Aroma Compounds and Their Odors

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.

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.

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.

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:

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.

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.

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.

• 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.