ODOR CHEMISTRY - 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. 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.

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Odor Chemistry

Aroma Compounds and Their Odors

All About Odor Chemistry

by Anne Marie Helmenstine, Ph.D.                                                                                       

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 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 compound do, in fact, 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	coconunt
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	scent of cork taint
diacetyl	butter scent/flavor
methylphosphine	metallic garlic

Among the "smelliest" of the odorants are methylphosphine and dimethylphosphine, 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 actually 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, 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 prior to 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.

Anne Marie Helmenstine, Ph.D.

·   Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

·   Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

·   ThoughtCo and About Education chemistry expert since 2001.

·   Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

Anne Marie Helmenstine, Ph.D.

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https://www.thoughtco.com/aroma-compounds-4142268