People put up with fishy-smelling fish because they’re probably thinking, Well, what else should it smell like? Odd as it may seem, though, fish needn’t smell like fish at all.
When they’re perfectly fresh, only a few hours removed from carousing around in the water, fish and shellfish have virtually no odor. A fresh “scent of the sea,” perhaps, but certainly nothing the least bit smelly. It’s only when seafood starts to decompose that it takes on that fishy aroma. And fish starts to decompose much faster than other kinds of meat.
Fish flesh, fish muscle, is made up of different kinds of protein from, say, beef or chicken.
It is not only tenderized more quickly by cooking but it is also more quickly decomposed by enzymes and bacteria; in other words, it spoils faster. That fishy smell comes from decomposition products, notably ammonia, sulfur compounds, and chemicals called amines that come from the breakdown of the amino acids in proteins.
The odors of these chemicals are noticeable long before the food gets downright unpleasant to eat, so a slight fishy smell indicates only that you’ve got a good nose and the fish isn’t quite as fresh as it might be, not necessarily that it’s bad.
Amines and ammonia are counteracted (Techspeak: neutralized) by acids; that’s why lemon wedges are often served with fish. If you have scallops that smell a trifle ripe, rinse them quickly in lemon juice or vinegar before cooking.
There’s another reason why fish spoils quickly. Most fish have the unfriendly habit of swallowing other fish whole, and they are therefore equipped with enzymes that digest fish. If any of these enzymes should escape from the guts by rough handling after a fish is caught, they’ll quickly go to work on its own flesh. That’s why fish should be gutted as soon as possible after being caught.
The decomposition bacteria in and on fish are also more efficient than those in land animals, because they’re designed to operate in the cold, cold seas and streams. To stop them from doing their dirty work, we have to cool them down a lot faster and a lot more than we do to preserve warmblooded meat. That’s why ice, which never gets above 32°F, is the fisherman’s best friend. Your home refrigerator is at about 40ºF.
A third reason that fish flesh spoils faster than land meat is that it contains more unsaturated fats. Unsaturated fats turn rancid (oxidize) much more readily than the saturated fats in beef, for example.
The oxidation of fats turns them into bad-smelling fatty acids, which contribute further to the fishy odor.