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Abstract Aquatic products are highly perishable because of the biological characteristics. So it is very important to study the preservation of aquatic products. Low temperature preservation technology is the earliest and most widely used technology. This paper introduced the research progress of low temperature preservation technology for aquatic products at home and abroad, and pointed out some problems and the future development trend of low temperature preservation. It provides a basis for the development of the aquatic product processing.
Key words Aquatic products; Low temperature preservation; Ice crystal
China is rich in aquatic resources and is one of the largest aquaculture and export countries in the world. Aquatic products are rich in nutrients, have enzyme high activity, and carry a large number of bacteria. After the fish body is inactivated, biochemical reactions will cause discoloration and odor, which reduces the edible quality and commodity value, and directly affects the storage, transportation and sales of aquatic products. The annual loss rate of aquatic products is about 15% in China. The average loss rate is only 2%-5% in developed countries such as Europe and America［1］. Fresh-keeping technology can maintain the quality, facilitate long-distance transportation and anti-season trade of aquatic products, and improve market competitiveness. Therefore, it is the concern what kind of fresh-keeping technology of aquatic products is chosen at home and abroad. Because of the difference of living area, species and fishing process, the types of surface microorganisms are different in aquatic products. Therefore, the fresh-keeping effect of aquatic products is related with the species, temperature and slaughter mode.
The Effect of Slaughter Mode
Different slaughter methods will cause different stress responses of fish body, which directly affects the quality of aquatic products. Most domestic fisheries companies use bloodletting to death or directly fall to death. Those methods can reduce the blood content in muscles, reducing bacterial growth. But aquatic products would struggle fiercely before they die and consume a lot of nutrients in their bodies. As a result, the stiffness period is shortened, the pH value is decreased, and the bacterial reproduction is accelerated, which is not conducive to preservation. Hultmann et al. ［2］confirmed that Atlantic cod had a higher level of stress was confirmed by blood physiology analyses at slaughter. This was further associated with significantly reduced muscle pH and somewhat elevated muscle collagenase-like activity in the stressed fish. After 5 d of iced storage, the stressed fish had significantly lower water holding capacity and reduced hardness. Yang et al.［3］investigated the effects of different lethal methods on the freshness of tilapia. The results showed that the lethal time of tilapia kept in 0 ℃ ice water was 30 min, and this method could prolong the time required to enter rigidity period, which was beneficial to the fresh-keeping of tilapia. By knocking on the head, the fish struggled less before death, and have less glycogen decomposition［4］. Electric shock and CO2 anesthesia can keep fish surface harmless, and electric shock can make fish tender which helps to improve the taste［5］. Low Temperature Preservation Technology of Aquatic Products
Low temperature preservation technology has become one of the most commonly used methods for fresh-keeping of aquatic products because of its low damage to raw materials and low cost. Common methods include ice storage, controlled freezing-point storage, superchilling, freezing and other fresh-keeping methods.
Ice storage
Ice storage is a kind of fresh-keeping method that bases on ice as medium, which reduces the temperature of fresh fish to near the freezing point without freezing. The temperature is 0-4 ℃. Ice storage can reduce the temperature fluctuation and water loss of aquatic products during storage, and the quality is closest to the biological characteristics of fresh fish. Ice storage can be divided into dry ice method and water ice method. Dry ice method refers to scattering crushed ice on the fish layer which form a layer of ice and a layer of fish, or mixing the crushed ice with fish for cooling. The water melted from ice can directly clean the surface of fish, removes mucus and bacteria, and prevents moisture loss and oxidation. However, there are some shortcomings, such as lax contact between fish and ice, easy crushing of lower fish, short storage time, etc. It is suitable for temporary storage during the transportation of fishing boat. The water ice method refers to cooling the fresh water or sea water with ice (fresh water 0 ℃, sea water -1 ℃), and then immersing the fish in ice water for cooling. When the fish body is cooled to 0 ℃, it is taken out and stored by dry ice method. The water ice method has fast cooling speed and less fish struggle, which is suitable for purse seine fishing boat with relatively concentration of fish catch and relatively single species or some canning factories. However, long-term soaking will cause the fish body to increase weight and smell, increase the salt content, and slightly discolored surface. Some scholars have combined some bacteriostatic gases with cold sea water to improve shelf life. Liu et al.［6］combined effects of CO2 and cold seawater on the quality of pacific white shrimp (Penaeus vanmamei) during the storage were investigated. The results showed that the shrimp in CO2-cold seawater was second-grade freshness (TVB-N value of 19.5 mg/100 g, K value of 20.3%) after 8 d of storage compared with serious spoilage of control. Activity of polyphenol-oxidase (PPO) was significantly inhibited by CO2-cold seawater. Meanwhile, melanosis of shrimp and deterioration of sense quality were delayed. Ice storage can be combined with biological preservatives to improve the quality of aquatic products. Yang［7］studied the preservation effects of chitosan combined with different concentrations of tea polyphenols (TP) on hairtail (Trichiutus haumela). The results showed that stored at (4±1) ℃, the hairtail covered with 1.0% chitosan combined with 0.4% TP had the best quality, the shelf life being extended from 4-5 d to 12-13 d. Controlled freezing-point storage
Controlled freezing-point storage (CFS) refers to the technique of keeping fresh at the temperature between 0 ℃ and the freezing point of the body. The temperature is 0--2.0 ℃. CFS is suitable for aquatic products with strong cold endurance and higher maturity. CFS can maintain the living characteristics of aquatic products as it keeps the original flavor and nutrition which is better than ice storage. Compared with superchilling and freezing storage, CFS consumes less energy and avoids protein denaturation and moisture loss by freezing. However, extremely small temperature fluctuations will cause recrystallization which can forms larger and uneven ice crystals, affecting the quality of aquatic products. Mi［8］found that compound (5% NaCl + 5% CaCl2 + 2.5% sorbitol) could effectively depress the freezing point and retard bacterial growth in the shrimps. An additional 2 d of shelf life at (-2.0±0.2) ℃ was achieved in shrimps when the freezing-point was depressed from -1.3 to -2.5 ℃. There was little difference in microstructure between fresh and freezing-point depressed shrimps after storage at (-2.0±0.2) ℃. Significant high water holding capacity, salt-sloution protein and Ca2+-ATP activity were obtained in freezing-point depressed shrimps. CFS can be combined with other preservation technologies. Gong［9］found that the freezing point of fresh grass carp slices decreased to -3.52 ℃ when added with 3% salt and 0.6% sucrose. The optimum treatment was immersing the slices in 3% citric acid solution for 10 min. 70% CO2 and 30% N2 package could control microbial growth and extend shelf life to 40 d.
Superchilling storage
Superchilling is a mild or partially freezing method which keeps the temperature slightly below the freezing point of fish (about -3 ℃). Superchilling temperature of aquatic products is slightly different because of fish species and superchilling methods, the temperature is -2--3 ℃. The temperature of superchilling is lower than ice storage and CFS storage, which overcomes the deficiency of metabolic aging of tissues by high- temperature storage. Gao et al.［10］studied the fresh-keeping effect of hairtail under the conditions of ice storage (4 ℃), CFS (-0.6 ℃) and superchilling (-3 ℃). The results showed that the shelf-life was 5 d at 4 ℃, was extended by 2 d at -0.6 ℃, reaching 7 d, and was up to 18 d at -3 ℃, showing a 3.6-fold increase compared with that at 4 ℃. Compared with freezing, superchilling avoids destroying cell structure by low temperature freezing, but the shelf life is shorter. Liu［11］observed the large yellow croaker which had be stored for 30 d at low temperature. Follow-up experiments revealed that the higher the freezing rate of fish (the freezing rates of -3, -6 and -20 ℃ were 30%, 65% and 90%, respectively) and the larger the area of ice crystals formed, the more damaged fish tissues were. The ice crystals of superchilling were fine and uniform, and the tissue damage was lower. The combination of superchilling and biological fresh- keeping agent can achieve better preservation effect by using hurdle factor. The results showed that tea polyphenols could effectively inhibit bacterial growth, reduce fat oxidation and prolong the shelf life of silver carp during superchilling storage. Freezing storage
Freezing storage is a fresh-keeping method of reducing the central temperature of fish to -18 ℃ and storing it at this temperature. It is suitable for long-term storage of aquatic products. According to the speed of freezing, it can be divided into slow freezing and quick-frozen. Quick-frozen passes through the maximal ice crystal production zone at a faster speed. The ice crystals are small, more numerous and evenly distributed, which is no obvious damage to tissues. However, the cost is high, so it is mainly used for precious aquatic products. The ice crystals are large, irregular and scattered by slow freezing. The drip loss is large during thawing, which leads to the decline of edible quality. Moisture loss, lipid oxidation and protein freeze denaturation will occur in aquatic products during freezing storage. Adding antifreeze agent can prevent protein freeze denaturation. Ma［13］found that the concentrations of trehalose, polydextrose and sodium lactate were 4%, 6%, and 5%, respectively as antifreeze agent. At -18 ℃, the quality changes of crisp grass carp by cryoprotectants combined with immersion chilling and freezing(ICF) group were studied and analyzed in the frozen storage for 6 months. Results showed that the content of myofibrillar protein, the Ca2+-ATPase activity, the total sulfhydryl, the water-holding power, antioxidant activity and texture of cryoprotectants plus ICF group were better than other groups. Cryoprotectants combined with ICF were benefit to maintain the stability of protein secondary during frozen storage. The synergistic effect of low temperature storage and biological preservative is better. Li［14］researched the effects of complex biological fresh-keeping agents on tuna at different freezing temperatures. Assured that compound biological preservative best portioning is tea polyphenol 6.55 g/L, chitosan 16.05 g/L and nisin 0.46 g/L. Compound biological preservative at -18 and -25 ℃ can both lengthen one months preservation length.
Cryogenic vitrification storage
The glass state refers to when the temperature of the macromolecular substance satisfies the condition of glass transition, the macromolecular materials do not have enough energy to cross the energy barrier of internal rotation, nor space for free activities. So the moveability is greatly reduced to special constructs of vitrification. For the glassy state storage of aquatic products, at first, it is necessary to accurately measure the glass transition temperature (Tg′) of different aquatic products. Tg′ has great differences in different aquatic products. When Tg′ is lower than -20 to -30 ℃, the thermodynamic phase diagram of system should be changed by no side effects additives which can improve Tg′. The water content of aquatic products has a great impact on Tg′. When the water content increases 1%, the Tg′ decreases 5-10 ℃ ［15］. Secondly, it is necessary to study the quantitative relationship between AT and various biochemical reaction rates, microbial reproduction, and enzyme activity during the glassy state storage. Super chilled storage
Super chilled storage refers to directly immersing fresh fish in brine at -10 ℃, and rapidly cooling the surface of the fish within 10 to 30 min, and then moving the fish to cryogenic cooling water (0--1 ℃) for storage. The slaughter and initial quick-frozen achieve simultaneously, which can hold within limits the biochemical changes after the fish died, and maintaining the original freshness and quality of the fish. The initial cooling of aquatic products is sufficient by super chilled storage, no ice is needed in circulation, and the shelf life is twice as long as ice storage. Unfortunately, there must be a special transport vehicle, and salt water freezing also directly impacts on the salinity of fish.
Suolian WU et al. Research Progress of Low Temperature Preservation Technology for Aquatic Products
Ice Crystal Formation
During the freezing process, the formation and content of ice crystals are closely related to the freezing temperature, the stability of temperature and the material temperature before freezing storage. Because of the rapid formation of ice crystals, the amount of heat increases dramatically which needs to be removed. However, the ice film of material surface hinders the heat transfer. The researchers speculate that the result of contradiction forms a large number of tiny ice crystals which leads to cell rupture, changes the cellular composition, and affects the shelf life and commodity value of the product［16］. The formation of ice crystals can be divided into nucleation and crystal growth.
Nucleation
The movement of water molecules slows down during temperature fall period. Under the gravity of directional alignment, the internal structure of water molecules gradually tends to form stable polymers which are similar to crystals. Temperature continues to lower. When stable nuclei appear, the polymers are converted to ice crystals. Latent heat is released during this process which promotes the temperature back to freezing point. The relationship between ice crystal formation and cell damage is the key to improve the quality of aquatic products. It has been the researchersdebating topic all the time about which one happens first between the formation of intracellular ice crystals and cell damage. Mazu［17］and Toner et al.［18］considered intracellular ice crystal formation is the main cause of cell damage. Intracellular ice crystal formation converts intracellular liquids into solids. The intracellular volume expansion by changes of chemical potential energy causes cell damage. Muldrew et al.［19］believe that cell damage precedes intracellular crystallization. Osmotic pressure of the extracellular solution is increased in the presence of ice. The osmotically driven water efflux that occurs in cells during freezing is viewed as the agent responsible for producing a rupture of the plasma membrane, thus allowing extracellular ice to propagate into the cytoplasm. Crystal growth
Crystal growth refers to the process that the water or vapor around ice crystal moves toward ice crystals, adheres and freezes on the ice crystal. When the storage temperature is high or the temperature fluctuation is large, the recrystallization will occur with the tiny ice crystal as the nucleus. When the frozen storage time is too long, recrystallization will occur even at constant temperature. The water molecules on the surface of small ice crystals cannot be tightly bound because of the higher surface free energy. Therefore, these water molecules tend to diffuse from the surface of small ice crystals, and deposit on the surface of large ice crystals. As a result, the larger ice crystals grow and the smaller ice crystals disappear. During the growth of ice crystals, the formation of solid ice crystals enlarges the specific surface area of muscle cells which can injure cells. Then a series of biochemical reactions will occur in tissues. It can be better delay the deterioration of the quality of cryogenic aquatic products by inhibiting the formation of nuclei, increasing freezing rate, or avoiding recrystallization of ice crystals during freezing storage.
Conclusion
Preservation by low temperature can satisfy with the requirements of commercial large-scale fresh-keeping, but the single method of low temperature has limitations. CFS has a small scope of application, superchilling is difficult to control temperature and requires high equipment. Freezing storage can preserve aquatic products for a long time, however, the ice crystals are easy to damage the tissue and aggravate the denaturation of proteins. The drip loss is large after thawing, which makes the meat taste worse. We need dig more deeply into the formation mechanism of ice crystals at the molecular level. At the same time, we must focus on the control of ice crystal formation during the preservation of aquatic products by combining new energy with freezing. It is the main development trend of fresh-keeping technology in the future by continuous improving and developing for fresh-keeping equipment, and integrating new packaging and cold sterilization technology to prolong the shelf life.
References
［1］LI JR. Research progress on fresh food preservation technology［J］. Journal of Chinese Institute of Food Science and Technology, 2010, 6(10): 17-19.
［2］HULTMANN L, PHU TM, TOBIASSEN T, et al. Effects of preslaughter stress on proteolytic enzyme activities and muscle quality of farmed Atlantic cod (Gadus morhua)［J］. Food Chemistry, 2012, 134(3): 1399-1408. ［3］YANG G, GUO J, LIN XD. Effects of different lethal methods and partial freezing treatment on fresh-keeping properties of tilapia ［J］. Food Science, 2009, 30(16): 278-281.
［4］NIU BW, REN Y, LUAN DL, et al. Effects of Different Slaughtering methods on preservation of turbot［J］. Fishery Modernization, 2008, 35(3): 38-41.
［5］LINES J, KESTIN S, Electric stunning of trout: power reduction using a two-stage stun［J］. Aquacult. Eng, 2005, 32: 483-491.
［6］LIU SL, ZHANG SS, LU F, et al. Combined effect of CO2 and cold seawater maintain quality pacific white shrimp［J］. Transactions of The Chinese Society of Agricultural Machinery, 2012, 43 (12): 158-164.
［7］YANG SP, XIE J, TONG Y, et al. Effects of chitosan combined with different concentrations of tea polyphenols on preservation of Trichiutus haumela［J］. Jiangsu Journal of Agricultural Sciences, 2010, 26 (4): 818-821.
［8］MI HB. Study on waterless preservation and controlled freezing-point storage of crucian carp and Chinese white shrimp (Fenneropenaeus chinensis)［D］. Hangzhou: Zhejiang University, 2014.
［9］GONG T. Study on controlled freezing-point storage and modified atmosphere packaging applied to the preservation of fresh grass carp slices［D］. Wuhan: Huazhong Agricultural University, 2008.
［10］GAO ZL, XIE J, SHI JB, et al. Quality changes of Trichiurus haumela under different storage conditions［J］. Food Science, 2013, 34 (16): 311-315.
［11］LIU MH. Studies on large yellow croaker in partial freezing［D］. Fuzhou: Fujian Agricultural and Forestry University, 2004.
［12］FAN WJ, SUN JX, CHEN YC, et al. Effects of tea polyphenols on freshness-keeping of partial-frozen silver carp in cold storage［J］. Transactions of the Chinese Society of Agricultural Engineering, 2009, 25(2): 294-297.
［13］MA XB. Effects of immersion chilling and freezing (ICF) on the quality characteristics of crisp grass crap［D］. Zhanjiang: Guangdong Ocean University, 2015.
［14］LI SS. Study on tuna biological preservation technology［D］. Zhoushan: Zhejiang ocean university, 2013.
［15］ATKINS AG. Food structure and behavior academic press［M］. London, 1987.
［16］MAGNUSSEN OM, HAUGLAND A, TORSTVEIT HEMMINGSEN, et al. Advances in superchilling of food-process characteristics and product quality［J］. Trends in Food Science and Technology, 2008, 19(8): 418-424.
［17］MAZUR P. Kinetics of water loss from cells at subzero temperatures and the likelihood of intracellular freezing［J］. Journal of General Physiology, 1963, 47(2): 347-369.
［18］TONER M, CRAVALHO EG, KAREL M. Thermodynamics and kinetics of intracellular ice formation during freezing of biological cells［J］. Journal of Applied Physics, 1990, 67(3): 1582-1593.
［19］MULDREW K, MCGANN LE. The osmotic rupture hypothesis of intracellular freezing injury［J］. Biophysical Journal, 1994, 66(2): 532-541.
Key words Aquatic products; Low temperature preservation; Ice crystal
China is rich in aquatic resources and is one of the largest aquaculture and export countries in the world. Aquatic products are rich in nutrients, have enzyme high activity, and carry a large number of bacteria. After the fish body is inactivated, biochemical reactions will cause discoloration and odor, which reduces the edible quality and commodity value, and directly affects the storage, transportation and sales of aquatic products. The annual loss rate of aquatic products is about 15% in China. The average loss rate is only 2%-5% in developed countries such as Europe and America［1］. Fresh-keeping technology can maintain the quality, facilitate long-distance transportation and anti-season trade of aquatic products, and improve market competitiveness. Therefore, it is the concern what kind of fresh-keeping technology of aquatic products is chosen at home and abroad. Because of the difference of living area, species and fishing process, the types of surface microorganisms are different in aquatic products. Therefore, the fresh-keeping effect of aquatic products is related with the species, temperature and slaughter mode.
The Effect of Slaughter Mode
Different slaughter methods will cause different stress responses of fish body, which directly affects the quality of aquatic products. Most domestic fisheries companies use bloodletting to death or directly fall to death. Those methods can reduce the blood content in muscles, reducing bacterial growth. But aquatic products would struggle fiercely before they die and consume a lot of nutrients in their bodies. As a result, the stiffness period is shortened, the pH value is decreased, and the bacterial reproduction is accelerated, which is not conducive to preservation. Hultmann et al. ［2］confirmed that Atlantic cod had a higher level of stress was confirmed by blood physiology analyses at slaughter. This was further associated with significantly reduced muscle pH and somewhat elevated muscle collagenase-like activity in the stressed fish. After 5 d of iced storage, the stressed fish had significantly lower water holding capacity and reduced hardness. Yang et al.［3］investigated the effects of different lethal methods on the freshness of tilapia. The results showed that the lethal time of tilapia kept in 0 ℃ ice water was 30 min, and this method could prolong the time required to enter rigidity period, which was beneficial to the fresh-keeping of tilapia. By knocking on the head, the fish struggled less before death, and have less glycogen decomposition［4］. Electric shock and CO2 anesthesia can keep fish surface harmless, and electric shock can make fish tender which helps to improve the taste［5］. Low Temperature Preservation Technology of Aquatic Products
Low temperature preservation technology has become one of the most commonly used methods for fresh-keeping of aquatic products because of its low damage to raw materials and low cost. Common methods include ice storage, controlled freezing-point storage, superchilling, freezing and other fresh-keeping methods.
Ice storage
Ice storage is a kind of fresh-keeping method that bases on ice as medium, which reduces the temperature of fresh fish to near the freezing point without freezing. The temperature is 0-4 ℃. Ice storage can reduce the temperature fluctuation and water loss of aquatic products during storage, and the quality is closest to the biological characteristics of fresh fish. Ice storage can be divided into dry ice method and water ice method. Dry ice method refers to scattering crushed ice on the fish layer which form a layer of ice and a layer of fish, or mixing the crushed ice with fish for cooling. The water melted from ice can directly clean the surface of fish, removes mucus and bacteria, and prevents moisture loss and oxidation. However, there are some shortcomings, such as lax contact between fish and ice, easy crushing of lower fish, short storage time, etc. It is suitable for temporary storage during the transportation of fishing boat. The water ice method refers to cooling the fresh water or sea water with ice (fresh water 0 ℃, sea water -1 ℃), and then immersing the fish in ice water for cooling. When the fish body is cooled to 0 ℃, it is taken out and stored by dry ice method. The water ice method has fast cooling speed and less fish struggle, which is suitable for purse seine fishing boat with relatively concentration of fish catch and relatively single species or some canning factories. However, long-term soaking will cause the fish body to increase weight and smell, increase the salt content, and slightly discolored surface. Some scholars have combined some bacteriostatic gases with cold sea water to improve shelf life. Liu et al.［6］combined effects of CO2 and cold seawater on the quality of pacific white shrimp (Penaeus vanmamei) during the storage were investigated. The results showed that the shrimp in CO2-cold seawater was second-grade freshness (TVB-N value of 19.5 mg/100 g, K value of 20.3%) after 8 d of storage compared with serious spoilage of control. Activity of polyphenol-oxidase (PPO) was significantly inhibited by CO2-cold seawater. Meanwhile, melanosis of shrimp and deterioration of sense quality were delayed. Ice storage can be combined with biological preservatives to improve the quality of aquatic products. Yang［7］studied the preservation effects of chitosan combined with different concentrations of tea polyphenols (TP) on hairtail (Trichiutus haumela). The results showed that stored at (4±1) ℃, the hairtail covered with 1.0% chitosan combined with 0.4% TP had the best quality, the shelf life being extended from 4-5 d to 12-13 d. Controlled freezing-point storage
Controlled freezing-point storage (CFS) refers to the technique of keeping fresh at the temperature between 0 ℃ and the freezing point of the body. The temperature is 0--2.0 ℃. CFS is suitable for aquatic products with strong cold endurance and higher maturity. CFS can maintain the living characteristics of aquatic products as it keeps the original flavor and nutrition which is better than ice storage. Compared with superchilling and freezing storage, CFS consumes less energy and avoids protein denaturation and moisture loss by freezing. However, extremely small temperature fluctuations will cause recrystallization which can forms larger and uneven ice crystals, affecting the quality of aquatic products. Mi［8］found that compound (5% NaCl + 5% CaCl2 + 2.5% sorbitol) could effectively depress the freezing point and retard bacterial growth in the shrimps. An additional 2 d of shelf life at (-2.0±0.2) ℃ was achieved in shrimps when the freezing-point was depressed from -1.3 to -2.5 ℃. There was little difference in microstructure between fresh and freezing-point depressed shrimps after storage at (-2.0±0.2) ℃. Significant high water holding capacity, salt-sloution protein and Ca2+-ATP activity were obtained in freezing-point depressed shrimps. CFS can be combined with other preservation technologies. Gong［9］found that the freezing point of fresh grass carp slices decreased to -3.52 ℃ when added with 3% salt and 0.6% sucrose. The optimum treatment was immersing the slices in 3% citric acid solution for 10 min. 70% CO2 and 30% N2 package could control microbial growth and extend shelf life to 40 d.
Superchilling storage
Superchilling is a mild or partially freezing method which keeps the temperature slightly below the freezing point of fish (about -3 ℃). Superchilling temperature of aquatic products is slightly different because of fish species and superchilling methods, the temperature is -2--3 ℃. The temperature of superchilling is lower than ice storage and CFS storage, which overcomes the deficiency of metabolic aging of tissues by high- temperature storage. Gao et al.［10］studied the fresh-keeping effect of hairtail under the conditions of ice storage (4 ℃), CFS (-0.6 ℃) and superchilling (-3 ℃). The results showed that the shelf-life was 5 d at 4 ℃, was extended by 2 d at -0.6 ℃, reaching 7 d, and was up to 18 d at -3 ℃, showing a 3.6-fold increase compared with that at 4 ℃. Compared with freezing, superchilling avoids destroying cell structure by low temperature freezing, but the shelf life is shorter. Liu［11］observed the large yellow croaker which had be stored for 30 d at low temperature. Follow-up experiments revealed that the higher the freezing rate of fish (the freezing rates of -3, -6 and -20 ℃ were 30%, 65% and 90%, respectively) and the larger the area of ice crystals formed, the more damaged fish tissues were. The ice crystals of superchilling were fine and uniform, and the tissue damage was lower. The combination of superchilling and biological fresh- keeping agent can achieve better preservation effect by using hurdle factor. The results showed that tea polyphenols could effectively inhibit bacterial growth, reduce fat oxidation and prolong the shelf life of silver carp during superchilling storage. Freezing storage
Freezing storage is a fresh-keeping method of reducing the central temperature of fish to -18 ℃ and storing it at this temperature. It is suitable for long-term storage of aquatic products. According to the speed of freezing, it can be divided into slow freezing and quick-frozen. Quick-frozen passes through the maximal ice crystal production zone at a faster speed. The ice crystals are small, more numerous and evenly distributed, which is no obvious damage to tissues. However, the cost is high, so it is mainly used for precious aquatic products. The ice crystals are large, irregular and scattered by slow freezing. The drip loss is large during thawing, which leads to the decline of edible quality. Moisture loss, lipid oxidation and protein freeze denaturation will occur in aquatic products during freezing storage. Adding antifreeze agent can prevent protein freeze denaturation. Ma［13］found that the concentrations of trehalose, polydextrose and sodium lactate were 4%, 6%, and 5%, respectively as antifreeze agent. At -18 ℃, the quality changes of crisp grass carp by cryoprotectants combined with immersion chilling and freezing(ICF) group were studied and analyzed in the frozen storage for 6 months. Results showed that the content of myofibrillar protein, the Ca2+-ATPase activity, the total sulfhydryl, the water-holding power, antioxidant activity and texture of cryoprotectants plus ICF group were better than other groups. Cryoprotectants combined with ICF were benefit to maintain the stability of protein secondary during frozen storage. The synergistic effect of low temperature storage and biological preservative is better. Li［14］researched the effects of complex biological fresh-keeping agents on tuna at different freezing temperatures. Assured that compound biological preservative best portioning is tea polyphenol 6.55 g/L, chitosan 16.05 g/L and nisin 0.46 g/L. Compound biological preservative at -18 and -25 ℃ can both lengthen one months preservation length.
Cryogenic vitrification storage
The glass state refers to when the temperature of the macromolecular substance satisfies the condition of glass transition, the macromolecular materials do not have enough energy to cross the energy barrier of internal rotation, nor space for free activities. So the moveability is greatly reduced to special constructs of vitrification. For the glassy state storage of aquatic products, at first, it is necessary to accurately measure the glass transition temperature (Tg′) of different aquatic products. Tg′ has great differences in different aquatic products. When Tg′ is lower than -20 to -30 ℃, the thermodynamic phase diagram of system should be changed by no side effects additives which can improve Tg′. The water content of aquatic products has a great impact on Tg′. When the water content increases 1%, the Tg′ decreases 5-10 ℃ ［15］. Secondly, it is necessary to study the quantitative relationship between AT and various biochemical reaction rates, microbial reproduction, and enzyme activity during the glassy state storage. Super chilled storage
Super chilled storage refers to directly immersing fresh fish in brine at -10 ℃, and rapidly cooling the surface of the fish within 10 to 30 min, and then moving the fish to cryogenic cooling water (0--1 ℃) for storage. The slaughter and initial quick-frozen achieve simultaneously, which can hold within limits the biochemical changes after the fish died, and maintaining the original freshness and quality of the fish. The initial cooling of aquatic products is sufficient by super chilled storage, no ice is needed in circulation, and the shelf life is twice as long as ice storage. Unfortunately, there must be a special transport vehicle, and salt water freezing also directly impacts on the salinity of fish.
Suolian WU et al. Research Progress of Low Temperature Preservation Technology for Aquatic Products
Ice Crystal Formation
During the freezing process, the formation and content of ice crystals are closely related to the freezing temperature, the stability of temperature and the material temperature before freezing storage. Because of the rapid formation of ice crystals, the amount of heat increases dramatically which needs to be removed. However, the ice film of material surface hinders the heat transfer. The researchers speculate that the result of contradiction forms a large number of tiny ice crystals which leads to cell rupture, changes the cellular composition, and affects the shelf life and commodity value of the product［16］. The formation of ice crystals can be divided into nucleation and crystal growth.
Nucleation
The movement of water molecules slows down during temperature fall period. Under the gravity of directional alignment, the internal structure of water molecules gradually tends to form stable polymers which are similar to crystals. Temperature continues to lower. When stable nuclei appear, the polymers are converted to ice crystals. Latent heat is released during this process which promotes the temperature back to freezing point. The relationship between ice crystal formation and cell damage is the key to improve the quality of aquatic products. It has been the researchersdebating topic all the time about which one happens first between the formation of intracellular ice crystals and cell damage. Mazu［17］and Toner et al.［18］considered intracellular ice crystal formation is the main cause of cell damage. Intracellular ice crystal formation converts intracellular liquids into solids. The intracellular volume expansion by changes of chemical potential energy causes cell damage. Muldrew et al.［19］believe that cell damage precedes intracellular crystallization. Osmotic pressure of the extracellular solution is increased in the presence of ice. The osmotically driven water efflux that occurs in cells during freezing is viewed as the agent responsible for producing a rupture of the plasma membrane, thus allowing extracellular ice to propagate into the cytoplasm. Crystal growth
Crystal growth refers to the process that the water or vapor around ice crystal moves toward ice crystals, adheres and freezes on the ice crystal. When the storage temperature is high or the temperature fluctuation is large, the recrystallization will occur with the tiny ice crystal as the nucleus. When the frozen storage time is too long, recrystallization will occur even at constant temperature. The water molecules on the surface of small ice crystals cannot be tightly bound because of the higher surface free energy. Therefore, these water molecules tend to diffuse from the surface of small ice crystals, and deposit on the surface of large ice crystals. As a result, the larger ice crystals grow and the smaller ice crystals disappear. During the growth of ice crystals, the formation of solid ice crystals enlarges the specific surface area of muscle cells which can injure cells. Then a series of biochemical reactions will occur in tissues. It can be better delay the deterioration of the quality of cryogenic aquatic products by inhibiting the formation of nuclei, increasing freezing rate, or avoiding recrystallization of ice crystals during freezing storage.
Conclusion
Preservation by low temperature can satisfy with the requirements of commercial large-scale fresh-keeping, but the single method of low temperature has limitations. CFS has a small scope of application, superchilling is difficult to control temperature and requires high equipment. Freezing storage can preserve aquatic products for a long time, however, the ice crystals are easy to damage the tissue and aggravate the denaturation of proteins. The drip loss is large after thawing, which makes the meat taste worse. We need dig more deeply into the formation mechanism of ice crystals at the molecular level. At the same time, we must focus on the control of ice crystal formation during the preservation of aquatic products by combining new energy with freezing. It is the main development trend of fresh-keeping technology in the future by continuous improving and developing for fresh-keeping equipment, and integrating new packaging and cold sterilization technology to prolong the shelf life.
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