The Chemistry of Fish

Legendary food scholar Harold McGee rocks us again

By Patricia Gadsby, Dwight Eschliman|Thursday, November 25, 2004
Ayu, or sweetfish, is a freshwater species that lives
on algae and moss, which give its flesh a watermelon
scent. Salt-grilled sweetfish is considered one of
Japan’s tastiest foods. Each fish typically grows to
about six inches.

At 6 a.m. workers are inspecting shipments of gleaming ice-packed fish atIMP Foods, a company in San Mateo, California, that suppliessushi-grade specimens to Japanese restaurants and a coterie of some ofthe most famous—and famously picky—American chefs. Harold McGee is inhis element. “Look at that, with that schnoz, and a whip comingout of its tail,” he says, stooping over what turns out to be acornetfish, a long, bony creature with a fluted tube for a snout, astrange rear end, and an altogether alarming red color. “In Japan we call it aka-yagara, which means ‘red arrow,’” says Glenn Sakata, IMP’s general manager. “It makes wonderful broth.”

A golden threadfin bream, itoyoriin Japanese, also catches McGee’s eye. Favored for sashimi, it’s quitelovely, with silver skin, luminous yellow stripes, a tail that blushesdeep pink. Sakata mentions in passing that both fish are bought by theFrench Laundry, Thomas Keller’s four-star temple to cuisine in Napa Valley. The sea bream shimmers with freshness in its box of shaved ice. McGee, who is wearing a regulation-issue hairnet, bows over it and draws a deep, appreciative breath.

McGee is the author of On Food and Cooking: The Science and Lore of the Kitchen,a doorstop of a book that first appeared in 1984 and became an instantclassic. Tall, bearded, and unapologetically bookish, he’s America’s premier food wonk. He likes nothing better than surfing journals the likes of Cereal Chemistry, Poultry Science, and The Bulletin of the Japanese Society of Scientific Fisheries.In fact, he’s made a career of turning huge amounts of arcane foodscience, centuries of history and culture, and wonderfully oddball,just-for-the-heck-of-it facts into a good read for curious cooks andeaters. Somehow he seems uniquely up to the task. How many people havestudied both physics and English literature at Caltech, earning ahybrid bachelor of science degree in literature? McGee has become thego-to guy for such questions as: How much oil can mayonnaise absorb?Why do red beans cause gas? How do you deal with an overdose of wasabi?When it came to fish, though, the original On Food and Cooking had little to say. Back then, meat research meant red meat. Fish barely got a mention.

Twentyyears later, “there’s been an explosion of information on the subject,”McGee says. Fishing has become “a more important and visible industry.”Fish stocks worldwide are under pressure as never before, “so nationalgovernments put more resources into research because of problems withsustainability and developing aquaculture.” Seafood consumption,meanwhile, is rising, driven in part by health concerns and America’s love affair with sushi. Sales of sushi in the United States have been booming, says Sakata. Now we’re buying tuna rolls in supermarkets.

So McGee’s new On Food and Cooking, 10 years in the making and almost twice the heft of the original, now includes an entire section on fish.

Fish,especially ocean fish, as McGee points out in his new book, live in avery different world from ours and that of the other animals we eat.The rules are different from those for cattle, pigs, and chickens. Tounderstand the character of fish as food—what makes it taste and smellthe way it does, why you like it, and why you sometimes don’t—you mustbear in mind the adaptations fish have made to life in water. Also,judging from McGee’s example, you have to follow your schnoz.

Asyou step through the door into the cool sanctuary of IMP’s fish market,there’s a slight briny edge to the air that instantly recalls theseaside. “Seacoast smell,” announces McGee. It’s the smell ofbromophenols synthesized by algae from the bromine in seawater. Incoastal areas, these bromophenols are propelled into the air by theaction of waves. “Think of waves dashed on rocks,” says McGee. Oceanfish accumulate the phenols in their tissues by eating algae or otheralgae eaters. A nice fresh fish is never smelly or “fishy,” says McGee.But its bromophenols can magically conjure up sea air.

Heruns his hands over the skin of an iced locally caught king salmon thatwill be dinner tonight for family and friends in nearby Palo Alto.Wild salmon has an aroma all its own that appears to arise from aparticular diet, notes McGee. He rubs his hands together to warm themand develop the traces of scent he has picked up from the fish, thencups his hands around his nose: “Unmistakably salmony.” The scent isbelieved to be partly due to their fats and partly to the pinkastaxanthins salmon acquire from eating crustaceans, which in turncreate them from the beta-carotene they’ve obtained from algae. Whenthe fish is cooked, astaxanthins give rise to fruity, floral tastes.

Ifa fish is spanking fresh, you can sometimes catch a surprisinglyplantlike aroma, says McGee, continuing the tutorial. Fish, especiallycold-water fish, have oily, unsaturated fats that remain fluid at lowtemperatures. Their fats are more like the polyunsaturated oils ofplants than the fats of other animals. What’s more, fish skin, likeplant leaves, contains lipoxygenases—and these enzymes break down thelong unsaturated fatty molecules of fish into many of the same smallaromatic fragments found in plants. In Japana legendary freshwater fish named ayu, or sweetfish, a native ofmountain streams, is said to have melon and cucumber aromas. “Do youhave 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.

Fleetingsea and plant aromas are delightful signs of freshness in a fish. Butafter a fish is caught and killed, other aromas develop. All meats, ofcourse, are perishable, but in fish the process occurs faster—largelybecause fish are designed for temperatures lower than those on dryland. Their unsaturated fats, an asset in cold ocean water, become aliability in our environment: They are easily oxidized—broken down byoxygen into fragments that smell and taste stale. Their cold-adaptedenzymes rev up in our warmer air. Even the bacteria normally clingingto their surfaces grow faster. Much of this enzymatic and bacterialactivity has odiferous consequences. If a fish is not properly chilled,kept on ice like the fish at IMP, it’s going to smell bad.

Theprincipal smell contributing to “fishiness,” says McGee, is a compoundcalled trimethylamine, or TMA. It is derived from trimethylamine oxide,or TMAO, which is not in itself objectionable. TMAO is one of severalamines and amino acids that ocean creatures accumulate inside theircells to buffer them against a fatal influx of sea salt. (Seawater is 3percent salt; the optimal level of dissolved minerals in animal cellsis about1 percent.) Some amino acids—sweet glycine and savory glutamate—turnout to be big contributors to seafood’s delicious repertoire offlavors. Not TMAO, though. The chemical is tasteless but is theprecursor of that unappetizing smell. Once a fish is dead, TMAO isgradually converted to TMA by bacteria proliferating on the surfaces ofthe fish. (Freshwater fish like ayu live in an environment less saltythan the inside of their cells, so they don’t accumulate amino acidsand amines. Their flesh is mild tasting and slower to turn smelly.)

Luckily,“fishiness” in an ocean fish past its prime can be reduced in thekitchen. A thorough rinsing in cold water helps minimize odors.“Oxidized fats, bacteria, and TMA on the surface can be rinsed off withtap water,” says McGee. “And acidic ingredients—lemon juice, vinegar,tomatoes—help in two ways.” They encourage stale-smelling fragments toreact with water and become less volatile, and they induce TMA to bondwith water and other molecules so they never escape the fish’s surfaceto assault your senses. As for truly spoiled fish, let’s just say thatby the time proteins are being broken down into skatole, putrescine,cadaverine, and hydrogen sulfide, you probably won’t want that fish inyour mouth.

Refrigerationretards spoiling, but it can’t stop decay. That’s especially true fordeep-sea fish with cold-adapted physiologies. “Enzymes and bacteriatypical of our warm-blooded meat animals normally work at 100 degreesFahrenheit and slow to a crawl in a refrigerator at 40°F. But the samerefrigerator feels perfectly balmy to deepwater fish enzymes andbacteria,” McGee writes in his new book. The key, then, to keeping fishas fresh as possible is ice. “Fish lasts nearly twice as long in a 32°Fslush as it does at typical refrigerator temperatures,” says McGee, whorecommends flaked or finely chopped ice because it makes better contactwith contours than large cubes or slabs.

AtIMP Foods, ice is everywhere. As fish get moved, unpacked, checked, andrepackaged, ice is involved every step of the way. You see ice and youhear ice—the crash of ice chips dumped on metal surfaces, the sloshingof slush, the soft noises made by shaved ice getting shoved around.

We end the morning in the fish market’s kitchen, where it’s warm. Sakata has good news. He has located an ayu.

The fish, though flown in from Japan,is still so firm, clear-eyed, and gleaming fresh, it appears lacquered.“It looks just caught,” says McGee. “Its mucus layer is completelyintact. Let’s see if I can get the gills to open up a little bit,” hesays, gently lifting the gill flap. “The gills are often the first partof a fish to go. Their red turns brown with oxidation. And since gillsare full of bacteria, because that’s where the water is filtered, theycan generate unpleasant aromas. So let’s take a sniff.”

He puts his nose to the gills, which are a healthy bright red.

“Ah, I think I smell the melon!” he exclaims. “Can you smell it? I’ve been reading about this fish for years, in the abstract, because I’ve never seen it in a market,” he says in a rush. “In Japan,they’ve analyzed the aroma, because that’s what people say they valuemost about this fish, and they’ve found some of its compounds are exactly the compounds you find in melons and cucumbers.”

“He knows more about this fish than me,” Sakata says.

“That’swhat’s so cool,” says McGee. “Once somebody points out to you that thisfish smells like watermelon, all of a sudden there’s another dimensionto the experience of eating it.”

Whichwe do then and there—the fish is simply sprinkled with sea salt andgrilled whole. Glenn Sakata splits the fish open like a book, and we goat its mild, surprisingly soft white flesh with chopsticks, enjoyingevery last sweet morsel. Ayu, the sweetfish. “Who could ask foranything more?” says Harold McGee.

On Food and Cooking: The Science and Lore of the Kitchen

An excerpt from the newly revised book by Harold McGee

Copyright 1984, 2004.

To be published by Scribner, a division of Simon & Schuster.

Peoplein many parts of the world enjoy eating ocean fish and shellfish raw.Unlike meats, fish have the advantage of relatively tender muscle and anaturally savory taste, and are easier and more interesting to eat raw.They offer the experience of a kind of primal freshness. The cook maysimply provide a few accompanying ingredients with complementaryflavors and textures, or firm the fish’s texture by means of lightacidification (ceviche), salting (poke), or both (anchovies brieflycured in salt and lemon juice). And raw preparations don’t require theuse of fuel, which is often scarce on islands and coastlines.

Probably the commonest form of raw fish is sushi, whose popularity spread remarkably in the late 20th century from its home in Japan. The original sushi seems to have been the fermented preparation narezushi; sushi means“salted” and now applies more to the flavored rice, not the fish. (Thefamiliar bite-sized morsels of raw fish and lightly salted andacidified rice are nigiri sushi, meaning “grasped” or “squeezed,” sincethe rice portion is usually molded by hand. The mass-produced versionof sushi found in supermarkets is formed by industrial robots.)

Many cultures from the Arcticto the tropics have recruited microbes to grow on fish and transformtheir texture and flavor. But the world center of fish fermentation iseastern Asia, where it has served two important purposes: to preserveand put to use the large numbers of small fish that inhabit the coastaland inland waters; and to provide a concentrated source ofappetite-stimulating flavors—above all the savory monosodium glutamateand other amino acids—for a diet dominated by bland rice.

Fish fermentation apparently arose several thousand years ago in the freshwaters of southwest China and the Mekong Riverregion. It then spread to the coastal deltas and was applied to oceanfish. Two broadly different techniques evolved: simply salting a massof small fish or fish parts and allowing it to ferment; and saltinglarger fish lightly, then embedding them in a fermenting mass of riceor other grains, vegetables, or fruits. In the simple fermentation, theproportion of salt is usually enough by itself to preserve the fishfrom spoilage, and bacteria are important mainly as flavor modifiers.But in the mixed fermentation, a smaller dose of salt preserves thefish for just a few weeks while the plant-based ingredients feed thesame microbes that sour milk or turn grape juice into wine. The fish isthen preserved by the microbes’ acids or alcohol, and flavored by themany byproducts of their growth.

Ofthe many Asian fermentations that mix fish and grains, one of the mostinfluential has been the Japanese narezushi, the original form ofmodern sushi. The best-known version is funa-zushi, made with rice andgoldfish carp (Carassius auratus) from Lake Biwa, north of Kyoto.Various bacteria consume the rice carbohydrates and produce a range oforganic acids that protect against spoilage, soften the head andbackbone, and contribute to the characteristic tart and rich flavor,which has vinegary, buttery, and cheesy notes. In modern sushi, madewith pristinely fresh raw fish, the tartness of narezushi survivesthrough the addition of vinegar to the rice.

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