The Scoville Heat Scale is based on Scoville heat units(SHU) which are used to measure the pungency of chili peppers. This unit was introduced in 1912 by the pharmacist Wilbur Scoville. It measures the level of heat in peppers, primarily caused by compounds known as capsaicinoids, with capsaicin and dihydrocapsaicin making up about 95% of these compounds. Capsaicin does not dissolve in water, but it dissolves in alcohol and fats. That’s why consuming whole milk, which contains butterfat, can help ease the burning sensation in the mouth, whereas drinking water is not as effective.

Scoville Heat Scale

Originally, the Scoville Heat Units were determined through a process that involved dissolving 1 gram of powdered pepper in 100 milliliters of ethanol. The ethanol extracts the capsaicin, which is then diluted with sweetened water. The solution is tested by a panel of human tasters who assess the heat level by detecting the point at which the pepper’s spiciness is no longer noticeable. The degree of dilution required to reach this threshold indicates the pepper’s heat level in Scoville units. For example, if a pepper’s solution needs to be diluted 50,000 times for the heat to be barely perceptible, it is rated at 50,000 Scoville units.

This traditional method (scoville heat scale ) has recently been replaced by a more precise technique known as high-pressure liquid chromatography (HPLC), which can directly measure the concentration of capsaicin and related compounds in parts per million (ppm). The results can be expressed in ASTA units (named after the American Spice Trade Association) or in Scoville Heat Units. To convert from ppm capsaicin to SHU, multiply by 16, as pure capsaicin has a pungency of around 16 million SHU. For example, if a pepper contains 1,000 ppm capsaicin, its heat level would be approximately 16 million SHU.

While breeders are continuously working to create even hotter pepper varieties, there is a theoretical upper limit to the Scoville Heat scale, which is the heat of pure capsaicin—16 million SHU. The practice of rating pepper heat goes back centuries. In fact, the Nahuatl language of the Aztecs had six distinct words to describe the various levels of spiciness in peppers.