The Chemistry of Fish
Legendary food scholar Harold McGee rocks us again
Patricia Gadsby, Discover Magazine 25 Nov 04
He runs his hands over the skin of an iced locally caught king salmon that will be dinner tonight for family and friends in nearby Palo Alto.
Wild salmon has an aroma all its own that appears to arise from a particular diet, notes McGee. He rubs his hands together to warm them and develop the traces of scent he has picked up from the fish, then cups his hands around his nose: “Unmistakably salmony.”
The scent is believed to be partly due to their fats and partly to the pink astaxanthins salmon acquire from eating crustaceans, which in turn create them from the beta-carotene they’ve obtained from algae. When the fish is cooked, astaxanthins give rise to fruity, floral tastes.
If a fish is spanking fresh, you can sometimes catch a surprisingly plantlike aroma, says McGee, continuing the tutorial.
Fish, especially cold-water fish, have oily, unsaturated fats that remain fluid at low temperatures. Their fats are more like the polyunsaturated oils of plants than the fats of other animals. What’s more, fish skin, like plant leaves, contains lipoxygenases—and these enzymes break down the long unsaturated fatty molecules of fish into many of the same small aromatic fragments found in plants.
In Japan a legendary freshwater fish named ayu, or sweetfish, a native of mountain streams, is said to have melon and cucumber aromas. “Do you have ayu?” asks McGee, hopefully. (Ayu rhymes with “yahoo.”) IMP Foods supplies the little fish to Charlie Trotter’s in Chicago, among other restaurants. But after checking, Sakata is sorry to say the market is all out.
Fleeting sea and plant aromas are delightful signs of freshness in a fish. But after a fish is caught and killed, other aromas develop.
All meats, of course, are perishable, but in fish the process occurs faster—largely because fish are designed for temperatures lower than those on dry land. Their unsaturated fats, an asset in cold ocean water, become a liability in our environment: They are easily oxidized—broken down by oxygen into fragments that smell and taste stale.
Their cold-adapted enzymes rev up in our warmer air. Even the bacteria normally clinging to their surfaces grow faster. Much of this enzymatic and bacterial activity has odiferous consequences. If a fish is not properly chilled, kept on ice like the fish at IMP, it’s going to smell bad.
The principal smell contributing to “fishiness,” says McGee, is a compound called trimethylamine, or TMA. It is derived from trimethylamine oxide, or TMAO, which is not in itself objectionable. TMAO is one of several amines and amino acids that ocean creatures accumulate inside their cells to buffer them against a fatal influx of sea salt. (Seawater is 3 percent salt; the optimal level of dissolved minerals in animal cells is about 1 percent.)
Some amino acids — sweet glycine and savory glutamate — turn out to be big contributors to seafood’s delicious repertoire of flavors.
Not TMAO, though. The chemical is tasteless but is the precursor of that unappetizing smell. Once a fish is dead, TMAO is gradually converted to TMA by bacteria proliferating on the surfaces of the fish. (Freshwater fish like ayu live in an environment less salty than the inside of their cells, so they don’t accumulate amino acids and amines. Their flesh is mild tasting and slower to turn smelly.)
Luckily, “fishiness” in an ocean fish past its prime can be reduced in the kitchen. A thorough rinsing in cold water helps minimize odors. “Oxidized fats, bacteria, and TMA on the surface can be rinsed off with tap water,” says McGee. “And acidic ingredients—lemon juice, vinegar, tomatoes—help in two ways.” They encourage stale-smelling fragments to react with water and become less volatile, and they induce TMA to bond with water and other molecules so they never escape the fish’s surface to assault your senses.
As for truly spoiled fish, let’s just say that by the time proteins are being broken down into skatole, putrescine, cadaverine, and hydrogen sulfide, you probably won’t want that fish in your mouth.
Refrigeration retards spoiling, but it can’t stop decay. That’s especially true for deep-sea fish with cold-adapted physiologies. “Enzymes and bacteria typical of our warm-blooded meat animals normally work at 100 degrees Fahrenheit and slow to a crawl in a refrigerator at 40°F. But the same refrigerator feels perfectly balmy to deepwater fish enzymes and bacteria,” McGee writes in his new book.
The key, then, to keeping fish as fresh as possible is ice. “Fish lasts nearly twice as long in a 32°F slush as it does at typical refrigerator temperatures,” says McGee, who recommends flaked or finely chopped ice because it makes better contact with contours than large cubes or slabs.