The heat in chili peppers is called pungency, and it comes from a group of compounds called capsaicinoids. Capsaicin is the primary heat-producing alkaloid present in peppers. The greater the concentration of capsaicin, the hotter the pepper tastes. The amount of capsaicin in a pepper also varies according to hereditary and environmental factors.
It's often said that seeds are the source of the heat in chili peppers, but while the seeds are hot, they don't cause the heat. The membrane, called the placenta, stores the heat -- the seeds are attached to the membrane, but the heat is transferred to the seeds, not stored in them.
The mouth's pain receptors -- not the taste buds -- transmit the heat sensation with the help of a neurotransmitter called substance P [source: University of Illinois]. A runny nose and watery eyes often follow the immediate wave of fire, and some people perspire profusely. Capsaicin also releases endorphins in the body, causing some people to feel exhilarated -- the same way that endorphins causes a runner's high. There's also an added bonus to enjoying an occasional fiery pepper: The capsaicin triggers thermogenesis, a fat-burning process.
Another myth is that chili peppers can cause ulcers, but evidence suggests that capsaicin can actually protect the stomach lining [source: Mateljan]. However, anyone taking anticoagulants, such as coumadin, should avoid large quantities of hot peppers because they may thin the blood [source: Graedon and Graedon].
If you've ever swallowed a searing chili pepper, you know that you instinctively want to down a jug of ice water to douse the flames. Not so fast! The heat-inducing compounds in peppers are fat soluble, so reaching for that glass of water is like throwing water on a grease fire -- it reignites the flames. To counteract the heat, reach for bread, chocolate, milk or other dairy products. Mexican dishes often include a side of sour cream, and Indian dishes are frequently served with a side of yogurt to tame the heat.
You may be wondering how to determine the relative pungency of different peppers. Keep reading to learn about the three basic methods for measuring heat.