With the adjustment of the structure of fishery industry in many countries, the transition from offshore fisheries to large marine fisheries. This also led to the development of the technology of seafood cold storage equipment in the tuna cold storage category. Tuna will be or will be enriching our table as a nutritious and valuable aquatic product. The domestic and international market prospects are very broad. It can be seen that the tuna cold storage has played a huge role here. High quality tuna must have good freshness and retain its distinctive bright red color. In the handling of catches, in addition to ensuring quality during deck treatment, it is necessary to do the freezing work of tuna in a particularly serious manner. The freezing work is very complicated. First of all, the basic knowledge of tuna freezing should be understood.
First, must use -60 ° C ultra-low temperature deep freezing knot
When the tuna is frozen, the water in the fish cells freezes to crystallize at -1.5 ° C, and the water in the cells freezes to crystals at -60 ° C. Therefore, the eutectic point of the tuna is -60 ° C, that is, the tuna is completely frozen at -60 ° C, and the reaction of the cell organism is maintained at a stop state, thereby maintaining the freshness of the tuna for a long time. If the freezing temperature does not reach -60 ° C, the part of the cell that has not frozen will be deteriorated, and the fish will be blackened after a long time.
Tuna generally takes more than half a year from capture to consumption. To maintain the freshness of tuna for a long time, the freezing temperature needs to reach -60 ° C. At such temperature, it can be well maintained for one and a half to two years.
Second, must use rapid freezing
When the tuna begins to freeze at -1.5 ° C to 5.5 ° C, its cell body becomes the largest volume of crystals, which destroys the cell membrane. When the fish body is thawed, the water will flow out of the fish body tissue, taking away the unique taste of the tuna, which will cause boring when consumed, greatly reducing its value. In order to ensure the quality of tuna, rapid freezing is adopted, and the passage time of "-1.5 °C~5.5 °C maximum ice crystal formation belt" is shortened to the minimum, that is, the freezing speed is increased, which is the most important work in tuna freezing.
Tuna is a warm-water migratory fish in the ocean. It is not polluted by the environment. The meat is soft and delicious, with high nutritional value and extremely broad market. Due to the special quality of tuna, the tuna freezing temperature is -60 ° C, and the long-term storage temperature is -50 ° C ~ -55 ° C.
Cryogenic refrigeration units have become the key to the development of tuna fisheries. At present, most of the tuna offshore fishing vessels use rapid-speed refrigeration, low-temperature refrigeration and refrigeration systems generally use piston-type single-stage two-stage units, using R404 refrigerant. Since the refrigeration unit uses R404 as the refrigerant, the two-pole compression system operates inefficiently. How to improve the efficiency of the system and ensure the stability of the refrigeration unit. At the same time, the low pressure suction side is always in the vacuum operation state. When the evaporation medium is at -60 °C, the absolute pressure is only 37.4 kPa. How to ensure the air in the system is running? There may be less access to the system to improve the economics and reliability of the system.
Throughout the entire logistics process of tuna processing. From the capture of tuna to retail, the perfect tuna ultra-low temperature chain is a necessary condition to ensure the development of the tuna industry. The cold chain includes tuna longline boats, tuna purse tugs, tuna refrigerated transport vessels, tuna land refrigerated warehouses, processing plants, transport vehicles, and freezer. Due to the ultra-low temperature requirements of tuna freezing (-55 ° C ~ -60 ° C), the tuna cold storage design and manufacturer of the core technology of speed wind refrigeration plays an important role.
In order to keep the color of the tuna unchanged, the lower the temperature of the cold storage, the longer the storage time.