Low temperature preservation can better preserve the original flavor and appearance of aquatic products.

　　The spoilage of aquatic products, on the one hand, is due to the fact that the contaminated microorganisms carried by the aquatic products themselves or during storage and transportation grow and reproduce under suitable conditions, and decompose the fish protein, amino acid, fat and other components to produce substances with different odor and toxicity. To cause deterioration of aquatic products; on the other hand, the enzymes contained in aquatic products can promote the spoilage of aquatic products under certain environmental conditions. Therefore, the preservation of aquatic products must control these two factors.

　　Conditions such as ambient temperature and moisture are closely related to the growth and reproduction of microorganisms. Moisture is an essential substance for microbial life activities and is an essential component of the body. The more water in aquatic products, the easier it is for bacteria to multiply. It is generally believed that bacteria can grow and multiply when the water is above 50%; when the water is below 30%, bacterial growth begins to be inhibited; when the water is below 12%, bacterial reproduction is difficult. Temperature is an important condition for the growth and reproduction of microorganisms, and the suitable growth temperature of various microorganisms is different. If it exceeds its optimum growth temperature range, it will stop growing or die.

　　The role of enzymes is also closely related to temperature. As the temperature increases, the ability of the enzyme to decompose gradually increases. At 37 ° C, the enzyme has the strongest decomposition ability and the fastest chemical reaction rate. When the temperature exceeds 37 ° C, the enzyme begins to be destroyed. By reducing the temperature, the enzymatic decomposition ability is weakened or lost, so that the microorganisms stop breeding or even die, and the fresh life of the aquatic product can be prolonged.

　　Therefore, low temperature preservation can effectively inhibit or slow down the activity of enzymes and bacteria growth of aquatic products, prevent spoilage and deterioration, thereby better retaining the original flavor, nutritional value and appearance quality of aquatic products, and improving economic benefits. Cold storage is an effective means for processing and preserving large quantities of fresh aquatic products.

　　Aquatic product preservation refers to production technology measures that take certain physical and chemical methods in the production and circulation of aquatic products and maintain their good freshness quality. The preservation methods mainly include low temperature preservation, chemical preservation, ionizing radiation preservation and modified atmosphere preservation. The most and most effective method currently used in production is low temperature preservation.

　　Low temperature preservation includes both frozen storage and non-frozen storage at low temperatures, commonly referred to as freezing and cooling. The bacteria causing spoilage of fresh aquatic products such as fish are mainly psychrophilic fungi, and the minimum temperature for growth is -7 ° C - 5 ° C, and the optimum temperature is 15 ° C - 20 ° C. If below the optimum temperature, the growth of the microorganism is inhibited; below the minimum temperature, the growth is stopped. Most bacteria delay after growing at around 0 °C. In the low temperature range, a slight decrease in temperature can significantly inhibit the growth of bacteria. Microorganisms are inhibited at freezing temperatures, in addition to the effects of low temperatures, and the water activity is reduced by freezing of the fish body.

　　In addition, the activity of various enzymes also decreases with a decrease in temperature, and is remarkably suppressed at about -20 ° C, and almost stops at -30 ° C or lower. For example, chemical changes after fish death, such as the rate of oxidation of oils, also decrease significantly with decreasing temperature. However, due to the differences in chemical composition and muscle structure of different aquatic products, the preservation effect at low temperatures is not completely the same.

　　The main methods of low temperature preservation: ice storage, cold brine preservation, micro-frozen preservation

　　The main methods for cryopreservation of aquatic products are ice storage, cold sea water or cold brine preservation, micro-frozen preservation and freezing preservation.

　　Ice storage is a widely used preservation method. When the ice melts, it can absorb a large amount of heat to reduce the temperature of the fish body. The melted water can also wash away the bacteria and dirt attached to the fish body. For fresh fish, small ice cubes or borneol are often used, and a layer of fish is preserved in layers of ice. The preservation time varies depending on the type of aquatic product and the preservation conditions, and is generally 1-2 weeks.

　　Cold seawater or cold brine preservation is mainly carried out by immersing fresh fish in cold sea water or cold salt water at a temperature of generally -1 °C. This method is mainly used in canning plants. When using on fishing vessels, it is necessary to use ice or refrigeration equipment to cool seawater or brine. If the fish body is immersed in cold sea water or cold salt water and cooled to 0 ° C, it is better to remove it and use ice preservation. The preservation period is about 10-20 days.

　　The micro-frozen preservation is mainly applied to fishing vessels, circulating in the fish body with low-temperature seawater or low-temperature brine, and preserved in the fish tank after micro-freezing. The micro-freezing temperature is -2 ° C -3 ° C, so that the water in the fish is partially frozen, and the storage temperature is about -3 ° C. The preservation period can range from 20 to 30 days. The use of cold sea water or cold brine preservation and micro-frozen preservation, the effect is better than ice, because the temperature is lower than ice, and the cooled fish body is more solid, which is conducive to transportation. The disadvantage is that it can cause the body to fade and the salt in the fish to increase; at the same time, the seawater or salt water is mixed with the blood, mucus and other pollutants of the fish, which is prone to foam and pollution.

　　Freezing and depositing is also called freezing. The freshwater products are frozen in the freezing device and then stored in a cold storage or cabin. The temperature at which fresh water products begin to freeze is -0.5 °C - 2 °C. About 80% or more of the water in the intramuscular tissue is frozen from the beginning of freezing to -5 °C. The freezing of fresh aquatic products generally adopts the quick freezing method, that is, the freezing speed should be above 2 cm/h, or the maximum ice crystal formation zone of 0 °C - 5 °C can be passed within 30 min. The storage temperature after freezing is generally required to be lower than -18 ° C, and a storage temperature of -30 ° C or lower may also be employed.

　　The low-temperature preservation methods for fresh-keeping cold storage aquatic products are as follows:

　　Cooling and preservation is a method of keeping the temperature of aquatic products close to freezing, but not freezing. The method of fresh-keeping cold storage is to spread the ice directly onto the surface of the fish and add a layer of ice to the top. When adding ice, pack the ice to be fine, spread the ice evenly, use enough ice, the ratio of fish to ice is generally 1:1, then soak the fish in water ice, use the amount of ice equal to the weight of the water plus the fish weight. The fresh-keeping cold water method is to reduce the water temperature to 0 °C with ice, multiply by the initial temperature of water, and divide by 80. This method is generally used for rapid cooling. When the fish body is cooled to 0 °C, it is removed and stored in a frozen state, so that the local water in the fish body is frozen. This method has a longer shelf life than the ice preservation method.

　　The fresh-keeping cold storage is kept at -2 °C - 3 °C, and the fresh-keeping effect is good, so that the temperature is lowered to 3 °C for storage. The ice-salt mixing method generally adds 3% salt to the crushed ice. The larger the ratio of salt to ice, the more the ice melts and the lower the temperature. If the ratio of salt to ice is 29%, the temperature can be lowered to 20 ° C, and the cooling rate is fast.

　　The fresh-keeping cold storage low-temperature brine micro-freezing method is a fresh-keeping method for immersing the aquatic product in a low-temperature brine of -1 ° C - 5 ° C. Due to the large heat transfer coefficient of the brine. The preservation methods stored below -18 ° C include a still air freezing method, a cryogenic gas freezing method, a dipping freezing method, a flat plate freezing method, and the like.

　　After ice storage, the frozen product can be stored for several months to one year. When the temperature is below -18 °C, bacterial growth and enzyme activity are completely inhibited. However, this method can not completely inhibit the oxidation and rancidity of fish body fat in the air; at the same time, the frozen storage will cause dry consumption of fish body water and affect its quality. Wash the fish body with clean water beforehand. Regardless of the low-temperature preservation method used, the fish should be cleaned promptly and quickly. According to the size of the variety, the damaged fish should be selected and the debris should be removed, and the perishable fish should be treated first.

　　The main methods for freezing seafood seafood at low temperature are as follows:

　　The impregnation or spray freezing method freezes the fish by dipping or spraying the fish with a cooled liquid or a refrigerant such as cold brine, liquid nitrogen, carbon dioxide, ethylene glycol or Freon. In this way, the heat transfer coefficient is large, and a faster freezing speed can be obtained. The contact freezing method is to pass a cooled liquid or a refrigerant into a flat plate freezing device, and then place the fish body between the plates of the freezing device to freeze.

　　The blow-by freeze is blown through the body of the fish with a forced flow of cold air to freeze it. The air velocity is generally 3 - 5 m / s. The freezing device of this type has two kinds of intermittent operation and continuous operation. The intermittent operation is to pack the fresh fish into the frozen fish pan and send it to the quick freezing room for freezing. The continuous operation is to continuously feed the fish from the conveyor to the freezing device and then to the other end of the device. In recent years, a fluidized bed type air blowing freezing method has been adopted, and even if the aquatic product is in a floating flow state under the action of a cold air flow, it is frozen. This method has a high freezing rate, but it is only suitable for freezing small aquatic products such as shrimp.

　　The changes associated with the quality of frozen preserved fresh fish are the occurrence of liquid separation, dry consumption, and lipid oxidation and protein denaturation. The rate at which the water in the fish forms ice crystals and the size of the ice crystals during the freezing process will affect the quality of the product. Generally, in the case of slow freezing, the water inside the fish body cells gradually shifts to the outside of the cell to form large ice crystals, which denatures the protein in the cells, and the water holding capacity of the fish meat decreases. When the time is long, irreversible changes occur, that is, the fish body is thawed. After that, the original water can no longer be absorbed, so that the liquid is discharged, and the water-soluble vitamins, minerals, and taste substances are lost. However, in the case of rapid freezing or quick freezing, water forms a large number of small ice crystals inside and outside the cell, which can reduce the denaturation of the intracellular protein and the destruction of the cell tissue to a certain extent, and better maintain the quality of the fish after thawing. Dry consumption is the dehydration and weight loss caused by evaporation of water and sublimation of ice crystals during freezing. It is caused by the vapor pressure difference between the fish body and the surface of the freezer, so that the water of the fish body can be continuously transferred to the frost surface of the freezer. Dry consumption makes the meat hard and discolored, and the surface of the fish is dry.

　　In order to prevent dry consumption, it is usually packaged in frozen fish or iced outside the fish, and the temperature of the refrigerator is uniform and constant, and the relative humidity around the fish body is maintained at 90% or more. Oxidative rancidity of fats and oils can be delayed by freezing, but it cannot be prevented. For this purpose, an antioxidant such as isoascorbic acid, dibutylhydroxytoluene (BHT), t-butyl p-hydroxyanisole (BHA) or the like may be impregnated, or an ice coat containing an antioxidant may be plated.

　　Low temperature preservation has strict requirements on cold storage

　　The freezer is used for short-term or long-term storage of cooled foods and frozen foods at different temperatures in different temperatures. Refrigeration is generally a food that will have such a need, in order to maintain its required temperature and quality storage. Because the temperature drops below -15 ° C, the food freezing rate is high, microorganisms and enzymes basically stop moving, growing, and oxidation is very slow. Therefore, the food can be stored for a long time and has a good quality of refrigeration. In addition, chilled foods also require a relatively stable temperature inside the store, and excessive temperature fluctuations will cause food spoilage. The refrigerator is mainly suitable for food storage such as meat and aquatic products. The temperature of the low temperature storage is below -20 °C, because seafood and cold drinks need not be deteriorated below -20 °C. If the temperature is less than -20 °C, the taste of seafood is completely different. The characteristic of the low temperature library is that the instrument is put into the cold storage from time to time. After a period of time, the temperature of the cold storage reaches -20 °C. There is no special requirement for this time, but there are strict requirements for the storage temperature, at -18 ° C - 23 Between °C is a typical low temperature library, storage of cold drinks containing dairy products, the library temperature should be set at -20 ° C - 28 ° C, these two different temperatures require different levels of equipment, equipment investment has a big gap.

　　The refrigeration temperature of the low-temperature refrigerator has a great influence on the quality of the stored items. The lower the temperature of the refrigerator, the better the quality of the stored items and the longer the relative storage time. According to the preservation time of the frozen storage fresh-keeping items in the refrigerator, and the tolerance of the equipment, the temperature of the refrigerator design is generally -18 ° C - 30 ° C, and the design temperature of the refrigerator used for storing aquatic products in China is generally At -18 ° C, it is through the aquatic cold storage, using frozen or refrigerated methods to preserve the freshness of aquatic products for a long time. In addition, the aquatic product freezer requires a temperature of -22 ° C -25 ° C to ensure that it will not deteriorate.

　　Cold storage generally refers to facilities that are cooled by various equipment, can be artificially controlled, and remain stable at low temperatures. Its basic components are: refrigeration system, electronic control devices, warehouses with certain thermal insulation properties, and ancillary buildings.

　　Aquatic product cold storage components: Aquatic frozen insulation board generally uses 100mm or 150mm polyurethane insulation board, double-sided color plate coating or stainless steel plate, color steel plate surface processed into invisible groove, its lightest weight, high strength, thermal insulation. Good performance, corrosion resistance, anti-aging, quick and easy assembly of the board, is the best choice for cold storage insulation.

　　The temperature control system of the aquatic product freezing room generally adopts advanced microcomputer control and advanced control methods, and the liquid crystal displays indoor temperature, boot time, fan delay time, alarm indication and various technical parameters. The operation is simple and the user is easy to get started.

　　The cold storage conditions of the meat are good, and whether the stored goods are safe is whether the temperature of the warehouse can be controlled well, that is, whether the refrigeration technology can meet the requirements. For example, if you store meat, if the temperature in the warehouse is only about -14 °C, it will be stored for up to 4-5 days, and the meat will deteriorate. Meat can be stored in the freezer for half a year or even longer. The general storage temperature of the freezer is -18 ° C - 23 ° C. The lower the temperature, the longer the storage time is. Usually, it can be stored for 6 months at -18 ° C. about. If you want to save longer, then the temperature should be lower. Of course, the lower the temperature, the higher the cost of the cold storage, not only the cost in the construction of the cold storage, but also the cost of operation and maintenance of the cold storage. When storing meat, aquatic products, etc., the storage temperature must be kept below -18 °C. Therefore, in general, it is necessary to use a refrigeration system with better performance. Fresh meat can be stored in the warehouse for more than half a year. The ideal temperature index of cold storage, different varieties, site conditions, etc. have certain differences. There are also different requirements for long-term storage, short-term storage, pre- and post-storage, and generally need to be finalized through the preservation test.

　　Cold storage fresh water products must be cold processed before storage

　　Low temperature preservation also has strict requirements on the raw materials and aquatic products of the imported aquatic products: all raw materials and aquatic products that have been preserved in the fresh-keeping cold storage must be fresh and good, complete in body shape, clean and hygienic, and meet the food safety requirements. Library. Aquatic products must be cold worked before entering the cold storage and meet the requirements of relevant regulations.

　　The first is the finishing of aquatic products. The raw materials of aquatic products have passed the inspection when they enter the processing plant. After meeting the food hygiene standards, they are sent to the finishing workshop for re-cooling of the aquatic products. Finishing processing before freezing, including washing, sorting, splitting, forming, weighing, loading, packaging and other processes. In the finishing workshop, the aquatic products should not be piled up and cannot be directly exposed to sunlight. When finishing operations, it should be sorted strictly according to the variety, specification and grade, carefully cleaned and finished, so that the plate is flat and accurate. The toxic fish that have been sorted must be stored separately and properly disposed of. The finishing operation should comply with the requirements of ZBB 53C03 "Technical Regulations for the Management of Fish Management". The division should be carried out in the processing workshop. The doors and windows of the workshop shall have anti-fly devices (such as air curtains, dark aisles, fly-proof nets, etc.), and the workshop facilities shall meet the corresponding food hygiene requirements. Air conditioning measures should be taken in the workshop to adjust the room temperature according to product requirements. Aquatic products that have not been processed in time should be stored in the cooling room with ice, and the time should not exceed 24 hours. The packaging of cooled and frozen aquatic products must meet the requirements of fresh-keeping and food hygiene. The packaging label should meet the requirements of GB7718 General Standard for Food Labeling.

　　Followed by cooling and freezing, the finished aquatic products should be cooled and frozen immediately. Cooling is to use the crushed ice or cold wind to quickly reduce the temperature of aquatic products to about 0 °C; the freezing operation should meet the requirements of ZBB53004 "Technical Regulations for Freezing Operation of Aquatic Products". When freezing, the temperature should be as low as possible.