Aquatic products are important food species. The world consumes over 2 billion tons of natural seafood and cultured seafood every year. With the increasing consumption of aquatic products, the demand of aquatic products quality in the consumer market is becoming higher and higher. Especially the aquatic products with better freshness are attractive and the price is good. Freshness has gradually become the most important index to evaluate the quality of aquatic products. However, if effective fresh-keeping measures are not taken immediately after fishing for aquatic products, corruption and deterioration will easily occur. Therefore, preservation of aquatic products is of great significance, and appropriate preservation technology can ensure the safety and good quality of aquatic products. We need make sure to adopt the appropriate and effective aquatic products seafood storage solution, as well as processing and packaging.

1. Aquatic Products Seafood Storage of Low Temperature Preservation

Corruption and deterioration of aquatic products are closely related to the activities of microorganisms and enzymes, and their activities are related to temperature. When the temperature decreases, microorganisms will stop breeding or even die, and enzymes will weaken their activities. Therefore, when aquatic products are in a low temperature environment, they can inhibit the growth of microorganisms and the role of enzymes, slow down biochemical reactions, and maintain high quality of aquatic products.

1.1 Cooling Preservation

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Cooling preservation is the most widely used and longest-standing traditional way of preservation, which is still in use today. This method is a method of keeping fresh aquatic products near freezing point without freezing them. The temperature is generally controlled at 0 – 4 C. The cooling medium is usually ice or cold sea water, so it is also called ice fresh or ice storage. However, this method has a short shelf life and is usually used on fishing boats or in short distance transportation.

Researchers studied the storage characteristics of crucian carp sashimi at different temperatures. It was found that the storage period of crucian carp sashimi was longer at 0 ℃, about 20 days; while at 4 ℃, the storage effect was the worst, the decline rate of pH value, elasticity, resilience, L* value was the largest, and the water holding capacity was the lowest, but the production of TBA was the largest. After storage for 10 days, the quality of the products showed a downward trend. After 20 days, there was a serious odor, loose fish and no resilience.

1.2 Ice-temperature Preservation

Ice-temperature preservation technology was put forward by Dr. Yamamoto in 1970s. This method is to control the storage temperature of aquatic products in the range of its ice-temperature zone (temperature range below 0℃ to tissue freezing point) to preserve them. This method keeps the unfrozen form of tissue cells in vivo, not destroying cells, but also slowing down the speed of corruption. Excellent quality of aquatic products.

Researchers studied ice-temperature preservation of Penaeus vannamei, there was no melanin deposit on the body surface and abdomen, white and bright muscle, elastic meat and no obvious odor. This was because ice-temperature treatment could better inhibit the activity of polyphenol oxidase, slow down the change of black rate, slow the change of pH value, TBC value and TVB-N value, and prolong the storage period by 1. Times. When studying the effect of ice-temperature preservation on the quality and physicochemical properties of carp meat, Researchers also pointed out that ice-temperature preservation technology and freezing point regulator can reduce the freezing point of carp meat, effectively inhibit the growth of microorganisms and the freezing denaturation of carp myofibrillar protein, and have good effects on maintaining the quality characteristics of carp meat, the structure and function of protein.

1.3 Microfreezing Preservation

Microfreezing preservation is a method of keeping aquatic products fresh at a temperature slightly lower than the freezing point of cell juice and stored at that temperature. Slow freezing can reduce the mechanical damage and cell rupture caused by ice crystals during freezing process. Cells are well preserved, and no deep thawing is needed when eating. The loss of soluble protein in cell protoplasm and juice is less, and the preservation rate of original freshness and nutrients of aquatic products is higher. In many cases, microfreezing technology is often combined with natural preservatives.

Researchers pointed out that the total number of bacteria in micro-frozen bighead carp has been increasing under micro-frozen conditions, and the preservation period is only 20-25 days. If combined with micro-frozen conditions and chitosan, the growth and reproduction of bacteria will be more effectively inhibited, TVB-N and TBARS values will also be reduced, and the decline rate of sensory quality index of bighead carp slices will be slowed down, after 10 g/L shell. The preservation period of fish fillets treated with polysaccharide solution is about 30 days. Research also shows that when silver carp was immersed in 0.1% tea phenol solution and then stored by micro-freezing technology, it could effectively inhibit bacterial life activities, slow down the rate of deterioration and greatly improve the shelf life of silver carp.

1.4 Freeze Preservation

If aquatic products are to be stored for a long time, they must be frozen. Freezing preservation is a method that the central temperature of aquatic products is lowered below – 15℃, and then preserved below – 18℃. At this time, most of the water in the tissues is frozen, which can keep aquatic products fresh for a long time. However, due to the low environmental temperature, the protein in aquatic products will deteriorate, and the phenomenon of juice loss will occur during thawing, which will affect the quality of aquatic products. Therefore, quick-freezing method is often used. The ice crystals formed in the frozen aquatic products are fine and uniform, the degree of protein denaturation is small, and the loss of juice during thawing has also been significantly improved.

Research showed that quick-freeze fish meat from -18℃ to -80℃, could effectively delay the decline of pH value of fish meat and inhibit the deterioration of meat. The lower the temperature, the slower the deterioration, the natural water loss rate and pressure water loss rate were 8.22% and 15.40%, respectively. The texture fluctuations of hardness, elasticity, chewiness and restorability were small, but with the prolongation of storage time, the separation of muscle fibers occurred. The phenomena of cleavage and coagulation are serious. The salt solubility of actin decreases and the water retention of muscle decreases accordingly. Researchers pointed out that, for sinonovacula,  the changes of Aw value, pH value and TVB-N value were relatively mild under freezing storage under -75℃, basically maintained at a fresh level, biochemical changes were almost stagnated, the shell was still gray, the foot was still translucent and elastic, the shell and meat were closely linked, no other odor, and no significant changes in texture characteristics. It can keep the fresh texture of sinonovacula meat.

2. Aquatic Products Seafood Storage Solution of Modified Atmosphere Packaging

Modified atmosphere packaging (MAP) is to achieve the purpose of preservation by adjusting the gas composition and proportion of aquatic products, inhibiting the respiration and growth of microorganisms in aquatic products and enzymatic reactions. Generally, inert gases (such as N2, CO2) are introduced to reduce the content of O2 and increase the content of CO2.

In ordinary package, after 3-4 days of storage, tuna began to have obvious fishy smell, and the same situation of tuna preserved by modified atmosphere packaging would delay at least 4 days, and the color of tuna treated by modified atmmosphere packaging was better than that of fish exposed to air. The change of pH value was relatively stable, and the increase of TVB-N value was slower. When researchers studied the preservation of tilapia by modified atmosphere packaging, it was found that when 20%-50% CO2 was inflated, cell respiration was inhibited, metabolism slowed down, weak acidity of the environment had obvious bacteriostatic effect. The hardness, elasticity and chewiness of the fillets were in a good state, and the more CO2 was added in this range, the better the preservation effect was. Oxymyoglobin is formed by the combination of myoglobin and proper amount of oxygen, which makes fish appear attractive red.

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3.Aquatic Products Seafood Storage Solution of Irradiation Preservation

Irradiation technology is cold treatment technology. The heat generated by the irradiation process of aquatic products is very small, which will not cause the sudden rise of internal temperature, and will not have corresponding negative impact on itself, and can maximize the maintenance of its unique flavor. Radiation radiation enters aquatic products in the form of electrotrophic waves, which directly or indirectly causes the breakage of chemical bonds in microbial DNA or RNA molecules, the change of bases in DNA sequences, the crosslinking of proteins and DNA molecules, damages the genetic material in cells, kills harmful microorganisms on the surface and inside of cells, and effectively avoids the problem of secondary pollution. At present, gamma-ray and high-speed electron radiation are mostly used.

Researchers explored the germicidal and fresh-keeping effect of fresh oysters irradiated by electron beam. The results showed that electron beam irradiation could effectively reduce the total number of colonies in fresh oysters. After 2.22kGy electron beam treatment, the total number of colonies decreased by 90%. Even before decay, the total number of colonies was in a very low level. TVB-N value increased with the prolongation of storage time, but with the irradiation. The increase of irradiation dose slowed down, so electron beam treatment could slow down the increase of TVB-N value of oysters during storage.

Scientists irradiated mussel with different doses of electron beam and gamma ray. The results showed that the contents of crude protein, crude fat, ash, water, unsaturated fatty acid and monounsaturated fatty acid in the mussel did not change significantly after irradiation, and the total number of colonies in the mussel decreased significantly. The content of amino acid gradually decreased with the increase of irradiation dose, but the proportion of flavor amino acid after electron beam irradiation was higher than that after gamma ray irradiation. Moreover, the preservation rate of polyunsaturated fatty acids is higher, and low dose electron beam irradiation can better maintain the pH value of the mussel, which shows that the mussel tastes more delicious after electron beam irradiation.

4. Preservation by High Voltage Electrostatic Field

High-voltage electrostatic field (HVEF) treatment technology is to place aquatic products between two electrodes and form a high-voltage electrostatic field between the two electrodes after power supply is switched on, which makes the microbial cell membrane in aquatic products unhealable perforation, endoplasmic loss, and ultimately leading to cell death. The aquatic products treated by this technology have little temperature rise, no change in their inherent properties, short sterilization time and remarkable effect, so it is considered as a potential cold sterilization and fresh-keeping technology.

Researchers treated tilapia fillets in high-voltage electrostatic field. It was found that high-voltage electrostatic can significantly inhibit the proliferation of microorganisms, slow down the corruption and bad odor of tilapia fillets, slow down the decline of muscle elasticity, slightly inhibit the breakage of muscle fibers, and improve the loss of fish juice. High voltage electrostatic treatment can delay the oxidation of fat and inhibit the denaturation of protein to a certain extent, which has a good effect on the quality of tilapia fillets.

5. Fence Technology

Fence technology is a scientific and reasonable combination of various preservation technologies in the seafood storage solution, through the synergy or interaction of different fence factors temporarily and permanently break the internal balance of microorganisms, thus inhibiting the growth and reproduction of microorganisms, preventing the occurrence of adverse changes, and making the preservation effect more lasting. The commonly used barrier factors are temperature, water activity, pH value, packaging, irradiation, competitive bacteria and natural preservatives (such as lysozyme, Streptococcus lactis, tea polyphenols, etc.).

When researchers try to determining the optimal fence combination of ready-to-eat abalone by single factor test and neural network test, it was pointed out that the pH value of abalone was adjusted to 5.86 by 0.15% citric acid after curing, and the Aw value was 0.918 after drying at 55 and 60 ~C for 1 h, while 0.39 g/kg Nisin and 1.6% sodium lactate were added to the film solution respectively, and sterilized at 89 ~C for 40 minutes. This combination of fences has better bacteriostasis effect, and the flavor and taste of the abalone products are not affected by any factors. The abalone products have golden color, compact and elastic meat quality, and have a unique fragrance. Similarly, researchers also used 0.2% citric acid and 0.054% lysozyme and 0.060% Nisin to preserve sea cucumbers. After sterilizing at 90 C for 30 minutes and storing at – 18 C for 90 days, the nutritional quality and edible quality of instant sea cucumbers lost little, the sensory changes were not great, and the same fresh taste was full.

 Conclusion

During the storage period of aquatic foods, there are many factors affecting their quality, and the factors are often mixed. The single fresh-keeping technology is often not perfect enough. Therefore, modern new fresh-keeping technology often requires several fresh-keeping technologies to be used simultaneously to improve the fresh-keeping effect. If we combine low temperature, MAP modified atmosphere packaging, antiseptic, ozone, ultra-high pressure and irradiation to preserve aquatic products, we can give full play to the advantages of various preservation technologies, improve the quality of aquatic products, extend their shelf life and maximize the economic value of aquatic resources.