Capsaicin

Most of the hot in hot peppers comes from capsaicin and a closely related compound, dihydrocapsaicin. It occurs in much lower quantities in oregano, cinnamon, and cilantro.

The compound's molecular weight is the highest of any of the vanilloids we've looked at so far, and the side chain contains a polar amide (-NHCO-) group. That makes capsaicin's volatility very low, and it is completely odorless. (A very good thing!)

Even without a telltale fragrance, capsaicin's presence in foods is hard to miss. A solution that contains only 10 parts per million produces a persistent burning sensation when placed on the tongue. It is tasteable at much lower concentrations. The intense flavor results from the molecule's long hydrocarbon tail. The chain allows it to bind very strongly with its lipoprotein receptor, which has some hydrocarbon side chains of its own (like dissolves like!) The fatty tail also allows the molecule to slip through lipid-rich cell membranes, making the burn more pervasive and persistent.

Several capsaicin-like compounds found in chiles have slight structural variations in the hydrocarbon tail, which changes their ability to bind to the receptors and their ability to penetrate layers of receptors on the tongue, mouth, and throat. That may explain why some chiles burn in the mouth, while others burn deep in the throat.

The perception that peppers are "hot" is not an accident. The capsaicin key opens a door in the cell membrane that allows calcium ions to flood into the cell. That ultimately triggers a pain signal that is transmitted to the next cell. When the cells are exposed to heat, the same events occur. Chile burns and heat burns are similar at the molecular, cellular, and sensory levels.

One expects that the long hydrocarbon tail will make capsaicin less water soluble than vanillin. This is indeed the case. Capsaicin is insoluble in cold water, but freely soluble in alcohol and vegetable oils. This is why drinking water after munching an habanero pepper won't stop the burning. A cold beer is the traditional remedy, but the small percentage of alcohol will not wash away much capsaicin. For relief from a chile burn, drink milk. Milk contains casein, a lipophilic (fat-loving) substance that surrounds and washes away the fatty capsaicin molecules in much the same way that soap washes away grease.

High concentrations are toxic. Exposure is painful and even incapacitating. Capsaicin prevents nerve cells from communicating with each other by blocking the production of certain neurotransmitters; at high concentrations it destroys the cells! Capsaicin's toxicity makes chiles more than just a food- they're also a weapon. The Mayans burned chiles to create a stinging smoke screen, and threw gourds filled with pepper extract in battle. Nowadays, capsaicin is the active ingredient in pepper sprays, used to ward off attacking muggers, dogs, and bears.

Paradoxically, capsaicin's ability to cause pain makes it useful in alleviating pain. Exposure to capsaicin lowers sensitivity to pain, and it is applied as a counter irritant in the treatment of arthritis and other chronically painful conditions.

People that eat lots of spicy capsaicin-rich foods build up a tolerance to it. The incentive: a small jolt of capsaicin excites the nervous system into producing endorphins, which promote a pleasant sense of well-being. The endorphin lift makes spicy foods mildly addictive (and for some, an obsession).

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