is defined as 'anything that can be smelled
', as opposed to 'something that smells bad'.
The human armpit observably produces at least 2 different food-like smells, which are: 1. a sweet fruit smell,
and 2. a meaty onion-like smell.
The chemicals that produce those smells have been identified in 2004. There is 1 main chemical for
each of such smells, which contains sulfur, but there are also several other, similar sulfur-containing chemicals that
occur in smaller amounts and contribute to the smell. Such chemicals are not produced directly by human metabolism,
but rather, the apocrine glands of the human armpit produces certain metabolites in large quantities, which are converted into the odor chemicals
via the bacterial enzyme cystathionine-b-lyase of
1 or more of 5 bacterial species, which are corynebacterium tuberculostearicum,
corynebacterium minutissimum, staphylococcus epidermidis, staphylococcus haemolyticus, and bacillus
licheniformis, with staphylococcus haemolyticus producing more of such chemicals than any of the other species.
The 2 main chemicals are nearly identical, as they have the same sequences of atom-to-atom
connections, but different possible angles of such connections,
meaning that they are stereoisomers. In fact, the 2 chemicals are so similar that they are
mirror forms of each other, meaning that they are enantiomers. The 2 similar chemicals are
3-methyl-3-sulfanylhexan-1-ol, which has the formula:
The enantiomer of the onion-like smell, which consists of approximately 75% of the total enantiomers by number, is called the S enantiomer, whereas the enantiomer of the fruit-like smell is called the R enantiomer.
Some of the minor food-like molecules are similar to the 2 major chemicals and are known to
occur naturally as onion and fruit flavors in certain foods. It is particularly notable that the chemical
3-sulfanylhexan-1-ol is a fruit flavor in red wine. The human nose is about 1000 times more sensitive to
molecules that contain thiol groups (-SH) than to most other molecules.
The 2 major food-like human armpit odor chemicals are produced from the human metabolite
cystathionine, which is an intermediate metabolite in the catabolism (i.e. break down) of the amino acid methionine.