Aroma Compounds and Their Odors

A woman smelling flowers
The odor of flowers is recognizable because of volatile molecules.

IAN HOOTON/SCIENCE PHOTO LIBRARY/Getty Images

An odor or odour is a volatile chemical compound that humans and other animals perceive via the sense of smell or olfaction. Odors are also known as aromas or fragrances and (if they are unpleasant) as reeks, stenches, and stinks. The type of molecule that produces an odor is called an aroma compound or an odorant. These compounds are small, with molecular weights less than 300 Daltons, and are readily dispersed in the air due to their high vapor pressure. The sense of smell can detect odors are extremely low concentrations.

How Odor Works

Organisms that have a sense of smell detect molecules by special sensory neurons called olfactory receptor (OR) cells. In humans, these cells are clustered at the back of the nasal cavity. Each sensory neuron has cilia that extend into the air. On the cilia, there are receptor proteins that bind to aroma compounds. When binding occurs, the chemical stimulus initiates an electric signal in the neuron, which transmits the information to the olfactory nerve, which carries the signal to the olfactory bulb in the brain. The olfactory bulb is part of the limbic system, which is also associated with emotions. A person may recognize an odor and relate it to an emotional experience, yet might be unable to identify the specific components of a scent. This is because the brain doesn't interpret single compounds or their relative concentrations, but the mix of compounds as a whole. Researchers estimate humans can distinguish between 10,000 and one trillion different odors.

There is a threshold limit for odor detection. A certain number of molecules need to bind olfactory receptors to stimulate a signal. A single aroma compound may be capable of binding to any of several different receptors. The transmembrane receptor proteins are metalloproteins, probably involving copper, zinc, and perhaps manganese ions.

Aromatic Versus Aroma

In organic chemistry, aromatic compounds are those that consist of a planar ring-shaped or cyclic molecule. Most resemble benzene in structure. While many aromatic compounds do have an aroma, the word "aromatic" refers to a specific class of organic compounds in chemistry, not to molecules with scents.

Technically, aroma compounds include volatile inorganic compounds with low molecular weights that can bind olfactory receptors. For example, hydrogen sulfide (H2S) is an inorganic compound that has a distinctive rotten egg scent. Elemental chlorine gas (Cl2) has an acrid smell. Ammonia (NH3) is another inorganic odorant.

Aroma Compounds by Organic Structure

Organic odorants fall into several categories, including esters, terpenes, amines, aromatics, aldehydes, alcohols, thiols, ketones, and lactones. Here is a list of some important aroma compounds. Some occur naturally, while others are synthetic:

Odor Natural Source
Esters
geranyl acetate rose, fruity flowers, rose
fructone apple
methyl butyrate fruits, pineapple, apple pineapple
ethyl acetate sweet solvent wine
isoamyl acetate fruity, pear, banana banana
benzyl acetate fruity, strawberry strawberry
Terpenes
geraniol floral, rose lemon, geranium
citral lemon lemongrass
citronellol lemon rose geranium, lemongrass
linalool floral, lavender lavender, coriander, sweet basil
limonene orange lemon, orange
camphor camphor camphor laurel
carvone caraway or spearmint dill, caraway, spearmint
eucalyptol eucalyptus eucalyptus
Amines
trimethylamine fishy
putrescine rotting meat rotting meat
cadaverine rotting meat rotting meat
indole feces feces, jasmine
skatole feces feces, orange blossoms
Alcohol
menthol menthol mint species
Aldehydes
hexanal grassy
isovaleraldehyde nutty, cocoa
Aromatics
eugenol clove clove
cinnamaldehyde cinnamon cinnamon, cassia
benzaldehyde almond bitter almond
vanillin vanilla vanilla
thymol thyme thyme
Thiols
benzyl mercaptan garlic
allyl thiol garlic
(methylthio)methanethiol mouse urine
ethyl-mercaptan the smell added to propane
Lactones
gamma-nonalactone coconut
gamma-decalactone peach
Ketones
6-acetyl-2,3,4,5-tetrahydropyridine fresh bread
oct-1-en-3-one metallic, blood
2-acetyl-1-pyrroline jasmine rice
Others
2,4,6-trichloroanisole the scent of cork taint
diacetyl butter scent/flavor
methyl phosphine metallic garlic

Among the "smelliest" of the odorants are methyl phosphine and dimethyl phosphine, which can be detected in extremely low amounts. The human nose is so sensitive to thioacetone that it can be smelled within seconds if a container of it is opened hundreds of meters away.

The sense of smell filters out constant odors, so a person becomes unaware of them after continuous exposure. However, hydrogen sulfide deadens the sense of smell. Initially, it produces a strong rotten egg smell, but binding of the molecule to odor receptors prevents them from receiving additional signals. In the case of this particular chemical, the loss of sensation can be deadly, as it is extremely toxic.

Aroma Compound Uses

Odorants are used to make perfumes, to add odor to toxic, odorless compounds (e.g., natural gas), to enhance the flavor of food, and to mask undesirable scents. From an evolutionary standpoint, a scent is involved in mate selection, identifying safe/unsafe food, and forming memories. According to Yamazaki et al., mammals preferentially select mates with a different major histocompatibility complex (MHC) from their own. MHC can be detected via scent. Studies in humans support this connection, noting it's also affected by the use of oral contraceptives.

Aroma Compound Safety

Whether an odorant occurs naturally or is produced synthetically, it may be unsafe, especially in high concentrations. Many fragrances are potent allergens. The chemical composition of fragrances is not regulated the same from one country to another. In the United States, fragrances in use before the Toxic Substances Control Act of 1976 were grandfathered in for use in products. New aroma molecules are subject to review and testing, under the oversight of the EPA.

Reference

  • Yamazaki K, Beauchamp GK, Singer A, Bard J, Boyse EA (February 1999). "Odortypes: their origin and composition."​ Proc. Natl. Acad. Sci. U.S.A. 96 (4): 1522–5.
  • Wedekind C, Füri S (October 1997). "Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity?". Proc. Biol. Sci. 264 (1387): 1471–9.