Putting out the Fire
Chiles are well named -- Capsicum comes from the Greek kapos, “to bite.” The pungent heat is concentrated in the inner membranes, or placenta, not in the seeds or flesh. The heat comes from a group of alkaloid chemicals called capsaicinoids, principally capsaicin (C 18H 27NO 3) and dihydrocapsaicin. Some chiles are so hot they can actually blister the skin, so it’s important to wear gloves when handling them, and be VERY careful to keep the juice out of your eyes.

These oily compounds are very soluble in fat and alcohol, but hardly at all in water. So to put the fire out, it may work better to take a swig of beer than a gulp of water—but milk or yogurt are far superior heat quenchers. That’s because certain protein compounds in milk literally work like detergent to strip the capsaicin from its receptor binding sites.

How hot is HOT?
Chiles evolved their hot taste as protection from mammalian predators whose digestive tract would destroy the seeds. Their flavor is affected by the genetic ancestry of each plant and by the environmental conditions under which it is grown. The most precise way of measuring a chile’s “heat,” or pungency, uses High Performance Liquid Chromatography. A less formal test involves diluting a sample until the heat can no longer be tasted. The results are popularly expressed as Scoville Heat Units. This comparative list is for fresh chiles – dried ones can be much hotter.