An aroma compound, also known as an odorant, aroma, fragrance or

flavoring A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the g ...

, is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

that has a smell or

odor An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...

. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently volatile for transmission via the air to the

olfactory system The olfactory system, or sense of smell, is the sensory system used for smelling ( olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an ...

in the upper part of the nose. As examples, various fragrant fruits have diverse aroma compounds, particularly strawberries which are commercially cultivated to have appealing aromas, and contain several hundred aroma compounds. Generally, molecules meeting this specification have

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

s of less than 310. Flavors affect both the sense of

taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...

and smell, whereas fragrances affect only smell. Flavors tend to be naturally occurring, and the term ''fragrances'' may also apply to synthetic compounds, such as those used in

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protec ...

. Aroma compounds can naturally be found in various

food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is in ...

s, such as fruits and their peels,

wine Wine is an alcoholic drink typically made from Fermentation in winemaking, fermented grapes. Yeast in winemaking, Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different ...

spices A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices are distinguished from herbs, which are the leaves, flowers, or stems of plants used for flavoring or as a garnish. Spices are ...

, floral scent,

perfume Perfume (, ; french: parfum) is a mixture of fragrant essential oils or aroma compounds (fragrances), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agreeable scent ...

fragrance oil Fragrance oils, also known as aroma oils, aromatic oils, and flavor oils, are blended synthetic aroma compounds or natural essential oils that are diluted with a carrier like propylene glycol, vegetable oil, or mineral oil. To allergic or otherw ...

s, and

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...

s. For example, many form

biochemical Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...

ly during the ripening of

fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...

s and other crops. Wines have more than 100 aromas that form as byproducts of

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...

. Also, many of the aroma compounds play a significant role in the production of compounds used in the food service industry to flavor, improve, and generally increase the appeal of their products. An

odorizer An odorizer is a device that adds an odorant to a gas. The most common type is one that adds a mercaptan liquid into natural gas distribution systems so that leaks can be readily detected. Other types have been used for carbon dioxide fire e ...

may add a detectable odor to a dangerous odorless substance, like

propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as ...

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...

, or

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

, as a safety measure.

Aroma compounds classified by structure

Esters

Linear terpenes

Cyclic terpenes

Note: Carvone, depending on its chirality, offers two different smells.

Aromatic

Amines

Other aroma compounds

Alcohols

Furaneol Furaneol, or strawberry furanone, is an organic compound used in the flavor and perfume industry. It is formally a derivative of furan. It is a white or colorless solid that is soluble in water and organic solvents. Odor and occurrence Althoug ...

(

strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...

) *

1-Hexanol 1-Hexanol (IUPAC name hexan-1-ol) is an organic alcohol with a six- carbon chain and a condensed structural formula of CH3(CH2)5OH. This colorless liquid is slightly soluble in water, but miscible with diethyl ether and ethanol. Two additiona ...

(

herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition o ...

, woody) * ''cis''-3-Hexen-1-ol (fresh cut

grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...

) *

Menthol Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature ...

(

peppermint Peppermint (''Mentha'' × ''piperita'') is a hybrid species of mint, a cross between watermint and spearmint. Indigenous to Europe and the Middle East, the plant is now widely spread and cultivated in many regions of the world.Euro+Med Plantb ...

)

Aldehydes

High concentrations of aldehydes tend to be very pungent and overwhelming, but low concentrations can evoke a wide range of aromas. *

Acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the ...

(ethereal) *

Hexanal Hexanal, also called hexanaldehyde or caproaldehyde is an alkyl aldehyde used in the flavor industry to produce fruity flavors. Its scent resembles freshly cut grass, like ''cis''-3-hexenal. It is potentially useful as a natural extract that pre ...

(green, grassy) * ''cis''-3-Hexenal (green

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

es) *

Furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occur ...

(burnt oats) * Hexyl cinnamaldehyde * Isovaleraldehyde – nutty, fruity, cocoa-like *

Anisic aldehyde 4-Anisaldehyde, or ''p''-Anisaldehyde, is an organic compound with the formula CH3OC6H4CHO. The molecule consists of a benzene ring with an formyl and a methoxy group. It is a colorless liquid with a strong aroma. It provides sweet, floral and ...

– floral, sweet, hawthorn. It is a crucial component of chocolate, vanilla, strawberry, raspberry, apricot, and others. * Cuminaldehyde (4-propan-2-ylbenzaldehyde) – Spicy, cumin-like, green

Esters

Fructone Fructone, also known as apple ketal and applinal, is a synthetic aroma compound with a fruity, apple-like smell with pineapple, strawberry, and woody aspects reminiscent of pine trees. Chemistry Fructone is usually synthesized from ethyl ace ...

(fruity, apple-like) *

Ethyl methylphenylglycidate Ethyl methylphenylglycidate, commonly known as strawberry aldehyde, is an organic compound used in the flavor industry in artificial fruit flavors, in particular strawberry. Uses Because of its pleasant taste and aroma, ethyl methylphenylglyci ...

(

Strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...

) * alpha-Methylbenzyl acetate (

Gardenia ''Gardenia'' is a genus of flowering plants in the coffee family, Rubiaceae, native to the tropical and subtropical regions of Africa, Asia, Madagascar and Pacific Islands, and Australia. The genus was named by Carl Linnaeus and John Ellis af ...

)

Ketones

* Cyclopentadecanone (musk-ketone) * Dihydrojasmone (fruity woody floral) * Oct-1-en-3-one (blood, metallic, mushroom-like) *

2-Acetyl-1-pyrroline 2-Acetyl-1-pyrroline (2AP) is an aroma compound and flavor that gives freshly baked bread, jasmine rice and basmati rice, the spice ''pandan'' (''Pandanus amaryllifolius''), and ''bread flowers'' (''Vallaris'' ''glabra'') their customary smell. ...

(fresh bread, jasmine rice) * 6-Acetyl-2,3,4,5-tetrahydropyridine (fresh bread, tortillas, popcorn)

Lactones

gamma-Decalactone ''gamma''-Decalactone is a lactone and aroma compound with the chemical formula C10H18O2. It has an intense-peach flavor. It is present naturally in many fruits and fermented products. It is particularly important in the formulation of peach ...

intense peach flavor *

gamma-Nonalactone ''gamma''-Nonalactone is a chemical compound found in bourbon whiskey. See also * delta-Nonalactone, δ-Nonalactone * Cyclotene References

Gamma-lactones {{organic-compound-stub ...

coconut odor, popular in suntan lotions *

delta-Octalactone ''delta''-Octalactone is a lactone and aroma compound with a creamy cocoa, coconut, and peach The peach (''Prunus persica'') is a deciduous tree first domesticated and cultivated in Zhejiang province of Eastern China. It bears edible juicy ...

creamy note * Jasmine lactone powerful fatty-fruity peach and apricot * Massoia lactone powerful creamy coconut *

Wine lactone Wine lactone is a pleasant smelling compound found naturally in apples, orange juice, grapefruit juice, orange essential oil, clementine peel oil and various grape wine Wine is an alcoholic drink typically made from fermented grapes. Ye ...

sweet coconut odor * Sotolon (

maple syrup Maple syrup is a syrup made from the sap of maple trees. In cold climates, these trees store starch in their trunks and roots before winter; the starch is then converted to sugar that rises in the sap in late winter and early spring. Maple tr ...

curry A curry is a dish with a sauce seasoned with spices, mainly associated with South Asian cuisine. In southern India, leaves from the curry tree may be included. There are many varieties of curry. The choice of spices for each dish in trad ...

fenugreek Fenugreek (; ''Trigonella foenum-graecum'') is an annual plant in the family Fabaceae, with leaves consisting of three small obovate to oblong leaflets. It is cultivated worldwide as a semiarid crop. Its seeds and leaves are common ingredients ...

)

Thiols

* Thioacetone (2-propanethione) A lightly studied organosulfur. Its smell is so potent it can be detected several hundred meters downwind mere seconds after a container is opened. * Allyl thiol (2-propenethiol; allyl mercaptan; CH₂=CHCH₂SH) (

garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...

volatiles and

garlic breath Garlic breath is halitosis (bad breath) resulting from the consumption of garlic. Causes The major volatile compounds responsible for garlic breath are allyl methyl sulfide, allyl methyl disulfide, allyl mercaptan, diallyl disulfide, dimet ...

) * (Methylthio)methanethiol (CH₃SCH₂SH), the "mouse thiol", found in mouse urine and functions as a semiochemical for female mice *

Ethanethiol Ethanethiol, commonly known as ethyl mercaptan, is an organosulfur compound with the formula CH3CH2SH. is a colorless liquid with a distinct odor. Abbreviated EtSH, it consists of an ethyl group (Et), CH3CH2, attached to a thiol group, SH. Its st ...

, commonly called ethyl mercaptan (added to

or other liquefied-petroleum gases used as fuel gases) * 2-Methyl-2-propanethiol, commonly called ''tert''-butyl mercaptan, is added as a blend of other components to natural gas used as fuel gas. * Butane-1-thiol, commonly called butyl mercaptan, is a chemical intermediate. * Grapefruit mercaptan (grapefruit) *

Methanethiol Methanethiol (also known as methyl mercaptan) is an organosulfur compound with the chemical formula . It is a colorless gas with a distinctive putrid smell. It is a natural substance found in the blood, brain and feces of animals (including humans ...

, commonly called methyl mercaptan (after eating

Asparagus Asparagus, or garden asparagus, folk name sparrow grass, scientific name ''Asparagus officinalis'', is a perennial flowering plant species in the genus '' Asparagus''. Its young shoots are used as a spring vegetable. It was once classified in ...

) *

Furan-2-ylmethanethiol Furan-2-ylmethanethiol (2-Furanmethanethiol) is an organic compound containing a furan substituted with a sulfanylmethyl group. It is a clear colourless liquid when pure, but it becomes yellow coloured upon prolonged standing. It possesses a stro ...

, also called furfuryl mercaptan (roasted

coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. Seeds of ...

) *

Benzyl mercaptan Benzyl mercaptan is an organosulfur compound with the formula C6H5CH2SH. It is a common laboratory alkylthiol that occurs in trace amounts naturally. It is a colorless, malodorous liquid. Preparation and occurrence Benzyl mercaptan can be prepare ...

(leek or garlic-like)

Miscellaneous compounds

Methylphosphine Methylphosphine is the simplest organophosphorus compound with the formula CH3PH2, often written MePH2. It is a malodorous gas that condenses to a colorless liquid. It can be produced by methylation of phosphide salts:W. L. Jolly “Methylphosphi ...

and dimethylphosphine (garlic-metallic, two of the most potent odorants known) *

Phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

( zinc phosphide poisoned bait) *

Diacetyl Diacetyl (IUPAC systematic name: butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH3CO)2. It is a yellow liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side). Diacet ...

(

butter Butter is a dairy product made from the fat and protein components of churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 80% butterfat. It is used at room temperature as a spread, melted as a condim ...

flavor) *

Acetoin Acetoin, also known as 3-hydroxybutanone or acetyl methyl carbinol, is an organic compound with the formula CH3CH(OH)C(O)CH3. It is a colorless liquid with a pleasant, buttery odor. It is chiral. The form produced by bacteria is (''R'')-acetoin. ...

(butter flavor) * Nerolin ( orange flowers) *

Tetrahydrothiophene Tetrahydrothiophene is an organosulfur compound with the formula (CH2)4S. The molecule consists of a five-membered saturated ring with four methylene groups and a sulfur atom. It is the saturated analog of thiophene. It is a volatile, colorle ...

(added to

) * 2,4,6-Trichloroanisole ( cork taint) *

Substituted A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...

pyrazine Pyrazine is a heterocyclic aromatic organic compound with the chemical formula C4H4N2. It is a symmetrical molecule with point group D2h. Pyrazine is less basic than pyridine, pyridazine and pyrimidine. It is a ''"deliquescent crystal or wax-li ...

Aroma-compound receptors

Animals that are capable of smell detect aroma compounds with their

olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give r ...

s. Olfactory receptors are cell-membrane receptors on the surface of sensory neurons in the

that detect airborne aroma compounds. Aroma compounds can then be identified by

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substanc ...

-olfactometry, which involves a human operator sniffing the GC effluent. In

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s, olfactory receptors are expressed on the surface of the olfactory epithelium in the

nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal ...

Safety and regulation

In 2005–06, fragrance mix was the third-most-prevalent

allergen An allergen is a type of antigen that produces an abnormally vigorous immune response in which the immune system fights off a perceived threat that would otherwise be harmless to the body. Such reactions are called allergies. In technical t ...

in patch tests (11.5%). 'Fragrance' was voted

Allergen of the Year Allergen of the Year is an annual award voted upon by the American Contact Dermatitis Society. This is "designed to draw attention to allergens that are very common, under-recognized, merit more attention because they are causing significant allergi ...

in 2007 by the American Contact Dermatitis Society. A recent academic study in the United States has shown that "34.7 % of the population reported health problems, such as migraine headaches and respiratory difficulties, when exposed to fragranced products". The composition of fragrances is usually not disclosed in the label of the products, hiding the actual chemicals of the formula, which raises concerns among some consumers. In the United States, this is because the law regulating cosmetics protects

trade secret Trade secrets are a type of intellectual property that includes formulas, practices, processes, designs, instruments, patterns, or compilations of information that have inherent economic value because they are not generally known or readily ...

s.Fragrances in Cosmetics
/ref> In the United States, fragrances are regulated by the

Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

if present in cosmetics or drugs, by the Consumer Products Safety Commission if present in consumer products. No pre-market approval is required, except for drugs. Fragrances are also generally regulated by the Toxic Substances Control Act of 1976 that "grandfathered" existing chemicals without further review or testing and put the burden of proof that a new substance is not safe on the

EPA The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...

. The EPA, however, does not conduct independent safety testing but relies on data provided by the manufacturer. A 2019 study of the top-selling skin moisturizers found 45% of those marketed as "fragrance-free" contained fragrance.

List of chemicals used as fragrances

In 2010, the International Fragrance Association published a list of 3,059 chemicals used in 2011 based on a voluntary survey of its members, identifying about 90% of the world's production volume of fragrances.

References

{{DEFAULTSORT:Aroma Compound Organic chemistry Olfaction Flavors Perfume ingredients