On the skin layer the lipid surface is made of free fatty acids, mono / di and tri glycerides, sterol and its esters. The difference between a fatty acid and a fatty aldehyde is … a chemical function (to keep simple I do not enter deep in chemistry). After several steps you can turn an acid into an aldehyde or vice versa. It is also one of the first industrial methods to obtain the so famous fatty aldehydes (C10, C11, C12 and so on) used in classic aldehydic perfumes.
While the fat/oil that often turn rancid in a bottle (with its particular smell) is something we want to get rid off … the aldehydes became a symbol for purity and clean.
(Many of those aldehydes are present in nature, in citrus or rose products)
When you iron a shirt or when you let to dry clothes in the sun (washed with unscented products!) there is a particular smell floating around. It smells like a dilution of 0,1-1% of C11 and C12L aldehyde! I'm not sure what are the real molecules in both cases (ironing and drying in the sun) but from this basic chemical reaction (of fats) we can explore an entire symbolism.
This side of the aldehydes was "recognized" many years after their first use.
Fatty aldehydes (there are more besides the classical No5 examples) are used today in many products to give a fresh and clean feeling. Of course in a different environement than that used in classical perfumery (no more indolic jasmine or chypre ambery). One of the best examples when their use is related to the name is - White Linen.
Aldehydes, lily of the valley/rosy and very green molecules are today the summum of cleanliness as used in many personal care products.
It is also interesting to quote one of the many stories around No5 - the clean note of the soap used by Emilienne d'Alençon - in the skin chemistry with aldehydes and oils.
The smell of aldehydes depends on dilution (as they are strong molecules) and context. Many fatty aldehydes have a waxy note than can make you suspicious about the clean aspect.
There are aldehydes (a chemical function) and aldehydic (a type of smell related to fatty aldehydes).
Aldehyde C9 (very green), Aldehyde C10 (orange, rose), Aldehyde C11 undecylic (classic rich aldehydic), Aldehyde C11 undecylenique, Aldehyde C12 laurique (iron shirt), Aldehyde C12 MNA (a fresh slightly ambery and incensy note but also ozonic), Aldehyde C13 (waxy grapefruit), Aldehyde Supra (9-undecenal, extremely clean and fresh citrusy), Mandarine aldehyde (2-dodecenal, very strong fresh mandarine), Intreleven aldehyde (isomerizes C11 undecylenic, less fatty),
From the many fresh aldehydes some more stable versions were prepared - the acetals and the nitriles. We find them in many personal care and functional products.
Some nitriles correspondent to the aldehydes like Ozonil, Clonal
And aldehydes from a different family than fat aldehydes: Farenal (2,6,10-trimethylundec-9-enal, leaf like and sea breeze), Florazon, Profarnesal
I know that clean and dirt can have many meanings because of culture, personal taste or experience. I will talk in next post about some perfumery notes that are clean/dirty for many people, beyond the personal belief.
There is an interesting book on human scents that I reccomend you:
The Scented Ape: The Biology and Culture of Human Odour by David Michael Stoddart
Further information on fatty acids (Wikipedia)
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Fragrance is the 8th Art - Octavian Coifan - Le Parfum est le 8ème Art