there is less bargaining now that the heat can be tested with predictability.

The American Spice Trade Association, an industry trade group, is a strong proponent of the machine. And the HPLC machine (High Pressure Liquid Chromatograph) measurements are expressed in ASTA units, the acronym of the trade group. But Scoville’s name has become so well established that the association has had as much success in making people adopt the scale as the US has in adopting the metric system. The companies that do employ the modern instrument use a conversion scale to express ASTA units in familiar Scoville units. Such is Scoville’s legacy.

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