What Chemicals make Chili Peppers Hot and Spicy and Why are some Hotter than Others?

The different sensory effects in hot and spicy food are caused by several different chemical compounds.

It would be much neater if we had individual descriptive words for each of these sensations, because they are indeed all different. Instead, we apply the words hot, peppery, spicy, pungent, piquant, biting, zingy, and sharp almost indiscriminately to all. But black pepper, chili peppers, ginger, mustard, horseradish, and wasabi are all distinguishable from one another by their distinctive brands of what I’ll generically call pungency, from the Latin pungere, meaning to prick or puncture.

The French piquer and the Spanish picar, meaning to prick or sting, give us the word piquancy, which is often used interchangeably with pungency but has the slightly broader connotation of an agreeable tartness or zest. A sauce, for example, may be piquant because of the pungent pepper it contains.

But “hot” spices give us much more than their “heat.” Like all foods, they have their own complex flavors. Different hot chili peppers, which are so often characterized simply by their relative heats, contribute unique earthy, fruity, smoky, sweet, or flowery nuances to our dishes. Mexican cuisine excels at using different types of chili peppers in dishes that will benefit from their different flavor profiles.

Let’s consider some of the “hot stuffs,” one at a time.

Black pepper comes from the Piper nigrurn plant, whose species name, suitably enough, is Latin for “black pepper.” When the plant’s berries are picked almost ripe and dried in the sun, enzymes turn them dark and they shrivel into our familiar black peppercorns, with their softly pungent flavor.

Green and white peppercorns are the same berry, but picked and processed differently. Green peppercorns are picked soft and unripe, then either dried or pickled in brine or vinegar, in which state they are often used instead of (and occasionally confused with) capers. White peppercorns are picked when ripe and red but then are allowed to ferment. Softened by the fermentation, the outer layer of skin can be washed off to expose the pale seed within. Alternatively, the outer skins of black peppercorns may be removed mechanically to uncover the white seed. After being dried, white peppercorns are somewhat less exciting than black ones, but they come in handy when you don’t want to add black specks to a white sauce.

The pungent chemical in these true pepper berries is piperine , the main aromatic ingredient released when the peppercorns are cracked open. As this flavorful oil slowly evaporates, the cracked or ground pepper loses its pungency. That’s why every recipe worth its salt and pepper specifies that the pepper be “freshly ground.”

In quite another category, the fiery-hot “peppers” discovered in the New World by Columbus and other Spanish explorers (yes, I know he was Italian, but Spanish pesetas were footing the bill) are not real pepper because they are not from the Piper nigrurn plant. They are members of the Solanaceae family of plants, specifically, varieties of Capsicum annuurn. A cynic might suspect that the explorers named these fruits pirniento because real pepper (pirnienta in Spanish) was such a valuable spice in Europe at the time that a little stretching of the truth might bring a better price back home. The explorers also called them chile and aji after their Aztec and Taino Indian names, and the world has been confused about what to call them ever since.

Here’s a quick rundown on this bewildering state of affairs.

In contemporary Spain, the capsicums are still called pimientos, while in Mexico they are still called chiles. But in other Latin American countries they are called aji, not to be confused with ajo, which is garlic. The British changed chile to chilli, while in the United States we spell it chili, reserving the word pirniento for another capsicum species, the sweet bell pepper, except that we sometimes spell pimiento pirnento, not to be confused with pirnenta, the tree from which we get allspice berries, which were so named because they taste like a combination of cloves, cinnamon, and nutmeg. (Got all that?)

The essential oils in chili peppers contain chemicals (alkaloids) called capsaicin (cap SAY-uh-sin) and dihydrocapsaicin plus a few closely related compounds, all collectively known as capsaicinoids. These are the burning, pungent components of hot chili peppers. About 80 percent of the capsaicinoids reside in the placenta, the fleshy ribs that fasten the seeds to the walls, not in the seeds themselves, as is commonly believed. Because of the way the seeds are attached, cooks who habitually scrape away “the seeds” with a spoon or knife blade have unwittingly been removing the real culprits, the ribs.

Capsaicinoids are odorless and are not detected by our noses or taste buds; the true flavors of the various capsicum fruits are contributed by other chemicals, just as in nonpungent fruits. Instead of being tasted or smelled, the capsaicinoids stimulate nerve endings in our skins and mucous membranes, specifically those of the trigeminal nerve, which among other jobs conveys sensations of pain and heat to the brain from the face, mouth, and nose. Hence, our brains persuade us to call these fruits “hot” and their pungency “heat,” and even to sweat when we eat them.

How hot is a hot pepper? The various species of capsicums are often listed in order of pungency, expressed in “Scoville units.” Back in 1912 , Dr. Wilbur Scoville, an American pharmaceutical chemist, devised his Scoville Organoleptic Test for measuring just how hot a pepper tastes to a panel of tasters. He mixed the ground-up pepper with sugar water and diluted it over and over again until the tasters could no longer distinguish any pungency. (Homeopathic capsaicin, anyone?) The number of times it had to be diluted is known as the number of Scoville Units of heat for that pepper.

The species we call bell peppers or sweet red or green peppers are capsicums that contain little if any capsaicin; they score zero on the Scoville scale. At the other end of the spectrum, the heat of pure capsaicin itself didn’t disappear until a dilution factor of about 16 million. Mexican anchos registered 1,000 to 2000 , jalapenos 2,500 to 5,000, and cayenne 30,000 to 50,000. Habaneros, at 200,000 to 300,000, are the widely acclaimed champs, although that record has recently been surpassed by the Capsicurn annuurn var. aviculare or tepin, a tiny (quarter-inch) wild pepper found in the mountains of northern Mexico. Scoville never saw one.

Note, however, that the same species of pepper can have different pungencies depending on growing conditions, so quoting exact Scoville ratings and arguing about who can eat the hottest pepper, an unfathomable point of pride among self-proclaimed chili-heads, is pointless.

Moving on from peppers both real and misnamed, we find several other assertive spices whose blends of pungent chemicals in their essential oils can kick our foods up a notch. (Apologies, Emeril.) You’ll note that the most pungent chemicals are in the mustard family of plants, the Brassicaceae.

No need to memorize the names; they won’t be on the final. But notice how isothiocyanates are the main pungent chemicals of the mustard family. They are formed when the plants’ seeds or roots are cut or crushed and the cells rupture, whereupon an enzyme in one part of the cells reacts with certain sulfur-containing compounds in another part to form the isothiocyanate. (Thioin the name of a chemical compound indicates that its molecules contain one or more sulfur atoms.)

Allyl isothiocyanate is the very potent “hot stuff” in horseradish, mustard seed, and wasabi. It is also the primary component of mustard oil, which is used very sparingly in cooking, notably in some Chinese stir-fries.

Contrary to some scare stories that have made the rounds on the Internet, mustard oil has no chemical relationship whatsoever to the mustard gas, actually not a gas but an oily liquid sprayed as a mist,  used by the Germans in World War I as a chemical weapon. It was called mustard gas only because of its sharp, acrid smell and because it irritates the skin like an old-fashioned mustard plaster. Of course, the irritation caused by mustard gas is infinitely worse: It kills rather than heals.

The capsaicinoids in hot peppers stimulate the same nerve endings in our mouths as heat does, but because they don’t create actual heat, sipping cold water to “cool” an overenthusiastic mouthful is useless.

Capsaicin oils are not very soluble in water, although they do dissolve in alcohol. But, alas!, beer, at about 5 percent, doesn’t contain enough alcohol to remove them from your burning tongue. Fortunately, however, tequila (at 40 percent alcohol or more) is better and is quite likely to be handy wherever too spicy foods are served.

Milk and sour cream are even better than tequila, because their protein molecules (mostly casein) are attracted to oils and drag them away, in much the same way as soap drags away oily dirt. But if you can’t bear to swap your tequila for milk (ply, carramba!), just chew a piece of bread or flour tortilla, which will abrade and absorb the oil from your tongue. And drink the beer anyway. Who needs an excuse?

Wilbur Scoville’s highly subjective method of determining pungency is gradually being replaced by more scientific methods. Using a technique known as high-performance liquid chromatography, or HPLC, chemists today can determine how much actual capsaicin and its relatives, dihydrocapsaicin, nordihydrocapsaicin, homocapsaicin, and homodihydrocapsaicin, a capsicum pepper contains. Capsaicin and dihydrocapsaicin together make up about 80 to 90 percent of the capsaicinoids in capsicum peppers. In pure form. Wilbur rated these two chemicals at 16 million Scoville units, while the other three scored a relatively wimpy 9,000 or so.