Scoville heat units

The Scoville scale is a measurement of the spicy heat of a chilli pepper, due to its capsaicin content. Capsaicin is a chemical compound that stimulates chemoreceptor nerve endings in the skin, especially the mucous membranes. The number of Scoville heat units (SHU) indicates the amount of capsaicin present. The scale is named after its creator, American chemist Wilbur Scoville, who developed a test for rating the pungency of chilli peppers. His method, which he devised in 1912, is known as the Scoville Organoleptic Test.

Scoville Organoleptic Test
In Scoville's method, an alcohol extract of the capsaicin oil from a measured amount of dried pepper is added incrementally to a solution of sugar in water until the "heat" is just detectable by a panel of (usually five) tasters; the degree of dilution gives its measure on the Scoville scale. Thus a sweet pepper or a bell pepper, containing no capsaicin at all, has a Scoville rating of zero, meaning no heat detectable. The hottest chilis, such as habaneros and nagas, have a rating of 200,000 or more, indicating that their extract must be diluted over 200,000 times before the capsaicin presence is undetectable. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity. Tasters taste only one sample per session.