Behind the Burn

There are 2 main chemicals responsible for the burning sensation that chili heads crave. The first, Capsaicin, is found in hot chili peppers and the other, allyl isothiocyanate, is found in foods like wasabi and horseradish. Both produce a different kind of burn and affect the body in different ways. Lets first consider Capsaicin the primary “burning” agent of chili peppers:

Capsaicin:

Molecular Formula: C18H27NO3

Molecular Weight: 305.41

A.K.A: Zostrix, Styptysat, capsaicine

Capsaicin is a member of a group of compounds called vanilloids. This group includes chemicals with very distinctive flavors and similar molecular structure. They all contain a hexagonal ring of carbons called a benzene ring. Other examples of vanilloids are vanillin (in vanilla), eugenol (in cloves) and zingerone (in ginger). Capsaicin contains a polar amide (-NHCO-) group that makes it odorless with a very low volatility. Even though you can not smell it, you sure can feel it!

The capsaicin molecule contains a long hydrocarbon (fatty) tail which allows it to strongly bind with the lipoprotein receptor of cells. This binding is what triggers the burning. The tail also allows the molecule to penetrate the lipid-rich cell membrane, extending and intensifying the burn. Once capsaicin binds to the receptors it “opens a door” in the cell membrane that allows for the flow of calcium ions into the cell, triggering a pain signal. This signal is then transmitted to the next cell just as if those cells were exposed to actual heat. Capsaicin excites heat receptors (pain fibers) in the skin called polymodal nociceptors. In doing so, it fools the central nervous system into triggering a neural response. Capsaicin does not trick the central nervous system into thinking it is in danger, as an extreme temperature would cause, but just that there is an intense stimuli producing a moderate warmth on the skin. The central nevous system then signals the body’s defense mechanism which widens the blood vessels, produces sweat and causes flushing (redness of skin). As many chili heads know spicy food can be addictive. In response to capsaicin, the central nervous system also triggers the release of endorphins, which promote a sense of pleasure. On molecular, cellular and sensory levels the body shows a similar reaction when faced with either chili burns or heat burns.

This reaction is not only contained to the mouth, in fact the “burn” is not a taste at all. It is a related, but distinct sensory experience. The pain receptors involved are located all over the body. This is why rubbing a habanero on your arm causes the same “burn” and warmth as if it were placed in the mouth. This type of chemical “trick” of the nervious system can also be seen, in a different way with menthol. Menthol tricks the body into sensing a cool burn, while capsaicin a hot burn. These chemicals are accidents of human physiology. They fool pain receptors who’s real function is to warn the body of dangerous events, like burns and inflammation. Luckily, we have been able to turn this accidental physiological response into something pleasurable.

Allyl Isothiocyanate:

Molecular Formula: C4H5NS

Molecular Weight: 99.1542

A.K.A: Mustard Oil, Redskin

The burning sensation and burning chemical from hot mustard, wasabi or horseradish is very different from that of peppers. While capsaicin is responsible for the burn in peppers, allyl isothiocyanate produces the nasal flaring sensation to which wasabi and horseradish are known. When either of these plants are chewed, grated or processed the plant cells are damaged. This damage releases the enzyme myrosinase which catalyzes the degradation of sinigrin, in horseradish or rhizome thioglucsides in wasabi, into allyl isothiocyanate. Allyl Isothiocyanate is very volatile (vaporizes easily) which is why the flavor hits the nose so strongly. The TRPA1 receptors in the nasal cavity recognize Allyl Isothiocyanate and sends a pain signal to the brain. Since it is in a vapor form and not bound as tightly to the receptors the burn subsides much more quickly than the burn from capsaicin. This chemical reaction in an evolutionary sense was supposed to ward of animals from eating them, which has not worked out all that well!