Have you ever wondered why store brought and frozen fish have a pungent fishy odour?
Water in the ocean is about 3% salt by weight. The amount of dissolved mineral including sodium chloride in the fish cells is less than 1%. In order to balance the saltiness in the seawater, ocean creatures fills their cells with amino acids and amines. There are 20 amino acids that make up proteins. The shellfish are filled with amino acid glycine, which is sweet and glutamic acid in the form monosodium glutamate, which is savory and filling. Whereas, the fish has some amino acid but it is mostly dominated with trimethylamine oxide. Now, when fish gets killed the bacteria and enzyme in the fish triggers an oxidation-reduction reaction. This results in trimethylamine and later in to ammonia causing the fish to have strong odour, as it gets older.