Most sea creatures contain a molecule called trimethylamine oxide (TMAO) in their flesh. The molecule helps balance their internal salt levels against the high concentration of salt in the water. It also helps with stabilizing proteins.
TMAO will break down (oxidize) when in contact with oxygen in the air. The oxidized chemicals produce the fishy smell for seafood that has been left out for too long and has spoiled.
Note that freshwater fish have much lower concentrations of TMAO so they don’t smell as strongly when left out.
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