12、How to preserve aquatic products

The power consumption of cold storage is a problem that many customers are more concerned about, and the power consumption of cold storage is actually not to be underestimated, so how should we calculate the power consumption of cold storage?

  1. Refrigeration and preservation

Refrigerated preservation is a preservation method that places slaughtered and washed fish bodies or divided fish bodies in a clean cooling room, uses cold air to cool the fish bodies, and stores them in a high-temperature cold storage at 0~4°C. Air cooling is generally carried out in a cooling room at −1~0°C. The evaporator in the cooling room can use exhaust pipes or air coolers. In actual refrigeration operations, it is generally necessary to lower the ambient temperature of the cooling room and maintain it at -10°C in advance. After placing the sample in the cooling room, it is necessary to continue to use cold air to cool the sample, quickly reduce the center temperature of the sample to 0°C, and then put it into the high temperature room. Store in cold storage or directly in the cooling room. Due to the small convection heat transfer coefficient and slow cooling speed of air, fish cannot be processed in large quantities, and cooling the fish body with cold air for a long time can easily cause dry consumption and oxidation of the fish body. Therefore, refrigeration preservation can be used for short-term storage of raw materials in aquatic product processing plants, and can also be used for short-term storage and preservation of divided and processed fresh aquatic products or prepared aquatic products.

  1. Keep fresh at ice temperature

Ice temperature preservation is a new food preservation technology discovered and developed by Dr. Akimi Yamane of Japan in 1970. Ice temperature preservation technology has been proven to be one of the best ways to maintain the freshness and quality of fresh aquatic products. The so-called ice temperature refers to the temperature area below 0℃ and above the freezing point. Its temperature is between refrigeration and slight freezing. Micro-freezing refers to the temperature range between the freezing point and -5℃, with -3℃ as the central temperature, and the aquatic products are partially frozen; while the storage temperature of ice-temperature preservation is above the freezing point, and the aquatic products are always in a fresh state without freezing. Therefore, the outstanding advantage of ice-warm preservation is that it can avoid a series of texture deterioration caused by freezing and maintain the freshness of aquatic products. Ice temperature preservation is a preservation method that is stored in a temperature area below 0℃ and above the freezing point. That is to say, the storage temperature range of ice temperature preservation (ice temperature area) is between 0℃ and freezing point. Fish muscle tissue contains chemical components such as protein, fat, sugar and salt, and its freezing point is generally 0.9–0.3℃. At present, the temperature control accuracy of cold storage is generally above ±1.0°C. Therefore, it is necessary to adjust the freezing point and widen the ice temperature area of ​​fish to effectively achieve ice temperature preservation. Adding an appropriate amount of salt, sucrose, polyphosphate and other freezing point regulators to fish meat can lower the freezing point of fish meat, widen the ice temperature area, and facilitate temperature control during ice temperature storage. The freezing point of fresh prepared aquatic products is generally -2.0~~1.0℃, and the freezing point of surimi products is -3.0~1.0℃. Ice-temperature preservation has high requirements for the storage temperature control accuracy of cold storage. The temperature fluctuation during storage should not be too large. It is necessary to choose a cold storage with even air distribution and good heat storage performance. The temperature control accuracy of ice-temperature preservation is required to be (±0.5 ~1.0)℃. In the preservation and storage operation, it is necessary to select the appropriate ice temperature storage temperature according to the characteristics and freezing point of different aquatic products. For conditioning fresh aquatic products and surimi products, (−1.0±0.5)℃ and (− 2.0±0.5)℃ as the ice temperature preservation temperature. Filling the packaging bag with CO can effectively inhibit the growth of cold-adapted bacteria, and the higher the concentration of CO, the stronger the antibacterial ability, and the more obvious the effect of extending the shelf life of fresh prepared aquatic products. When inflating packaging, it is generally necessary to evacuate first, then fill with gas and seal. If only CO. is filled, CO will dissolve in water and create a vacuum in the sealed bag, causing juice in the muscles to leak out and affect the product. Exterior. Therefore, the ratio of CO2 to N can be controlled at 70%~75%:30%~25%. Micro-freezing preservation Micro-freezing preservation is a mild freezing or partial freezing preservation method that preserves aquatic products below the freezing point (around -3°C). It is also called supercooling or partial freezing preservation. In 0~ In the temperature range of 10°C, the temperature coefficient of growing microorganisms is generally 5; while below 0°C, the temperature coefficient of the growth and reproduction rate of most microorganisms is 1.5~2.5. The storage performance of slightly frozen aquatic products is 2.0~2.5 times that of refrigerated freshness; and in the slightly frozen state (−3~-2)°C, part of the water in the fish freezes, resulting in a reduction in water activity, and the bacterial cell juice is partially Freezing and concentrating change its physiological and biochemical processes, and the activities of most psychrophilic bacteria are inhibited. It can be seen that the ability of micro-freezing to inhibit microorganisms is 4 times that of refrigeration, which allows fish to remain fresh for a longer period of time without spoilage. The micro-frozen shelf life varies depending on the fish species, but is generally 2027 days, which is 1.5 to 2 times longer than the refrigerated shelf life.

There are certain differences in the freezing point of different species of fish due to their different chemical compositions. The freezing point of freshwater fish is generally -0.7~0.2℃. At present, the micro-freezing temperature adopted by various countries is generally 3~2℃, and this temperature range is exactly in the maximum ice crystal formation temperature zone (-5~-1℃). Therefore, using the quick freezing method to quickly change the temperature zone is a measure that needs to be taken to preserve freshness by micro-freezing. In recent years, Chinese scholars have done some research on micro-freezing preservation of seabass, sardines, tilapia, crucian carp, etc. The results show that micro-freezing can effectively inhibit the growth of the total number of bacteria, maintain low T-VBN and K values, and extend the shelf life of fresh fish. There are three main types of micro-freezing methods for fish preservation: ice-salt mixed micro-freezing method, blast cooling micro-freezing method and low-temperature salt water micro-freezing method. The first two methods are suitable for micro-freezing preservation of freshwater fish and their products.

  1. Freeze and keep fresh

The use of refrigerated preservation and micro-freezing preservation technologies can inhibit the enzyme activity and bacterial growth in fish to a certain extent, but the storage and preservation period is short, generally only 7 to 10 days and 15 to 20 days. In order to achieve long-term storage goals, the fish body temperature must be lowered to below -18°C and stored at a temperature below -18°C. This type of preservation method is the frozen storage method. The frozen preservation method is applicable to all aquatic products. It can be used to preserve processed raw fish, as well as primary processed products and prepared aquatic products.

When frozen fish enter the cold storage, they should be stacked separately according to variety, specification, grade and batch number.

The stacking should be stable, and each fish should be marked with the variety, grade, quantity, storage time and other necessary instructions.

There should be a large gap between stacks of frozen fish of different varieties, not less than 0.7m, to facilitate the differentiation of different varieties. There should be a forklift channel with a width of about 2m in the freezer room to facilitate the entry and exit of goods. In actual operations, it is necessary to ensure that goods are first in, first out.

The quality of frozen fish products mainly depends on factors such as the quality of raw fish, pre-freezing treatment, packaging, freezing methods, and the temperature and time that frozen products experience during the circulation process. The impact of freezing storage temperature on the quality of frozen products is greater than the freezing rate. The lower the freezing temperature, the better the quality of the fish will be maintained, the longer the storage period, and the higher the daily operating costs will be. For most frozen foods, storage at -18°C not only has a shelf life of one year, but is also the most economical.

There are three freezing methods for aquatic products.

(1) Blowing freezing is a method of freezing food by using forced circulation of flowing cold air. Salt water immersion freezing, direct contact freezing, immersing aquatic products in salt water or spraying brine on the aquatic products to freeze. The brine used is a saturated sodium chloride solution, and its temperature is lowered to -18°C before freezing. When the center temperature of the aquatic product drops to -15°C, the freezing is completed. Then remove the aquatic products, quickly rinse with clean water, package and freeze. If the freezing method of immersing in salt water is used, the salt water is flowing, and the aquatic products should be pre-cooled before freezing. The advantage of this method is that it freezes quickly; the disadvantage is that it is easy to damage the skin and scales of aquatic products, resulting in poor appearance, the meat tastes salty, accelerated fat oxidation during storage, equipment in contact with salt water is easy to corrode, and salt water is affected by blood, minced meat, etc. The pollution needs to be replaced frequently.



(2) The salt water used for indirect contact freezing is calcium chloride aqueous solution. Through the forced action of the stirrer, the salt water continuously circulates in the pool and is cooled by the evaporator, so that the salt water in the pool is at a low temperature, and the frozen aquatic products , washed, put into buckets, and immersed in salt water pools for freezing. Because the eutectic point of calcium chloride brine is low, it is usually lowered to 30~20℃ for freezing. The freezing time of aquatic products is about 6~8 hours. The advantage of this method is that the freezing speed is faster than air freezing, and it avoids the penetration of salt into aquatic products; the disadvantage is that all containers and equipment in contact with salt water are susceptible to corrosion.

(3) Plate freezing is a freezing method that uses the freezing plate of the plate machine to directly contact the aquatic products for heat exchange. Plate freezers are divided into two types: vertical and horizontal. The horizontal plate freezer consists of a refrigeration system including a compressor and a hydraulic lifting device. Each plate freezer is equipped with several or more than ten freezing plates, which are the evaporators in the refrigeration system. There is a liquid supply and a return air main pipe behind or on both sides of the plate. Each plate is connected with a rubber hose. Or stainless steel pipe connection so that the plate can move up and down. The freezing time is about 4~5 hours. The structure of the vertical plate freezer is basically similar to that of the horizontal plate freezer, but the plates are upright and parallel. The fish plates are not used during freezing, but are poured in bulk. The advantage of the plate freezing method is that it freezes quickly; the disadvantage is that the horizontal method is labor-intensive and cannot freeze large fish. Although the vertical method can reduce labor intensity, aquatic products are easily deformed due to bulk packaging, which affects the appearance.

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