What are Scoville Units?

A pharmacist named Wilbur Scoville invented the Scoville Organoleptic Scale in 1912 to measure the heat of peppers. A "Scoville Unit" is actually a measure of capsaicin (the chemical in hot peppers that is responsible for their heat).

Scoville's test was a comparative taste test that is considered subjective by today's standards. A more sophisticated method is in use today, but in honor of Wilbur Scoville, the unit of measure is still called the Scoville.

The capsaicin level in peppers can vary from plant to plant due to local environmental conditions. This means that a pepper's rating is an average measure.

The hottest pepper on record is the Habanera/Scotch Bonnet. Some claim it is one variety, while others claim that the Habanera and Scotch Bonnet are slightly different varieties. Habanera Peppers are rated at 100,000 to 350,000 Scoville units. By contrast, the Serrano Pepper comes in at about 5,000 to 15,000 Scoville units.

What Heats Up That Hot Sauce
What causes the “heat” in peppers? All hot peppers belonging to the genus capsicum, which includes red peppers, Tabasco's, habaneras, and paprika, contain capsaicinoids that produce a burning sensation in the mouth by acting directly on the pain receptors in the mouth and throat. At higher levels, they cause the eyes to water and the nose to run, and they often induce perspiration. There are five common naturally occurring capsaicinoids. The primary capsaicinoid, capsaicin or trans-8-methyl-N-vanillyl-6-nonenamide, is so hot (rated at 16 million Scoville units) that a single drop diluted in 100,000 drops of water will produce a blistering of the tongue. Capsaicin is 70 times hotter than piperine, the spicy principal in black pepper, and 1000 times stronger than zingerone, the active ingredient of ginger. It is barely soluble in water but is very soluble in oils or alcohols.

The second most common capsaicinoid, dihydrocapsaicin (DC), is just as hot. Together, these two comprise 80–90% of the total capsaicinoids found in peppers (typically, 0.01–1% by wt content). Others are nordihydrocapsaicin (NDC), homocapsaicin (HC), and homodihydrocapsaicin (HDC), with Scoville ratings ranging from 6.9 million to 9.3 million units.

Testing in the Future
So what’s in the future for “heat” testing? No doubt an HPLC  (High Performance Liquid  Chromatography) is faster, more cost-effective, and easier to handle than a panel of tasters; and it never suffers from taste fatigue.

Pure Capsaicin measures 16,000,000 Scoville units! That is so hot  that a single drop diluted in 100,000 drops of water will produce a blistering of the tongue. Capsaicin is 70 times hotter than piperine, the spicy principal in black pepper, and 1000 times stronger than zingerone, the active ingredient of ginger. It is barely soluble in water but is very soluble in oils or alcohols.

Note:

When peppers are dehydrated, they tend to increase in "heat" by about 10 times.