Fresh-keeping Refrigeration Technology and Application of Fishing Boats

For a long time, offshore fishing vessels have generally adopted the ice-preservation method for catches. However, with the adjustment of fishing vessels, fisheries have moved outwards, fishing operations have been farther and longer, and the demand for fish products has been increasing. The traditional way of ice preservation has become increasingly unsuitable for the preservation of existing fishery production. Therefore, it has become necessary to extensively promote the use of new refrigeration technologies on fishing boats, and a variety of low-temperature preservation methods for fish have also emerged.

In fact, the spoilage of aquatic products is mainly caused by the life activities of microorganisms and the biochemical reactions carried out by enzymes in aquatic products. This is because after the fish loses its life, because the cell tissue in the body has already died, it cannot inhibit the action of the enzyme and resist the invasion of the microorganisms, which ultimately leads to spoilage and deterioration of aquatic products. However, regardless of the growth and reproduction of microorganisms, or the catalytic action of enzymes, it is necessary to have appropriate temperature and sufficient moisture as conditions. When environmental conditions are not met, microorganisms will stop breeding or even die, and enzymes will lose their catalytic ability and even be destroyed. In general, as temperatures decrease, the effects of microorganisms and enzymes gradually become weaker. At the same time, the speed of chemical reactions such as oxidation will also be significantly slowed down, so that the freshness of aquatic products can be maintained for a longer period of time without spoilage. This is the principle of low temperature preservation of aquatic products, and is also the theoretical basis for the application of refrigeration technology.

Cooling preservation Fresh-keeping cooling refers to reducing the temperature of the fish body to a temperature close to the freezing point of the liquid in the fish body, thereby suppressing and slowing the activity of enzymes and microorganisms, and keeping the fish body in a good period of time. Theory and practice have proved that in the cooling process of fish, only rapid cooling is used to achieve good results. The faster the cooling rate, the slower the biochemical process and the lower the consumption of cold. Therefore, in order to maintain a good freshness of fish, the focus of preservation should be on the boat, that is, when the fish is caught, it should be cooled immediately, which is the key to the preservation of fish.

Cooling preservation on the way of its refrigeration, can be divided into micro-freezing and ice preservation combined with cold water preservation of two.

Micro-frozen and ice-preserving: Refrigeration methods of micro-freezing and ice-keeping have emerged in early 1992. They are mainly used in the export of fresh-keeping vessels, generally equipped with two sets of refrigeration units, one of which is used as a backup. Because the required cabin temperature is between 0°C and -5°C, the equipped refrigeration compressor only needs single-stage compression, so the supporting equipment is relatively simple and easy to operate. With the advent of semi-hermetic compressors and the application of temperature control technology, the operation is more concise and reliable. However, the disadvantage is that when the fish is loaded, a considerable proportion of ice must be added, which, to a certain extent, increases the crew's labor intensity.

Cold Seawater Preservation: Cold Seawater Preservation is to place catches in fresh seawater at 0°C~-1°C for cooling. The required cooling capacity of seawater is provided by the refrigeration unit. The unit is composed of refrigeration compressors, cooling coils, etc. It can be divided into two methods: direct supply of thermal expansion valve and indirect cooling of ethylene glycol. Because the latter adopts indirect cooling, the temperature of the seawater is easy to control, and it is beneficial to eliminate the problem of icing 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, there has been an electronic expansion valve. This expansion valve can directly supply liquid to the cooling coils of the fishing tanks, so that the evaporation temperature of the refrigerant can be well controlled so that the seawater temperature in the seawater tanks is not lower than Its freezing point can effectively eliminate the ice-hanging phenomenon, which not only improves the heat dissipation effect of the coil, but also simplifies the operation procedure and is highly praised by users. In addition, in seine-bound working vessels where the catch is highly concentrated, the preservation of fresh cold water is playing a huge role. This type of operation has greatly reduced the crew's labor intensity due to the use of the suction pump to load and unload cargo.

However, the biggest drawback of cold seawater preservation is that the fish body swells in the seawater due to excessive salt and water absorption, and the fish meat is slightly salty and has a slight discoloration on the body surface. Therefore, how to improve the preservation effect of cold seawater will remain a subject that needs further study now and in the future.

Freezing and preservation As the construction technology of large-capacity fishing boats matures, the endurance of fishing boats continues to increase, coupled with the scarcity of fishery resources, ocean fishing has become a general trend, but how to improve refrigeration technology can not only satisfy offshore fishing operations but also ensure the quality of fish bodies. What? Freezing preservation is the best solution to this problem.

Due to the limitation of storage time, the cooling and fresh-keeping can only be used for the temporary storage of fresh fish during transportation, processing and sales. In order to obtain a long-term storage and preservation effect, about 90% of the water in the fish body must be frozen into ice, so that the temperature of the fish body can be lowered to a lower level, which is to freeze and keep fresh. Practical studies have shown that, under certain conditions, the lower the temperature of the fish body, the more conducive to long-term storage. At -12°C, although the reproduction rate has dropped to a very low level, the effect of enzymes and non-enzymes is still present. Therefore, the recommended temperature for freeze-preservation of fish in general is -18°C.

However, under such temperature conditions, a single-stage refrigeration compressor obviously cannot meet the low temperature requirement of -18°C, and a two-stage refrigeration compressor becomes an inevitable choice. The system unit mainly includes a two-stage refrigeration compressor, a condenser, a liquid reservoir, a heat exchanger, a fish tank evaporator, and a freezer. The freshly caught fish is put into the fish bowl and sent to the freezer for quick freezing. When the fish body temperature drops to -15°C, it is moved to the fish tank for storage, and the temperature of the fish tank is kept below -18°C. This kind of preservation method is able to better guarantee the quality of the fish body by adopting the quick freezing method (freezing time does not exceed 7 hours).

The inside of the freezer is composed of many flat plates. The fish plate is placed between two flat plates. The plate is filled with refrigerant. Each freezer has a fan at the beginning and end. The plate in the traditional freezer is mostly made of low-carbon steel material. Although it is inexpensive and easy to process, its corrosion resistance is poor. Even if the surface is galvanized, its corrosion resistance is difficult to meet the production requirements. With the appearance of calendared aluminum alloy flat plates, the corrosion problem of the flat plate is well resolved. The flatness of the inner and outer plates is very high. Using argon arc welding, a special groove structure is formed on both sides of the flat plate, which also solves the large flat plate. Freezer structural strength issues.

With the large-scale deployment of gold-fished gunboats, cryogenic refrigeration technology has gradually been applied. Although this low-temperature (up to -40°C) 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 ultra-low temperature technology popular in China. It has become possible.

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