You are lying on the bed, pants rolled down to your ankles and arms spread apart. Beside you, the tablet is still showing bonjour madame's marvels: you are in state of supreme ecstasy that, you know, won't last long. Something bothers you: a poignant smell of chlorine arising from the humid toilet paper you've just tossed to the floor.
You try to ignore it, but sperm's smell is not to be ignored.
Forums on the internet are full of dirty chats about the taste and smell of human sperm. It is debated whether its taste changes or not based on what you eat (BBC actually tested the thing); on the contrary, most people agree that sperm's odour resembles that of bathroom detergents containing chlorine.
One more brilliant comic by Cyanide and Happiness @ Explosm.net
Human sperm contains pretty much everything – sugar (fructose), salt, hormones, proteins and of course cells (sperm cells) – but it lacks what gives chlorine its peculiar smell: sodium hypochlorite.
Therefore, the smell must be produced by something else.
As early as in 1677 - when chemists were shamelessly experimenting with organic fluids to figure out what they contained - some crystals were discovered, forming spontaneously in sperm soon after ejaculation; but it took some more centuries before the molecule making up those crystals was identified, and christened with the name it still carries: spermine.
Spermine can be found all over the body, performing important and still partially unknown functions; it is especially concentrated in sperm (where it performs important and still partially unknown functions...).
Nowadays, it is a common ingredient of many face creams.
Spermine itself has no special odour, but in the ejaculate its molecule is subjected to a series of chemical reactions which finally convert it into spermidine, or into putrescine. The latter has quite a pungent - and, as you may guess, not very pleasant - smell: putrescine actually forms in decomposing bodies (through bacterial degradation of some amino acids) and it is responsible (at least partially) of their fetid odour.
However, sperm does not smell like carrion (well, at least it shouldn't: if you thought you finally understood why your ejaculates stink like decomposing bodies, you'd better see a doctor!).
The reason is that both putrescine and spermidine are further metabolised by enzymes found in the sperm and are transformed in1-pyrroline, reaction's final product, as well as "spermous" aroma itself.
The spermous smell conveyed by 1-pyrroline can be perceived by most, but not all, people: 20% of the population is not able to recognize it.
(this would explain why some women claim their man's sperm has no odour at all...)
In conclusion, let's have a short detour into the world of plants.
It seems that the tree Pyrus Calleryana, of Chinese origin but common in Europe, stinks like sperm.
Many plants contain spermine, spermidine and putrescine; however, 1-pyrroline produced through their metabolism does not usually accumulate, as it undergoes further reactions and becomes something different. Clearly, Pyrus Calleryana must be an exception.
Sperm tree depicted by Danilo Battaglia: will a condom protect us from its smell?
The tree is actually a very beautiful one; therefore, if you don't mind the smell, go on and plant it in your yard. Be aware, though, that Pyrus Calleryana is a very resistant and invasive tree: you may never be able to get rid of it, and of its stink.
P.S. There is a facebook page called “I hate Trees that smell like Sperm”, and it has far more Likes than biocomiche's page. If you also think it is a shame, than be ashamed (it's your fault!) and try to make it up by clicking here.
Other sources and Further reading