Fish and seafood.
In discussing the food uses of fishes, the term ‘fish’ refers to edible species of finfish, molluscs, and crustaceans coming from the marine or freshwater bodies of the world, either by capture fisheries or by aquaculture. Accordingly, ‘fishery products’ means any human food product in which fish is a characterizing ingredient, such as dried, salted, and smoked fish, marinated fish, canned seafood, minced fish flesh such as surimi, and miscellaneous products.
Fish have always been important in the diets of those communities living close to the sea, rivers, and lakes. The development of on-board refrigeration and freezing on fishing vessels as well as refrigerated transport, has improved both the quality and shelf-life of fish and its availability to the general consumer. The development of attractive processed products has also contributed to the widening of fish consumption. According to Food and Agriculture Organization figures, capture fisheries and aquaculture supplied the world with approximately 110 million tons of food fish in 2006, providing an apparent per capita supply of 16.7 kg. Of this total, aquaculture accounted for 47%.
Edible fish muscle contains 18–20% protein and 1–2% minerals; the percentage of lipids varies from less than 1% to more than 20% (in high-fat finfish), and fish has the added advantage of being low in saturated fat. In general, lean fish is not an important source of calories, which are mostly obtained from the staple carbohydrates in the diet. Fatty fish, however, is a significant energy source in many fish-consuming communities in both the developed and the developing worlds. Today it is recognized that fish is probably more important as a source of micronutrients, minerals, and particularly essential fatty acids than for its energy or protein value. The essential micronutrients and minerals in fish include vitamins A and D, calcium, phosphorus, magnesium, iron, zinc, selenium, fluorine, and iodine (in marine fishes).
Several studies have demonstrated beneficial effects of a diet including two or three servings of fish per week on recognized cardiovascular risk factors, such as a reduction of plasma triacylglycerol concentrations, blood pressure, and platelet aggregation (thrombogenesis). These benefits have been attributed to the long-chain omega-3 fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which are found primarily in fatty fish such as salmon, tuna, herring, mackerel, and sardines.
Food-borne diseases attributed to fish can result from the fish itself (i.e., toxic species, allergies) or from bacterial (i.e., Clostridium botulinum, Listeria monocytogenes, Salmonella, Vibrio, and Staphylococcus), viral (i.e., hepatitis, Norwalk gastroenteritis), or parasitic (i.e., Anisakis and related worms) contamination. Also, naturally occurring seafood toxins (i.e., scombrotoxin, ciguatoxins, shellfish poisoning from toxic algae) can cause food-borne illnesses. In recent years, reports of contamination of fish by chemical residues have raised concerns about the healthfulness of certain fish for vulnerable groups of the population.
General Characteristics of Finfish
A very large number of species of finfish are used for food by the world’s population. The carcass yield of finfish (60–70%) is similar to that of beef, pork, or poultry. The percentage of edible tissue in the dressed carcasses of finfish (without head, skin, and viscera) is higher than that of other food animals, because fishes contain less bone, adipose tissue, and connective tissue. There are three main categories of vertebrate fish that are widely used as foods. The bony fishes (teleosts) provide two compositional categories: white fishes (or lean fishes) and fatty fish. The third category is the cartilaginous elasmobranch fishes.
The flesh of these fishes is very low in fat and consists primarily of muscle and thin layers of connective tissue. The concentrations of most of the B vitamins are similar to those in mammalian lean meats, although fish may contain higher amounts of vitamins B6 and B12. The mineral levels are also similar, although the very fine bones that are eaten with the fish flesh can raise the calcium content; fish is also a significant source of iodine. These fishes (i.e., cod) accumulate oils only in their livers, which are a rich source of vitamin A (retinol), vitamin D, and long-chain PUFAs in their triacylglycerols.
These fishes (i.e., mackerel, herring) have fat in their flesh, which is usually much darker than that of white fishes, with similar blocks of muscle and connective tissue. The amount of fat is related to the breeding cycle of the fish, so that the fat content falls considerably after breeding. The flesh of fatty fishes is generally richer in the B vitamins than that of white fishes, and significant amounts of vitamins A and D are present. The mineral concentrations are not very different, but fatty fish is a better source of iron. The oil of these fishes is particularly rich in very-long-chain PUFA, especially those of the omega-3 (n-3) series such as EPA, and DHA. These fishes accumulate oils in their muscles, belly flap, and skin (subdermal fat).
The cartilaginous fishes include the sharks and rays, whose flesh is rich in connective tissue and relatively low in fat, although they do accumulate oils in their livers. The concentrations of vitamins and minerals are very similar to those in white fish. These fishes contain urea in relatively large amounts, and so protein values based on total nitrogen are overestimated. The ammonia smell of cooked sharks and rays is not an indication that the fish is spoiled but rather is the result of enzymatic degradation of urea.
General Characteristics of Shellfish
The term ‘shellfish’ includes any aquatic invertebrate, such as molluscs or crustaceans, which has a shell or shell-like exoskeleton. The cephalopods have an internal shell (as in squids) or no shell (as in octopods). Owing to the presence of the tough exoskeleton, the edible portion in shellfish (approximately 40%) is less than that in finfish, with the exception of cephalopods, whose edible yield is 70–75%. The lipid content of the edible parts of most shellfish is low, as bivalves store their energy surplus as glycogen and not as depot fat, whereas crustaceans and cephalopods store their fat in their digestive glands (hepatopancreas). In many fish-eating communities, these foods are very highly valued gastronomically.
Molluscs and Cephalopods
A wide range of molluscs are eaten by man, including bivalves (such as mussels, oysters, and scallops), gastropods (such as winkles and whelks), and cephalopods (such as squids and octopuses). The flesh is muscular with low levels of fat, although the fat is more saturated and richer in cholesterol than that of finfish. The mineral levels in shellfish are usually somewhat higher than those in finfish, and the vitamin concentrations are low. Bivalves and gastropods are often eaten whole after boiling or sometimes raw; usually, only the muscular mantles of cephalopods are eaten. In some cultures, only selected parts are eaten; for example, only the white adductor muscle of the scallop is eaten in North America. Bivalve molluscs are filter feeders and can be contaminated by toxins and pathogenic organisms from the sea water.
Crustaceans include a range of species, both freshwater (such as crayfish) and marine (such as crabs, shrimps, prawns, and lobsters). These animals have a segmented body, a chitinous exoskeleton, and paired jointed limbs. The portions eaten are the muscular parts of the abdomen and the muscles of the claws of crabs and lobsters. The flesh is characteristically low in fat and high in minerals, with vitamin levels similar to those found in finfish.
Nutritional Value of Fish and Shellfish
Fish and shellfish are excellent sources of protein. A 100 g cooked serving of most types of fish and shellfish provides approximately 18–20 g of protein, or about a third of the average daily recommended protein intake. The fish protein is of high quality, containing an abundance of essential amino acids, and is very digestible by people of all ages. Seafood is also loaded with minerals such as phosphorus, magnesium, iron, zinc, and iodine in marine fish.
The caloric value of fish is related to the fat content and varies with species, size, diet, and season. Seafood is generally lower in fat and calories than beef, poultry, or pork. Most lean or low-fat species of fish, such as cod, hake, flounder, and sole, contain less than 100 kcal (418 kJ) per 100 g portion, and even fatty fish, such as mackerel, herring, and salmon, contain approximately 250 kcal (1045 kJ) or less in a 100 g serving. Most crustaceans contain less than 1% fat in the tail muscle because depot fat is stored in the hepatopancreas, which is in the head region.
In fish, depot fat is liquid at room temperature (oil) and is seldom visible to the consumer; an exception is the belly flaps of salmon steaks. Many species of finfish and almost all shellfish contain less than 2.5% total fat, and less than 20% of the total calories come from fat. Almost all fish has less than 10% total fat, and even the fattiest fish, such as herring, mackerel, and salmon, contains no more than 20% fat (Table 1 below). In order to obtain a good general idea of the fat contents of most finfish species, flesh color might be considered. The leanest species, such as cod and flounder, have a white or lighter color, whereas fattier fishes, such as salmon, herring, and mackerel, have a much darker color.
Low (< 2.5% fat), less than 20% of total calories from fat
Medium (2.5–5% fat), between 20% and 35% of total calories from fat
High (> 5% fat), between 35% and 50% total calories from fat
|Most flatfishes (flounder, sole, plaice||Tuna (yellowfin)||Sardine|
|Shark, skate||Tuna (bluefin)|
The triacylglycerol depot fat in edible fish muscle is subject to seasonal variation in all marine and freshwater fishes from all over the world. Fat levels tend to be higher during times of the year when fishes are feeding heavily (usually during the warmer months) and in older and healthier individual fishes. Fat levels tend to be lower during spawning or reproduction. When comparing fat contents between farmed and wildcaught food fish, it should be remembered that farmed species have a tendency to show a higher proportion of muscle fat than their wild counterparts. Also, the fatty-acid composition of farmed fish depends on the type of dietary fat used in raising the fish. Cholesterol is independent of fat content and is similar in wild and cultivated fishes.
Most protein-rich foods, including red meat and poultry as well as fish, contain cholesterol. However, almost all types of fish and shellfish contain well under 100 mg of cholesterol per 100 g, and many of the leaner types of fish typically have 40–60 mg of cholesterol in each 100 g of edible muscle. It is known that most shellfish also contain less than 100 mg of cholesterol per 100 g. Shrimp contain somewhat higher amounts of cholesterol, over 150 mg per 100 g, and squid is the only fish product with a significantly elevated cholesterol content, which averages 300 mg per 100 g portion. Fish roe, caviar, internal organs of fishes (such as livers), the tomalley of lobsters, and the hepatopancreas of crabs can contain high amounts of cholesterol.
Omega-3 PUFA in Fish and Shellfish
The PUFA of many fish lipids are dominated by two members of the omega-3 (n-3) family, C20:5 n-3 (EPA), and C22:6 n-3 (DHA). They are so named because the first of several double bonds occurs three carbon atoms away from the terminal end of the carbon chain.
All fish and shellfish contain some omega-3, but the amount can vary, as their relative concentrations are species specific (Table 2).
Low-level group (< 0.5 g per 100 g)
Medium-level group (0.5–1 g per 100 g)
High-level group (> 1 g per 100 g)
|Cod, Haddock, Pollock||Halibut||Mackerel|
|Most flatfishes||Red Snapper||Salmon (most species)|
Generally, the fattier fishes contain more omega-3 fatty acids than the leaner fishes. The amount of omega-3 fatty acids in farm-raised products can also vary greatly, depending on the diet of the fishes or shellfish. Many companies now recognize this fact and provide a source of omega-3 fatty acids in their fish diets. Omega-3 fatty acids can be destroyed by heat, air, and light, so the less processing, heat, air exposure, and storage time the better for preserving omega-3 in fish. Freezing and normal cooking cause minimal omega-3 losses, whereas deep frying and conditions leading to oxidation (rancidity) can destroy some omega-3 fatty acids.
Research has shown that EPA and DHA are beneficial in protecting against cardiovascular diseases, through the reduction in serum triacylglycerides, blood pressure and platelet aggregation (thrombogenesis). Brain, nervous tissue membranes, and the retina of the eye contain a high proportion of DHA. Limited evidence from studies of human infants suggests that learning ability and retinal function may be impaired if there is an insufficient intake of DHA in the mother’s diet during pregnancy and lactation.
Both finfish and shellfish are highly valuable sources of proteins in human nutrition, supplying approximately 7.9% of the world’s protein requirements and 15.3% of the total animal protein.
The protein content of fish flesh, in contrast to the fat content, is highly constant, independent of seasonal variations caused by the feeding and reproductive cycles, and shows only small differences among species.
Table 3 summarizes the approximate protein contents of the various finfish and shellfish groups.
|Fish group||g per 100 g|
|White finfish||16 - 19|
|Fatty finfish||18 - 21|
|Crustaceans||18 - 22|
|Bivalves||10 - 12|
|Cephalopods||16 - 18|
Fatty finfish and crustaceans have slightly higher than average protein concentrations. Bivalves have the lowest values if the whole body mass is considered (most of them are usually eaten whole), whereas values are roughly average if specific muscular parts alone are consumed; this is the case with the scallop, in which only the adductor muscle is usually eaten.
The essential amino acid compositions of fish and shellfish are given in Table 4.
Fish proteins, with only slight differences among groups, possess a high nutritive value, similar to that of meat proteins and slightly lower than that of egg. It is worth pointing out the elevated supply, relative to meat, of essential amino acids such as lysine, methionine, and threonine. In addition, owing in part to the low collagen content, fish proteins are easily digestible, giving rise to a digestibility coefficient of nearly 100.
The recommended dietary allowance (RDA) of protein for human male and female adults is in the range of 45–65 g per day. In accordance with this, an intake of 100 g of fish would contribute 15–25% of the total daily protein requirement of healthy adults and 70% of that of children. A look at the dietary importance of the Mediterranean diet is convenient: one of its characteristics is the high consumption of all kinds of fish, chiefly fatty fish. In many Mediterranean countries, fish intake averages greater than 50 g per day (edible flesh); thus, fish protein contributes greater than 10% of the total daily protein requirements steadily over the whole year in those countries.
Nonprotein Nitrogen (NPN) Compounds in Fish
NPN compounds are found mostly in the fiber sarcoplasm and include free amino acids, peptides, amines, amine oxides, guanidine compounds, quaternary ammonium molecules, nucleotides, and urea. NPN compounds account for a relatively high percentage of the total nitrogen in the muscles of some aquatic animals, 10–20% in teleosts, approximately 20% in crustaceans and molluscs, and 30–40% (and in special cases up to 50%) in elasmobranches. In contrast, NPN compounds in land animals usually represent no more than 10% of total nitrogen.
Most marine fishes contain trimethylamine oxide (TMAO); this colorless, odorless, and flavorless compound is degraded to trimethylamine, which gives a ‘fishy’ odor and causes consumer rejection. This compound is not present in land animals and freshwater species (except for Nile perch and tilapia from Lake Victoria). TMAO reductase catalyzes the reaction and is found in several fish species (in the red muscle of scombroid fishes and in the white and red muscle of gadoids) and in certain microorganisms (Enterobacteriaceae, Shewanella putrefaciens).
Migratory marine species such as tuna, characterized by a high proportion of red muscle, have a high content (approximately 1%) of free histidine. The presence of free histidine is relevant in several fish species because it can be microbiologically decarboxylated to histamine, which cannot be inactivated by cooking, thus becoming a hazard to consumers. The symptoms of the resulting illness (scombroid poisoning) are itching, redness, allergic symptoms, headache, diarrhea, and peppery taste. Scombroid poisoning is most common after ingesting mahi-mahi, tuna, bluefish, mackerel, and skipjack.
Nucleotides and related compounds have a noticeable participation in flavor; moreover, some of them may be used as freshness indices. Adenosine triphosphate (ATP), adenosine diphosphate, and adenosine monophosphate decompose quickly leading to a build-up of inosine and hypoxanthine. As this corresponds well to a decline in freshness, the ratio of the quantity of inosine and hypoxanthine to the total quantity of ATP and related substances is called the K-value and used as a freshness index of fish meat.
Guanosine is an insoluble compound that gives fish eyes and skin their characteristic brightness. It is degraded to guanine, which does not have this property; therefore, brightness decreases until it completely disappears.
The NPN fraction contains other interesting compounds, such as small peptides. Most of them contribute to flavor; besides this, they have a powerful antioxidant activity. Betaines are a special group of compounds that contribute to the specific flavors of different aquatic organisms: homarine in lobster and glycine-betaine, butiro-betaine, and arsenic-betaine in crustaceans. Arsenic-betaine has the property of fixing arsenic into the structure, giving a useful method for studying water contamination.
The vitamin content of fish and shellfish is rich and varied in composition, although somewhat variable in concentration. In fact, significant differences are neatly evident among groups, especially regarding fat-soluble vitamins. Furthermore, vitamin content shows large differences among species as a function of feeding regimes.
Of the fat-soluble vitamins, vitamin E (tocopherol) is distributed most equally, showing relatively high concentrations in all fish groups, higher than those of meat. However, only a part of the vitamin E content is available as active tocopherol on consumption of fish, because it is oxidized in protecting fatty acids from oxidation. The presence of vitamins A (retinol) and D is closely related to the fat content, and so they are almost absent in most low-fat groups. Appreciable but low concentrations of vitamin A are found in fatty finfish and bivalve molluscs, whereas vitamin D is very abundant in fatty fish. In fact, 100 g ofmost fatty species supply over 100% of the RDA of this vitamin.
Water-soluble vitamins are well represented in all kinds of fish, with the sole exception of vitamin C (ascorbic acid), which is almost absent in all of them. The concentrations of the rest are highly variable; however, with few exceptions, they constitute a medium-to-good source of such vitamins, comparable with, or even better than, meat. The contents of vitamins B2 (riboflavin), B6 (pyridoxine), niacin, biotin, and B12 (cobalamin) are relatively high. Indeed, 100 g of fish can contribute up to 38%, 60%, 50%, 33%, and 100%, respectively, of the total daily requirements of those vitamins. Fatty fish also provides a higher supply of many of the water-soluble vitamins (namely pyridoxine, niacin, pantothenic acid, and cobalamin) than does white fish or shellfish. Crustaceans also possess a relatively higher content of pantothenic acid, whereas bivalve molluscs have much higher concentrations of folate and cobalamin.
A Mediterranean diet rich in fish – and especially in fatty finfish – contributes steadily over the year to an overall balanced vitamin supply. The supply of vitamins D, B2, B6, B12, and niacin from this particular diet is more than 15% of the daily requirements; all other vitamins, except ascorbic acid, are supplied to a lesser, but significant, extent.
The first point to note is that all kinds of finfish and shellfish present a well-balanced content of most minerals, either macrominerals or trace elements, with only a few exceptions. Sodium content is low, as in other muscle and animal origin foods. However, it must be remembered that sodium is usually added to fish in most cooking practices in the form of common salt; also, surimi-based and other potassium and calcium levels are also relatively low, though the latter are higher in fish than in meat; in addition, small fish bones are frequently eaten with fish flesh, thus increasing the calcium intake. Fish is a good source of magnesium and phosphorus, at least as good as meat. These elements are particularly abundant in crustaceans; fatty finfish show elevated levels of phosphorus, and bivalve molluscs have high amounts of magnesium.
Fish is a highly valuable source of most trace elements. Fatty fish provides a notable contribution to our iron supply, similar to that of meat, whereas shellfish have higher concentrations of most dietary minerals. In particular, crustaceans and bivalve molluscs supply zinc, manganese, and copper concentrations well above those of finfish. Worth mentioning is the extraordinary dietary supply of iodine in all kinds of finfish and shellfish; however, this depends on the concentration present in feed, particularly in planktonic organisms.
In summary, 100 g of fish affords low levels of sodium and medium-to-high levels of all the remaining dietary minerals. In fact, it can contribute 50–100% of the total daily requirements of magnesium, phosphorus, iron, copper, selenium, and iodine. A Mediterranean diet, rich in fatty fish and all kinds of shellfish, can lead to an overall balanced mineral supply, which may well reach greater than 20% of daily requirements of phosphorus, iron, selenium, and iodine.
Chemical Contaminants in Fish
Several chemical contaminants can find their way into fish and some of them can bioaccumulate. These compounds can be divided into three major groups:
- Toxic metals: mercury, cadmium, lead, and tin. These elements are present in water both from natural sources and as a result of human activities, such as emissions from industrial processes, biocides, and paints. These metals are taken up by marine organisms and tend to accumulate in organisms such as predatory fish which are higher up the food chain.
- Halogenated organic compounds: polychlorinated dibenzodioxins, polychlorinated dibenzofurans, polychlorinated biphenyls (PCBs), polybrominated biphenyls, and insecticides (chlorinated hydrocarbons). This is a very diverse group with a chemical stability that allows them to bioaccumulate and persist in the environment.
- Processing-related compounds: polycyclic aromatic hydrocarbons (PAHs), sulfites (used in shrimp processing), nitrosamines, and residues of drugs used in aquaculture (e.g., antibiotics or hormones).
As regards mercury in fish, methylmercury is the chemical form of most concern and can make up more than 90% of the total mercury in fish and seafood. Fish accumulate mercury as a result of its natural presence in the environment and from pollution. Large predatory fish, such as swordfish, tuna, and sharks, accumulate higher levels of methylmercury through intake over a long life-time. Large predatory species are often migratory and it is not possible to exclude fish from particular waters where background levels of mercury contamination might be high. In addition to the setting of maximum levels, targeted consumer advice is an appropriate approach for protecting vulnerable groups of the population. In particular, the advice is aimed at women of child-bearing age and young children as methylmercury can affect the neurodevelopment of the unborn child and young children. It is recommended that pregnant women and nursing mothers should avoid certain species of fish and limit their consumption of other fish to an average of 400 g of cooked fish per week.
Organotin compounds such as tributyltin are organic substances containing the metal tin. There is evidence for marine contamination and organotin compounds bioaccumulate in organisms which is a cause for concern.
Certain fish species such as herring, salmon, and others, originating from contaminated regions of the world may contain high levels of polychlorinated dioxins and furans and PCBs, which are bioaccumulative toxic compounds. Longterm exposure may result in negative effects on the developing nervous system, as well as disruption of the endocrine system. Toxic levels in affected regions should be monitored on a regular basis, and consumers should be informed of the dietary recommendations concerning restrictions on consumption of contaminated fish, especially by identified vulnerable groups of the population.
As regards PAHs, benzo(a)pyrene can be used as a marker for the occurrence and effect of carcinogenic PAHs in food. These toxic compounds can contaminate fish during smoking processes and heating and drying processes that allow combustion products to come into direct contact with fish. In addition, environmental pollution may cause contamination with PAH in fish and fishery products. manufactured foods contain high amounts of added sodium.
Aquatic marine molluscs with a prominent head and tentacles, such as squids, cuttlefishes, and octopuses.
Invertebrate animals that have a hard exoskeleton and two pairs of antennae, such as lobsters, shrimps, crabs, and barnacles.
Dietary reference intakes (DRIs)
Set of four reference values related to the daily dietary intake of nutrients of healthy individuals in a particular life stage and gender group: Estimated average requirements (EAR), recommended dietary allowances (RDA), adequate intakes (AI), and tolerable upper intake levels (UL).
Aquatic vertebrate with fins and a cartilaginous skeleton, such as sharks, rays, and skates.
Invertebrate animals with a soft unsegmented body, usually enclosed in a calcareous shell, such as clams, oysters, and whelks. Surimi A fish-based food product processed to resemble the texture, color, and flavor of the meat of lobster, crab, and other seafood.
Aquatic vertebrate with fins and a bony skeleton, such as cod, tuna, salmon, sole, and sardines.
Any of a group of lipids formed from glycerol and three fatty acids, which can be saturated, monounsaturated, or polyunsaturated.
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