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To say that the understanding of fish, we have a long tradition of gastronomic culture in the country, do have pride, the well-known "fish and bear paw" allusion in Our country has been circulated for more than 2,000 years, the fish listed as the first of the seafood, and as the head of the mountain treasure of the bear paw comparison, it shows that people have a long consensus on the taste of fish delicious and nutritious.
In China's current dietary structure, from the daily per capita per capita share of heat, protein and fat alone, it is close to the world average, the lack of aquatic animal protein, its proportion is significantly lower than the world average, increasing the proportion of aquatic products in "meat" food has become a development trend and a new global fashion. Fish meat is not only delicious meat, but also high nutritional value, especially fish meat is rich in DHA, which can promote children's brain development and enhance memory. The biggest advantage of fish meat is that it is high in protein, low in fat, and the saturated fatty acid content is much lower than that of beef and lamb. The Dietary Guidelines for Chinese Residents recommend that adults eat 280 to 525 grams of fish per week.
First, the muscle tissue of the fish body
The muscular tissue of the fish body is the main edible part, symmetrically distributed on both sides of the spine, generally referred to as the body side muscles. During the movement, the fish body can be swung forward by the expansion and contraction of the longitudinal fibers on the left and right side of the body. The overall structure of the fish muscles is shown in Figure 1, from which you can see the left and right side muscles with the spine as the axis and separated by a vertical diaphragm: each body side muscle is then divided into dorsal and abdominal muscles by the horizontal diaphragm. From the lateral muscle tissue of the fish body after the cortex is removed, many muscle segments in the shape of ≥ can be seen continuously from the front to the tail. Each muscle segment is formed by a myriad of parallel muscle fibers arranged longitudinally and connected anteriorly and posteriorly on many membranes of connective tissue called myostia. The number of muscle segments in various fish bodies is certain. From the cross-section of the lateral muscles of the body (Figure 2), the nodes of the dorsal and abdominal muscles are arranged in concentric circles.
Fig. 1 Lateral muscles and sarcomere of fish
Muscle fibers
The muscle segments of fish meat are made up of countless muscle fibers parallel to the body axis. On the outside of each muscle fiber is a layer of muscle fiber membrane, and many muscle fibers are combined with each other by the muscle membrane of the connective tissue to form muscle fiber bundles. Most muscle fiber bundles are reassembled to form myoganglionic segments. Between the bundles of muscle fibers there is a layer of extramuscular membrane formed by more connective tissue. Muscle fibers are a multinucleated cell tissue consisting of many parallel myofibrils. Filled with muscle plasma between myofibrils, and the presence of mitochondria, fat balls, glycogen granules, etc. Muscle plasma is composed of water-soluble muscle plasma proteins, low-molecular nitrogen and free amino acids, organic acids, water-soluble vitamins, inorganic salts, etc.
Fig. 2 Fish body lateral muscle tissue section
Myofibers
Myofibroblasts are made up of many myofibrils. When viewed under an optical and electron microscope, the severed head of the dense myofibrils in the cross-section of the myofiber can be seen, while in the longitudinal section of the myofiber, the alternating striations of light and dark on the myofibrils, the so-called striated muscles, can be seen. Myofiber fibers consist of many parallel parallel parallel thick and thin filaments that cross anteriorly and anteriorly. The coarse filament is equivalent to the part of the myofibrilloid band, also known as the A-band, which is composed of fibrous molecules of myosin. Filaments are equivalent to the part of the myofibrilloid band, also known as the I belt, which consists of actin and regulatory protein molecules.
Fig. 3 Translucent fish meat, yellow-banded trevally
Dark flesh with red flesh
There is a muscle tissue of dark flesh near the horizontal diaphragm from the front to the tail of the fish body (Figures 1 and 2), which is dark red and is significantly different from the color of other light-colored ordinary flesh (white flesh) on the back and abdomen. This is mainly because dark meat contains more respiratory pigment proteins such as hemoglobin and myoglobin than ordinary meat. For example, the dark meat of true seabream and carp has a pigment protein content of 520 mg and 350 mg per 100 grams, respectively, while in ordinary meat it is only 6 mg and 53 mg respectively.
Fig. 4 Orange-red salmon (Atlantic salmon)
The amount of dark flesh of the fish varies depending on the species. Generally active pelagic fish, such as herring, mackerel, sardines, bonito, tuna, etc., have a lot of dark flesh, extending from the lateral line of the fish body along the outside of the sides of the horizontal diaphragm to the periphery of the spine. The ones distributed on the outside are called superficial dark flesh, and those near the spine are called deep dark flesh. Less active demerit fish have less dark flesh and are limited to a small number of dark skins on the surface, such as cod, flounder, seabream, carp, etc. In the common meat of migratory fish with strong movement, such as bonito, tuna, etc., it also contains quite a lot of pigmented proteins such as myoglobin and cytochrome, so it also has different degrees of red, generally called red meat, and sometimes this fish is called red meat fish. Fish with light-colored ordinary meat or white meat are called white meat fish. Dark meat than ordinary meat in addition to containing more pigment protein, and contain more lipids, glycogen, vitamins and enzymes, etc., in the physiological can adapt to slow and sustained migratory movement, while ordinary meat is the opposite, mainly suitable for hunting, jumping, avoiding enemies and other rapid movement. In terms of edible value and processing and storage performance, dark meat is lower than white meat.
Fig. 5 Carp meat
The general law of fish color is shown in Figure 6 below, the dark red part is red muscle, and the light part is white muscle. Usually in order to adapt to the needs of long-term cruising, out-of-the-oceanic fish not only develop slow red muscle, but also have a high myorubin content in white muscle, the flesh color is dark red, the coastal fish travel distance is shorter, the red muscle is less, the heme content in white muscle is low, and the flesh color is pink or almost white.
Fig. 6 General law of fish meat color
The muscle tissue of vertebrates such as fish is composed of striated muscles, and in aquatic invertebrates other than fish, shrimp, crabs, etc. are also striated muscles. In the muscle tissues of other shellfish, squid, octopus, etc., there are both striated muscles, twill muscles and striated muscles (smooth muscles). The closed shell muscles such as scallops are composed of striated muscles. The mantle membrane of the squid and the translucent closed shell muscle of the oyster are mainly composed of twill muscles. There are also bright and dark bands in the myofel fibers and striated muscles of the twill muscle, but the difference is that the light and dark bands on the muscle fibers as a whole show a certain angle of inclination.
Second, the chemical composition of fish meat
The general chemical composition of the meat of fish and other aquatic animals (including shrimp, crabs and shellfish). The content of these constituents mostly ranges from 70% to 85% water, 10% to 20% crude protein, 0.5% to 10% crude fat, less than 1% carbohydrates, and 1% to 2% inorganic salts (Table 1). Compared with terrestrial animal meat, fish and other aquatic animal meat have more water content and less fat content, and the protein content is relatively more than that of terrestrial animal meat.
There are many species of fish and other aquatic animals, so there are some different characteristics of the muscular chemical composition between different fish and between fish and other aquatic animals. In addition, different parts of the fish body, different ages, different seasons, etc. also bring different changes to the chemical composition of the meat of fish and other aquatic animals.
Table 1 General chemical composition of common fish and other aquatic animals (%)
Differences in types
Among fish, marine migratory pelagic fish such as mackerel, herring, sardines, tuna, etc. are mostly fatter than those of bottom fish such as turbot, cod, seabream, and yellow croaker. The former is generally called fatty fish, and its fat content can reach 20% to 30%: the latter is called less fat fish, and the content is more than 5% (there are also called less fat fish with a fat content of less than 1%, and 1% to 5% are called medium fat fish). Turbot and cod are as low as 0.5%. The fat content of fish is also negatively correlated with the moisture content, with less water content having more fat content and vice versa. The protein content of different fish is different from fat and water, and the difference is small, generally between 15% and 22%. In addition, the carbohydrates contained in fish are mainly very small amounts of glycogen, which, like the content of inorganic salts, differ very little from species to species.
The fat content of shellfish and mollusk muscles such as squid and octopus in aquatic animals is small and stable, more than 1%; the water content of shellfish is mostly 80% to 85%, or even higher, significantly higher than that of fish. Therefore, the protein content of shellfish is low, mostly in the range of 10% to 15%. In addition, the protein content of squid and other shrimp and crab meat in the fat season are basically the same as those in fish meat, while the glycogen of shellfish and the ash content of shellfish, shrimp, crab, etc. are higher than those of fish.
Differences in fish body parts and ages
The chemical composition of the muscles of the same fish varies depending on the body part, age and weight of the fish. In general, the fat content of the surface muscles of the head, abdomen and fish body is more than the fat content of the tail, back and deep muscles of the fish body; the fat content of fish with large age and weight is more than that of age and weight. Correspondingly, the parts with high fat content and the fish with large age and weight have a relatively small water content. Proteins differ little from glycogen, inorganic salts, and other components (Table 2). In addition, dark meat has a higher fat content than white meat.
Table 2 Chemical composition of muscles in different parts of seabream (%)
Differences between seasons
Fish and various aquatic animals, due to different temperature changes in different seasons of the year and physiological and ecological changes such as growth, reproduction, migration and food sources, will cause significant changes in fat, water, and even protein components. Generally in the season of high temperature and abundant bait, the fish grows fast, the body fat accumulation increases, and gradually decreases in winter. In addition, the fat content before spawning is high and decreases significantly after spawning. Shrimp, crab and shellfish, etc., are manifested by seasonal increases and decreases in glycogen and protein content. The seasonal variation in the fat content of migratory fatty fish in fish is the largest (Figure 7), and the seasonal change in protein, glycogen, and water content of oysters and the like is also obvious (Figure 8).
Figure 7 Annual changes in fat content of pseudosardines
Figure 8 Changes in water, protein, and glycogen during oyster growth
The amount of bait affects the change of muscle composition such as fish as shown in Table 3. This difference is clearly reflected between natural and farmed species, and the fat of farmed eels in Table 3 is significantly higher than that of natural eels. The moisture content is the opposite, and other components such as proteins change very little.
Table 3 Comparison of chemical composition of natural eels and farmed eels (%) (Zou Shengxiang)
Among the many meats, fish not only retains the advantages of meat food, but also makes up for the lack of most meat foods. Chen Shunsheng, a professor at the Food College of Shanghai Ocean University, said that the protein in fish meat is the highest quality, not only the content is higher than that of white meat such as chicken and duck, but also the proportion of essential amino acids in protein is higher than that of red meat, which is one of the main sources of high-quality protein. Moreover, fish meat has less muscle matrix protein, short muscle fiber, and tender meat, which is softer and tenderer to eat than poultry meat and red meat, and it is easier to digest and absorb. Due to the different growing environments, fish contain more vitamins and minerals than land animals, such as calcium, iron, zinc, iodine, vitamin A, vitamin D, B vitamins and so on. In addition, fish is also rich in biofunctional ingredients such as DHA, glutathione, and taurine. That's why fish is so much more beneficial to health!
(Synthesis: Southwest Fishery Network)
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