23 February 2011

The boquet of 18 chemicals

Smells and other odours are sweeter in the air at some distance, than near the nose; as hath been partly touched heretofore. The cause is double: first, the finer mixture or incorporation of the smell: for we see that in sounds likewise, they are sweetest when we cannot hear every part by itself. The other reason is, for that all sweet smells have joined with them some earthy or crude odours; and at some distance, the sweet which is the more spiritual, is perceived, and the earthy reacheth not so far.

Francis Bacon
From this book
For Valentine's Day, my wife gave me a nice bottle of cologne. The ingredient list on the box includes more than 20 chemicals. If denatured alcohol, water, propylene glycol, which are the solvents, and green 5, yellow 5 and yellow 6, which are the colorants, are excluded, there remains about 18 chemicals that I suspect all contribute to the perceived fragrance of this eau de toilette.

For a chemical to be odoriferous it needs to be volatile: if not enough molecules travel from a substance to the inside of our noses thru the air, we will not smell it*.

While reading an article today on the recreation of flower smells in the laboratory, a question popped in my mind. If many natural and artificial odors are mixtures of several volatile chemicals, how do our noses detect them more or less simultaneously to perceive a single odor that is always the same?

What happens is that as the volatile molecules in a fragrant flower or a layer of perfume applied to one's skin evaporate, they begin to fill the air space above. I suspect that at ordinary temperatures, the relative concentrations of the molecules in a small volume of air above a source producing the molecules, i.e., a flower or a liquid, is always roughly the same. Thus, the nose receives the same gaseous mixture on every occasion. The perceived odor itself results from the simultaneous interactions of all the different molecules with the receptors in the nose and the resultant electrical outputs sent to the brain.

*Of course, we can also smell the chemicals that are not volatile but are in solution. When that happens, for example, during the consumption of a food, the odor is usually perceived as a flavor.

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