First, cooling and preservation
Cooling and keeping fresh is a method of keeping the temperature of the fish down to near freezing, but not freezing. Generally, the temperature is between 0 and -4 °C, which is a widely used method for prolonging the storage of aquatic products. After the fish is caught, it can be preserved for about 1 week by the cooling method. The lower the cooling temperature, the longer the preservation period. The quality and shelf life of the cooled fish depends on the quality of the raw materials, the duration of the cooling of the cooling process and the preservation conditions.
Due to the low temperature bacteria in the water attached to the fish at the cooling storage temperature, the proliferation and decomposition of the low temperature bacteria are still slow. Therefore, this method is stored for a long time, and fish spoilage may occur.
(1) Ice preservation
1. Sprinkling ice method This is a fresh-keeping method of sprinkling crushed ice directly on the surface of the fish. This method is simple and melts the ice water and can hold the surface of the fish body, remove bacteria and mucus, and lose weight. Specific operation of fish body cleaning → fish management → ice boxing (sprinkle ice should be even, layer of ice fish). For special fish or large fish, you can remove the viscera after laparotomy, ice in the abdomen, and then ice box (the bottom of the container, the wall and the surface should be evenly shed). A small opening at the bottom of the container allows the ice water to flow out.
The amount of ice used for chilled fish is determined based on temperature, thermal insulation conditions, and refrigeration equipment and chilled time. Generally, the ratio of fish to ice is 1:1. When the temperature is high and there is no heat insulation, the proportion of ice should be increased.
2, water ice method first use ice to cool the water to 0 ° C (clear sea water -1 ° C), and then soak the fish in water ice. The advantage is that the cooling rate is fast. It is used for straight fish or large catches such as squid and sardines. Calculate with the following amount of ice:
Ice volume = (water weight + fish weight) × initial temperature of water / 80
Due to the introduction of external heat, it is directly heated after death, and the actual amount of ice added is higher than the calculated value. The water ice method is generally used for rapid cooling. When the fish body is cooled to 0 °C, it is taken out and replaced with ice. Generally, the whole process of fresh-keeping is not based on the water ice method, because the soaking time is long, the fish absorbs water and swells, and it is easy to deteriorate.
3. Precautions for cooling and fresh-keeping 1 Wash the fish body with clean water as soon as possible after the fish is caught, remove the viscera and the internal organs, wash the blood and dirt, and prevent bacterial contamination. 2 The fish should be promptly and quickly sorted according to the size of the variety, select the broken fish, remove the inedible and toxic fish, and treat the perishable fish first to avoid backlog in the temperature regulation for a long time. 3 Add ice as soon as possible, use enough ice, the ice should be fine, the ice should be even, 1 layer of fish, not dehydrated, the top part should be added 1 layer of ice. 4 fish goods to avoid excessive accumulation. If the accumulation is too high, the fish below will be crushed. It is best to use the movable shelf to pile up the fish in the bulk. If it is not used, it can only pile up to 3 layers, and then pile up the shelves. 5 Cooling the melted ice water to the surface of the contaminated fish on the fish body. The fish can be separated by strips or bins with sulfuric acid paper or cellophane, and it is necessary to ensure that the melt water can be discharged from the container and the fish tank.
4, seawater spray instead of ice preservation new technology on board the use of seawater frozen fish goods is not a new concept, but has been very slow for years. Most of the research focused on seawater immersion and the fish were placed in a sink filled with chilled seawater. The seawater immersion tank can only be built at the time of shipbuilding, and the existing ship has a high cost and is difficult to rebuild. The seawater immersion method is limited to large fishing boats because the pool needs ample space. On this basis, New Zealand has further panicked the seawater spraying system. This device is specially designed for the small fishing boats in the southern Pacific Ocean. Since the beginning of marine fishing, the frozen fish with ice has been very successful, but because of the higher cost of equipment for purchasing water and storing ice, Use is limited. The seawater spraying method solves the above problems well and can be used for fishing at any time. The freezing speed is fast and the fish waste is less. This system overcomes the shortcomings of using frozen fish for a long time and is easily broken, and can store a large number of fish goods quickly and labor-savingly.
The principle of seawater spraying is to pass the circulating seawater through a special freezing device to lower the temperature to -1.5 ° C, and then spray the water on the fish body in a special container. Under ideal conditions, the temperature of fish stored with ice is not easy to reach below 0 °C. The minimum temperature of many commercial fishing vessels is limited to 1.7 °C. If the air is blown by a blower, the fish will be stored in the air at -1.5 °C. The body surface will be very dry.
The main advantage of the seawater spray system is the ability to continuously wash the fish with a slow stream of water and eliminate bacteria. It generally only has 350-500 liters of circulating water, and the pressure circulation rate is 14000-23000 liters per hour. It can change seawater at any time without affecting the temperature of the fish body. Because the seawater transported to the fish tank is stable, it is sprayed with sea water. The system is able to control the temperature of the fish more accurately than with ice.
On-board seawater spray freezing methods are continuing to be researched and developed, and New Zealand fishing industry personnel believe that this system is promising and can bring great benefits to fisheries.
(2) Cold seawater preservation Fresh seawater preservation is a method of immersing the catch in the cooling seawater of 0--1 °C. The cold sea water has two kinds of ice-cooled seawater and machine-cooled seawater. Ice-refrigerated seawater is produced by mixing crushed ice and seawater. Machine-cooled seawater is produced by mechanical cooling to cool seawater. Cold seawater preservation is mainly used in fishing boats or canneries.
The cold seawater fresh-keeping device on board usually consists of a Freon refrigeration unit, a seawater cooler, a fish tank, a seawater circulation pipeline, a water pump, etc. In order to prevent the introduction of external heat, the fish tank is insulated from the upper and lower sides. When the hull is shaken, it will cause fish body abrasions. When operating, the cold sea water is required to be filled, and the hatch cover is also required to be watertight. The cold sea water fresh-keeping device for fishing vessels is suitable for the combination of refrigeration machine and crushed ice, which not only saves power consumption, but also facilitates the effective use of space on board.
The cold sea water has a fast cooling rate and good fresh-keeping effect. It can handle a large amount of fish in a short time, and is especially suitable for a seine-operated fishing boat with a single species and a high concentration of catch. The shortcoming of cold seawater preservation is that the fish body is soaked in cold sea water. The fish body is swollen due to seepage and water, the fish meat is slightly salty and the surface is slightly discolored, the fish protein is easy to lose, and it will decay early in the future circulation. . The hull shaking will also damage or descale the fish body. When the blood water is too much, the seawater will be foamed with millions of pollution. The fish body freshness will fall faster than the ice fish at the same temperature. In addition, the cold sea water preservation device needs certain equipment, and the production requirements of the cabin are required. The high reason has affected the promotion and application of cold seawater preservation technology to a certain extent. Foreign cold seawater preservation is mainly used for purse seine fishing, pre-cooling of upper fish preservation and trawler fish before freezing.
In recent years, foreign studies have achieved certain results in the introduction of carbon dioxide deposits in cold seawater. Because the main cause of fish spoilage is bacterial action. Under the same temperature conditions, cold seawater preserved fish is more corrupted than ice fish, because seawater circulation expands bacterial pollution. When carbon dioxide is introduced into the cold seawater, the pH value of the seawater is reduced to inhibit the growth of the bacteria and prolong the shelf life of the catch. However, this method must overcome the corrosive effect on metals in order to promote its application. Some fishing vessels in Japan have liquid nitrogen pipes at the bottom of the cooling seawater tank. When the liquid is bubbled into the air, it can speed up the cooling of the fish and drive away the oxygen in the seawater, making the fatty fish difficult to oxidize and deteriorate.
Second, micro-frozen preservation
The micro-frozen preservation is a preservation method in which the water content of the fish body is partially frozen under the cooling of -2--3 ° C. It has a longer preservation period than ice and cold sea water, and has a good preservation effect. In particular, the preservation of benthic fish and freshwater fish with poor freeze resistance is relatively complete. There are several methods for micro-freezing:
(1) Ice-salt mixing method When ice and salt are mixed, the ice and salt melt rapidly, and a large amount of heat is absorbed in a short time, so that the temperature is rapidly lowered. The greater the ratio of salt to ice in a certain range, the faster and the faster the ice melts and the lower the temperature. When the salt ratio is 29%, the temperature can be lowered to -20 °C. In production, 3% salt is usually added to crushed ice to reduce the temperature to -3% for transport or storage. However, due to the rapid melting of ice, the cooling temperature is low, the ice water absorbs heat after the ice melts, and the temperature of the fish also rises. In the preservation process, it is not as long as the water alone is maintained at a low temperature for a long time. Therefore, the ice salt preservation is used for short-term preservation, or it is more suitable for pre-cooling of the fish before the ice storage season.
(2) Low-temperature brine micro-freezing method Because the heat transfer coefficient of the brine is large, the catch is immersed in the low-temperature brine of -1--5 ° C, and the cooling rate is fast. The main equipment of the salt water micro-frozen ship includes three parts: the brine micro-frozen tank, the insulated fish tank and the refrigeration system. When operating, pre-prepare a certain amount of cold brine, then pump the clean seawater into the micro-freezing chamber, add salt to make the seawater reach about 10 waves. The brine temperature was lowered to -5 ° C by cooling while the fish tank was cooled to -3 ° C. The rinsed catch is then loaded into a mesh bag that is rotated in a salt water tank for micro-frozen, and when the brine temperature rises, it continues to cool down to -5 ° C to a fish body center temperature of -2 ° C. After the micro-freezing, the catches are transferred into the insulated fish tanks for bulk storage, and the air is kept cool to maintain the temperature of the fish tank at about -3 °C. The salt water should be measured after each micro-freezing, and the corresponding salt amount should be added. The salt water should be replaced in time according to the degree of pollution.
(3) Before the ice micro-frozen fishing vessel sails, the temperature of the fish tank is maintained at -7 °C by the refrigeration unit. The caught fish immediately immersed in the ice with a ratio of 1:3 fish to ice. The cabin temperature should be kept at -2 °C when the fish body is cooled back.
Third, ice preservation
It is a preservation method that stores fish at a temperature below -18 °C after freezing. Bacterial growth and enzymatic activity were completely inhibited at this temperature, but oxidative rancidity in fish air was not completely inhibited. At the same time, the moisture of the fish body will produce dry consumption during the frozen storage, which will affect the oxidation of the ice or the vacuum packaging to prevent oxidation and shrinkage. Frozen products can be stored for several months to one year, and the storage time varies depending on the species and storage temperature. The frozen storage methods mainly include air freezing, contact freezing, immersion freezing, and boiling liquid freezing.
(I) Static air freezing method This freezing method refers to freezing in low temperature still air. The upper part of the freezing chamber is provided with a cooling coil, and the lower part is a cooling tube holder. Fish and the like are placed on the tube rack, and the fish body is naturally convected by air at -25--30 °C. This method has a slow freezing rate, a large dry fish body, and poor quality of frozen products. But the equipment is simple. Suitable for small freezer use. In order to accelerate the freezing of the fish body, the freezing method of circulating the cold air at a wind speed of 1-2 meters per second by using a blower is called a half blow freezing method.
(2) The tunnel freezing method is the air blowing freezing method. The circulating cooling air is sent to the rectangular insulated tunnel type ice junction chamber to freeze the aquatic product. The upper air passage of the freezing room is equipped with an evaporative cooling pipe and a blower, and the lower part is equipped with a mobile fish cart and a fish plate. Air supply -35--45 ° C, wind speed 3-5 meters per second. The freezing speed is faster than the static air method, and it can be operated continuously, but when the wind speed is high, the frozen product is easy to dry and discolor. This method is one of the most commonly used freezing methods on land.
(3) The flat plate freezing method is a contact freezing method in which materials are pressed between low temperature metal freezing plates for heat exchange. The freezing device consists of a number of metal plates that can be moved to adjust the spacing. The inside of the plate is cooled by a refrigerant (or cooled brine) to a cooling temperature of -25 to -40 °C. The freezing distance between the plates during freezing is 5-10 cm. The plate freezes quickly and prevents dryness and discoloration. It is a common method of quick freezing.
(4) Immersion freezing method A freezing method in which fish or the like is placed in direct contact with a solvent such as cooled brine or propylene glycol. The brine has a sodium chloride solution or a calcium chloride solution, and is typically circulated in a freezing tank at a rate of 0.04 - 0.09 meters per second using a sodium chloride solution at -16 °C. The freezing speed is fast, and it is mostly used for freezing large ships such as squid and tuna.
(5) Cryogenic gas freezing method Deep freezing is to expose the product to a liquefied gas at a low temperature of -60 ° C to freeze it. This type of freezing device mainly uses two kinds of gases, liquid nitrogen and liquid carbon dioxide, and the product is placed on a conveyor belt to be rapidly frozen by cryogenic gas. The product sprayed onto the conveyor belt with -196 ° C liquid nitrogen gas, the used waste nitrogen is discharged to the atmosphere at a temperature of -30--100 ° C. The liquid nitrogen consumption is 1.2-2 kg per 1 kg of product frozen. The freeze cost is about 8 times higher than the blower freeze. Liquid carbon dioxide gas can be used instead of liquid nitrogen to freeze the product, and carbon dioxide can be stored in atmospheric pressure vessels at atmospheric pressure.
The boiling liquid freezing method is a boiling liquid freezing system designed for rapid freezing of a monomer, and the frozen product enters the flowing freezing liquid and freezes in direct contact with the boiling refrigerant. This refrigerant is purified Freon 12 with a boiling point of -30 °C. The freezing device includes an insulated enclosure, a feed conveyor, a product feed conveyor that is lowered into the boiling refrigerant, and heat exchange between the product and the refrigerant, causing the surface of the product to be immediately frozen. The product is then placed on a horizontal conveyor at the bottom of the unit and the chilled liquid is sprayed while advancing until it is completely frozen and sent to the discharge conveyor for transport. This method freezes almost no weight loss
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