What Makes Peppers Hot?

I got the idea for this week's show while eating dinner. I was eating at a spicy meal at a Chinese restaurant, and it suddenly occurred to me that the "hot" taste of the Chinese mustard was very different from the "hot" taste of the Kung Pow Chicken.

To get a taste of an interesting area of science, you will need several of the following:

• hot cinnamon candy
• hot sauce
• horseradish
• ginger
• peppermint candy
• wintergreen candy
• spearmint candy

What do all those things have in common? They all stimulate your trigeminal nerve. This nerve branches to your mouth, nose and eyes and it is very sensitive to certain chemicals. Depending on the chemical, the sensation can register as heat, cold, or pain. Depending on how concentrated the chemicals are, you can get the pleasant cooling of an after dinner mint, or the blazing pain of a mouth full of very hot peppers.

The hot sensation of my Kung Pow Chicken and the hot and sour soup came from peppers. You can get a similar sensation by tasting some hot sauce. Peppers contain capsaicinoids, a group of chemicals that are concentrated in the seeds and the white membranes on the inside of the pepper. Capsaicinoids stimulate the thermal and pain receptors in the trigeminal nerve, producing a sensation that can range from pleasantly warm to very hot. The capsaicinoids are dissolved in oil, which is why water does not seem to help much in
getting rid of the heat from a really hot pepper. Water and oil don't mix, so the water does not remove the chemical. Milk works better, as the capsaicinoids will dissolve in the milk fat, and it can then carry the irritant away.

The hot mustard that came with my egg roll produced a very different sensation. Instead of a sensation of heat on my tongue, the mustard sent a strong, almost painful sensation into my nose and sinuses. Mustard and horseradish contain isothiocyanates. These chemicals vaporize and are easily carried into the nose. There they stimulate the same group of nerves, but with a very different sensation. It is not really heat, but more of a pain or "bite." Since they vaporize so easily, the sensation does not last nearly as long as the heat from peppers.

Mints act on the same nerves in a very different way. Instead of registering as heat, mints contain menthol, which stimulates the trigeminal nerve with the sensation of cold. Menthol dissolves in oil like the capsaicinoids from a pepper, but it also vaporizes like the isothiocyanates from horseradish. This means that you feel the mint in your mouth and your nose. This can be mild or almost as painful as a hot pepper, depending on how concentrated it is.

Other foods contain chemicals that stimulate the trigeminal nerve, such as ginger (gingerols), onions (Diallyl sulfide) and black pepper (piperine). Since it is not really a taste or a smell, many sources list it as a separate sense, the trigeminal sense. The next time you want to investigate your sixth sense, head for your nearest Chinese restaurant. Be sure to have some
steamed dumplings for me. Yum!

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