Preservation and refrigeration technology and appl

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For a long time, offshore fishing vessels have generally used the ice coating method to preserve their catches. However, with the adjustment of fishing vessel operation mode, the fishing ground is moved out, the fishing operation range is getting longer, the time is lengthening, and consumers' requirements for fish products are increasing, the traditional method of keeping fresh with ice has become increasingly unsuitable for the existing requirements of fishery production and preservation. Therefore, it is inevitable to widely promote and apply new refrigeration technology on fishing boats, and a variety of low-temperature preservation methods of fish products have also emerged

in fact, the deterioration of aquatic products is mainly caused by the life activities of microorganisms and biochemical reactions carried out by enzymes in aquatic products. This is because after the death of fish, the cell tissue in the body has died, which can not inhibit the role of enzymes and resist the invasion of microorganisms, and finally lead to the deterioration and corruption of aquatic products. However, both the growth and reproduction of microorganisms and the catalysis of enzymes require appropriate temperature and sufficient water. When environmental conditions do not adapt, microorganisms will stop reproducing, or even die, and enzymes will lose their catalytic ability, or even be destroyed. Generally speaking, as the temperature decreases, the role of microorganisms and enzymes will gradually become very weak. At the same time, the speed of chemical reactions such as oxidation will also be significantly slowed down by sticking with supramolecular glue, so the freshness of aquatic products can be maintained for a long time without corruption and deterioration. This is the principle of low-temperature preservation of aquatic products and the theoretical basis for the application of refrigeration technology

cooling and preservation cooling and preservation refers to reducing the temperature of the fish body to close to the freezing point of the liquid in the fish body, so as to inhibit and slow down the activity of enzymes and microorganisms, so that the fish body can maintain good freshness for a certain period of time. Theory and practice have proved that in the cooling process of fish products, only rapid cooling can achieve good results. The faster the cooling speed is, the slower the biochemical process is, and the less the cooling consumption is. Therefore, in order to keep the fish fresh, the focus of fresh-keeping should be placed on the ship, that is, when the fish is caught, it should be cooled immediately, which is the key to the fresh-keeping of fish

cooling and fresh-keeping can be divided into micro freezing and ice fresh-keeping and cold sea water fresh-keeping

micro freezing and ice preservation: the refrigeration method of micro freezing and ice preservation appeared in early 1992 and is mainly used on export fresh-keeping ships. Generally, two sets of refrigeration units are equipped, one of which is used as a standby. Since the cabin temperature is required to be between 0 ℃ ~-5 ℃, the prepared cold compressor only needs single-stage compression, so the supporting equipment is relatively simple and easy to operate. With the emergence of semi closed compressor and the application of temperature control technology, the operation is more concise and reliable. But the disadvantage is that a considerable proportion of ice needs to be added when loading fish, which increases the labor intensity of the crew to a certain extent

cold sea water preservation: cold sea water preservation is to place the catch in 0 ℃ ~-1 ℃ cooled sea water for preservation. The cooling capacity required for seawater is provided by the refrigerating unit. The unit is composed of refrigeration compressor and cooling coil, which can be divided into two ways: direct liquid supply by thermal expansion valve and indirect cooling with ethylene glycol as refrigerant. Due to indirect cooling, the seawater temperature of the latter is easy to control, which is conducive to eliminating the problem of freezing on the surface of the cooling coil. However, the supporting equipment in this way is relatively cumbersome, and the operation and maintenance are also troublesome. With the development of refrigeration technology, electronic expansion valve has appeared. This expansion valve can directly supply liquid to the cooling coil of the fishing cabin, so it can well control the evaporation temperature of the refrigerant, make the seawater temperature in the seawater cabin not lower than its freezing point, and effectively eliminate the phenomenon of ice hanging. It not only improves the heat dissipation effect of the coil, but also simplifies the operation procedure, which is highly praised by users. In addition, cold sea water preservation plays a great role in the enclosed operation ships with highly concentrated catches. Due to the use of fish suction pumps for loading and unloading goods, this kind of operation greatly reduces the labor intensity of the crew

but the biggest defect of cold sea water preservation is that the fish body swells due to excessive salt and water absorption in the sea water, the fish meat is slightly salty, and the body surface is slightly discolored. Therefore, how to improve the fresh-keeping effect of cold seawater will still be a subject that needs further research at present and in the future

freezing preservation with the maturity of the construction technology of high-powered fishing boats, the endurance of fishing boats continues to improve, coupled with the gradual lack of fishery resources, ocean fishing has become a general trend, but how to improve the refrigeration technology, which can not only easily lead to electric shock hazards, but also ensure the quality of fish? Freezing preservation is the best solution to this problem

due to the limitation of storage time, cooling and preservation can only be used for temporary storage during the transportation, processing and sales of fresh fish. To obtain the effect of long-term storage and preservation, about 90% of the water in the fish must be frozen into ice, so that the temperature of the fish can be reduced to a lower level, which is freezing and preservation. Practical research has proved that under certain conditions, the lower the fish body temperature, the more conducive to long-term storage. At -12 ℃, although the reproduction rate of microorganisms has decreased to a very low level, in fact, the role of enzymes and non enzymes still exist. Therefore, the recommended temperature for freezing and preservation of fish is -18 ℃

however, under the condition that the PCU components of such temperature bar are worn very lightly, the single-stage refrigeration compressor obviously cannot meet the low temperature requirement of -18 ℃, and the two-stage refrigeration compressor has become an inevitable choice. This system unit mainly includes two-stage refrigeration compressor, condenser, liquid reservoir, heat exchanger, fishing tank evaporator and freezing machine. Put the newly caught fish into the fish pan and send it to the freezing machine for quick freezing. When the temperature of the fish drops to -15 ℃, move it to the fishing cabin for storage and keep the temperature of the fishing cabin below -18 ℃. This kind of fresh-keeping method can better ensure the quality of fish body because of the quick freezing method (the freezing time is no more than 7 hours)

the interior of the freezing machine is composed of many layers of flat plates. The fish dish is placed between the two layers of flat plates, and the flat plates are filled with refrigerant. A fan is set at the head and tail of each freezing machine. The plates in the traditional freezing machine are mostly made of low carbon steel. Although the price is low and easy to process, the corrosion resistance is poor. Even if the surface is galvanized, its corrosion resistance is difficult to meet the production requirements. With the emergence of calendered aluminum alloy plates, the corrosion problem of the plates has been well solved, and the internal and external surfaces of the plates are highly polished. Argon arc welding is adopted, and the two sides of the plates are made into special groove structures, which also solves the problem of structural strength of large plate freezing machines

with the large-scale of the golden spear fishing boat, the ultra-low temperature elimination method: please check whether the sensor selection in the online setting is correct, and the refrigeration technology is gradually popularized and applied. Although this low-temperature (up to -40 ℃) preservation method puts forward higher requirements for refrigeration equipment, with the rapid development of domestic refrigeration technology, domestic equipment is gradually replacing imported equipment, making the popularization and application of ultra-low temperature technology in China possible

source: Chinese agriculture

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