Here is a collection of compounds with noticeable flavors or aromas. In addition to sweeteners like aspartame and sodium cyclamate are fragrant compounds like menthol and camphor. Many of these are terpenes, a class of volatile alkene compounds consisting of rearranged multiples of the isoprene unit, the monomer of natural rubber. There are some musk compounds used in perfumery, and the spicy compounds in peppers and mustard. Finally there are some rather unpleasant-smelling chemicals like putrescine and cadaverine, which arise in rotting meat. Formal sugars can be found on the sugars page.
Most of these chemicals are generated by plants. In some cases there is an evolutionary reason for this, either to attract animals or insects by an attractive scent or to deter them with an unpleasant one. Other compounds may serve antibiotic or antifungal ends in the vegetable host. In other cases they may prevent dehydration or provide a pigment to encourage pollination. Still others may be artifacts of some other chemical process.
Esters are organic compounds arising from a combination of a carboxylic acid and an alcohol, giving the R-CO-O-R linkage shown. Low-molecular weight esters give fruits their characteristic odors and flavors. It is interesting that almost all the low-weight esters smell pleasant, while their acid counterparts are acrid and repellent. This is simply an artifact of our evolution; not surprisingly, a selective advantage was gained by organisms which turned up their noses at rotting foods but ate fresh ones. The coevolution of flowering plants (angiosperms) and humans is an interesting story in its own right.
The relation of the smell brain (rhinencephalon) to intelligence is an old story. It is argued that one reason for mammals developing their reasoning powers was that our verminous ancestors, small rats rummaging around for dinosaur eggs, were primarily nocturnal. Along with the development of sensitive olfactory organs – tiny molecular analyzers – they had to interpret the smells. This is a lot harder on a neurological level than the task the brains of the dinosaurs were charged with, which amounted to “chase and eat.” The development of the cerebrum directly parallels that of the smell brain. To this day, smells remain extremely strong triggers of emotions, more so than sights or sounds.
Aromatic compounds (arenes) contain a benzene or benzene-like ring structure. The delocalized pi bonding in this uniquely stable structure accounts for the chemical and physical properties of the liquid: most arenes are volatile liquids at room temperature. This pi-electron cloud also contributes to the olfactory response. Often this is perceived as a cloying sweetness, redolent of ketones, which also contain delocalized double bonds. The nose is a remarkable chemical analyzer whose detailed workings, like those of the other senses, are incompletely understood.
The other major division of organic hydrocarbons is aliphatic. Aliphatic compounds may contain rings, but not resonance-stabilized pi bonds.
Terpenes are volatile plant compounds with distinctive scents. Breaking a twig or crushing a leaf releases these compounds; depletion results in a stale taste. Fenchone, camphor and menthol are counterirritants; adding menthol to tobacco reduces throat irritation.
Essential oils are extractions (solutions) of plant materials, often rich in terpenes. Many of the terpenes impart CNS depressant effects (as do other organic solvents); they interact with neuronal phospholipid membranes and alter the function of ion gates and receptors. Absinthe, a distillate from wormwood, at one time contained thujone, and was a favorite drink of van Gogh’s (see the sedatives page).
Many of these compounds are also antiseptic; Listerine, as it is presently constituted, contains thymol, eucalyptol and menthol in addition to alcohol.
non-nutritive sweeteners & flavor enhancers
Saccharin is the only truly non-calorific sweetener in common use. First isolated from coal tar, it is a proven carcinogen, though in doses many times what one would get from food. Aspartame, a variation on phenylalanine, has about the same number of calories per gram as sugar (4 per gram) but is 180 times as sweet. It has less of the bitter aftertaste of saccharin. The cyclamates were removed from the US market over 20 years ago as suspected carcinogens; their taste was reputedly much closer to that of sugar than saccharin or aspartame.
Sucralose is a halogenated form of sucrose; appending chlorine groups where hydroxy (-OH) groups occur, in three positions, results in a molecule that the body’s enzymes cannot metabolize. [Compare to olestra, an altered form of fat.] Sunett (acesulfame potassium) is a saccharin-like compound with similar taste. Newer philosophies of application often involve mixtures of artificial sweetener and sugars. Equal, which contains dextrose and maltodextrose as well as NutraSweet, is one example.
Monellin is a polypeptide about 3,000 times as sweet as sucrose, from the serendipity berry. Thaumatin is another polypeptide, about 100,000 times sweeter than sucrose.
The red pepper principle capsaicin initially causes intense irritation followed by desensitization to pain. The mechanism for this is probably through substance P or endorphin release. These compounds modulate pain impulses both in the CNS and peripherally. Capsaicin has found recent application in topical preparations and has been said to elevate mood. Receptors in the digestive tract respond to capsaicin by releasing cytoprotective substances such as prostaglandins and mucus. Capsaicin also works as a natural decongestant. In addition to cayenne, all hot peppers contain capsaicin, as does the spice paprika. Capsaicin is also used as natural insecticide/pest deterrent.
Whole peppers and their extracts contain many other chemicals besides capsaicin, of course, abundant carotenoids (vitamins A), thiamine, vitamin C, gingerol, and assorted other organic alcohols/terpenes.
Other hot spices have their own irritant chemistries. The black pepper principles are chavicine (cis form) and piperine (all trans form). In any peppercorn there is a mixture of these two forms, with chavicine being a little sharper to the tongue. When pepper goes stale, the percentage of chavicine decreases, which the tongue perceives as staleness. Grinding the pepper facilitates the isomerization process by reliving local steric constraints that maintain the higher-energy cis forms in the peppercorn. Methysticin is another pepper compound which resembles kavain, the active ingredient of kava. Cubebine, from Java pepper, resembles the black pepper constituents.
Black mustard contains sinapine, also present as a fragment of a larger chemical (sinalbin) in yellow mustard. Sinigrin is present in horseradish as well as black mustard.
Garlic contains several sulfurous compounds with antibacterial properties including allin and allicine. Ajoene is also an antithrombitic (blood thinning agent).
1-methylpropyl-1-propenyl-disulfide is one of several organosulfur compounds from asafetida (“devil’s dung”), a rare spice. Butyl mercaptan and dicrotyl sulfide are the major odiferous principles in skunk spray.
Finally, several isothiocyanates and indoles (mustard oils; not shown) in cauliflower are responsible for the burning sensation at the core of that vegetable and several related cruciferous vegetables including broccoli; these principles may convert estrogenic steroids to more benign forms (cancer-preventative).
Musks are exocrine compounds of mammals, used to mark territory or as pheromones. These include muscone and exaltolide. Synthetic chemicals such as musk baur, which stimulate the olfactory receptors in similar ways, have been created for the perfume industry. These are typically easier to synthesize than the genuine compounds, which contain large rings (macrolides). Needless to say, there is a whole industry behind odor molecules which is byond the current scope 🙂
Decomposition of proteins leads to many malodorous organic amides including putrescine and cadaverine. Spermidine and spermine are polyamines formed from putrescine; these compounds were first detected in semen but occur widely. They are used in biochemical research: they interact with DNA. Neurine, a choline putrefaction product, has a fishy odor, and is present in fish. Pellitorine is a plant insecticide.
Oxidized fats, prevalent in fried foods, pose a health risk over the long term. 3-(1,3,5,7,9-dodecapentanyloxy)-1,2-propanediol, a potent mutagen present in human feces, has been implicated in colon cancer.