Fish are notorious for their strong — and sometimes unpleasant — odor. But what causes that distinctly fishy smell, and is there a way to avoid it while cooking?
Fish odor is the result of microbial and chemical reactions that occur as fish proteins and fats are exposed to heat and oxygen over time.
Fresh fish should have a mild scent similar to the salty smell of the ocean. It should "smell kind of like seaweed," said Eric Decker, a professor in the Department of Food Science at the University of Massachusetts Amherst.
"It's kind of like an antifreeze for fish that live in cold water," Decker told Live Science.
These psychrophilic — or cold-loving — bacteria convert TMAO to trimethylamine (TMA), the molecule responsible for the signature fishy, ammonia-like smell. Cold-water species, like cod and haddock, may develop this smell from TMA faster than others, Decker said.
Fishy odor may also come from enzymes that break down once the fish is caught. Bacteria in fish flesh convert lysine, an amino acid, into cadaverine, a molecule that's associated with decomposing animals, according to the American Chemical Society.
To slow down the bacterial growth — and thus the fishy smell — focus on two factors. "Time is number one; temperature is number two," Carl Batt, a professor of food science at Cornell University, told Live Science.
To continue this cold chain to the home kitchen, Decker recommended mimicking a good fishmonger. "Put some ice in a bowl, put the fish on the ice and put it in your refrigerator," he said. "That's going to slow the bacterial growth."
Low-fat fish — like cod, haddock and flounder — are best for freezing because microbial spoilage will likely happen first. Freezing delays that process, Decker said . But particularly fatty fish — like mackerel, blue fish and herring — are usually not frozen because, despite the cold temperature, "they're going to oxidize pretty fast," he said, and thus start to spoil and smell unless they are stored in a low oxygen container.
That's why those latter species are often canned: because that preservation technique removes most oxygen and stops lipid oxidation.
Even after you cook fresh fish, odors could grow if leftovers aren't eaten quickly. "Cooking any meat makes oxidation go faster," Decker said. That doesn't mean the fish is bad, but the flavor will probably be affected after a day or two as the fish amino acids continue to oxidize in the fridge.
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 fishy smell may be a delicacy in another culture," Batt said, referring to fermented fish products such as garum, a fish sauce popular in ancient Rome.
But to err on the side of safety, you can check the Danish National Food Institute's Food Spoilage and Safety Predictor before you dig in.
