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An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor. A chemical compound has a smell or odor when two conditions are met: the compound needs to be volatile, so it can be transported to the olfactory system in the upper part of the nose, and it needs to be in a sufficiently high concentration to be able to interact with one or more of the olfactory receptors.

Aroma compounds can be found in food, wine, spices, perfumes, fragrance oils, and essential oils. For example, many form biochemically during ripening of fruits and other crops. In wines, most form as byproducts of fermentation. Odorants can also be added to a dangerous odorless substance, like natural gas or hydrogen, as a warning. As well many of the aroma compounds plays a significant role in the production of flavorants, which are used in the food service industry to flavor, improve and increase the appeal of their products.

Aroma compounds classified by structure

Esters

Compound name	Fragrance	Natural occurrence	Chemical structure
Methyl formate	Ethereal
Methyl acetate	Sweet, nail polish Solvent
Methyl butyrate Methyl butanoate	Fruity, apple Pineapple
Ethyl acetate	Sweet, solvent	Wine
Ethyl butyrate Ethyl butanoate	Fruity, orange Pineapple
Isoamyl acetate	Fruity, banana Pear
Pentyl butyrate Pentyl butanoate	Fruity, pear Apricot
Pentyl pentanoate	Fruity, apple
Isoamyl acetate	Fruity, banana Pear
Octyl acetate	Fruity, orange

Terpenes

Compound name	Fragrance	Natural occurrence	Chemical structure
Myrcene	Woody, complex	Verbena, bay
Geraniol	Rose, flowery	Geranium, lemon
Nerol	Sweet rose, flowery	Neroli, lemongrass
Citral, lemonal Geranial, neral	Lemon	Lemon myrtle, lemongrass
Citronellal	Lemon	Lemongrass
Citronellol	Lemon	Lemongrass, rose Pelargonium
Linalool	Floral, sweet Woody, lavender	Coriander, sweet basil Lavender
Nerolidol	Woody, fresh bark	Neroli, ginger Jasmine

Cyclic terpenes

Compound name	Fragrance	Natural occurrence	Chemical structure
Limonene	Orange	Orange, lemon
Camphor	Camphor	Camphor laurel
Terpineol	Lilac	Lilac, cajuput
alpha-Ionone	Violet, woody	Violet
Terpineol	Lilac	Lilac, cajuput
Thujone	Minty	Cypress, lilac Juniper

Aromatic

Compound name	Fragrance	Natural occurrence	Chemical structure
Benzaldehyde	Almond
Eugenol	Clove	Clove
Cinnamaldehyde	Cinnamon	Cassia Cinnamon
Ethyl maltol	Cooked fruit Caramelized sugar
Vanillin	Vanilla	Vanilla
Anisole	Anise	Anise
Anethole	Anise	Anise Sweet basil
Estragole	Tarragon	Tarragon
Thymol	Thyme	Thyme

Amines

Compound name	Fragrance	Natural occurrence	Chemical structure
Trimethylamine	Fishy Ammonia
Putrescine Diaminobutane	Rotting flesh	Rotting flesh
Cadaverine	Rotting flesh	Rotting flesh
Pyridine	Fishy
Indole	Fecal Flowery	Feces Jasmine
Skatole	Fecal	Feces

Other aroma compounds

Alcohols

• Furaneol (strawberry)
• 1-Hexanol (herbaceous, woody)
• cis-3-Hexen-1-ol (fresh cut grass)
• Menthol (peppermint)

Aldehydes

• Acetaldehyde (pungent)
• Hexanal (green, grassy)
• cis-3-Hexenal (green tomatoes)
• Furfural (burnt oats)

Esters

• Fructone (fruity, apple-like)
• Hexyl acetate (apple, floral, fruity)
• Ethyl methylphenylglycidate (strawberry)

Ketones

• Dihydrojasmone (fruity woody floral)
• Oct-1-en-3-one (blood, metallic, mushroom-like)^[1]
• 2-Acetyl-1-pyrroline (fresh bread, jasmine rice)
• 6-Acetyl-2,3,4,5-tetrahydropyridine (fresh bread, tortillas, popcorn)

Lactones

• gamma-Decalactone intense peach flavor
• gamma-Nonalactone coconut odor, popular in suntan lotions
• delta-Octalactone creamy note
• Jasmine lactone powerful fatty fruity peach and apricot
• Massoia lactone powerful creamy coconut
• Wine lactone sweet coconut odor
• Sotolon (maple syrup, curry, fenugreek)

Thiols

• Ethanethiol, commonly called Ethyl mercaptan (added to propane or other liquefied petroleum gases used as fuel gases)
• Grapefruit mercaptan (grapefruit)
• Methanethiol, commonly called Methyl mercaptan (Durian or leek)
• 2-Methyl-2-propanethiol, commonly called tertiary-butyl mercaptan is added as a blend of other components to natural gas used as fuel gas.

Miscellaneous compounds

• Methylphosphine and dimethylphosphine (garlic-metallic, two of the most potent odorants known)^[1]

• Nerolin (orange flowers)
• Tetrahydrothiophene (added to natural gas)
• 2,4,6-Trichloroanisole (cork taint)
• Substituted pyrazines

Aroma compound receptors

Animals which are capable of smell detect aroma compounds with olfactory receptors. Olfactory receptors are cell membrane receptors on the surface of sensory neurons in the olfactory system which detect air-borne aroma compounds.

In mammals, olfactory receptors are expressed on the surface of the olfactory epithelium in the nasal cavity.

Safety

'Fragrance' was voted Allergen of the Year in 2007 by the American Contact Dermatitis Society. The composition of fragrances are usually not disclosed in the label of products, hiding the actual chemicals of the formula, which raises concerns between some consumers.^[2]

References

1. ^ ^{a b} D. Glindemann, A. Dietrich, H. Staerk, P. Kuschk, (2005). "The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines". Angewandte Chemie International Edition 45 (42): 7006–7009. doi:10.1002/anie.200602100.  
2. ^ Toxic chemicals linked to birth defects are being found at alarming levels in women of childbearing age

See also

• Flavour and Fragrance Journal
• Odor
• Olfaction
• Olfactory system
• Olfactory receptor
• Pheromone
• Odor detection threshold
• Aroma of wine