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netted fish struggle and usually die from asphyxiation, which may produce certain metabolic changes that are rather disadvantageous to the shelf life of the fish as food.

        Nutritional Composition and Quality  

        A. Southgate classified fish used as foods into the following:

1.The bony fish-the Teleosts fall into two compositional groups: (a) white fish such as cod, haddock, halibut, lemon sole, plaice ( and most other flat fish), saithe, and whiting and (b) fatty fish, such as eels, herring, pilchards, salmon, sardines, sprats, trout, tuna, and whitebait

2.The cartilaginous Elasmobranch fish, such as dogfish, shark, and skate 

1.White Fish

The white fish are very low in fat and have muscle blocks surrounded by thin sheets of connective tissue. They have lower concentrations of most of the B group vitamins than mammalian muscle, except perhaps vitamin B6. The mineral matter content of white fish is similar to that of mammalian muscle. The consumption of very fine bones of these fish with the flesh may raise the calcium content. Like most other marine organisms, the fish accumulate trace elements from seawater, and thus are a rich source of iodine. They may, however, be contaminated with certain toxic metals taken from heavily polluted waters (16). White fish accumulate fats (oils) in their livers which are a rich source of vitamin A (retinol) and vitamin D, as well as long-chain polyunsaturated fatty acids in their triglycerides.  

the fatty fish is normally richer in the B complex vitamins than the white fish, with significant amounts of vitamin A and D. The minerological composition is similar to that of the white fish. The fat has a higher proportion of very long chain polyunsaturated fatty acids, making the fat very prone to develop oxidative rancidity. Therefore, fatty fish are often smoked or pickled to preserve them.  

         3.Cartilagious Fish  

        These are the marine fish, such as sharks and rays. The flesh has low fat content, though their livers are rich in oil content, the content of vitamins and minerals being similar to that of the white fish. They remarkably maintain the osmolality of their extracellular fluids by increasing the urea content, thus giving an overestimate of the protein values based on total nitrogen content (6). The nutritional composition of some fish, molluscs, and crustaceans given in Table 11 shows that within the major groups of these marine species, there is a considerable similarity with respect to their fat and protein content.

The muscle of fish, molluscs, and crustacea is a good source of protein, providing similar amounts of lean meats. The protein quality in terms of amino acid composition is very similar in most fishes (Table 12), though the molluscan and crustacean proteins appear to be rich sources of essential amino acids (Table 11) (13). Their low fat content, however, has a high proportion of long-chain polyunsaturated fatty acids (Table 13). The mineral matter content of fish and the concentration of inorganic nutrients is comparable with other meats, with the exception of calcium content of fine bones in fish like herring. The levels of N a and intercellular elements, K and P, are higher than in meats, but the Fe and Zn levels tend to be lower, except in shellfish. Oysters are particularly rich sources of Zn, with levels as high as lOO mg/100 g. Fish are also a major source of iodine.       

        4. Invertebrate Seafoods

The popular shellfish species include two major phyla, the Mollusca (true shellfish) and the Arthropoda, Order Crustacea which include species such as crabs, shrimps, prawns, and lobsters.

a. Mollusc.s'. The variety of molluscs eaten by humans include bivalves such as mussels, oysters, and scallops, gastropods such as winkles and whelks, and other molluscs who have lost their external shells, retaining an inner pen, the squids and octopuses. The true shelled molluscs may be eaten whole after boiling or occasionally raw. The muscular flesh has low fat and vitamin content, but the mineral levels are generally higher than in true fish. Being filter feeders, they often accumulate trace elements, both essential and contaminant, from the seawater. They are also rather prone to contamination from aqueous pathogenic organisms, necessitating stringent regulations regarding the sites of their catch. Some rules require that these animals should be "rested" in unpolluted water for a period before they are sold. The muscular squids and octopuses are generally eaten after cooking.  

 species. These animals tend to accumulate trace elements from the water, their vitamin levels being similar to those of white fish .

The average consumption offish in the United Kingdom in 1991 was about 29 g/person/day, of which about 18% was frozen convenience products , shellfish accounting for 2.5% of the total fish consumption. The consumption of fish in other European countries such as Spain, Portugal, and Italy is much higher than in the United Kingdom, probably because of their access to a wider range of species used as fresh foods. Thus the contribution of fish and other ocean foods to the nutrient intakes in a country such as the United Kingdom is relatively minor, providing only about 1% of energy, 4.7% of protein, and 1.3% of fat (3.2% of polyunsaturated fatty acid intakes). The only vitamin provided in significant quantity is vitamin D (14% of the total by the fish).  

B.Processing of Fish and Seafoods

Fish being a highly perishable product, various processing methods have been developed to preserve them. First they are washed with a clean water to remove slime, blood stains, and so on. Larger fish may be gutted (i.e., all the internal organs or viscera are removed and the body cavity is washed). The fish are often processed by drying, salting, smoking, freezing, and canning.

        1. Drying

Fish are sun-dried to remove moisture from the tissues and to arrest bacterial and enzymatic putrefaction. In India, about 35% to 40% of the total sea catch is cured in the sun. This method is not hygienic and there are significant losses due to spoilage. Also, the dried fish develop a pecu-

liar odor.  

        2. Salting

Salting or pickling is widely followed in countries such as India; both dry-salting and wet-salting methods are adopted. For dry-salting, fish are first rubbed with salt powder, and then packed in tubs with dry salt powder sprinkled in between layers of fishes. After a period of about 10 to 20 hours, the fish are removed, washed in brine, and dried in the sun for 2 to 3 days. In wet-salting, cleaned fish are packed in large vats containing concentrated salt solution, and stirred daily till properly pickled. Large-sized fish may be cut longitudinally to produce slits to facilitate penetration of salt. After 7 to 10 days of pickling, the salty water oozing out from the fish is drained off and fish is sold without drying. This fish does not keep long and must be disposed of within 2 to 3 months. The fishes soaked in salt solution may be taken out and smoked.

The products of sun-drying and salting are unattractive; these methods are often associated with case-hardening, development of rancidity, color changes, mold growth and attack of insects and mites. Preservatives, such as acids, sodium benzoate, ethylene oxide or an antibiotics like aureomycin can help to prolong shelf -life of fish. However, many countries do not permit the use of these preservatives. Irradiation of fish with gamma-radiation prolongs the shelf life of fish by

20 to 25 days.

         3. Freezing and Canning

These are the current methods of preserving high-quality fish, greatly extending the storage life and maintaining freshness. If the fish is gutted and frozen down to -29°C within 2 hours of its catch, its storage life can be extended as long as 2 years. In some cases, clean whole fish is frozen. Fin fish are usually frozen as fillets (lengthwise cuts ), steaks ( cross-cut section), or sticks (lengthwise or crosswise cut from fillet or steaks ). Large fish are frozen by the sharp freeze, a comparatively slow freeze, and small fish, fillets, and steaks are quick frozen. Slow freezing can result in protein denaturation, if proper care is not taken. Freezing may also result in desiccation or drying caused by the transfer of moisture from fish surface to the cold surface of freezing equipment. Frozen fatty fish may undergo oxidative rancidity more quickly than lean ones. Desiccation and oxidation can be prevented by properly protecting fish with suitable wrappers prior, to freezing.

The fat fish are more suitable for canning ( e.g., salmon, tuna, sardine, herring, lobster, shrimp ). In the cases of salmon, tuna, sardine, and mackerel, an additional quantity of fish or vegetable oil is usually added to the fish prior to its canning, whereas shrimps are often canned in brine. Canning retains the natural flavor of the fish. Shellfish, however, can turn dark or discolored during canned storage, owing to the release of hydrogen sulfide from its sulfur compounds. The latter reacts with the iron of the can to produce black iron sulfide. This can be prevented by using an enamel containing zinc, because zinc sulfide is white in color.

C. Fish Products

A range of products made from fish and other seafoods include fish meal used as animal feed, fish flour used for protein enrichment of human food, fish oil used for feeding and industrial purposes, fish glue (isinglass), and a high-grade fish collagen used to clarify wines, beer, and vinegar.

        1. Fish Meal

Fish meal is prepared from parts of fish not used as human food and sometimes the whole fish is of low quality. The material is ground, usually with removal of some oil followed by dehydration. Fish meal contains about 55 to 70% protein, 2 to 5% fat, 10 to 12% ash, and 6 to 12% moisture. It is used as animal and poultry feed. A low-grade fish meal can also be used as a manure for plantation crops.

        2. Fish Flour

After the solvent extraction of oil and fatty substances from the ground fish tissues to remove all the fishy odor, the tissue is dehydrated and milled to produce a bland nutritious powder rich in protein and mineral matter. Fish flour produced under appropriate bacteriological and sanitary control can be utilized in human food, as fish flour or fish protein concentrate. It can be incorporated at 3 to 10% level into a variety of dishes without altering their acceptability. Fish flour has about 85 to 90% of high quality protein.

        3. Fish Oils

Two kinds of fish oils, viz., liver oils and fish body oils, are produced. Liver oil is the principal natural source of vitamin A and, to a lesser extent, vitamin D. Fish species such as cod, halibut, tuna, and shark are good sources of fish liver oils. Both the oil and vitamin A contents vary in different fishes. Body oil is obtained from fishes such as sardine, herring, and salmon, which are fatty fishes (Table 11).

Liver oil can be obtained by cooking good quality minced fish liver at 85°-95°C which results in disintegration of liver cells and release of oil. The oil floating on the steam condensate can then be skimmed off or separated by centrifugation.

Fish body oils are produced along with the fish meal by first grinding fishes to a pulp and steaming. The oil and water become separated from the protein. The cooked flesh is then pressed to produce oil and presscake. The latter is used for fish meal. The press liquor ( or stick water) is concentrated and the oil is recovered.