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Odor
Chemistry
Aroma
Compounds and Their Odors
All About Odor Chemistry
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.
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 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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