When someone buys fish at the fresh fish market, they should smell the fish to test for freshness, the less “fishy” the smell the fresher the fish.
I am supposed to explain the source and chemistry, including reactions, structures, and names of the compounds, behind the development of the “fishy” odor.
The "fishy" odor that develops in fish is primarily caused by a compound called trimethylamine (TMA). TMA is a volatile organic compound that has a distinct fish-like smell. It is naturally present in fish as an amino acid derivative called trimethylamine oxide (TMAO).
When a fish is caught and killed, the enzyme responsible for converting TMAO to TMA becomes activated. This enzyme is called trimethylamine N-oxide reductase. Once activated, it breaks down TMAO into TMA, resulting in the characteristic fishy smell.
The chemical reaction occurring during the breakdown of TMAO to TMA involves the transfer of oxygen atoms. TMAO has three methyl (CH3) groups attached to the nitrogen atom, whereas TMA has only one. This conversion involves the removal of two methyl groups and the release of one molecule of water.
Structurally, TMAO consists of an amine group (NH2) bonded to three methyl groups (CH3) and an oxygen atom (O), forming a tetrahedral shape around the nitrogen atom. On the other hand, TMA has a similar structure but with only one methyl group bonded to the nitrogen atom.
The odor of TMA is considered unpleasant by many people, hence the term "fishy" odor. However, the intensity of the fishy smell can also depend on the freshness of the fish. As fish decomposes, other compounds may also contribute to the overall odor.
In summary, the "fishy" odor in fish is mainly caused by the breakdown of trimethylamine oxide (TMAO) into trimethylamine (TMA) during the enzymatic activity in the fish. The release of TMA gives fish its distinct smell.