Scoville Heat Units (SHU) are the units used to rate the heat of peppers. The Scoville scale is the ranking of the heat of chili peppers or other spicy foods. It is a measurement of the concentration of capsaicin in the peppers and is recorded in Scoville heat units.

Capsaicin produces a burning sensation in any tissue it touches. Capsaicin, along with several related compounds, are called capsaicinoids and are produced by chili peppers. Capsaicin is the compound in chile peppers that makes them hot.

The Scoville Unit was named after american Wilbur L. Scoville (January 22, 1865 – March 10, 1942). Wilbur first tried to measure the heat of peppers in 1912. He was a pharmacologist for the Parke-Davis Company and was best known for his creation of the Scoville Organoleptic Test which is now standardized as the Scoville scale. Prior to 1912, there was no generally accepted methodology for measuring and rating chili pepper heat.

Wilbur’s original Scoville Organoleptic Test consisted of a panel of 5 tasters who would systematically taste a precise solution of chile extract and slightly sweetened water. The extract (capsaicin oil) was extracted from a pepper using an alcohol solution. The tasters diluted the solution with more sugar water until it no longer had a detectable heat sensation. When 3 out of 5 tasters could no longer perceive any heat a Scoville heat unit value was assigned.

A typical Jalapeno pepper is about 4,500 Scoville units. This means that 4,500 parts of sugar water are required to dilute one part Jalapeno extract until its heat can no longer be felt. Dilute it any further and you would not be able to taste any hotness.

As you can imagine, this testing method was highly imprecise. It was also very subjective due to the use of human testers whose palates could vary significantly with regards to heat tolerance. Also, the human palate becomes desensitized to capsaicins after a while so it is inaccurate to compare a number of samples of the same chili. As a result of this, this testing method was discontinued. However, chile heat is still given in Scoville units.

Today, high-pressure liquid chromatography (HPLC) machines measure a pepper's heat. This methodology was developed with the help of Dr. Paul Bosland, a professor of horticulture at New Mexico State University. Chili peppers are dried, ground, then water filtered through them to extract the capsaicinoids. The result is tested by the HPLC machine to measure how many parts per million of heat-causing alkaloids are present in a given chile pepper.

Although this method takes out the guess work, it only rates the heat of the sample pepper being tested, and not the absolute fire power of every chile in that variety. The actual heat of any chile pepper varies considerably based on the environment, climate, seed heritage, soil, weather, geography, harvest time, etc. SO, the Scoville Unit Scale is still just a guideline.