Why Does Fish Smell Fishy and Where Does the Odor Come From?

Silly-sounding question, maybe, but with several interesting answers.

People tend to put up with fishy smelling fish in markets and restaurants because they’re thinking, Well, what else should it smell like? But fish needn’t smell like fish at all. Not if it’s perfectly fresh.

When they’re only a couple of hours removed from the water, fish and shellfish have virtually no odor. A fresh “scent of the sea,” perhaps, but certainly nothing the least bit unpleasant. It’s only when seafood starts to decompose that it takes on that “fishy” aroma. And fish decomposes much faster than other kinds of meat.

Fish flesh, fish muscle, is made of a different kind of protein from, say, beef or chicken. It breaks down more quickly, not only in cooking, but also under the action of enzymes and bacteria. In other words, it spoils faster. That fishy smell comes from the products of decomposition, notably ammonia, various sulfur compounds, and chemicals called amines that result from the breakdown of amino acids.

The human nose is remarkably sensitive to these chemicals. The odors are noticeable long before the food gets downright unhealthy to eat, so a slight fishy smell indicates only that the fish isn’t as fresh, or as enjoyable, as it could be, not necessarily that it’s dangerous.

Amines and ammonia are bases, which are counteracted by acids. That’s why lemon wedges, which contain citric acid, are often served with fish.

If you buy scallops that smell a trifle ripe, rinse them in lemon juice or vinegar before cooking. But don’t let them soak, because scallops absorb water like sponges and will then steam themselves when you try to grill or sauté them. The very best way to test seafood for freshness is to ask as politely as possible to sniff the merchandise before buying, although in certain Mediterranean markets with scrupulously high standards, this can be taken as a grave insult.

A second reason why fish spoils more quickly than other meats is that, in the wild, most fish have the unfriendly habit of swallowing smaller fish. (It’s a jungle down there.) They are therefore equipped with digestive enzymes that are exquisitely effective digesters of fish flesh. After the fish is caught, if any of these enzymes should escape from the guts through rough handling, they’ll quickly go to work on the fish’s own flesh. That’s why gutted fish will keep longer than whole ones.

A third reason: The decomposition bacteria in and on fish are more efficient than those on land because they’re designed to operate in the cold seas. Warm them up slightly, and they’ll really go to town. To stop them from doing their dirty work, we have to cool the fish down a lot more quickly and thoroughly than we do to preserve warmblooded meat.

That’s why ice is the fisherman’s best friend, lots and lots of it. Ice not only lowers the temperature, but it keeps the fish from drying out. Fish don’t appreciate being dry, even after they’re deceased.

Reason number four: In general, fish flesh contains more unsaturated fats than land-animal flesh. That’s one of the reasons why we value it in these cholesterol-dreading times. But unsaturated fats turn rancid (oxidize) much more readily than those saturated fats that are so delicious in beef. The oxidation of fats turns them into foul-smelling organic acids, which contribute further to the unappealing aroma.

If you walk into a seafood restaurant and it smells fishy, depart immediately in search of the nearest hamburger.