Capsaicin gives peppers their hotness.
The compound is used in a wide range of concentrations in the production of pharmaceuticals and spicy food products. SHUs are based on the amount of dilution in an alcohol and sugar solution that is required to make the hotness undetectable to a majority of human testers. After a wash step, substrate is added, which reacts with the bound conjugate to produce blue color. More blue color means less capsaicin. A stop solution is then added, which changes the blue color in the wells to yellow.