Foods derived from salt water are considered “seafood” whereas all food derived from water environments, be they fresh or salt, are considered marine foods. the principal marine foods are salt water fish, crustaceans and shellfish such as shrimp, lobster, crab, clams, and oysters, and certain freshwater fish and crustaceans. Seafood also includes other sea animals and plants such as seaweed and sea cucumber. Seafood are also converted into large quantities of manufactured of processed foods, most of which are frozen or canned. Examples include recooked, battered, breaded, and frozen filters, fish sticks, and shrimp, as well as canned tuna, salmon, and sardines. In addition, fish are salted, smoked, pickled, or dried. Currently, Americans consume about 16 lb. of seafood per capita per year, compared to 112 lb. of red meats and 64 lb. of chicken. In other parts of the world, seafood makes up the major source of protein in the diet. Although the United States is a major exporter of fish, over 60% of the seafood consumed in this country are from imports, reflecting market demand for selected species.
There are about 50 major species of fish, crustaceans, and shellfish available in the United States, varying with season. In 1989, the U.S. Food and Drug Administration published its Guide to Acceptable Market Names for Food Fish Sold in Interstate Commerce and now requires that fish be appropriately named in accordance with this list to facilitate informed choice by consumers. For the last 30 years the three most important seafood on an economic basis have been shrimp, salmon, and tuna. The teen is toward greater consumption of fresh or frozen fish, shrimp, and shellfish, whereas consumption of canned, smoked, salted, and pickled fish is decreasing.
Americans consume only certain parts of most fish, principally the muscles. Remaining parts plus large amounts of other fish such as menhaden, a fish not commonly used for human food, go into animal feeds. This represents about 50% of the weight of the U.S. fish catch, some of which returns in the forms of red meat, poultry, and egg.
FISH PROCUREMENT
the usual methods of procuring fish for food are quite different from other food-gathering methods and significantly affect the quality of fish foods. modern farmer, for example, have considerable control over plants and animals; they harvest and slaughter at times and in places largely of their own choosing. Hogs and cattle are fed under strict control and then brought to the place of slaughter according to schedule so that subsequent processing and refrigeration may follow immediately under optimized conditions. Food animals are also genetically bred for specific attributes. Not so with most seafood animals. Most fish are pursued and hunted and may be caught in the wild at great distance from processing facilities. These fish may lie poorly iced in the fishing vessel for a week or two, which generally is insufficient refrigeration, before being returned to port-such “fresh fish” is not fresh all; its quality is quite poor and becomes much worse before it reaches the consumer. This is aggravated by the fact that fish tissue is generally more perishable than most animal tissue under any circumstances, for reasons which will be indicated later. This practice is, however, changing to some degree. Oysters, lobsters, trout, catfish, salmon, and a few other species are now commonly “farmed” and this practice is on the increase worldwide. “Farming” fish is called aquaculture. Breeding programs and husbandry practices are being utilized in many cases. In the past, the waters of coastal nations were lightly fished by small boats. Fish were plentiful and boats returned to port in a day or so. Under such conditions, iced fish could be of high quality. In more recent years, with increased fishing pressure, improved methods of locating fish such as sonar and use of helicopters, and large trawler operations, vessels must travel farther to find fish. Under such conditions, since fish would have to remain in the ship’s hold for several weeks or more, the need to prevent spoilage by processing on board has become essential.
Such processing is now being done on factory ships (Fig. 15.1) that accompany fleets of smaller fishing vessels. These have automatic equipment for eviscerating and cleaning fish and for converting livers to fish oil. They also have fileting machines, quick-freezing facilities, and fish meal processing plants to convert less edible portions of the fish to by-products. This is the current trend in the fishing industry of major fishing countries and will improve fish quality in the years ahead. Unfortunately, large numbers of vessels are not yet so equipped, and one of the major problems faced by food scientists is related to maintaining fresh quality.
MARINE FISH
One useful division of saltwater fish separates them into two groups depending on the depth of water in which they are found, which is correlated with great differences in the fish’s fat content. Fish found in the middle and surface water layers of the sea are called pelagic fish and include herring, mackerel, salmon, tuna, sardines, and anchovies. This group includes many of the fatty fish, which in some instances have muscle containing 20% fat. The other group, called demersal fish, is found at or near the bottom of the sea. Ordinarily on the continental shelves, and includes cod, haddock, whiting, flat fish such as flounder and halibut, ocean perch, shrimp, oysters, claims, and crab. Demersal fish usually have less than 5% fat and sometimes less than 1% fat in the muscle.
Composition and Nutrition
The composition and nutritional properties of the edible muscle of fish of a given species are quite variable, depending on season of the year, degree of maturity, and other factors. The herring, for example, may vary in muscle fat from about 8% to 20% with changes in the season and available food supply. The composition of most fish falls in the ranges of about 18-35% total solids, 14-20% protein, 0.2-20% fat, and 1.0-1.8% ash (see Table 14.1).
Nutritionally fish proteins are highly digestible and at least as good as red meat with respect to content of essential amino acids. Consequently, the most important function of fish in all major flash-eating countries, of the world is to provide high quality protein.
Because the fats of fish also are readily digestible and rich in unsaturated fatty acids, nutritionists frequently emphasize the importance of fish in the diet. But like all unsaturated fats, those in fish are highly susceptible to oxidation and the development of off-flavors and rancidity.
Fish are rich in vitamins. The fat of fish is an excellent source of vitamins A and D, and this was the reason for giving cod liver oil to children before multivitamin tablets were common. Fish muscle is a fair to good source of the B vitamins. Generally, shellfish and crustaceans are still richer in B vitamins than finfish.
Seafoods are a good source of important minerals and an excellent source of iodine in particular. Fish are lower in iron than most meats. Canned fish with the bones, such as salon and sardines, are excellent sources of calcium and phosphorus.
Spoilage Factors
Fish tissue generally is more perishable than animal tissue, even under conditions of refrigerated or frozen storage. Beyond this generalization, it is difficult to make broad statements about the storage life of freshly caught fish because of the many variables that are encountered. Differences in tissue compositions of species, influence of season of the year on composition, differences between freshwater and saltwater fish and the effects of salt on the normal microflora of these variables. Further, there presently are no rigid criteria that adequately differentiate such terms as truly “fresh,” “good” or “acceptable,” although grading systems based on taste panel results and selected chemical analyses can make useful distinctions. Certainly, the quality of a fish begins to change as soon as it is taken from the water, and “fresh” fish that is quite acceptable commercially is not fully the equivalent of the product when caught.
The stability data in Table 15.1 reflect commercial acceptability. Fresh fish held at a moderate temperature 16C remain good for only about 1 day or less. On ice at 0C, finfish may remain good for periods up to about 14 days, but this is not true of all species. In contrast, beef may be aged at 2C for several weeks to improve its texture and flavor. Even with mild salting and smoking, as in the case of finnan haddie and kippers, fish may remain good at 0C for only a few weeks. Heavy salting and drying will, of course, preserve fish for long periods. There are several important reasons why fresh fish spoil rapidly, and these are microbiological, physiological, and chemical in origin.
microbiological
Although the flesh of healthy live fish is bacteriologically sterile, there are large numbers of many types of bacteria in the surface slime and digestive tracts of living fish. When a fish is killed, these bacteria rapidly attack all constituents of the tissues. Furthermore, since these bacterial live on the cold-blooded fish at rather low ocean temperatures, they are well adapted to cold and continue to grow even under common refrigeration conditions.
Physiological
Fish struggle when caught and use up virtually all of the glycogen in their muscles, so little glycogen is left to be converted to lactic acid after death. Thus, preservative effect of muscle lactic acid to slow bacterial growth is limited. This is in contrast to animal meat where animals are rested before slaughter to build up glycogens reserves.
Chemical
Associated with the fat of fish are phospholipids rich in trimethylamine. Trimethylamine split from phospholipids by bacterial and natural fish enzymes has a strong characteristic fishy odor. It is interesting to note that fish as taken from the water have little or no odor. Yet virtually all fish products that consumers encounter have a fishy odor and this is evidence of some deterioration. The fishy odor from liberated trimethylamine is further augmented by odorous products of fat degradation. The fats of fish are highly unsaturated and become easily oxidized, resulting in additional oxidized and rancid off-flavors.
Preservation Methods
Because of the great tendency of fish to spoil, a number of methods of preservation have been developed over the years. The most basic methods are smoking and/or salting with subsequent drying. This is effective, but such preserved fish are not accepted in all cultures. Other societies find such preserved seafoods highly desirable.
Chemical preservatives such as sodium benzoate or sorbic acid can prolong storage life. In the United States, it used to be permissible to incorporate the antibiotic aureomycin in low levels into the ice used for packing fish on shipboard and in transit; the melting ice then held down bacterial growth. But this application is no longer allowed. Sodium nitrate and nitrite are permitted in a limited number of applications, as are salts of sorbic acid.
In recent years, irradiation with gamma rays to pasteurization doses has received much interest as an effective means of prolonging storage life of fresh refrigerated fish by 2-3 weeks. Experimental irradiation facilities have been tested to the determine commercial feasibility and to establish data to support petition to the FDA for this irradiation application. But the current methods of greatest importance by far in preserving quality fish products still are refrigeration, freezing, and canning.
Shipboard Operations
Freezing can give excellent or poor quality results depending on how quickly the freezing is done after the fish is caught and on the freezing and storage temperatures. Ideally, fish should be gutted and frozen to -30C within 2 h of being caught and held at this temperature. Unfortunately, this is costly and often unfeasible, and even supermarket frozen food cabinets generally are not kept below the -18 to -15C range.
Figure 15.2
When fish are not processed and hard-frozen on board ship, and they generally are not excepting on the newer factory-type vessels, they commonly are stored with layers of ice at 0C or in refrigerated chilled seawater at about -1C. When the catch is held this way for a week or longer, it is no longer truly fresh on arrival at port. In Fig. 15.2 halibut is seen being unloaded from the cold hold of a fishing vessel. Not all fish held on ice or in chilled seawater is gutted on board ship, frequently this is done when the fish reaches port. Ungutted tuna often are chilled in ship brine wells down to -12C, which partially freezes the fish. The tuna is then thawed and gutted weeks later in processing plants.
Processing Plant Operations
Fish that are not processed at dockside facilities commonly are packed with ice for transport to a processing plant. Here the fish are washed, gutted if not previously done, and then scaled, skinned, or filleted. There are machines for gutting, skinning, and filleting, but much of this work is still done by hand since machines need to be set for a given size of fish and size varies considerably.
Freezing
Fillets may be wrapped a few to a package for retail trade or packaged in sizable boxes and frozen. Small packages may be frozen in contact freezers of the type used to freeze retail portions of vegetables. Large boxes are frozen in room and tunnel blast freezers, preferably to a temperature of -30C or below.
Cleaned whole fish also are frozen in the round. This often is done with larger fish, which after freezing are sawed perpendicular to their length into fish steaks; salmon and halibut are often handled this way. When individual fish are frozen, they are sometimes glazed with layers of ice by dipping in cold water. Dipping and freezing may be repeated several times to build up a thick glaze to protect the frozen fish from surface oxidation and from freezer burn (drying out) during frozen storage. Glazing also is practiced with frozen shrimp. When steaks are cut from large frozen fish, the newly cut surface are sometimes glazed to improve their keeping quality. It is common to add an antioxidant to the glazing water. Glazed frozen fish still require protective packaging in wrapping materials that are air-tight and moisture-tight.
Much fish goes into the preparation of prebreaded, precooked, and frozen fish sticks and individual fish portions. Generally, fish fillets that have been block-frozen in large boxes are used for this; they are cut to fish stick or portion size on band saws from the frozen blocks. The frozen portions are then battered and breaded and automatically conveyed to deep-fat fryers (Fig. 15.3). The fried piece is then cooled, packaged, and refrozen.
The storage life of quality frozen fish further depends on its fat content. High quality, low-fat fish stored in the frozen state at -21 to 23C may retain its quality for as long as 2 years.
Canning
Whereas high-fat fish do not store as well as low-fat fish in the frozen state, fish which contain more oil are more suitable for canning. Important examples are salmon, tuna, sardines, and herring. In the case of salmon, tuna, and sardines, additional fish oil, vegetable oil, or water commonly is added to the fish prior to can closure. Vegetable extracts are often added to help improve flavor. Canned fish products generally have a shelf life of several years.
Briefly, canning tuna involves the following steps:
1. Thaw the partially frozen tuna received from the fishing vessel.
2. Eviscerate, clean, and sort tuna for size.
3. Precook whole tuna in steam ovens (Fig. 15.4) to soften flesh for easy separation.
4. Let cool overnight, and then separate the light and the dark meat. The white meat gets the premium price, and the separation is still largely a hand operation which may involve 100 workers in a large plant.
5. Compact the tuna meat by machine into a cylindrical shape, cut off portions, and automatically fill into cans.
6. Add salt and vegetable oil or water to the cans.
7. Vacuum-seal the cans and sterilize them in a retort. The color of the flesh may darken somewhat due to browning reactions on heating.
Inspection and Grading
In the United States there is a voluntary fish inspection service and there are grade standards for fish and fishery products much like those of the USDA for agricultural products.
These and other standards –covering sanitation, quality, identity of fishery products, and processing plant operations –are administered by the National Marine Fisheries Service of the Department of Commerce. This agency also cooperates with the FDA, USDA, and state agencies, especially in matters relating to food safety and honest representation.
SHELLFISH
The term shellfish as generally used refers to the true shellfish (e.g. oysters and clams) and the crustaceans (e.g., lobsters and crabs). The high degree of perishability of finfish is shared by shellfish, expect that most shellfish are even more perishable.
Lobsters and crabs, for example, are best kept alive up to the point of their cooking or freezing, otherwise they deteriorate in quality in a matter of a day or less.
Shrimp
A favorite U.S. seafood, shrimp are caught in large trawling nets near U.S coastal waters of the South Atlantic, Pacific, and the Gulf of Mexico, with additional quantities being imported from central and south America where they are often farmed in aquaculture. Many are frozen in the raw state and in the breaded and precooked condition. Many are canned and some are freeze-dried.
Figure 15.4
After capture, heads are removed the sooner the better for quality. This often is done on the shrimp boats. The landed iced shrimp are then unloaded at the processing plant where they are washed and sorted according to size. They then may be inspected, packed, and frozen in the shell without deveining. On the other hand, shrimp for breading and precooking prior to freezing are automatically peeled of shell and mechanically deveined by splitting and washing out the vein like intestine.
Shrimp should be consumed or processed within 5 days of being caught even when they are iced. In addition to continued bacterial activity, iced shrimp will darken in color and may become black due to natural polyphenol oxidase enzymes contained in the shrimp (Fig 15.5). This discoloration can be inhibited by addition of ascorbic acid or citric which can be added to the ice or used as a dip or spray. Shrimp quality is especially favored by very rapid freezing; such as is achieved with liquid nitrogen. Texture, color, and flavor are superior and drip loss is minimized when cryogenic freezing is used. However, these advantages are largely lost when shrimp are frozen by slower methods at sea and held on ice prior to arrival at the processing plant. To avoid this, some processors are supplying liquid nitrogen to shrimp boats. The Ph. of shrimp tissue is a fairly good index of shrimp quality. Freshly caught shrimp have a flesh ph. of about 7.2, which increases even in ice storage. Quality remains generally good up to a ph. of about 7.7. Above a ph. of 7.9, shrimp become progressively spoiled.
Oysters and Clams
Oysters and clams remain essentially fixed in their environments and are harvested by raking the bottom or digging the mud close to shore. The U.S. oyster industry is concentrated in the Chesapeake Bay area and Long Island Sound, with lesser amounts of oysters coming from coastal waters of the south Atlantic and Pacific Northwest states. The clam industry is more widely scattered along the entire seacoast.
Figure 15.5
Live oysters and clams are removed from the shell by hand, washed, sorted for size, and then further processed. Processing may simple involve packing in cans or jars and shipping in ice to market. Oysters and clams also are canned and sterilized in retorts and to a lesser extent are frozen. The frozen products may later be used in the manufacture of stews and chowders.
The condition of the water from which the clams and oysters and harvested is important. The oysters and clams can become infected with bacteria or virus from water which has become polluted from sewage. This is particularly serious because oyster and clams are frequently eaten raw. Outbreaks of infectious hepatitis and gastroenteritis have been traced to uncooked clams and oysters. For these reasons, commercial plant handing oysters and clams operate under regulations of the U.S. Public Health Service –FDA and usually also under state regulations.
Crabs
Crabs of major importance in the United States are the blue crab, Alaskan king crab, Dungeness crab, and tanner crab. For meat removal, crabs are cooked prior to the flesh being hand-picked. Meat is removed from the larger king crab with water jets and by passing legs through rubber rollers to squeeze out the meat. Crabmeat is canned or pressed into blocks and frozen. Whole cooked crabs and king crab legs with the shell are frozen also.
The yield of meat from crabs is low, averaging about 12% (20% for king crab). Disposal of the remaining shellfish “wastes” has become a difficult problem since recent environmental regulations prohibit the dumping of untreated shellfish wastes into coastal waters and treatment can be costly. This has focused research efforts on conversion of “wastes” into useful products such as chitin and chitin derivatives from the shells (also from the shells of shrimp) and enzymes rom other tissues.
FISH BY-PRODUCT
Parts of fish not used for human food such as the intestines, heads, and gills, as well as whole fish of less favored species, have been ground up, dehydrated, and converted to fish meal for animal and poultry feed. Such fish meal generally had a fishy odor and a high bacterial content and was not considered suitable for human food. Fish meal has also been used as fertilizer.
More recently, ways have been found to extract oils and fatty substances from ground can be heated, stripped of solvent, and then dehydrated and milled to a bland, highly nutritious powder rich in high quality protein and minerals. When produced under proper bacteriological and sanitary control from selected species of fish, the product is known as fish protein concentrate (FPC) and can be used as human food.
Fish protein concentrate when properly manufactured and packaged can be readily incorporated into a wide variety of basic foods as an enrichment without adverse effects on acceptability of these items. Fish protein concentrate can contain 85-92% of high quality protein. It has been estimated that for a cost of approximately a cent, per day per person, FPC could balance protein-deficient diets around the world. However, as yet, this product has acquired but limited commercial importance.
CONTAMINENTS IN FISH
Many foods taken from marine environments are near the top of the food chain and can concentrate undesirable pollution from that environment. Unfortunately, human activity has resulted in pollution of many marine environments with biological and chemical pollutants, so it is not surprising that seafoods harvested from polluted water contain pathogenic microorganisms and toxicants. There are also naturally occurring toxicants which can be concentrated in seafoods.
Mercury is an example of a contaminant which has concentrated in seafoods. Mercury occurs naturally but usually not in toxic forms or amounts. It is also used in several industrial processes and may be unnaturally high due to industrial pollution. Mercury accumulation in fish depends not only on the level of mercury in the water but also on the type of fish, its natural food chain, and the age of the fish. Older and larger fish of a given species tend to have higher mercury levels, believed due to the longer time they have had to concentrate it in their tissues from their food supply.
Because mercury in sufficient concentration is toxic, regulatory agencies of various countries have set upper limits of mercury permitted in foods. before 1979, the U.S. Food and Drug Administration’s recommended maximum level for mercury in fish was 0.5 mg/kg; in some other countries this level was higher. Such levels are established on the basis of toxicological data plus an appropriate safety factor. At the present time, however, no one knows the absolute threshold level that is toxic in humans. In 1979 the U.S. action level was increased from 0.5 to 1.0 mg/kg of mercury in seafoods, and in 1984 this action level was made specific for methyl mercury rather than total mercury. The changes reflect newer information on toxicology of the compounds of mercury and economic considerations.
In addition to mercury, other pollutants reaching lakes, rivers, and the ocean can accumulate in fish. For example, PCBs (polychlorinated biphenyls) have been implicated as a human health risk, especially from lakes and rivers near sites where PCBs were used in various manufacturing processes. Other chemicals such as dioxin, pesticides such as DDT, endrin and diel Drin that may leach from soils, and heavy metals such as cadmium and lead can become, pollutants. The situation is aggravated when the pollutant is not readily biodegradable, as is the case for more to the above materials.
it is not possible to immediately remove such pollutants from marine environments so regulatory agencies usually set limits on the amount of pollutant that seafood can contain and often advise people to limit the amount of a given fish they should consume.
Seafoods can also be contaminated by toxins and organisms of natural origin. Red tide which occurs without predictability is an example. The red color is due to immense numbers of the protozoa Gymnocardium brevis, which can infect fish and shellfish. Consumption of infected seafood can cause respiratory irritation in man, and such seafood has had to be withheld from the market on occasion.
Some fish themselves contain toxins which can cause death. Puffer fish, considered a delicacy in some Asian cultures, contain a potent toxin. The toxin is contained in the gonads of the fish and can be removed, but great are must be practice.
Yet another problem is known as scromboid fish poisoning and is due to the formation of histamine resulting from microbiological decay when fish is held under poor storage conditions (above 0 to 6C). Cooking does not provide protection against such toxins if already formed in the fish.
Also, fish may harbor parasitic tapeworms and round worms that can infect humans. Such infections may result from the consumption of raw fish. These parasites are readily destroyed by common cooking and freezing procedures and by salting and/or smoking. Eating raw seafood which has not been frozen or cooked can be risky.
NEWER PRODUCT IN SEA FOOD
Seafoods, with only limited exceptions, are unique in that they are almost exclusively harvested from the wild. Humans have learned through agriculture to domesticate most other food source, but seafoods depend to a large extent on nature. As such, it is recognized that the food from the sea is not an inexhaustible resource. Several new sources of seafood and new processes to better utilize existing seafoods have been developed in recent years. Oysters, salmon, catfish, and others are commonly farmed. Species which return to their spawning grounds after roaming free are sometimes “ranches.” Ranching fish means hatching fry and turning them lose form a designated place. After growing in the wild, the fish will return to spawn and can be harvested.
Machines similar to those used to separate meat from bone have been successfully used to obtain minced fish flesh from filleting wastes and from underutilized species. This minced fish flesh is now being used in the processed seafoods of several countries. One newer process which provides high quality food from minced fish yields surimi. Minced fish is highly washed to remove soluble including pigments and flavors, leaving an odorless and flavorless high-protein flesh. This is combined with other ingredients to give the fish muscle emulsion texture and frozen stability. Flavors and color are added and the product extruded into shapes resembling other products such as crabmeat or lobster meat. Surimi production is a highly technical process requiring considerable skill. In the United states this process is mostly used to make artificial crab products. Surimi is especially popular in parts of Asia including Japan.
Figure 15.6
In order to provide a more constant and reliable source of certain seafoods, there has been a growth in various form of aquaculture, involving both freshwater and saltwater species. Presently, about 15% of the world’s seafood and 5% of U.S. seafood is produced under varying of control, utilizing ponds, tanks, cages, nets and other forms of confinement (Fig. 15.6). In the southern part of the United States, for example, growing catfish in freshwater ponds has become successful. In northern Europe, raising salmon represents a large and growing business. Research has been concerned with fish genetics, nutrition, disease control, yields, fish qualities for food use, and development of markets. In aquaculture as well as ocean fishing there is no doubt that an enormous potential for food remains to be developed.
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