Fresh fish should have a mild smell. Strong fish y smell is the first sign to go bad. How does the fishy smell come from?
It can be several days from when the fish are caught to when they reach the supermarket. In that time, bacteria that grow naturally in fish start to consume a substance called trimethylamine N-oxide(TMAO) in fish. These bacteria change TMAO into trimethylamine(TMA), the substance responsible for the fish y smell. Bacteria in fish can also change lysine(赖氨酸) into cadaverine(尸胺),a substance that's associated with breaking down the fish once they are caught and giving off fishy smell.
Chemical reactions can also lead to fishy smell. This happens through the oxidation(氧化) of fat. Fish are an important source of omega-3 fatty acids. When these fats are exposed to oxygen, they oxidize and break down into the substance that you can smell.
To slow down the fishy smell, what is beyond question is that the less time between when the fish are caught and when they reach the kitchen, the better. But today, fish are often flown across the globe. To keep smell-producing bacteria at bay, the fish must be frozen or kept at the low temperature possible as soon as they are caught and cleaned.
Controlling fat oxidation can function as well, especially for fattier fish species. While freezing slows bacterial growth, it does not stop fat oxidation. This reaction will occur as long as oxygen is present. Fatty fish are usually not frozen because, despite the cold temperature, they're going to oxidize pretty fast unless they are stored in a low oxygen container. That's why those species are often canned.
It's also important to remember that smell is not always an indicator of safety, especially in processed fish products. "What you might consider the fish y smell may be a delicacy in another culture," said Carl A. Batt, a professor of food science at Cornell University.
