On Food And Cooking Part 21

Neurotoxic sh.e.l.lfish poisoning

Clams, oysters Clams, oysters

Paralytic sh.e.l.lfish poisoning

Clams, mussels, oysters, scallops, c.o.c.kles Clams, mussels, oysters, scallops, c.o.c.kles

Ciguatera poisoning

Barracuda, grouper, snapper, other reef fish Barracuda, grouper, snapper, other reef fish

Type of Poisoning

Toxins Toxins

Diarrhetic sh.e.l.lfish poisoning

Okadaic acid Okadaic acid

Amnesic sh.e.l.lfish poisoning

Domoic acid Domoic acid

Neurotoxic sh.e.l.lfish poisoning

Brevetoxins Brevetoxins

Paralytic sh.e.l.lfish poisoning

Saxitoxins Saxitoxins

Ciguatera poisoning

Ciguatoxins Ciguatoxins

Anisakid and Cod Worms These species of These species of Anisakis Anisakis and and Pseudoterranova Pseudoterranova can be an inch/2.5 centimeters or more long, with a diameter of a few human hairs. Both often cause only a harmless tingling in the throat, but they sometimes invade the lining of the stomach or small intestine and cause pain, nausea, and diarrhea. They're commonly found in herring, mackerel, cod, halibut, salmon, rockfish, and squid, and can be contracted from sus.h.i.+ or lightly marinated, salted, or cold-smoked preparations. Farmed salmon are much less likely to be infected than wild salmon. can be an inch/2.5 centimeters or more long, with a diameter of a few human hairs. Both often cause only a harmless tingling in the throat, but they sometimes invade the lining of the stomach or small intestine and cause pain, nausea, and diarrhea. They're commonly found in herring, mackerel, cod, halibut, salmon, rockfish, and squid, and can be contracted from sus.h.i.+ or lightly marinated, salted, or cold-smoked preparations. Farmed salmon are much less likely to be infected than wild salmon.

Tapeworms and Flukes Larvae of the tapeworm Larvae of the tapeworm Diphyllobothrium latum, Diphyllobothrium latum, which can grow in the human intestine to as long as 27 feet/9 meters, are found in freshwater fish of temperate regions worldwide. Notable among these is the whitefish, which caused many infections when home cooks made the traditional Jewish dish gefilte fish and tasted the raw mix to correct the seasoning. which can grow in the human intestine to as long as 27 feet/9 meters, are found in freshwater fish of temperate regions worldwide. Notable among these is the whitefish, which caused many infections when home cooks made the traditional Jewish dish gefilte fish and tasted the raw mix to correct the seasoning.

More serious hazards are a number of flukes, or flatworms, which are carried by fresh- and brackish-water crayfish, crabs, and fish. They damage the human liver and lungs after being consumed in such live Asian delicacies as "jumping salad" and "drunken crabs."

Potential Carcinogens Formed During Fish Preparation Certain cooking processes transform the proteins and related molecules in meat and fish into highly reactive products that damage DNA and may thereby initiate the development of cancers (p. 124). So the rule for cooking meat also holds for cooking fish: to minimize the creation of potential carcinogens, steam, braise, and poach fish rather than grilling, broiling, or frying it. If you do use high, dry heat, then consider applying a marinade, whose moisture, acidity, and other chemical qualities reduce carcinogen production. Certain cooking processes transform the proteins and related molecules in meat and fish into highly reactive products that damage DNA and may thereby initiate the development of cancers (p. 124). So the rule for cooking meat also holds for cooking fish: to minimize the creation of potential carcinogens, steam, braise, and poach fish rather than grilling, broiling, or frying it. If you do use high, dry heat, then consider applying a marinade, whose moisture, acidity, and other chemical qualities reduce carcinogen production.

Life in Water and the Special Nature of Fish As a home for living things, the earth's waters are a world apart. The house rules are very different than they are for our cattle and pigs and chickens. The adaptations of fish and sh.e.l.lfish to life in water are the source of their distinctive qualities as foods.

The Paleness and Tenderness of Fish Flesh Fish owe their small, light bones, delicate connective tissue, and large, pale muscle ma.s.ses to the fact that water is much denser than air. Fish can attain a neutral buoyancy - can be almost weightless - simply by storing some lighter-than-water oils or gas in their bodies. This means that they don't need the heavy skeletons or the tough connective tissues that land animals have developed in order to support themselves against the force of gravity.

A Health Inconvenience: Waxy Fish Waxy FishThere's an unusual digestive consequence to eating the fish called escolar and walu (Lepidocybium flavobrunneum and and Ruvettus pretiotus Ruvettus pretiotus). They, and to a lesser extent the orange roughy, acc.u.mulate substances called "wax esters," which are an oil-like combination of a long-chain fatty acid and a long-chain alcohol. Humans lack the digestive enzymes necessary to break these molecules into their smaller, absorbable parts. The wax esters therefore pa.s.s intact and oily from the small intestine into the colon, where a sufficient quant.i.ty will cause diarrhea. Restaurants are the best place to experience these luscious fish - the flesh is as much as 20% calorie-free "oil" - because they usually limit the serving size to a tolerable amount.

The paleness of fish flesh results from water's buoyancy and its resistance to movement. Continuous cruising requires long-term stamina and is therefore performed by slow-twitch red fibers, well supplied with the oxygen-storing pigment myoglobin and fat for fuel (p. 132). Since cruising in buoyant water is relatively effortless, fish devote between a tenth and a third of their muscle to that task, usually a thin dark layer just under the skin. But water's resistance to movement increases exponentially with the fish's speed. This means that fish must develop very high power very quickly when accelerating. And so they devote most of their muscle ma.s.s to an emergency powerpack of fast-twitch white cells that are used only for occasional bursts of rapid movement.

In addition to red and white muscle fibers, fish in the tuna family and some others have intermediate "pink" fibers, which are white fibers modified for more continuous work with oxygen-storing pigments.

The Flavor of Fish and Sh.e.l.lfish The flavors of ocean and freshwater creatures are very different. Because ocean fish breathe and swallow salty water, they had to develop a way of maintaining their body fluids at the right concentration of dissolved substances. Water in the open ocean is about 3% salt by weight, while the optimum level of dissolved minerals inside animal cells, sodium chloride included, is less than 1%. Most ocean creatures balance the saltiness of seawater by filling their cells with amino acids and their relatives the amines. The amino acid glycine is sweet; glutamic acid in the form of monosodium glutamate is savory and mouthfilling. Sh.e.l.lfish are especially rich in these and other tasty amino acids. Finfish contain some, but also rely on a largely tasteless amine called TMAO (trimethylamine oxide). And sharks, skates, and rays use a different substance: slightly salty and bitter urea, which is what animals generally turn protein waste into in order to excrete it. The problem with TMAO and urea is that once the fish are killed, bacteria and fish enzymes convert the former into stinky TMA (trimethylamine) and the latter into kitchen-cleanser ammonia. They're thus responsible for the powerfully bad smell of old fish.

Fish muscle tissues, shown in cross-section. Below left: Below left: Most fish swim intermittently, so their muscle ma.s.s consists mainly of fast white fibers, with isolated regions of slow red fibers. Most fish swim intermittently, so their muscle ma.s.s consists mainly of fast white fibers, with isolated regions of slow red fibers. Center: Center: Tuna swim more continuously and contain larger ma.s.ses of dark fibers, while even their white fibers contain some myoglobin. Tuna swim more continuously and contain larger ma.s.ses of dark fibers, while even their white fibers contain some myoglobin. Right: Right: Soles, halibuts, and other bottom-hugging flatfish swim on their side. Soles, halibuts, and other bottom-hugging flatfish swim on their side.

Freshwater fish are a different story. Their environment is actually less salty than their cells, so they have no need to acc.u.mulate amino acids, amines, or urea. Their flesh is therefore relatively mild, both when it's fresh and when it's old.

The Healthfulness of Fish Oils Why should fish and not Angus steers provide the highly unsaturated fats that turn out to be good for us? Because oceanic waters are colder than pastures and barns, and most fish are cold-blooded. Throw a beefsteak in the ocean and it congeals; its cells are designed to operate at the animal's usual body temperature, around 100F/40C. The cell membranes and energy stores of ocean fish and the plankton they eat must remain fluid and workable at temperatures that approach 32F/0C. Their fatty acids are therefore very long and irregular in structure (p. 801), and don't solidify into orderly crystals until the temperature gets very low indeed.

The Perishability of Fish and Sh.e.l.lfish The cold aquatic environment is also responsible for the notorious tendency of fish and sh.e.l.lfish to spoil faster than other meats. The cold has two different effects. First, it requires fish to rely on the highly unsaturated fatty acids that remain fluid at low temperatures: and these molecules are highly susceptible to being broken by oxygen into stale-smelling, cardboardy fragments. More importantly, cold water requires fish to have enzymes that work well in the cold, and the bacteria that live in and on the fish also thrive at low temperatures. The enzymes and bacteria typical of our warm-blooded meat animals normally work at 100F/40C, and are slowed to a crawl in a refrigerator at 40F/5C. But the same refrigerator feels perfectly balmy to deep-water fish enzymes and spoilage bacteria. And among fishes, cold-water species, especially fatty ones, spoil faster than tropical ones. Where refrigerated beef will keep and even improve for weeks, mackerel and herring remain in good condition on ice for only five days, cod and salmon for eight, trout for 15, carp and tilapia (a freshwater African native) for 20 days.

The Sensitivity and Fragility of Fish in the Pan Most fish pose a double challenge in the kitchen. They are more easily overcooked to a dry fibrousness than ordinary meats. And even when they're perfectly done, their flesh is very fragile and tends to fall apart when moved from pan or grill to plate. The sensitivity of fish to heat is related to their perishability: muscle fibers that are specialized to work well in the cold not only spoil at lower temperatures, they become cooked at lower temperatures. The muscle proteins of ocean fish begin to unfold and coagulate at room temperature!

Though overcooked fish gets dry, it never gets tough. The fragility of cooked fish results from its relatively small amounts connective-tissue collagen, and from the low temperature at which that collagen is dissolved into gelatin.

The Unpredictability of Fish Quality The quality of many fish and sh.e.l.lfish can vary drastically from season to season. This is because they live out life cycles that typically include one phase during which they grow and mature, acc.u.mulating energy reserves and reaching their peak of culinary quality, and a subsequent phase during which they expend those reserves to migrate and create ma.s.ses of eggs or sperm for the next generation. And most fish don't store their reserves in layers of fat, as land animals do. Instead they use the proteins of their muscle ma.s.s as their energy pack. During migrations and sp.a.w.ning, they acc.u.mulate protein-digesting enzymes in their muscle and literally transform their own flesh into the next generation. Then and afterward, their muscle is meager and spent, and makes a spongy, mushy dish.

Because different fish have different cycles, and can be in different phases depending on the part of the world in which they've been caught, it's often hard to know whether a given wild fish in the market is at its prime.

The Anatomy and Qualities of Fish Fish and sh.e.l.lfish have many things in common, but anatomy is not one of them. Fish are vertebrates, animals with backbones; sh.e.l.lfish are boneless invertebrates. Their muscles and organs are organized differently, and as a result they can have very different textures. The anatomy and special qualities of sh.e.l.lfish are described separately, beginning on p. 218.

Fish Anatomy For about 400 million years, beginning well before reptiles or birds or mammals had even made an appearance, fish have had the same basic body plan: a streamlined bullet shape that minimizes the water's resistance to their movement. There are exceptions, but most fish can be thought of as sheets of muscle tissue anch.o.r.ed with connective tissue and the backbone to a propulsive tail. The animals push water behind them, developing thrust by undulations of the whole body and flexing of the tail.

Skin and Scales Fish skin consists of two layers, a thin outer epidermis and a thicker underlying dermis. A variety of gland cells in the epidermis secrete protective chemicals, the most evident of which is mucus, a proteinaceous substance much like egg white. The skin is often richer than the flesh, averaging 510% fat. The thick dermis layer of the skin is especially rich inconnective tissue. It's generally about one-third collagen by weight, and therefore can contribute much more thickening gelatin to stocks and stews than the fish's flesh (0.33% collagen) or bones. Moist heating will turn the skin into a slick gelatinous sheet, while frying or grilling enough to desiccate it will make it crisp. Fish skin consists of two layers, a thin outer epidermis and a thicker underlying dermis. A variety of gland cells in the epidermis secrete protective chemicals, the most evident of which is mucus, a proteinaceous substance much like egg white. The skin is often richer than the flesh, averaging 510% fat. The thick dermis layer of the skin is especially rich inconnective tissue. It's generally about one-third collagen by weight, and therefore can contribute much more thickening gelatin to stocks and stews than the fish's flesh (0.33% collagen) or bones. Moist heating will turn the skin into a slick gelatinous sheet, while frying or grilling enough to desiccate it will make it crisp.

Scales are another evident form of protection for the fish skin. They are made up of the same hard, tough calcareous minerals as teeth, and are removed by sc.r.a.ping against their grain with a knife blade.

Bones The main skeleton of a small or moderate-size fish, consisting of the backbone and attached rib cage, can often be separated from the meat in one piece. However, there are usually also bones projecting into the fins, and fish in the herring, salmon, and other families have small "floating" or "pin" bones unattached to the main skeleton, which help stiffen some of the connective-tissue sheets and direct the muscular forces along them. Because fish bones are smaller, lighter, and less mineralized with calcium than land-animal bones, and because their collagen is less tough, they can be softened and even dissolved by a relatively short period near the boil (hence the high calcium content of canned salmon). Fish skeletons are even eaten on their own: in Catalonia, j.a.pan, and India they're deep-fried until crunchy. The main skeleton of a small or moderate-size fish, consisting of the backbone and attached rib cage, can often be separated from the meat in one piece. However, there are usually also bones projecting into the fins, and fish in the herring, salmon, and other families have small "floating" or "pin" bones unattached to the main skeleton, which help stiffen some of the connective-tissue sheets and direct the muscular forces along them. Because fish bones are smaller, lighter, and less mineralized with calcium than land-animal bones, and because their collagen is less tough, they can be softened and even dissolved by a relatively short period near the boil (hence the high calcium content of canned salmon). Fish skeletons are even eaten on their own: in Catalonia, j.a.pan, and India they're deep-fried until crunchy.

Fish Innards The innards of fish and sh.e.l.lfish offer their own special pleasures. Fish eggs are described below (p. 239). Many fish livers are prized, including those of the goatfish ("red mullet"), monkfish, mackerel, ray, and cod, as is the comparable organ in crustaceans, the hepatopancreas (p. 219). The "tongues" of cod and carp are actually throat muscles and a.s.sociated connective tissue that softens with long cooking. Fish heads can be 20% fatty material and are stuffed and slow-cooked until the bones soften. And then there are "sounds," or swim bladders, balloons of connective tissue that such fish as cod, carp, catfish, and sturgeon fill with air to adjust their buoyancy. In Asia, fish sounds are dried, fried until they puff up, and slowly cooked in a savory sauce. The innards of fish and sh.e.l.lfish offer their own special pleasures. Fish eggs are described below (p. 239). Many fish livers are prized, including those of the goatfish ("red mullet"), monkfish, mackerel, ray, and cod, as is the comparable organ in crustaceans, the hepatopancreas (p. 219). The "tongues" of cod and carp are actually throat muscles and a.s.sociated connective tissue that softens with long cooking. Fish heads can be 20% fatty material and are stuffed and slow-cooked until the bones soften. And then there are "sounds," or swim bladders, balloons of connective tissue that such fish as cod, carp, catfish, and sturgeon fill with air to adjust their buoyancy. In Asia, fish sounds are dried, fried until they puff up, and slowly cooked in a savory sauce.

Fish Muscle and its Delicate Texture Fish have a more delicate texture than the flesh of our land animals. The reasons for this are the layered structure of fish muscle, and the spa.r.s.eness and weakness of fish connective tissue.

Muscle Structure In land animals, individual muscles and muscle fibers can be quite long, on the order of several inches, and the muscles taper down at the ends into a tough tendon that connects them to bone. In fish, by contrast, muscle fibers are arranged in sheets a fraction of an inch thick ("myotomes"), and each short fiber merges into very thin layers of connective tissue ("myosepta"), which are a loose mesh of collagen fibers that run from the backbone to the skin. The muscle sheets are folded and nested in complex W-like shapes that apparently orient the fibers for greatest efficiency of force transmission to the backbone. There are about 50 muscle sheets or "flakes" along the length of a cod. In land animals, individual muscles and muscle fibers can be quite long, on the order of several inches, and the muscles taper down at the ends into a tough tendon that connects them to bone. In fish, by contrast, muscle fibers are arranged in sheets a fraction of an inch thick ("myotomes"), and each short fiber merges into very thin layers of connective tissue ("myosepta"), which are a loose mesh of collagen fibers that run from the backbone to the skin. The muscle sheets are folded and nested in complex W-like shapes that apparently orient the fibers for greatest efficiency of force transmission to the backbone. There are about 50 muscle sheets or "flakes" along the length of a cod.

Connective Tissue Fish connective tissue is weak because its collagen contains less structure-reinforcing amino acids than beef collagen does, and because the muscle tissue also serves as an energy store that's repeatedly built up and broken down, whereas in land animals it is progressively reinforced with age. Meat collagen is tough and must be cooked for some time near the boil to be dissolved into gelatin, but in most fish it dissolves at 120 or 130F/5055C, at which point the muscle layers separate into distinct flakes. Fish connective tissue is weak because its collagen contains less structure-reinforcing amino acids than beef collagen does, and because the muscle tissue also serves as an energy store that's repeatedly built up and broken down, whereas in land animals it is progressively reinforced with age. Meat collagen is tough and must be cooked for some time near the boil to be dissolved into gelatin, but in most fish it dissolves at 120 or 130F/5055C, at which point the muscle layers separate into distinct flakes.

Succulence from Gelatin and Fat Both gelatin and fat can contribute an impression of moistness to fish texture. Fish with little collagen - trout, ba.s.s - seem drier when cooked than those with more - halibut, shark. Because the motion for steady swimming comes mostly from the back end of the fish, the tail region contains more connective tissue than the head end, and seems more succulent. Red muscle fibers are thinner than white fibers and require more connective tissue to join them with each other, so dark meat has a noticeably finer, more gelatinous texture. Both gelatin and fat can contribute an impression of moistness to fish texture. Fish with little collagen - trout, ba.s.s - seem drier when cooked than those with more - halibut, shark. Because the motion for steady swimming comes mostly from the back end of the fish, the tail region contains more connective tissue than the head end, and seems more succulent. Red muscle fibers are thinner than white fibers and require more connective tissue to join them with each other, so dark meat has a noticeably finer, more gelatinous texture.

The fat content of fish muscle runs a tremendous range, from 0.5% in cod and other white fish to 20% in well-fed herring and their relatives (p. 184). Fat storage cells are found primarily in a distinct layer under the skin, and then in the visible sheets of connective tissue that separate the myotomes. Within a given fish, the belly region is usually the fattiest, while muscle segments get progressively leaner toward the back and tail. A center-cut salmon steak may have twice the fat content of a slice from the tail.

Fish anatomy. Unlike the muscles of land animals (p. 120), fish muscles are arranged in layers of short fibers, and organized and separated by sheets of connective tissue that are thin and delicate.

Softness Certain conditions can lead to fish flesh becoming unpleasantly soft. When fish flesh is depleted by migration or by sp.a.w.ning, their spa.r.s.e muscle proteins bond to each other only very loosely, and the overall texture is soft and flabby. In extreme cases, such as "sloppy" cod or "jellied" sole, the muscle proteins are so tenuously bonded that the muscle seems almost liquefied. Some fish come out mushy when thawed after frozen storage, because freezing disrupts the cells' compartments and liberates enzymes that then attack the muscle fibers. And enzyme activity during cooking can turn firm fish mushy in the pan; see p. 211. Certain conditions can lead to fish flesh becoming unpleasantly soft. When fish flesh is depleted by migration or by sp.a.w.ning, their spa.r.s.e muscle proteins bond to each other only very loosely, and the overall texture is soft and flabby. In extreme cases, such as "sloppy" cod or "jellied" sole, the muscle proteins are so tenuously bonded that the muscle seems almost liquefied. Some fish come out mushy when thawed after frozen storage, because freezing disrupts the cells' compartments and liberates enzymes that then attack the muscle fibers. And enzyme activity during cooking can turn firm fish mushy in the pan; see p. 211.

Fish Flavor The flavor of fish may well be the most variable and changeable among our basic foods. It depends on the kind of fish, the salinity of its home waters, the food it eats, and the way it is harvested and handled.

Fish Taste In general, seafood is more full-tasting than meats or freshwater fish, because ocean creatures acc.u.mulate amino acids to counterbalance the salinity of seawater (p. 188). The flesh of ocean fish generally contains about the same amount of salty sodium as beef or trout, but three to ten times more free amino acids, notably sweet glycine and savory glutamate. Sh.e.l.lfish, sharks and rays, and members of the herring and mackerel family are especially rich in these amino acids. Because the salt content of seawater varies substantially - it's high in the open ocean, lower near river mouths - the amino-acid content and therefore taste intensity of fish varies according to the waters they're caught in. In general, seafood is more full-tasting than meats or freshwater fish, because ocean creatures acc.u.mulate amino acids to counterbalance the salinity of seawater (p. 188). The flesh of ocean fish generally contains about the same amount of salty sodium as beef or trout, but three to ten times more free amino acids, notably sweet glycine and savory glutamate. Sh.e.l.lfish, sharks and rays, and members of the herring and mackerel family are especially rich in these amino acids. Because the salt content of seawater varies substantially - it's high in the open ocean, lower near river mouths - the amino-acid content and therefore taste intensity of fish varies according to the waters they're caught in.

An additional element of fish taste is contributed indirectly by the energy-carrying compound ATP (adenosine triphosphate). When a cell extracts energy from ATP, it is transformed into a series of smaller molecules, one of which, IMP (inosine monophosphate), has a savory taste similar to that of glutamate. However, IMP is a transient substance. So the savoriness of fish increases for some time after its death as IMP levels rise, then declines again as IMP disappears.

Fish Aroma Fresh and Plant-like Few of us get the chance to enjoy the experience, but very fresh fish smell surprisingly like crushed plant leaves! The fatty materials of both plants and fish are highly unsaturated, and both leaves and fish skin have enzymes (lipoxygenases) that break these large smellless molecules down into the same small, aromatic fragments. Nearly all fish emit fragments (8 carbon atoms long) that have a heavy green, geranium-leaf, slightly metallic smell. Freshwater fish also produce fragments that are typical of freshly cut gra.s.s (6 carbons), and earthy fragments also found in mushrooms (8 carbons). Some freshwater and migratory species, especially the smelts, produce fragments characteristic of melons and cuc.u.mbers (9 carbons). Few of us get the chance to enjoy the experience, but very fresh fish smell surprisingly like crushed plant leaves! The fatty materials of both plants and fish are highly unsaturated, and both leaves and fish skin have enzymes (lipoxygenases) that break these large smellless molecules down into the same small, aromatic fragments. Nearly all fish emit fragments (8 carbon atoms long) that have a heavy green, geranium-leaf, slightly metallic smell. Freshwater fish also produce fragments that are typical of freshly cut gra.s.s (6 carbons), and earthy fragments also found in mushrooms (8 carbons). Some freshwater and migratory species, especially the smelts, produce fragments characteristic of melons and cuc.u.mbers (9 carbons).

Smell of the Seacoast Ocean fish often have an additional, characteristic aroma of the seacoast. This ocean aroma appears to be provided by compounds called bromophenols, which are synthesized by algae and some primitive animals from bromine, an abundant element in seawater. Bromophenols are propelled into the seacoast air by wave action, where we smell them directly. Fish also acc.u.mulate them, either by eating algae or by eating algae eaters, and the fish can thus remind us of the sea air. Farmed salt.w.a.ter fish lack the oceanic aroma unless their artificial feed is supplemented with bromophenols. Ocean fish often have an additional, characteristic aroma of the seacoast. This ocean aroma appears to be provided by compounds called bromophenols, which are synthesized by algae and some primitive animals from bromine, an abundant element in seawater. Bromophenols are propelled into the seacoast air by wave action, where we smell them directly. Fish also acc.u.mulate them, either by eating algae or by eating algae eaters, and the fish can thus remind us of the sea air. Farmed salt.w.a.ter fish lack the oceanic aroma unless their artificial feed is supplemented with bromophenols.

Muddiness Freshwater fish sometimes carry an unpleasant muddy aroma. It's most often encountered in bottom-feeding fish, especially catfish and carp that are raised in ponds dug directly in the earth. The chemical culprits are two compounds that are produced by blue-green algae, especially in warm weather (geosmin and methylisoborneol). These chemicals appear to concentrate in the skin and the dark muscle tissue, which can be cut away to make the fish more palatable. Geosmin breaks down in acid conditions, so there is a good chemical reason for traditional recipes that include vinegar and other acidic ingredients. Freshwater fish sometimes carry an unpleasant muddy aroma. It's most often encountered in bottom-feeding fish, especially catfish and carp that are raised in ponds dug directly in the earth. The chemical culprits are two compounds that are produced by blue-green algae, especially in warm weather (geosmin and methylisoborneol). These chemicals appear to concentrate in the skin and the dark muscle tissue, which can be cut away to make the fish more palatable. Geosmin breaks down in acid conditions, so there is a good chemical reason for traditional recipes that include vinegar and other acidic ingredients.

Fis.h.i.+ness The moment fish are caught and killed, other aromas begin to develop. The strong smell that we readily identify as "fishy" is largely due to the salt.w.a.ter-balancing compound TMAO (p. 188), which bacteria on the fish surfaces slowly break down to smelly TMA. Freshwater fish generally don't acc.u.mulate TMAO, and crustaceans acc.u.mulate relatively little, so they don't get as fishy as ocean fish. In addition, the unsaturated fats and fresh-smelling fragments (aldehydes) produced from them slowly react to produce other molecules with stale, cheesy characters, some of which accentuate the fis.h.i.+ness of TMA. And during frozen storage, the fish's own enzymes also convert some TMA to DMA (dimethylamine), which smells weakly of ammonia. The moment fish are caught and killed, other aromas begin to develop. The strong smell that we readily identify as "fishy" is largely due to the salt.w.a.ter-balancing compound TMAO (p. 188), which bacteria on the fish surfaces slowly break down to smelly TMA. Freshwater fish generally don't acc.u.mulate TMAO, and crustaceans acc.u.mulate relatively little, so they don't get as fishy as ocean fish. In addition, the unsaturated fats and fresh-smelling fragments (aldehydes) produced from them slowly react to produce other molecules with stale, cheesy characters, some of which accentuate the fis.h.i.+ness of TMA. And during frozen storage, the fish's own enzymes also convert some TMA to DMA (dimethylamine), which smells weakly of ammonia.

Fortunately, the fis.h.i.+ness of fish past its prime can be greatly reduced a couple of simple treatments. TMA on the surface can be rinsed off with tap water. And acidic ingredients - lemon juice, vinegar, tomatoes - help in two ways. They encourage the stale fragments to react with water and become less volatile; and they contribute a hydrogen ion to TMA and DMA, which thereby take on a positive electrical charge, bond with water and other nearby molecules, and never escape the fish surface to enter our nose.

The aromas of cooked fish are discussed on p. 208.

Flavor Compounds in Raw Fish and Sh.e.l.lfishThe basic flavors of fish and sh.e.l.lfish arise from their different combinations of taste and aroma molecules.

Source Amino acids: sweet, savory Amino acids: sweet, savory Salts: salty Salts: salty IMP: savory IMP: savory

Terrestrial meats + +.

Freshwater fish + +.

Salt.w.a.ter fish +++ +++.

Sharks and rays +++ +++.

Molluscs +++ +++.