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In studying amines, I read that lemon juice is also used to wash fish because it reacts with the amines on and in the fish to convert the amines to its salt, just reducing the "fishy smell". I was thinking that it must be the citric acid in lemon juice that somehow reacts with the amines to convert them to their salts.
What is the general chemical reaction of citric acid with amines in fish, and why does this reaction actually reduce the "fishy" smell?
There are two cases, both related to acid-base reactions. Both are also partial reasons why so many fish recipes use lemon juice.
Fish, especially sea fish, naturally contain trimethylamine-N-oxide $\ce{(CH3)3N-O}$ that, after death, gets enzymatically reduced to trimethylamine $\ce{(CH3)3N}$, the source of the ammonia-like fish odour.
Trimethylamine N-oxide is an osmolyte found in saltwater fishes, sharks, rays, molluscs, and crustaceans. It is considered a protein stabilizer that may serve to counteract urea, the major osmolyte of sharks, skates and rays. It is also higher in deep-sea fishes and crustaceans, where it may counteract the protein-destabilizing effects of pressure.TMAO decomposes to trimethylamine (TMA), which is the main odorant that is characteristic of degrading seafood.
TMAO is biosynthesized from trimethylamine, which is derived from choline.
In acidic environments, TMA forms water-soluble, non-volatile, odourless trimethylammonium salts, which eliminates the volatile smelly trimethylamine.
$$\ce{(CH3)3N + H+ <=>> (CH3)3NH+ }$$
It is the direct analogy to the smelly volatile ammonia and odourless ammonium:
$$\ce{NH3 + H+ <=>> NH4+}$$
Another source of fishy smell/taste, especially of some sweet water species like carp or catfish, is geosmin, a polycyclic alcohol, being a derivate of decahydronaphthalene(decaline). It is released after the death of some Streptomyces or cyanobacteria cells which gives a muddy or earthy taste, and the human smell sensors are extremely sensitive to it (5 ppt).
Geosmin is also responsible for the smell of the soil after rain, of some beetroots and even of drinking water of surface origin. There is speculation the nose sensitivity for geosmin is an evolutionary response to the need for efficient searching for water.
Geosmin reportedly breaks down in acidic environments.
