The best I have found is the following:
"The house-permeating “fishy” aroma of cooked fish appears to involve a group of volatile molecules formed by fatty-acid fragments reacting with TMAO (trimethylamine oxide). "
However the answer is not clear to me, maybe because I forgot chemistry. TMAO exist in fish regardless of overcooking as do fatty acids.
Is it simply that heat/prolonged heat causes the fatty acids and TMAO to react creating the new compounds, thus decreasing heat/cooking time will causes less formation of these molecules?
The excerpt talks as though this smell is unavoidable, is this the case?
Best Answer
Trimethylamine N-oxide (TMAO) is actually odorless, freshly caught fish doesn't smell fishy at all, however it breaks down to trimethylamine (TMA) which is responsible for that infamous rotting smell.
As the the tissues containing TMAO is exposed to air, the usual suspects, the bacteria start breaking down the TMAO into TMA; thus causing the fishy smell...
I'm not aware of a mechanism of TMAO breaking down to TMA with heat, however TMA is a gas at room temperature and above. If the particular fish you're cooking is not so fresh, or loaded with TMAO (thus TMA eventually)... With heat, the TMA trapped in the tissues of the fish might dissipate and result in "The house-permeating 'fishy'" smell.