Pisces class- this is the largest group of modern vertebrates, which unites more than 25 thousand species. Fish are inhabitants of the aquatic environment; they breathe through gills and move with the help of fins. Fish are distributed in different parts of the planet: from high mountain reservoirs to ocean depths, from polar waters to equatorial ones. These animals inhabit the salty waters of the seas and are found in brackish lagoons and the mouths of large rivers. They live in freshwater rivers, streams, lakes and swamps.
External structure of fish
The main elements of the external body structure of a fish are: head, operculum, pectoral fin, ventral fin, body, dorsal fins, lateral line, caudal fin, tail and anal fin, this can be seen in the figure below.
Internal structure of fish
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Fish organ systems |
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1. Skull (consists of the braincase, jaws, gill arches and gill covers) 2. Skeleton of the body (consists of vertebrae with arches and ribs) 3. Skeleton of fins (paired - pectoral and abdominal, unpaired - dorsal, anal, caudal) |
1. Brain protection, food capture, gill protection 2. Protection of internal organs 3. Movement, maintaining balance |
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Musculature |
Wide muscle bands divided into segments |
Movement |
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Nervous system |
1. Brain (divisions - forebrain, middle, medulla oblongata, cerebellum) 2. Spinal cord (along the spine) |
1. Movement control, unconditioned and conditioned reflexes 2. Implementation of the simplest reflexes, conduction of nerve impulses 3. Perception and conduction of signals |
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Sense organs |
3. Hearing organ 4. Touch and taste cells (on the body) 5. Lateral line |
2. Smell 4. Touch, taste 5. Feeling the direction and strength of the current, the depth of immersion |
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Digestive system |
1. Digestive tract (mouth, pharynx, esophagus, stomach, intestines, anus) 2. Digestive glands (pancreas, liver) |
1. Capturing, chopping, moving food 2. secretion of juices that promote food digestion |
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swim bladder |
Filled with a mixture of gases |
Adjusts immersion depth |
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Respiratory system |
Gill filaments and gill arches |
Carry out gas exchange |
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Circulatory system (closed) |
Heart (two-chambered) Arteries Capillaries |
Supplying all body cells with oxygen and nutrients, removing waste products |
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Excretory system |
Kidneys (two), ureters, bladder |
Isolation of decomposition products |
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Reproduction system |
Females have two ovaries and oviducts; In males: testes (two) and vas deferens |
The figure below shows the main systems of the internal structure of fish
Fish class classification
Living fish today are divided into two main classes: cartilaginous fish and bony fish. Important distinguishing features of cartilaginous fish are the presence of an internal cartilaginous skeleton, several pairs of gill slits that open outward, and the absence of a swim bladder. Almost all modern cartilaginous fish live in the seas. Among them, the most common are sharks and rays.
The vast majority of modern fish belong to the class of bony fish. Representatives of this class have an ossified internal skeleton. A pair of external gill slits are covered with gill covers. Many bony fish have a swim bladder.
Main orders of Pisces
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Orders of fish |
The main characteristics of the detachment |
Representatives |
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Cartilaginous skeleton, no swim bladder, no gill covers; predators |
Tiger shark, whale shark, katran |
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Manta ray, stingray |
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Sturgeon |
Osteochondral skeleton, scales - five rows of large bone plates, between which there are small plates |
Sturgeon, beluga, sterlet |
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Dipnoi |
They have lungs and can breathe atmospheric air; the chord is preserved, there are no vertebral bodies |
Australian cattail, African scalefish |
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lobe-finned |
The skeleton mainly consists of cartilage, there is a notochord; poorly developed swim bladder, fins in the form of fleshy outgrowths of the body |
Coelacanth (the only representative) |
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Carp-like |
Mostly freshwater fish, there are no teeth on the jaws, but there are pharyngeal teeth for grinding food |
Carp, crucian carp, roach, bream |
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Herring |
Most are schooling sea fish |
Herring, sardine, sprat |
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cod |
A distinctive feature is the presence of a mustache on the chin; the majority are cold-water marine fish |
Haddock, herring, navaga, burbot, cod |
Ecological groups of fish
Depending on their habitat, ecological groups of fish are distinguished: freshwater, anadromous, brackish and marine.
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Ecological groups of fish |
Main features |
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Freshwater fish |
These fish constantly live in fresh water. Some, such as crucian carp and tench, prefer standing water. Others, such as the common gudgeon, grayling, and chub, have adapted to life in the flowing waters of rivers. |
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Migratory fish |
This includes fish that move from sea water to fresh water to reproduce (for example, salmon and sturgeon) or from fresh water to breed in salt water (some types of eels) |
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Salty fish |
They inhabit desalinated areas of the seas and the mouths of large rivers: such are many whitefish, roach, goby, and river flounder. |
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Sea fish |
They live in the salty water of seas and oceans. The water column is inhabited by fish such as anchovy, mackerel, and tuna. Stingrays and flounder live near the bottom. |
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A source of information: Biology in tables and diagrams./ Edition 2, - St. Petersburg: 2004.
Beluga, crucian carp, herring, trout, carp, silver carp, carp are well-known fish. This list can be continued endlessly. And their commercial importance is difficult to overestimate. And indeed, very diverse. Modern taxonomy includes more than 20 thousand species of these aquatic animals. Thanks to what structural features did they manage to master this habitat and occupy a dominant position in it? What class do fish that differ in their structure belong to? You will find the answer to these and other questions in our article.
Signs of fish
It’s not for nothing that they say about self-confident people: “They feel like a fish in water.” Scientists know that the first fish lived in the Silurian period. Outwardly, they were similar to modern sharks with movable jaws on which sharp teeth were located. Millions of years have passed, and in the process they have changed and acquired a number of new adaptive characteristics.
As aquatic animals, they all have a streamlined body shape, fully or partially covered with scales, various types of fins located on the body, and gills as respiratory organs. These are common characteristics for all representatives of a given systematic unit. But what class fish belong to can be answered by considering their significant differences. At the moment there are two of them: Bone and Cartilaginous.
Features of the external structure
The body of absolutely all fish is covered with scales. It protects the skin of aquatic inhabitants from excessive water friction. After all, most of them spend most of their lives on the move. An additional protection against friction is the large amount of mucus that the skin is rich in. This helps many species survive in unfavorable conditions of temporary drought. Not all fish species have a completely covered body with scales. For example, in sharks it is located in one row along the surface of the body, resembling their teeth in appearance. The same can be said about numerous representatives of the Sturgeon order. Most bony fish are protected by scales, like a durable shell. It also performs additional functions: camouflage from predators, warning coloring in predatory and poisonous species, sexual identification in water.
Fin structure
The next characteristic feature of fish is the presence of fins. These formations serve as limbs for movement in water, and some ancient species are even able to crawl with their help. The fins are divided into two groups. The first are the paired ones: abdominal and thoracic. They help maintain the balance of fish in the water column. The caudal, anal and dorsal are unpaired. They work like a rudder, guiding the body of aquatic animals in the desired direction. As a result of evolution, the limbs of reptiles were formed from the fins of fish.
You can easily see the lateral line on the body of the fish. This is a unique organ of balance and touch, characteristic only of fish.
Internal structure of fish
The organ systems of these animals also have their own characteristics associated with the aquatic habitat. The musculoskeletal system is represented by the skeleton. Depending on the class, it is formed by cartilage or bone tissue. All bones of the head skeleton are connected motionlessly, except for the lower jaw. This allows fish to easily capture prey. This section of the skeleton also includes gill covers and arches, the latter of which are attached to the respiratory organs of fish - gills. consists of individual vertebrae connected to each other and to the skull motionlessly. The ribs are attached to the trunk of the spine. The skeleton of the fins is represented by rays. They are also formed by bone tissue. But the paired fins also have belts. Muscles are attached to them, causing them to move.
Through type. It begins with the oropharyngeal cavity. Most fish have sharp teeth on their jaws, which are used to capture and tear food. Enzymes from the liver and pancreas also take part in the digestion process. In the processes of excretion and salt metabolism, the main role in the fish body is played by paired kidneys. They open to the outside with the help of the ureters.
Fish are cold-blooded animals. This means that their body temperature depends on changes occurring in the environment. This sign is determined by the circulatory system. It is represented by a two-chambered heart and a closed structure of blood vessels. During its movement, venous and arterial blood mix.
The nervous system is represented by the brain and spinal cord and nerves. And its peripheral part is made up of nerve fibers. In the brain, the cerebellum reaches special development. This part determines the fast and coordinated movements of fish. The sense organs are capable of perceiving any irritation possible in the aquatic environment. Since the lens of fish eyes does not change its shape and position, animals see well only at a short distance. But at the same time they are able to distinguish both the shape and color of various objects. The organ of sound perception is represented by the inner ear and is associated with the structure responsible for balance.
Fish reproduction also has its own characteristics. These animals are dioecious, with external fertilization.
What is spawning
The process of fish reproduction is also called spawning. It happens in water. The female lays eggs, and the male waters her with seminal fluid. As a result, a fertilized egg is formed. As a result of successive mitotic divisions, an adult individual develops from it.
Sometimes fish reproduction is associated with spawning migrations and significant changes in the behavior and structure of fish during this period. For example, pink salmon form large herds, in which they move from the seas to the upper reaches of rivers. During this journey, they have to overcome many obstacles, moving against the current. These fish develop a hump on their backs, and their jaws become twisted and twisted. Having lost a lot of strength, after the fertilization process, adult individuals die. Surprisingly, the young fry return independently to the same habitat.
Groups of fish
The enormous species diversity necessitated the classification of this fish. Currently, scientists have precisely identified the characteristics by which the class of Fish can be classified. Systematic affiliation is determined by the presence of gill slits or covers and the type of scales. This way you can distinguish between bony and cartilaginous fish. There are other characteristics by which these animals are grouped. For example, fish that move to other habitats to spawn are called migratory. But, taking into account the scope of application, a distinction is made between commercial and ornamental representatives of these aquatic animals.
Cartilaginous fish
What class do fish that have a cartilaginous skeleton and gill slits that open outward belong to? It's not hard to guess. These are cartilaginous fish. They lack a swim bladder, so they either live on the bottom or are constantly on the move. Sawfish, white, giant, whale sharks, stingrays... You know such fish. The list of dangerous predators can be continued with the sea devil, electric stingray and These sea inhabitants pose a great danger to the lives of animals and humans. Although there are quite innocent specimens among cartilaginous fish. Thus, it feeds on fish and crustaceans. Apart from its terrifying appearance, it does not pose any danger to humans.
Bony fish
Perhaps every schoolchild will answer the question of which class the fish that are most numerous belong to. Their skeleton consists entirely of bone tissue. The swim bladder, located in the body cavity, allows its owners to stay in the water column. The gills are covered with gill covers, and do not open outward with separate openings. Bony fish have these characteristics.
The meaning of fish
Representatives of this superclass of vertebrate animals are primarily of commercial importance. People eat their nutritious meat and protein-rich caviar. And the number of recipes for preparing different types does not know the count. Fish oil has long been used as a treatment for bacterial and viral respiratory diseases. Man annually catches a huge number of individuals and breeds them on his own. Flour is also obtained from meat and bones. It is used as fertilizer and feed for many domestic animals.
Recently, sport fishing has become increasingly popular, attracting participants from different countries. And certainly every one of us dreams of catching a goldfish that makes all our wishes come true!
Thus, which class fish belong to can be determined by the characteristics of their structure, organization and way of life.
Depending on their economic use, fish are divided into commercial, pond, and aquarium (exotic). Commercial fish are groups of fish that are the object of fishing (harvesting) in natural reservoirs of various types, for which breeding biotechnology has not been developed. The basis of the fishery is marine migratory fish and, in smaller quantities, freshwater fish. The group of pond fish is not numerous. They are bred and grown in artificial reservoirs to obtain marketable live food.
fish (carp, salmon, catfish and some types of sturgeon) The group of aquarium fish includes mainly tropical, coral, many freshwater and marine fish (about 3 thousand species).
Superclass Jawless. Class Cyclostomes. It includes two subclasses: Hagfish and Lamprey. These are the most primitive vertebrates that live in marine and fresh water bodies. The body is serpentine, devoid of scales. The skin secretes a large amount of mucus. There are no jaws. There are no paired fins. The mouth is surrounded by a round suction funnel equipped with horny teeth. The powerful gimlet tongue also has teeth. The gills look like bags. Only lampreys are of commercial importance.
The Lamprey family includes 8 genera (about 20 species). Among them there are marine, anadromous and freshwater species. The waters of Russia are inhabited by the European river, or Neva; Pacific, or Ice Sea; Caspian lamprey and other species
Lampreys breed in fresh water. The eggs are small and sticky. After 2 weeks, the larva (sandworm) hatches. The larval period lasts 4-5 years. Then, over the course of about six months, the process of metamorphosis occurs, and in the spring, young lampreys 8-15 cm long roll into the sea, where they spend one or two years.
Lamprey meat is delicious. They catch it mainly at night while moving into rivers. The catches are small.
Class Cartilaginous fish. Sharks are of greatest commercial importance, rays are of minor importance. They are characterized by a cartilaginous skeleton, placoid scales or a bare body, lamellar gills (5-7), not covered by the operculum, and the absence of a swim bladder. Most sharks are ovoviviparous (white and fox sharks), polar and cat sharks are oviparous, dogfish, herring and blue sharks are viviparous. Fertility ranges from 3 fry for the herring to 500 eggs for the polar shark. Embryo development lasts up to 2 years (katran, frilled shark)
Sharks are marine fish, but some species enter fresh waters and even live there permanently. About 300 species are known. These are mainly heat-loving fish that live in tropical and subtropical oceans, but are also found in cold waters (polar shark). Off the coast of Russia live the herring shark, polar shark (Barents Sea), cat shark (Black Sea) and katran shark (Barents, Black and Far Eastern seas). Most sharks are predators, eating fish, squid, and crustaceans. About 50 species of sharks are dangerous to humans. The size of sharks is from 15 cm (dwarf) to 20 m (whale). They live for about 40 years.
Many species are of commercial importance, and above all, the katran, polar, herring, blue, cat, marten and other sharks. Sharks are especially prized in Japan, South Korea and Italy. In other countries they are almost never caught. Shark meat contains a lot of urea, which gives it an unpleasant odor. It is removed by soaking in salt water. Shark oil contains a lot of vitamins A, D, etc., and a special enzyme has been discovered that has anti-carcinogenic properties. Therefore, sharks probably do not develop malignant tumors.
Class Bony fish. Representatives of the subclass Ray-finned, superorders Cartilaginous ganoids and Bony fishes are of greatest commercial importance. These include the majority of commercial, pond and aquarium fish, which are the main objects of veterinary supervision.
They are characterized by a skeleton that is partially or completely ossified, ganoid or bony scales, comb-like gills covered with covers, and the presence of a swim bladder. These are oviparous species with external fertilization. The eggs (eggs) are small, not covered with a cornea.
Superorder Cartilaginous ganoids. They are the remnants of ancient groups that preceded the emergence of teleost fish. Cartilaginous ganoids retained a number of primitive features: ganoid scales, non-ossified notochord, absence of vertebral bodies, cartilaginous skull, spiral valve in the intestine, etc. Currently living fish are represented by one order Sturgeon, the families Sturgeon and Paddlefish.
Family Sturgeon. Includes 4 genera - Beluga, Sturgeon, Shovelnose and False Shovelnose. The most valuable of them are representatives of the Beluga and Sturgeon genera (Fig. 21).
Sturgeons are characterized by a spindle-shaped body, covered with five rows of bony plates (bugs) or bare. The snout is elongated, conical or spatulate, with four antennae on the underside; lower retractable mouth; teeth are missing.
Most sturgeon are anadromous fish, the rest are semi-anadromous and freshwater. They have a long life cycle, late sexual maturation, and spring-summer spawning. The caviar is bottom, sticky. They feed on benthos; large individuals are predators. Many species of anadromous sturgeon have spring and winter races. Winter races enter rivers in the fall, spawn in the spring and then slide into the sea. Spring races enter rivers and spawn in the spring.
Genus Beluga. Includes two species: beluga and kaluga. Beluga is an anadromous fish that inhabits the basins of the Caspian, Azov and Black Seas; goes to spawn in the rivers Volga, Ural, Kura, Don, Kuban, etc. Kaluga lives in the river. Amur, semi-through form.
Beluga is the largest commercial fish, reaching a weight of 1 ton and a length of about 4.2 m (commercial weight 50-120 kg). Beluga lives over 100 years. Females reach sexual maturity at the age of 16-18 years, males - 12-14 years. She does not spawn annually, approximately once every 5 years. Fecundity depends on the size of the female - 0.5-7 million eggs. It spawns in April-May and lays its eggs on stones. The incubation period for eggs at 12-13 °C is about 8 days. After hatching, the larvae roll into the sea. An adult beluga is a typical predator that feeds on fish in the sea (spit, herring, gobies).
Genus Sturgeon. Includes 16 species, of which sturgeon, stellate sturgeon, thorn and sterlet are of greatest economic importance.
The Russian sturgeon lives in the basins of the Caspian, Azov and Black seas. Migratory fish; sometimes it also forms a residential form. Spawns in the Volga, Ural, Terek, Danube, Dnieper, Don and Kuban rivers. Puberty occurs in females at 10-14 years, in males at 8-9 years. Maximum length 230 cm, weight up to 80-120 kg (average fishing weight 12-24 kg). Spawns in May - early June on rocky soils, laying 70-800 thousand eggs. The duration of egg incubation is about 4 days. The hatched larvae roll into the sea, and some of the larvae linger in rivers for up to a year. Sturgeon are bred in fish hatcheries, growing up to a year old. Juveniles feed on invertebrates, adults - mollusks and fish.
The Siberian sturgeon is a semi-anadromous fish that lives in the rivers of Siberia from the Ob to the Kolyma. In Baikal and the Upper Ob it forms a freshwater residential form. It feeds in the Ob and Yenisei bays and rises to the upper reaches of rivers to spawn.
Reaches a length of 2 m and a weight of 200 kg. Puberty occurs in females at 15-18 years, in males at 11-15 years. Fertility 80-600 thousand eggs. It spawns once every 2-4 years in May-June. The duration of incubation of eggs is 3-8 days, depending on the water temperature (15-20 C). It feeds on invertebrates, mollusks and fish. Siberian sturgeon is bred in fish hatcheries and warm water farms.
The thorn is an anadromous fish that lives in the basins of the Caspian and Black Seas. Spawns mainly in the river. Ural. Sexual maturity occurs in females at 12-14 years, in males at 6-9 years. Reaches a length of 2 m or more. Spawns in April-May at a temperature of 10-15 "C. Fertility averages 600 thousand eggs. Feeds on fish and shellfish.
Stellate sturgeon is an anadromous fish that lives in the basins of the Caspian, Azov and Black Seas. It goes to spawn in the Ural, Volga, Kura and other rivers. It is a numerous valuable commercial fish, reaching a length of about 2.2 m and a weight of 6-8 kg (average commercial weight is 7-8 kg). Female stellate sturgeon reach sexual maturity at 12-17 years, males at 9-12 years. The fertility of females is 20-400 thousand eggs. Spawning takes place from May to August. The duration of incubation of eggs at 23 °C is about 2-3 days. The juveniles roll into the sea at the age of 2-3 months.
Adult stellate sturgeon feeds mainly on chironomid larvae, crustaceans, and fish. In terms of catches, it ranks second among sturgeon after Russian sturgeon. Stellate sturgeon are bred at fish hatcheries in the Volga, Kuban and Don, growing up to the age of one year.
Sterlet is a freshwater fish that lives in rivers and reservoirs of the European part of Russia, found in the Ob and Yenisei. Commercial dimensions of sterlet: length 30-65cm, weight 0.5-2kg. Males reach sexual maturity at 4-5 years, females at 7-9 years. Fertility 6-140 thousand eggs. Caviar is sticky. Spawning takes place in May with fast currents on pebble soil once every 2 years.
The sterlet is a typical benthos feeder - it feeds on insect larvae, especially chironomids. It forms hybrid forms with sturgeon, stellate sturgeon, and beluga (see Fig. 21).
Bester is a promising hybrid of beluga and sterlet (see Fig. 21), which is used as a pond fish for cultivation in ponds, cages, pools, as well as in lakes and reservoirs. Bester inherited from the beluga rapid growth and a predatory lifestyle, and from the sterlet - early puberty, the ability to live in fresh water.
The growth rate of bester is high: fingerlings reach a weight of 50-100 g, two-year-olds - 800-1000 g, three-year-olds - 2 kg, four-year-olds - up to 5-6 kg. Sexual maturity occurs in males at 3-4 years, in females at 6-8 years. Work is underway to obtain recurrent beluga x bester hybrids.
Family Paddlefish. Paddlefish are more primitive in structure than sturgeons: their body is naked, their snout is very elongated, in the shape of an oar, with two antennae (see Fig. 21). They live in the fresh waters of America (Mississippi River). In Russia it has been acclimatized since 1974 for cultivation in pond farms.
Paddlefish reach a length of 2 m and a weight of 50-75 kg. Sexual maturity occurs at 4-7 years. Fecundity ranges from 80 to 200 thousand eggs. The caviar is sticky and dark. Spawning takes place on rocky soil in the spring at a water temperature of about 14-15 °C. The duration of egg development is 7-10 days. The paddlefish feeds on zoo-
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plankton, partly phytoplankton and detritus, grows rapidly: fingerlings reach a mass of 200-900 g, two-year-olds - 2.5-3.0 kg, three-year-olds - 4-5 kg, four-year-olds - 6 kg.
Superorder Bony fishes. Most modern fish belong to bony fish - the most developed, advanced group of fish. They are characterized by the presence of a bony skeleton with a completely dissected spine. Their scales are bony, lamellar, cycloid or ctenoid in shape. The swim bladder is connected to the intestine (open vesical) or isolated (closed vesical); There is no spiral valve in the intestine.
Bony fishes include about 40 orders and more than 18 thousand species that predominate over other fish in marine, brackish and fresh waters, and also form anadromous and semi-anadromous forms.
Fish are animals that constantly live in water and move with the help of fins. Eating fish animals and food. There are two classes of fish: cartilaginous and bony fish.
Due to the uniqueness of the mouth, cartilaginous fish are called transverse. These are mainly marine fish. Their skeleton consists of cartilaginous tissue, their skin is covered with special tooth-like scales, and their mouth contains enamel-covered teeth. Combined into the cartilaginous class fish two subclasses: whole-headed and elasmobranchs.
Bony fish- the most prosperous class of vertebrates. The main feature of these creatures is that their skeleton, as well as the skull, are formed mainly from real bone tissue. None of the predecessors of fish had bones. Bony fish appeared 400 million years ago and populated all water bodies on the globe. They live in both fresh and salt water oceans.
Here is the list fish descriptions available in the Internet encyclopedia "Hyperworld":
Description of fish
Alphabetical index
Of all the inhabitants of the ocean, sharks seem to be the most famous and the most notorious. For the most part, these are large creatures with a long body, which has the most streamlined shape, which allows them to develop impressive speeds and make long journeys.
The third family of herring-like fish - anchovies - is worth talking about separately. These are small schooling fish that form huge aggregations. They live near the coast in the tropical and temperate zones of the World Ocean. Anchovies are distinguished from other herrings by their excessively large mouth, large eyes and cylindrical body shape. Anchovies are colored silver-white and appear translucent.
The white-eye has a laterally compressed body, somewhat more elongated in length than that of the bream. The snout is thick, convex, the mouth is retractable, semi-inferior. The eyes are large (up to 30% of the length of the head) with a white-silver iris.
Beluga is one of the largest migratory fish. In the past, it reached a length of more than 5 m and a weight of more than 1000 kg. The lifespan of such large specimens apparently exceeded 100 years.
Of the bony fish, the most dangerous are warts. These small, slow, clumsy creatures live in the shallow waters of tropical seas and while away their days among the fragments of coral, huddled in a crevice or buried in the sand.
Bystryanka is similar to an ordinary bleak, but at first glance it differs from it by two dark stripes running along the middle of the body, on the sides of the lateral line, and by the fact that it is noticeably wider and more humpbacked. The holes on the lateral line are bordered at the top and bottom with black dots, so a dotted double stripe runs along the lateral line. In addition, the bystryanka has a thicker head than the bleak, the lower jaw does not protrude above the upper, like the latter, the dorsal fin lies closer to the head and the number of pharyngeal teeth is smaller.
The body of the round goby is roll-shaped, laterally compressed, with a high caudal peduncle and a steep forehead.
The characteristic features of the tsutsik goby are the following: the anterior nasal openings are elongated into antennal-shaped tubes hanging down over the upper lip, the gill covers are bare except for their upper part, the bases of the pectoral fins and the back of the throat are covered with cycloid scales.
Rational use of fish resources for food, medicinal, and feed products is possible only on the basis of in-depth knowledge of the chemical composition of fish.
This composition is characterized by the content of complete proteins, on average 14-22%, easily digestible biologically active fats - 0.2-33%, mineral substances, practically according to the group nomenclature of D. I. Mendeleev’s table - 1-2%, extractive substances - 1, 5–3.9% and even up to 10% (shark meat), fat- and water-soluble vitamins A, D and group B and other substances. Water accounts for 52–85% of the fish’s mass. The chemical composition of only the edible parts of the fish is considered.
Compared to the meat of slaughtered animals, fish muscles have large individual deviations from the average chemical composition. These differences are associated with lifestyle (pelagic, bottom, anadromous, semi-anadromous), habitat (marine, freshwater), species characteristics, metabolic characteristics, sex, age, physiological state of the fish and other factors.
The chemical composition of fish is subject to significant fluctuations, but within one family there is relative constancy in the content of basic substances.
The most constant value is the total content of water and fat in the meat of fish of various species, close to 80%. Let us denote this quantity by the letter K.
However, this relatively constant value can change for fish of different groups classified by protein content:
1) low-protein fish (up to 10% protein (coal)) have K = 90.7%;
2) medium protein (10–15% (notothenia)) – 85.5%;
3) protein (more than 15%, up to 20% (herring)) – 80.4%;
4) high-protein (more than 20% (mackerel)) – 76.6%.
Q fat = K-Q moisture.
1) skinny fish (cod, etc.) – less than 2%;
2) medium fat content (bream, carp, etc.) – 2–8%;
3) fatty (sturgeon, salmon, etc.) – 8-15%;
4) especially fatty (eel, halibut, white fish) - more than 15%. Particularly significant changes in the fat content in fish meat are associated with spawning. After spawning, the fish is so exhausted that it turns out to be an inferior raw material in terms of commercial food, and some fish die immediately (blackback herring, Far Eastern salmon, etc.). During the spawning period, fish lose up to 30% of all nutrients. Nutritional value after spawning is restored for different fish in 20–60 days.
There are species differences in the distribution of fat in the body of fish. For example, in herrings, fat is evenly distributed under the skin with some predominance in the abdominal part; cod meat contains no more than 1% fat, but all the fat is deposited in the liver (up to 70% of its mass); catfish have an accumulation of fat in the tail; in cyprinids and perches, fat during the period
fish feeding increases in the mesentery (intestinal loops), sometimes reaching 50% of the mass of the internal organs; In salmon and sturgeon, fat layers the muscle tissue, giving it particularly high taste. For most fish, there is an increase in fat content and fleshiness on the abdominal part in the direction from the head to the anus and along the dorsal part in the opposite direction - from the tail to the head. Dark fish meat contains more fat than white meat. Dark meat is located along the lateral line along the entire length of the carcass. The exception is tuna and some other scombroid fish, whose dark meat is less fatty.
Fish oil is characterized by the presence of unsaturated fatty acids with an increased number of double bonds: linolenic C 17 H 29 COOH (three double bonds), arachidonic C 19 H 31 COOH (four double bonds), clupanadonic C 21 H 33 COOH (five double bonds). connections). Unsaturated fatty acids form the basis of fish oil (up to 84% of the total fatty acids), which explains its liquid consistency and easy digestibility. At the same time, due to the high unsaturation of fatty acids, fish oil is easily oxidized with the accumulation of oxidation products (peroxides, hydroperoxides) and decomposition (aldehydes, ketones, low molecular weight fatty acids, alcohols, etc.), which significantly worsen the taste and smell of not only the fat , but also the fish products themselves, being at the same time toxic elements for the human body.
Freshwater and marine fish differ in their fatty acid composition. Freshwater fish oil contains up to 60% of the total amount of fatty acids with the number of carbon atoms C 16 and C 18 (palmitooleic, oleic, linoleic, linolenic), approaching in this regard to poultry fat. Marine fish oil contains up to 65% of more highly unsaturated fatty acids such as C 18, C 20, C 22 (oleic, linoleic, linolenic, archidonic, clupanadonic).
For example, herring fat contains: oleic acid - 7-8%, linoleic and linolenic acid - 10-18%, archidonic acid - 18-22%, clupanadonic acid - 7-15%. The content of clupanadonic fatty acid is almost a species characteristic of herrings. The very name of clupanadonic fatty acid comes from the Latin Clupea - “herring” and is associated with the quantitative content of acid in herring meat. Due to the high unsaturation of this acid, herring fat oxidizes especially quickly, which leads to darkening of the meat when cutting salted herring for consumption as a cold snack.
Proteins (nitrogenous substances) are the most important component of the edible parts of fish.
High-protein fish are marine pelagic (schooling, living in the surface layers of water), anadromous, semi-anadromous, with an average protein content - sea bottom and freshwater fish.
In terms of nutritional value, fish meat is among the most valuable food products. Thus, 1 kg of pike perch meat in France is accepted as the standard of value for protein products of animal origin.
Proteins, unlike other organic compounds, contain nitrogen in their composition, which is why they are called nitrogenous substances. In addition to protein nitrogenous compounds, fish also contain non-protein nitrogenous substances. The nitrogenous substances of bony fish consist of 85% proteins (protein nitrogen) and 15% various non-protein compounds (non-protein nitrogen). In cartilaginous fish, protein nitrogen accounts for 55–65% and non-protein nitrogen – 35–45%.
The areas of fish processing are largely related to the composition of nitrogenous substances. For example, the high content of non-protein nitrogen (urea) in the meat of some sharks requires pre-soaking it in water, soda and other solutions so that it is nutritionally complete, i.e. without a characteristic odor, other undesirable tastes, odors, and to eliminate excessive rigidity. Only after such processing can meat be used for the production of dried and smoked balyk, hot smoked products, salted-dried, fresh-dried, pickled fish, fried, boiled, culinary products, etc.
Opinions have been expressed about the possibility of using the “nitrogen volatile base” indicator as one of the decisive factors on the issue of classifying shark meat of different species (about 300 species and 19 families are known) as food or non-food fish.
Fish meat proteins contain all essential amino acids. This determines the special value of fish as one of the highest quality sources of protein nutrition.
In fish, proteins of muscle tissue, proteins of connective tissue, gonads (the sex products of eggs and milt), and bone tissue can be distinguished.
Muscle tissue proteins: myofibrillar (myosin, actin, actomyosin, etc.), sarcoplasmic proteins (myogen, albumin, globulin, etc.), sarcolemma proteins - the sheath of the muscle fiber and the associated connective tissue endomysium and peremysium (collagen, elastin), proteins of the muscle fiber core (nucleoproteins, phosphoproteins).
Myofibrillar proteins are salt-soluble. They are characterized by complete biological usefulness and are distinguished by high moisture-holding capacity. Their content reaches 75–80% of the total amount of muscle tissue proteins. The high content of hygroscopic proteins explains the reason for the low loss of moisture during heat treatment of fish, which ensures fairly good juiciness and digestibility of culinary fish products (boiled, baked, fried fish, etc.).
Sarcoplasmic proteins (cytoplasms) are water-soluble. Most of them are enzymes and accelerate biochemical processes during fish storage. Their content in muscle tissue is 18–20% of the total amount of proteins.
When producing minced fish from fish of small size and low nutritional value, determining its structural and mechanical properties and water-holding capacity, a coefficient showing the ratio of salt-soluble proteins to water-soluble ones is taken into account.
According to the value of this coefficient, all low-value fish can be divided into three groups: K< 1 (0,58-0,79), К = 1 (0,8–1,15) и К >1 (1.16-1.25). With an increase in the coefficient, the quality of minced meat and its rheological properties improve, a cohesive structure is formed in blanched minced meat products, and the shelf life of minced meat is extended. Therefore, sarcoplasmic proteins must be removed by washing the minced meat.
The proteins of the sarcolemma (shell) of the muscle fiber, the proteins of the connective tissue organically associated with the membrane (endomysiel), and the proteins of the septa (the stronger connective tissue of the perimysium) are represented by collagen and elastin. These are incomplete proteins, since they do not contain the essential amino acid tryptophone. There is very little elastin (0.1%), and therefore the connective tissue of fish is represented almost exclusively by collagen. These proteins are resistant to various solutions. But under the influence of heat, collagen is destroyed, turns into a more soluble substance - glutin, and in the form of an aqueous solution is well absorbed by the human body. Fish broths (like meat broths), rich in glutin (sol), form a jelly (gel) when cooled. Collagen is a source of those amino acids that are scarce in complete proteins, and this is its nutritional value. It is believed that glutinized collagen solutions strengthen the human heart muscle.
Glutinized collagen has a very high hydrophilicity, and therefore fish does not lose moisture during cooking or frying, which provides the product with a delicate structure and juicy consistency.
The connective tissue of different species of fish contains unequal amounts of collagen of different structures, more dense in large fish (sharks) and more delicate in small, especially freshwater fish. The collagen content in different fish ranges from 1.7% (sterlet) to 10% (shark).
The muscle tissue proteins discussed above are classified as simple proteins. However, muscle tissue also contains complex proteins (proteins), which are compounds of proteins with other substances (carbohydrates, fats, nucleic acids, etc.): nucleoproteins, phosphoroproteins, glucoproteins, lipoproteins.
Phospho- and nucleoproteins are concentrated in the core of the muscle fiber. The latter consist of nucleic acids, phosphoric acid residues and nitrogenous compounds (purine, pyrimidine bases). Nucleoproteins and phosphorus proteins are the main sources of protein phosphorus, which causes high irritability of the cells and tissues in which it is included. The content of protein phosphorus (in terms of phosphorus anhydride) ranges from 0.26 (“sturgeon”) to 0.63 (“flounder”) by weight of meat.
Lipoproteins contain fats, not only simple ones (triglycerides), but also complex ones (phosphatides). The most common phosphatide is lecithin. Muscle tissue cells contain structural lipoproteins, including lecithin, rich in phosphorus. Consequently, lipoproteins are a source of lecithin phosphorus: from 1.16 (“sturgeon”) to 0.64% (“cod”) of meat weight, calculated as phosphorus anhydride.
Glucoproteins (mucins, mucoids) include carbohydrates and, upon hydrolysis, release glucose, which explains the sweetish taste of fish meat in comparison with the meat of warm-blooded animals. Due to the high carbohydrate content (1–1.5%) in fish, during its culinary processing, more table salt is used than during similar processing of animal and poultry meat. There is a saying “fish loves salt”, which is added not only for the purpose of preserving, but also to eliminate the sweetish taste.
Gonads (caviar, milk) contain simple proteins (protamines, histones), which are characterized by a simplified composition of amino acids with a predominance of basic diamino acids, which increases the pH of the environment and makes these products less stable during storage than fish meat. In addition, fish reproductive products also contain complex proteins (lipoprotein and glucoprotein complexes), which provide the viscosity of caviar. Of the phosphoproteins in caviar, the protein ichthulin should be noted, the content of which is 10–25% of the total protein composition.
Bone tissue proteins are represented by ossein, which is similar in amino acid composition and properties to collagen. The chemical bond between ossein and the mineral composition of fish bone is less strong than in the bone tissue of animals and birds. This becomes especially noticeable during the heat treatment of fish, when the process of glutinization of ossein occurs and the structural and mechanical properties (strength) of the bone decrease. For example, in the past, the degree of doneness of canned fish was determined by crushing the bone between the fingers. The crumbling consistency of the bone (vertebrae) indicated that the canned food was ready for consumption, and in this form the bone was not dangerous to the human digestive tract.
The protein and amino acid composition of fish proteins has some features compared to proteins in the meat of warm-blooded animals and birds:
1) first of all, these are individual species deviations in protein content (from 9 to 23%) and even within a species, depending on the geographical feature: Caspian, White Sea, Pacific herring, Azov-Black Sea mackerel, Atlantic, Pacific, Far Eastern and European salmon, etc. d.;
2) the presence of a large number of complex proteins (proteids) and their concentration in individual organs (for example, in caviar);
3) almost complete absence of myoglobin protein, which explains the white color of muscle tissue (with rare exceptions);
4) more myofibrillar proteins, which have a high hydrating ability, which explains the low loss of moisture during heat treatment, however, during the rigor stage of fish, less actomyosin is formed, and therefore (and also due to the low content of connective tissue and high enzyme activity) the rigor stage of fish proceeds fast;
5) there are fewer water-soluble proteins (sarcoplasm), but they have high enzymatic activity and reduce the shelf life of fish;
6) more complete proteins – up to 93–97%, for comparison: animal meat – 75–85%, poultry meat – 90–93%;
7) connective tissue of fish, almost 100% consisting of collagen (little elastin). Therefore, the fabric easily boils when collagen glutination occurs and in this form retains moisture, significantly reducing its loss.
8) the unequal amino acid composition of fish proteins of different species, which determines the specificity of the taste and smell of fish products and the direction of the most rational technological processing to obtain the most gastronomically valuable products, taking into account national priorities, traditions, habits, tastes: some types of fish are better blanched, boiled, others - for frying, baking, others - for smoking, drying or drying, fourth - for the production of sterilized canned food or processed by salting, fifth - universal in technological processing, etc.;
9) the presence of diamino acids of the type RCOOH(NH 2) 2 in fish proteins - up to 25% of the total, therefore the pH of fish tissue juice is in the range of 6.3–6.6 and only in some fish is 6.0–6, 1. This is a slightly acidic environment in which putrefactive microbes easily develop. Therefore, chilled fish spoils faster (maximum shelf life is 5 days) than chilled animal meat (shelf life is up to 15 days or more);
10) dicarboxylic amino acids (type R(COOH) 2 NH 2) no more than 10% of the total amount. Many sulfur-containing amino acids: cystine, cysteine, methionine. Therefore, fish meat is a good source of sulfur. When storing fish, sulfur-containing proteins decompose with the release of H 2 S (hydrogen sulfide). This is used in assessing the freshness of fish. The degree of freshness of the fish is assessed by the amount of H2S formed: fresh, questionable freshness, stale;
11) during deamination of amino acids
R (COOH) 2 NH 2 + H 2 – RCH 2 COOH + NH 3
NH 3 (ammonia) is formed, the qualitative reaction to the content of which is also an indicator of the freshness of the fish: a negative reaction - the fish is fresh, a weakly positive reaction - the fish is of suspicious freshness, a positive reaction - the fish is stale, a strongly positive reaction - the fish is spoiled;
12) decarboxylation of amino acids (RCOOHNH 2 + CO 2) produces amines, the quantitative content of which is a sign of fish freshness or spoilage. Nitrogenous non-protein compounds are always present in fish tissues as products of constant transformation (metabolism) of proteins. Some proteins break down, others are modified, others are synthesized, and at the same time, individual protein fragments are released that contain nitrogen and are called extractives. They are extracted (extracted) with warm water from the tissues of the fish. Their content is small - 1.5–3.9% of the mass of fish of different species (in the meat of some species of sharks - up to 10%). However, they are significantly
affect the organoleptic characteristics (taste, smell) of fish, promote the enzymatic activity of the digestive juices of the human body when consuming fish, but at the same time, as low-molecular compounds, they are the object of nutrition for microorganisms and, thus, reduce the shelf life of fish products.
When storing fish, the amount of nitrogenous non-protein compounds increases, as enzymatic and microbiological breakdown of proteins occurs. To a certain extent, this improves the taste and consumer properties of the product (it ripens), and then the taste and smell gradually, with the accumulation of extractive substances, become unacceptable for the food product, i.e., it deteriorates.
Fresh fish contains 1.5–3 times more extractive substances than the meat of warm-blooded animals, and due to the high activity of fish enzymes, the amount of non-protein nitrogenous compounds increases rapidly during fish storage. Therefore, constant consumption of fish products “tires” a person’s taste and olfactory organs, and he wants to switch his attention to other food. The increased content of extractive substances reduces the dietary value of fish. Unlike fish, animal meat is almost always consumed with gusto.
Restrictions on meat consumption are more likely related to a person’s health status, age, and other factors, but not to nutritional characteristics.
Poultry meat contains more extractive substances than animal meat, and it becomes boring faster. Game meat contains so many extractive substances that broths are not prepared from it, but are consumed fried.
These examples are given to understand the role of extractives in the formation of taste and aroma characteristics of products and their shelf life.
All extractive substances of fish can be classified into several groups according to their belonging to certain classes of organic compounds and nutritional value: volatile nitrogenous bases, ammonium bases, phosphorus-containing substances, free amino acids and peptides, various substances.
A specific feature of fish extractive compounds are volatile nitrogenous bases. These include ammonia (NH 3) and di-, trimethylamines (DMA, TMA) - NH(CH 3) 2 and N(CH 3) 3. Ammonia is formed from the breakdown of urea (NH 2) 2 CO. Trimethylamine (TMA) can be formed by replacing the hydrogen atom in the NH 3 molecule with a methyl group according to the scheme:
NH 3 → H 2 CH 3 → NH(CH 3) 2 → N(CH 3) 3
monomethylamine dimethylamine trimethylamine
or from physiologically inactive trimethylamine oxide (TMAO):
NO (CH 3) 3 → N(CH 3) 3.
The quantitative content of volatile bases is determined by assessing the freshness of chilled, frozen fish along with determining the presence of H 2 S and NH 3 . When determining this indicator, the content of TMA as the most toxic component is isolated from the total amount of volatile nitrogenous bases. In fresh fish that has just fallen asleep, the content of volatile bases is 15–17 mg%, including TMA up to 2.5 mg% in sea fish and up to 0.5 mg% in freshwater fish. It should be noted, however, that the amount of volatile bases (VB) for different fish species is strictly individual. The accumulation of these substances in meat causes an unpleasant odor.
Trimethylamine oxide (TMAO) – NO (CH 3) 3 – belongs to the group of ammonium bases. In sea fish its content is higher (up to 470 mg% in cod) than in freshwater fish (5-92 mg% - perch, bream, pike), in shark meat - up to 900 mg%. This compound is believed to be non-toxic. But if it decomposes during storage of fish products or during
During heat treatment, a specific fishy smell appears. Rusting of the inside of cans is caused by the presence of TMAO.
Rice. ATP breakdown scheme
The accumulation of hypoxanthine improves the taste of fish broth (fish soup). During the breakdown of proteins, free amino acids are formed, which also affect the organoleptic characteristics of fish products. These include histidine, arginine, creatine. Histidine is present in large quantities in the meat of freshwater fish. During the process of spoilage of fish meat, histidine is decarboxylated to form histamine, a toxic substance that causes food poisoning. Arginine for crustaceans and mollusks, creatine for fish are physiologically important muscle components. Creatine can turn into creatine, which gives the fish a bitter taste when it loses freshness.
Carnosine and anserine are natural dipeptides, i.e. compounds consisting of two amino acids that do not enter into chemical bonds with other amino acids. Anserine is found in the meat of marine fish, carnosine - in the meat of cod and sturgeon.
Various extractive substances in fish meat include urea, the content of which in the meat of sharks reaches 2000 mg%, sturgeon - up to 550 mg%; traces are present in the meat of other fish species. Urea (NH 2) 2 CO is a product of ammonia synthesis. From two molecules of ammonia one molecule of urea is formed, thus preventing poisoning of a living organism. The high urea content in the meat of certain shark species makes it impossible to use it as food after heat treatment without first soaking the raw material. To eliminate the ammonia smell of shark meat, it is crushed, washed and processed into minced meat products, subjected to various heat treatments. If shark meat is processed by smoking, then washing and soaking of raw materials is excluded from the technological process.
Carbohydrates in fish muscle exceed 1% and are represented mainly by glycogen (animal starch). The breakdown of glycogen (hydrolysis or phosphorolysis) produces glucose, pyruvic and lactic acids. Glycogen is involved in the processes of fish ripening during post-mortem changes, salting, and drying. The more glycogen, the more complete the ripening process, the more aromatic and tastier the finished product.
Glucose is a product of the breakdown of glycogen; as a reducing monosaccharide, it can react with amino acids - products of protein hydrolysis, to form complex chemical complexes - melanoidins. This is usually observed during the heat treatment of fish: when boiling fish soup, drying, drying fish. Melanoidins give a darkish color to the surface of the product (in contact with oxygen), a pleasant aroma and a sweetish
taste of fish broths. Therefore, simple carbohydrates are classified as extractive compounds of fish.
Mineral substances in fish meat are very diverse in composition, but in quantity they constitute only 1.2–1.5%. Ocean fish have a particularly rich mineral composition, since sea water contains almost all the minerals known to us. Fish selectively accumulate minerals from their environment in their body and organs. The predominant minerals of fish: macroelements - sodium, potassium, chlorine, calcium, phosphorus, magnesium, sulfur, trace elements, iodine, copper, iron, manganese, bromine, aluminum, fluorine; ultramicroelements: zinc, cobalt, strontium, uranium.
Minerals are represented by ions, salts in the composition of proteins, vitamins, enzymes, and hormones. Complex proteins (proteids) contain phosphorus, iron, calcium, magnesium, potassium, sodium, sulfur, etc. Complex enzymes as part of the prosthetic group contain microelements (copper, iron, manganese, etc.), which sharply activates their biochemical activity . Many vitamins, especially group B, hormones also include micro- and ultramicroelements.
Sea fish is especially rich in iodine. The meat of fish from the cod family has an iodine flavor, valued by gastronomes. People who constantly eat sea fish do not have thyroid diseases.
The specific taste and aroma of fish is largely expressed by its mineral composition. Some types of fish of low consumer value produce excellent, aromatic broths due to the transfer of mineral substances into them, but their meat itself is not very attractive after cooking. When cooking heads and bone tissue, more mineral substances pass into the broth than when cooking muscle tissue. Therefore, extractive, rich broths are obtained by cooking unheaded gutted fish.
Vitamins are found in various parts and organs of fish. Fat-soluble vitamins (A, D, K) predominate in those parts and organs where fats accumulate. This is primarily the liver. Fish oil (medical) with a high content of vitamins is produced from the liver of cod and sharks. Fish oil contains essential fatty acids (linoleic, linolenic, arachidonic), which together form vitamin F. It is believed that this vitamin is a preventative against cancer, reduces cholesterol levels in the liver and ensures the elasticity of blood vessels.
Of the water-soluble vitamins, sufficient levels of vitamins B 1 (thiamine) and B 2 (riboflavin) were noted in muscle tissue. The internal organs of fish contain vitamin B 12, which is a hematopoietic catalyst, the absence of which can lead to pernicious anemia.
Fish enzymes play an extremely important role in the processes occurring in the post-mortem period in all tissues and organs of fish, also in various methods of processing fish raw materials, especially during salting, drying, cold smoking, and the production of preserves.
The organs and tissues of fish contain enzymes of all six classes according to the systematic nomenclature of the Commission on Enzymes of the International Biochemical Union from 1961: oxidoreductases (redox), transferases (transfer enzymes), hydrolases (cleavage enzymes with the participation of water), lyases (cleavage enzymes without participation of water), isomerases (conversion enzymes), ligases (synthesis enzymes).
Redox and hydrolytic enzymes are of greatest importance in shaping the consumer properties of fish products.
The processes of ripening of fish after death (from suffocation), as well as the biochemical processes of ripening of salted and dried fish, occur primarily with the participation of enzymes of these classes. Redox enzymes are the most numerous
a class of more than 220 items; they are divided into several groups. The first group is dehydrogenases, which act as hydrogen carriers. Dehydrogenases are two-component systems, the active part (coenzymes) of which are NAD (nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate). During the initial maturation of fish, carbohydrates undergo changes. During lactic acid fermentation, NAD hydrogen (reduced hydrogen coenzyme dehydrogenase) reduces pyruvic acid to lactic acid. The resulting lactic acid creates an acidic environment unfavorable for the development of putrefactive microbiological processes, muscle proteins swell, harden, and the stage of rigor mortis begins in freshly dead fish, which indicates the impeccable freshness of the fish.
At subsequent stages of fish maturation, hydrolytic enzymes come to the fore: proteolytic (proteases), which catalyze the breakdown of proteins and peptides; esterases (lipases), causing hydrolysis of carboxylic acid esters (fats); amylolytic (amylases), hydrolyzing glucose bonds of starch, dextrins; phosphatases that hydrolyze phosphoric acid esters (glucose-1-phosphate, etc.).
Hydrolases are especially active in acidified environments. Therefore, after the formation of lactic acid, the activity of hydrolytic enzymes increases. Proteolytic enzymes (trypsin, pepsin, cathepsin, etc.) cause the breakdown of protein molecules according to the following scheme:
proteins → peptones → polypeptides → tripeptides → dipeptides → amino acids
Amino acids are the final structural element of the enzymatic breakdown of proteins. The more protein breakdown products are formed, especially low molecular weight ones (dipeptides, amino acids), the brighter the taste and aroma of the product. In production practice, the process of ripening of chilled, frozen, salted, dried fish is determined by the amount of amino acids formed (by the content of amino-ammonia nitrogen). It is believed that 30% of amino-ammonium nitrogen (of the total nitrogen included in both proteins and non-proteins) characterizes the product as fully ripe and fresh. A further increase in this indicator indicates overripeness of the fish and subsequent spoilage.
During further storage of fish, low-molecular-weight protein breakdown products (primarily amino acids) become food sources for microorganisms. Moreover, depending on the type of microorganisms, amino acids can decompose to form various end products of metabolism according to the scheme presented in Figure 4.
The accumulated substances have toxic properties and give the fish an unpleasant odor. Proteolytic enzymes hydrolyze proteins much more actively than similar enzymes of terrestrial animals, so the process of ripening fish proceeds much faster than meat from slaughtered animals. Moreover, the action of fish proteases occurs in a fairly wide pH range: from an acidic environment (pH 3.5–4.5), where activity is maximum, to alkaline (pH 8), where activity is 5–10% of activity at pH 3.5– 4.5. At a natural pH for fish of 6.6–7.0, enzyme activity is 310 times lower than at pH 3.5–4.5.
Significant fluctuations in the level of activity of muscle proteases (peptide hydrolases) were noted depending on the size of the fish and the fishing season.
Sodium chloride (NaCl) even at a concentration of 3% causes partial inactivation of enzymes; at a 5% concentration an inhibitory effect is provided, and 10%
Rice. Scheme of microbiological breakdown of amino acids
A high concentration of table salt inactivates muscle peptide hydrolases almost completely.
In the technology of processing uncut fish by salting, cold smoking, drying, as well as when storing chilled fish, it is necessary to take into account the activity of enzymes of internal organs (intestines, pyloric appendages), represented by pepsin and trypsin, which in terms of optimal pH are close to the digestive enzymes of terrestrial animals, however have differences. The digestive enzymes of fish have a temperature optimum that is much lower, and their ability to break down proteins is higher than that of terrestrial animals.
Their activity varies depending on the season and type of fish. The action of table salt causes an inhibitory effect, but the residual activity of enzymes in fish entrails is higher than the activity of proteolytic enzymes in muscle tissue. This circumstance explains the need for a detailed study of the digestive enzymes of fish in order to establish a technological processing process taking into account the variability of the activity of proteolytic enzymes depending on various factors.
In parallel with the proteolytic processes during fish ripening, hydrolysis of fats occurs under the action of enzymes - lipases according to the following scheme:
triglycerides → diglycerides → monoglycerides → free fatty acids and glycerol.
The end products of this hydrolysis (free fatty acids) increase the acid number of fat, which leads to its spoilage, but this is not always reflected in organoleptic characteristics. For example, when drying fish, fats undergo not only hydrolysis, but also oxidative changes, but the taste and smell of the fish only improve, i.e. there is no direct relationship between the breakdown of fats and the consumer value of the product.
Simultaneously with changes in proteins and fats during the ripening of fish products, significant transformations are observed in the carbohydrate part.
As noted above, the maturation process actually begins with phosphorolysis and hydrolysis of fish glycogen. Under the influence of redox enzymes, glycogen undergoes breakdown according to the following scheme:
glycogen (animal starch) → glucose-1-phosphate → fructose-1,6-phosphate → phosphotrioses (phosphodioxyacetone and phosphoglyceraldehyde) → pyruvic acid (CH 3 COCOOH) → lactic acid (H 3 CHOHCOOH).
Approximately 90% of all glycogen breaks down according to this pattern, which ultimately leads to an increase in titratable acidity.
At the same time, hydrolysis of glycogen is observed under the action of amylolytic enzymes according to the following scheme:
glycogen (C 6 H 10 O 5) n → dextrins (of different molecular weights) → maltose (C 12 H 22 O 11) → glucose (C 6 H 12 O 6).
Among phosphates, attention should be paid to enzymes that cause the hydrolysis of nucleotides (ATP, etc.) with the formation of purine (adenine, guanine, etc.) or pyrimidine (cytosine, uracil, thymine) bases, ribose or deoxyribose sugars and phosphoric acid . This breakdown of nucleotides increases the amount of extractive substances and enhances the taste and aroma of fish products. But at the same time it expands the nutrient medium for microorganisms and makes the product less stable during storage.
Water in the tissues and organs of fish is in a free and bound state. Free water is a liquid in the intercellular space, in blood plasma and lymph, in addition, retained mechanically in macro- and microcapillaries due to surface tension forces, and also osmotically retained in cells by the pressure of solutions. There is also chemically bound water, which is part of the molecule of the substance.
Free water is a solvent of organic and mineral substances, and all biochemical and microbiological processes take place in it. This is ordinary water: it freezes at 0 o C and boils at 100 o C, it is easily pressed out and evaporates when dried.
Bound water is adsorptively retained in colloids (proteins, glycogen) by forces of electrical attraction. Bound water, being difficult to separate, provides tissue density to a certain extent along with colloids (primarily proteins). It does not take part in enzymatic or microbiological reactions and thereby contributes to the preservation of the product. It does not freeze at temperatures used for freezing fish, does not leak when defrosted, remaining a permanent agent of tissues, forms their structure along with other components. The more bound water, the more stable the product during storage.
The ratio of free and bound water in the muscle tissue of fish of different species is not the same. The total moisture content is from 52 to 85%, of which free up to 75.5% and less bound up to 9.5% or more. With different methods of fish processing (thermal, freezing, grinding, etc.), this ratio, as well as the total moisture content, may vary slightly. For example, freezing and drying reduces the total moisture content as free water is lost (evaporates, sublimates). During heat treatment, free moisture is partially lost, but the amount of bound water increases slightly due to the watering of meat proteins.
The use of different saltings (dry, wet, mixed) can lead to either a loss of moisture (with dry strong) or an increase in moisture (with wet, weak and medium strength) in the salted product.
Chemical composition of fish meat
Fish meat consists mainly of the muscles of the body along with the loose connective and adipose tissue adjacent to them. The consistency of fish meat of different species, other things being equal, depends on the content of connective tissue formations, fat, protein substances, water and the nature of the connection between water and proteins. There is less connective tissue in fish meat than in the meat of land animals, so its consistency is more tender.
According to the chemical composition and functional significance, organic and inorganic substances included in fish meat are divided into energy, plastic, metabolic and functional.
