So, What are SHUs?
In 1912, Wilbur L. Scoville, a pharmacologist with Parke Davis, devised the first modern technique for measuring a chilli’s bite. Officially, his method was known as the Scoville Organoleptic Test.
Scoville tried first to measure pungency by studying how chilli 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 chillies that is responsible for their heat) dissolved in a solution a million times its volume - no laboratory test could detect such a low concentration.
Scoville’s method was simple. He soaked each variety of chilli separately in alcohol overnight. Because capsaicin is soluble in alcohol, the soaking extracted the pungent chemical. This solution was then diluted with sugar water until it was no longer detectable to a group of tasters' palate.
For example, if the dilution required was 1,000 units of water to 1 unit of alcohol solution then the sample was said to measure 1,000 Scoville Units. Of the panel of five taste testers, at least three members had to agree before a value was assigned. A Capsicum, or other sweet chilli - which contains no capsaicin - has a Scoville rating of zero, or no detectable heat, even when it's undiluted. On the other end of the spectrum, the hotter chillies, such as habanero chillies, have a rating of 200,000 - 300,000 (even higher), which indicates their extract has to be diluted 200,000 - 300,000 times before the capsaicin heat is undetectable.
Although the development of this method was innovative in classifying the chillies according to heat, it was highly subjective and imprecise because it involved human testers.
Today, a High Pressure Liquid Chromatograph (HPLC) machine, a sophisticated analytical method, is used to measure capsaicin content in chillies. This method is much more accurate and precise. It measures the capsaicin levels in parts per million (mg/L).
The American Spice Trade Association, an industry trade group, is a strong proponent of this machine; and the HPLC machine measurements are expressed in ASTA units, the acronym of the trade group.
But Scoville’s name has become so well established that companies that do employ the modern instrument use a conversion scale to express ASTA units in the all-familiar Scoville units.
Such is Scoville’s legacy.