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Violets Ionone and Orris The Flower That Disappears From Your Nose

Violets, Ionone and Orris: The Flower That Disappears From Your Nose is about Viola odorata and the compound that carries its smell. The violet is the single best case study in all of olfaction, because it demonstrates three things at once that no other flower demonstrates so cleanly: a scent that switches itself off while you are smelling it, a scent that some people genetically cannot smell at all, and a scent that is not made by the plant everyone associates with it. It also founded the modern fragrance industry. Alpha.

1. The compound

• The violet note is carried by the ionones — α-ionone and β-ionone — with dihydro-β-ionone and relatives contributing.
• They are C13-norisoprenoids: fragments left over when carotenoids are cleaved. β-Ionone is literally a degradation product of β-carotene, which is why it appears across rose, tea, tobacco, raspberry, apricot and wine, and why the same chemistry underlies the rose ketones. See Functional Groups, Polarity and Separation: Indoles, Phenols, Rose Ketones and How Aroma Chemistry Works.
• In the plant they are formed by carotenoid cleavage dioxygenases (CCDs) acting on carotenoids — an enzymatic cut, not a harsh one.
• α-ionone reads more violet and powdery; β-ionone more woody, raspberry, cedar.

2. The vanishing act

• Smell a violet and the scent is there — then within seconds it is gone. Wait a little and it returns. Smell again and it vanishes again.
• The cause is rapid receptor adaptation: β-ionone binds the receptor and desensitises it almost immediately. The flower never stopped emitting; the receptor stopped reporting. It resets in moments and the scent reappears.
• This is not ordinary olfactory fatigue, which takes minutes for most odours. Ionone does it in seconds, and reversibly, over and over.
• The effect was famous long before the chemistry. Shakespeare has Laertes describe the violet as "sweet, not lasting, / The perfume and suppliance of a minute" (Hamlet I.iii) — an accurate description of receptor adaptation written around 1600.
• Perfumers have always known it: a violet accord has to be rebuilt around the problem, because the wearer stops perceiving the very note they bought.

3. The flower some people cannot smell

• β-Ionone sensitivity is close to a single-gene trait in humans, which is extraordinarily rare for an odour.
• The receptor is OR5A1. A single variant, rs6591536, changes one amino acid (N183D) in the second extracellular loop, and that one change explains more than 96% of the variation in how strongly people smell β-ionone (Jaeger et al., Current Biology, 2013).
• Carry at least one G allele and you are sensitive: the smell is pleasant and floral.
• Be AA homozygous and you are roughly 100× less sensitive, and at higher concentrations you do not smell violets at all — you smell something sour, pungent, vinegary.
• So two people can smell the same flower, or eat the same raspberry, and genuinely disagree about what it smells like — not as a matter of taste, but of receptor affinity. The effect carries into food choice, which is what the original study measured.
• This is the clearest known example of a specific anosmia with a mapped cause, and it is the standard teaching case for why "what does it smell like" is not a question with one answer. It belongs beside the other heritable perception differences in Skin, Hair and Eye Colour: Ancestry and Genetics.

4. The violet that is not a violet

• Here is the commercial twist: violet flowers yield almost no oil. Extraction is so poor and so costly that true violet-flower absolute is effectively a specialty curiosity.
• What perfumery actually calls "violet" comes from two other places:

** Orris root — the rhizome of Iris pallida and Iris germanica. Its violet character comes from the irones (α-, β- and γ-irone), which are not present in the fresh root. They form slowly as precursors called iridals oxidise during three to five years of drying and ageing. The rhizome is then ground and distilled to orris butter. That multi-year wait makes orris one of the most expensive materials in perfumery, historically rivalling gold by weight.

** Synthetic ionones — cheap, abundant, and the reason a violet scent exists on an ordinary shelf at all.

• Violet leaf is a separate material entirely and smells nothing like the flower: green, cucumber, wet earth. Its character compound is 2,6-nonadienal ("violet leaf aldehyde"). Violet leaf absolute is a real and widely used material — the leaf gives what the flower will not.

5. The compound that started an industry

• In 1893 Ferdinand Tiemann and Paul Krüger synthesised ionone in Berlin while trying to reproduce the smell of violets from orris constituents.
• It was the first time a prized natural scent could be made cheaply at scale, and it broke the dependence of perfumery on botanical supply.
• Jacques Guerlain built Jicky (1889) and later houses built Après l'Ondée (1906) and Vol de Nuit on these new synthetics; Aimé Guerlain and his contemporaries treated them as a new palette rather than a substitute.
• Industrially, ionones are made by an aldol condensation of citral with acetone to give pseudoionone, followed by acid-catalysed cyclisation — and which acid is used decides whether you get mostly α- or mostly β-ionone. It is a textbook demonstration that reaction conditions, not just reagents, determine the product.

6. The violet elsewhere

• As a chemical indicator. Syrup of violets was one of the earliest acid–base indicators. Robert Boyle used it in the 1660s: the anthocyanin pigment turns red with acid and green with alkali. A sweet made from flowers was, for a while, a laboratory instrument.
• As food. Candied violets, Parma violet sweets, violet syrup and crème de violette. The flavour is the same ionone story, which is why some people find violet sweets perfumed and delicious and others find them like soap — see the OR5A1 section above.
• As medicine. Viola odorata has a long record as a demulcent for coughs and sore throats, usually as a syrup, and in traditional use for headaches and sleeplessness. It contains mucilage, saponins and salicylates — the last being a plausible basis for the headache use.
• As a symbol. The Greeks made it the flower of Athens. Napoleon took it as his emblem and was nicknamed Caporal Violette; his supporters wore violets to identify themselves while he was exiled, with the promise he would return "with the violets in spring". In the Victorian language of flowers it stood for modesty and faithfulness.
• Sweet violet versus African violet. Viola odorata is the scented violet of this page. The ubiquitous houseplant "African violet" (Saintpaulia, now Streptocarpus) is not a violet at all and has no scent.

7. Why this page matters to the rest of the shelf

• It is the cleanest proof that smell is a property of the perceiver as much as the molecule — one base-pair changes what the flower is.
• It shows that a scent can be real and unsmellable at the same time, which is a caution for anyone evaluating an extract by nose.
• It shows that what a material is called has little to do with where it comes from: the violet of commerce is an iris root and a synthetic.
• And it is another case of trace carotenoid fragments carrying an entire identity, exactly as with the rose ketones and the sulfur compounds in Hops, Cannabis and the Chemistry of "Dank": Terpenes and Volatile Sulfur Compounds.

Sources

• Jaeger S. R. et al., "A Mendelian Trait for Olfactory Sensitivity Affects Odor Experience and Food Selection", Current Biology 23 (2013) 1601–1605 — OR5A1, rs6591536, N183D.
• Knaapila A. et al., sensory characterisation of β-ionone stimuli by rs6591536 genotype, Food Quality and Preference (2014).
• Tiemann F. and Krüger P., Berichte der deutschen chemischen Gesellschaft 26 (1893) — the synthesis of ionone.
• Ohloff G., Scent and Fragrance (1994); Arctander S., Perfume and Flavor Materials of Natural Origin (1960) — orris, irones, violet leaf.
• Roberts D. D. et al. and the irone/iridal literature on orris ageing.
• Boyle R., Experiments and Considerations Touching Colours (1664) — syrup of violets as an indicator.
• Shakespeare W., Hamlet I.iii.

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