Why does fresh cooked fish smell fishy the next day?

Freshly cooked fish doesn't actually smell "fishy" the next day. The smell you're noticing is actually a result of oxidation.

Here's how it works:

* Trimethylamine Oxide (TMAO): Fish contain a compound called trimethylamine oxide (TMAO). TMAO is a natural part of the fish and doesn't have a strong smell.

* Breakdown: When fish is cooked and then left exposed to air, the TMAO breaks down into trimethylamine (TMA). TMA is the compound that creates the strong "fishy" smell.

* Oxidation: This breakdown process is accelerated by oxygen, which is why the smell gets stronger over time.

So, the "fishy" smell isn't the original smell of the fish, but a new smell that develops as a result of chemical changes after cooking.

Here are some things that can contribute to a stronger fishy smell:

* Fat content: Fattier fish like tuna and mackerel have higher levels of TMAO, so they will tend to smell "fishy" more quickly.

* Storage: Storing cooked fish in the refrigerator in an airtight container can help slow down the oxidation process.

* Time: The longer the cooked fish is exposed to air, the more pronounced the "fishy" smell will become.

In summary, the "fishy" smell you notice in cooked fish the next day is not the natural smell of the fish but a byproduct of oxidation. It's not necessarily a sign of spoilage, but it can be unpleasant.