Gradient quick-freezing process method suitable for aquatic products

Abstract

The invention discloses a gradient quick-freezing process method suitable for aquatic products. The method sequentially comprises the following steps: immersing a to-be-frozen aquatic product into brine ice, and circularly pre-cooling until the center temperature is lower than 4 DEG C; draining and blow-drying until no visible free water exists on the surface; liquid nitrogen spraying is carried out, the surface temperature is monitored, and when the surface temperature is lower than-35 DEG C and is continuously stable for at least 3 seconds, spraying is stopped to form a surface freezing layer; the temperature is buffered and equalized under the condition of constant wind speed at-30 DEG C; two-stage main freezing is carried out at the temperature of-38 DEG C, the temperature from the first stage to the center temperature ranges from-8 DEG C to-12 DEG C, and in the second stage, the air speed is increased till the center temperature is lower than-18 DEG C; and then cryogenic stabilization is carried out in an environment of-42 DEG C to-48 DEG C until the central temperature is lower than-20 DEG C. According to the method, the temperature state variable is used as a stage switching control condition, the process controllability is high, and the method is suitable for quick-freezing processing of fillets and whole shrimps of different specifications.

Description

Technical Field

[0001] This invention relates to the field of aquatic product processing and freezing preservation technology, specifically a gradient quick-freezing process method suitable for aquatic products. Background Technology

[0002] Aquatic products (such as fish fillets and whole shrimp) are prone to quality deterioration during freezing, mainly manifested as increased juice loss, tissue structure damage, and decreased texture after thawing. The reasons are usually related to the formation and growth of ice crystals during freezing, especially when the product stays in the maximum ice crystal formation zone (usually near freezing point to several sub-zero temperatures) for a long time, the ice crystals are prone to grow and cause mechanical damage to the cell structure.

[0003] In existing processes, common methods include: 1) Single high-speed wind-cooled quick-freezing: The equipment is highly versatile, but when there is a large temperature difference between the surface and the center of the product and the freezing front is uneven, it can easily lead to local stress concentration, tissue damage and batch fluctuations. 2) Direct cryogenic freezing with liquid nitrogen throughout the entire process: High cooling intensity, but high energy consumption and operating costs, and is more sensitive to different specifications of loading and surface conditions; 3) Multi-stage complex enhanced nucleation / auxiliary field rapid freezing (such as ultrasonic nucleation): may improve ice crystal distribution, but the system is complex, and the investment and maintenance costs are high, which is not conducive to the upgrading of small and medium-sized processing lines.

[0004] Therefore, there is an urgent need for a quick-freezing method for aquatic products that can ensure freezing quality while taking into account process stability and cost controllability, especially suitable for common categories such as fish fillets and whole shrimp. Summary of the Invention

[0005] The purpose of this invention is to provide a gradient quick-freezing process for aquatic products. This process combines pre-cooling nucleation preparation, rapid surface freezing with liquid nitrogen, and staged main freezing / deep cryogenic stabilization. It also uses temperature state variables to control the stage switching, thereby improving the consistency and controllability of the freezing process and reducing the impact of batch differences.

[0006] The technical solution adopted by this invention is as follows: To achieve the above objectives, this invention adopts the following technical solution: S1. Enhanced precooling and nucleation preparation: Before entering the quick-freezing stage, aquatic products awaiting freezing require thorough pre-cooling. This stage employs an ice-salt water immersion system with a temperature controlled between -2°C and -4°C, and a saline concentration of approximately 3%. The products are completely immersed in the ice-salt water, and a circulating pump drives the saline water flow, creating forced convection heat transfer on the product surface. This convection not only rapidly lowers the overall temperature of the product, but more importantly, within the temperature range near freezing point but not yet largely frozen, the turbulent disturbance and temperature fluctuations of the saline water induce numerous tiny nucleation sites on the surface and near-surface of the product tissue. Although these nucleation sites are not as uniform and dense as those generated by ultrasonic cavitation, they still effectively limit the growth space of individual ice crystals during subsequent rapid cooling, thus laying the foundation for the formation of fine ice crystals.

[0007] The termination criterion for the pre-cooling stage should not be based on a fixed time, but rather on the core temperature of the product. For thin products such as shrimp, soaking can be stopped when the core temperature drops below 4°C, typically for 3 to 5 minutes. For thicker products such as salmon and tuna chunks, the soaking time should be extended to 8 to 12 minutes to ensure the core temperature also drops below 4°C. This temperature setting is based on the fact that 4°C is the temperature point near the upper limit of the maximum ice crystal formation zone for aquatic products. At this temperature, the product's interior has not yet begun large-scale freezing, but the tissue has been sufficiently cooled and a preliminary nucleation base has been established, creating favorable conditions for rapid traversal of the maximum ice crystal formation zone.

[0008] The mechanism of ice-salt precooling includes three aspects: First, the addition of salt lowers the freezing point of the solution, allowing it to remain liquid at sub-zero temperatures, thus achieving a stronger cooling capacity than ordinary ice water; second, the turbulent disturbances generated by forced circulation can break the temperature boundary layer in the static liquid, significantly improving heat exchange efficiency and causing the product temperature to drop rapidly and uniformly; third, in the temperature range close to the freezing point, the small disturbances and local supercooling phenomena in the solution induce the formation of preliminary ice crystal nucleation points on the surface of the product.

[0009] S2. Surface treatment: After soaking, the product undergoes a simple drip-drying process. This step uses a vibrating conveyor belt in conjunction with a low-speed airflow drying device (approximately 1 m / s) to remove the adhering water film from the product surface within about 30 seconds, achieving a stable state free of visible water droplets and liquid residue. The removal of free surface water is crucial for the subsequent liquid nitrogen spraying stage: if water droplets or a water film remain on the surface, they will instantly form a locally thick and hard frozen shell under the ultra-low temperature of liquid nitrogen. This uneven frozen shell will generate thermal stress concentration due to the temperature difference between the inside and outside, leading to quality problems such as microcracks, tissue tearing, or increased juice loss after thawing. Therefore, although drip-drying preparation is only a brief auxiliary step, it is a key link in ensuring the consistency of subsequent freezing quality.

[0010] S3. Rapid freezing of the surface layer by liquid nitrogen spraying: After pre-cooling and drip-drying, the products then enter the liquid nitrogen spraying section. This section uses liquid nitrogen spray pipes arranged above and to the sides of the conveyor belt to provide short-term forced heat exchange to the products. Liquid nitrogen has a boiling point of -196℃ at atmospheric pressure. When it is sprayed onto the product surface in droplet form, it rapidly absorbs heat from the product surface and vaporizes, generating a very strong convective heat transfer effect. Within 8 to 15 seconds of spraying, the surface temperature of the product can drop sharply from about 4℃ to below -35℃, and the tissue within 2 to 3 millimeters of the surface layer freezes almost simultaneously, forming a uniform, rapidly frozen layer. This frozen layer serves multiple purposes: First, it can quickly lock in the moisture on the product surface, preventing moisture migration from the interior to the surface during subsequent freezing, thus preventing dehydration and weight loss; second, it provides a stable boundary condition for internal freezing, allowing the freezing front to advance from the surface to the center under a more consistent outer constraint, thereby improving the overall uniformity of freezing; third, the rapidly formed surface freezing can fix the tissue structure as early as possible, reducing the mechanical damage to cell membranes caused by ice crystal growth during freezing.

[0011] The liquid nitrogen spraying process employs a state feedback mechanism rather than fixed-time control. Specifically, during spraying, the product surface temperature is monitored in real time using a non-contact infrared thermometer. When the surface temperature drops to -35°C and remains stable for 3 seconds, liquid nitrogen spraying stops and the process switches to the next step. This control method effectively addresses differences in initial temperature, thickness, and loading density between different batches of products, ensuring that each batch has the same surface freezing state before entering the main freezing section, thus significantly reducing batch-to-batch quality fluctuations. Compared to the original micro-mist spraying solution, the equipment cost of ordinary liquid nitrogen spraying is only about 30% of the former, and maintenance is simpler. Furthermore, provided the surface is fully dripped dry, its freezing effect is not significantly different from micro-mist spraying, with only a slight loss of uniformity on extremely thin or irregularly shaped products.

[0012] S4, Buffer Temperature Equalization: After liquid nitrogen spraying, the product does not immediately enter the main freezing section, but first passes through a brief buffer zone. This buffer zone is set at approximately -35°C, with a low wind speed of 1.5 m / s, where the product remains for about 30 seconds. The purpose of this buffering step is to mitigate the extreme temperature difference caused by the liquid nitrogen spraying: immediately after the spraying, the product's surface temperature is extremely low (below -35°C), while the internal temperature remains between -5°C and 0°C. If it immediately enters the strong convection main freezing environment of 10 to 12 m / s, a steeper temperature gradient will form between the surface and near-surface layers, leading to a sudden increase in thermal stress and thus increasing the risk of cracking and structural damage. By briefly stopping in the buffer zone with low wind speed and moderate temperature, the surface temperature can slightly recover, and the near-surface temperature can further decrease, mitigating the temperature difference and achieving stress release and initial temperature field homogenization before entering the main freezing section. Furthermore, the nitrogen mist generated during the liquid nitrogen spraying process is gradually dispersed within the buffer zone, avoiding interference with the uniformity of the wind field in the main freezing section.

[0013] The heat exchange mechanism of liquid nitrogen spraying is based on the rapid vaporization and heat absorption of liquid nitrogen. The latent heat of vaporization of liquid nitrogen is approximately 200 kJ / kg, far exceeding that of ordinary refrigerants, thus enabling it to remove a large amount of heat from the product surface in a very short time. When liquid nitrogen droplets come into contact with the high-temperature product surface, they instantly boil and vaporize, forming a nitrogen vapor layer. This vapor layer continuously absorbs heat from the surface during its renewal process, causing the surface temperature to drop rapidly at a rate exceeding 10°C per second. This ultra-rapid cooling allows the surface structure to traverse the maximum ice crystal formation zone (-1°C to -5°C) in less than one second; ice crystals are solidified almost before they have time to grow, resulting in a fine and uniform ice crystal structure. The buffer zone's function is based on the principles of heat conduction and thermal stress release: when the surface temperature is extremely low and the internal temperature is high, a large temperature gradient exists between the two. This gradient causes tensile or compressive stress within the material, especially at the interface between the frozen and unfrozen layers. By staying in the buffer zone, the surface can absorb a small amount of heat from the surrounding environment and warm up slightly, while the internal cold energy is also transferred to the surface, making the temperature gradient gentler and the stress partially released, thereby reducing the risk of cracking during subsequent strong convective heat transfer.

[0014] S5, Two-stage main freezing: After buffering, the product enters the main freezing section, which is the longest and most impactful part of the entire quick-freezing process. The main freezing section uses a -40°C low-temperature environment, with variable frequency fans controlling the airflow speed in stages. The process consists of two main stages: the first stage is the pre-freezing phase, where the airflow speed is set at a moderate level of 7-8 m / s, lasting approximately 3 minutes. The purpose of this stage is to steadily advance the freezing front inwards while avoiding secondary impact on the product that has just completed liquid nitrogen surface freezing. Since a frozen layer has already formed on the surface, heat exchange mainly occurs at the interface between the frozen and unfrozen layers. A moderate airflow speed ensures sufficient heat exchange intensity without causing new stress concentration on the surface due to excessive convection.

[0015] When the product's core temperature drops to approximately -10°C, it enters the mid-stage of primary freezing. At this point, the product has largely traversed the zone of maximum ice crystal formation, and most of its internal structure is frozen, although some areas in the central region remain partially frozen. During this stage, the wind speed increases to a high level of 10 to 12 m / s to accelerate the advance of the freezing front towards the center. The strong convective heat transfer brought about by the high wind speed significantly shortens the product's residence time in the -10°C to -18°C range, which is crucial for inhibiting further ice crystal growth and reducing freezing damage. Core temperature can be monitored through random sampling using an insertion temperature probe or estimated using a heat transfer model combined with surface temperature and known product specifications. When the core temperature reaches below -18°C, the primary freezing stage is essentially complete, and the entire interior of the product is frozen.

[0016] The basis for using staged wind speed control in the main freezing section lies in the changes in heat transfer characteristics during the freezing process. In the early stage of freezing, a frozen layer has formed on the product surface. The thermal conductivity of the frozen layer is about four times higher than that of the unfrozen tissue. Therefore, the advancement speed of the freezing front is mainly controlled by the thermal resistance of the unfrozen layer and the release rate of latent heat of phase change. At this time, a medium wind speed can ensure the steady advancement of the freezing front while avoiding stress caused by excessive external heat exchange leading to a large temperature difference between the surface and the interior. When the freezing front approaches the center and most of the tissue has frozen, the thickness of the unfrozen layer is very small, and the thermal resistance decreases. At this time, increasing the wind speed can significantly accelerate the freezing of the central area and shorten the residence time of the product in the critical temperature zone.

[0017] S6, Cryogenic Stability: After primary freezing, the product enters the cryogenic stabilization phase. This phase is set at -45°C, with wind speed reduced to a moderate level of 6-7 m / s. The goal of cryogenic stabilization is not only to further lower the product's core temperature to below -20°C, but more importantly, to maintain this lower temperature for a certain period (usually 5-10 minutes) to homogenize the internal temperature field and solidify the ice crystal structure. If aquatic products are immediately transferred to a conventional -18°C cold storage after freezing, ice crystal recrystallization may still occur during storage due to temperature fluctuations. This means that small ice crystals gradually disappear, crystal boundaries migrate, and ice crystal size increases, leading to textural degradation and increased juice loss. Cryogenic stabilization lowers the product's core temperature to -20°C or even lower, and maintains this temperature for a period of time, significantly reducing the thermal motion of molecules and inhibiting ice crystal boundary migration and recrystallization. This allows the product to maintain a more stable microstructure and better quality during subsequent long-term storage.

[0018] The mechanism of the cryogenic stabilization stage is based on solid-state diffusion kinetics: the recrystallization process of ice crystals is driven by grain boundary migration, and the migration rate is exponentially related to temperature. At -18°C, molecules still possess some thermal mobility, causing ice crystal boundaries to migrate slowly, leading to the gradual disappearance of small crystals and the continued growth of larger crystals. However, at -20°C or even lower temperatures, molecular thermal motion is significantly suppressed, and the grain boundary migration rate decreases by more than an order of magnitude, effectively delaying the recrystallization process. This allows the product to maintain a fine ice crystal structure and good textural quality during long-term storage.

[0019] Once cryogenic stabilization is complete, the product should proceed to the cryogenic packaging process as soon as possible. The packaging environment temperature should ideally be controlled between -20°C and -25°C, and the exposure time from exiting cryogenic stabilization to vacuum sealing should be strictly controlled within 3 minutes. This is because the product surface rapidly absorbs heat from the environment after leaving the cryogenic environment, causing the surface temperature to rise, which can lead to surface condensation, increased desiccation, and a higher risk of microbial growth. Rapid packaging and vacuum sealing maximize the preservation of the product's low-temperature state and surface quality, ensuring the overall effectiveness of the quick-freezing process continues into storage and transportation.

[0020] Compared with the prior art, the beneficial effects of the present invention are: 1. Pre-cooling with ice-salt water circulation ensures uniform cooling of the product before it enters the critical freezing stage, thus establishing an initial nucleation basis; 2. By spraying liquid nitrogen, a surface freezing boundary is formed in a short time, providing stable outer boundary conditions and reducing the migration of internal moisture to the surface and the desiccation of the outer layer; 3. Reduce the instantaneous temperature difference and thermal stress mutation caused by rapid cooling of liquid nitrogen through buffering and temperature equalization; 4. Through two-stage wind speed primary freezing, the freezing front advances from the surface to the center in a gradient manner, and shortens the residence time in the key temperature zone; 5. Deep cryogenic stabilization reduces the risk of recrystallization caused by subsequent storage temperature fluctuations; 6. Temperature state variable control is used instead of fixed time control to improve adaptability and consistency to changes in initial temperature, specification thickness, and loading density of different batches.

[0021] The technical solution of the present invention will be further described in detail below through embodiments. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present application, the technical solutions in specific embodiments of the present application will be clearly and completely described below. Unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by those skilled in the art.

[0023] Example 1 This embodiment uses whiteleg shrimp as the processing target, with a shrimp size of 50-70 shrimp per kilogram, and an individual shrimp weight of approximately 15-20 grams. Before processing, ensure that the raw materials are in good freshness and that the shrimp are intact and undamaged.

[0024] During the pre-cooling stage, a 3% salt solution at -3℃ is prepared and circulated using a circulating pump at a flow rate of 2-3 m / s to create a forced convection heat transfer environment. Whole shrimp are completely immersed in the circulating salt solution for approximately 4 minutes, with the core temperature checked every minute during this period. Pre-cooling ends when the core temperature drops to 3-4℃. In practice, it was found that the circulation flow rate significantly affects pre-cooling efficiency. When the flow rate is below 1.5 m / s, the pre-cooling time needs to be extended to more than 6 minutes; while a flow rate above 3.5 m / s, although accelerating pre-cooling, can cause slight mechanical damage to the shrimp surface. Therefore, a flow rate range of 2-3 m / s is preferred. After pre-cooling, the internal temperature distribution of the shrimp is relatively uniform, with the temperature difference between the surface and the core controlled within 2℃.

[0025] Once pre-cooled, the whole shrimp are immediately transferred to the draining and drying process. A stainless steel mesh belt with a vibration frequency of 25-30 Hz is used for draining, with a vibration duration of approximately 10 seconds, effectively removing moisture from the surface and folds of the shrimp. Subsequently, surface drying is carried out in an airflow environment at approximately 10℃ and a wind speed of 1 m / s, with the drying time controlled at 40-50 seconds. The drying criteria are that there are no obvious water droplets on the shrimp surface and it feels dry to the touch.

[0026] Whole shrimp, after surface preparation, enter the liquid nitrogen spraying section. The spraying system uses a multi-nozzle arrangement, with three spray nozzles above and on each side of the conveyor belt. The liquid nitrogen flow rate is controlled at 15-20 liters / minute. During spraying, a non-contact infrared thermometer monitors the surface temperature of the shrimp's back in real time, with the measurement point selected at the thickest part of the shrimp. After the liquid nitrogen spraying begins, the surface temperature drops rapidly at a rate of approximately 8-10°C per second, reaching -35°C after about 10 seconds. The system is set to automatically stop liquid nitrogen spraying when the surface temperature reaches below -35°C and remains stable for more than 3 seconds. At this point, a uniform frozen layer approximately 2.5 mm thick is formed on the shrimp's surface. This frozen layer is translucent, hard, and effectively locks in the shrimp meat structure. The liquid nitrogen consumption is approximately 0.6-0.8 times the shrimp's weight, saving about 70% of liquid nitrogen compared to full liquid nitrogen flash freezing.

[0027] After the liquid nitrogen spraying was completed, the shrimp were immediately placed in a buffer temperature equalization zone. The temperature in this zone was controlled at -35℃, and temperature uniformity was maintained by a low-speed fan (approximately 1.5 m / s). The shrimp remained in this zone for about 30 seconds. During the buffer period, monitoring showed that the surface temperature of the shrimp gradually rose from around -36℃ to -32℃, while the temperature near the surface (3-5 mm from the surface) dropped from around -8℃ to -15℃. The temperature difference between the surface and near-surface layers decreased from an initial 28℃ to approximately 17℃, effectively mitigating the sharp temperature gradient.

[0028] After buffering, the shrimp are transferred to the main freezing tunnel. The temperature inside the tunnel is stably controlled at -40℃, using a variable-frequency driven axial flow fan for forced convection heat transfer. The main freezing process involves two stages with finely controlled wind speed: the first stage sets the wind speed at 7.5 m / s for approximately 2.5 minutes, during which the shrimp's core temperature drops from approximately -2℃ to -10℃, with the freezing front steadily advancing from the surface towards the center. By pre-implanting a micro-temperature probe in the shrimp's center for real-time monitoring, the system automatically switches to the second stage when the core temperature enters the -8℃ to -12℃ range. In the second stage, the wind speed is increased to 11 m / s for approximately 1.5 minutes, causing the shrimp's core temperature to rapidly drop below -18℃. This two-stage wind speed control shortens the shrimp's residence time in the maximum ice crystal formation zone (-1℃ to -5℃) to approximately 40 seconds, significantly inhibiting the formation of large ice crystals. After thawing and slicing, it was found that the ice crystals in the shrimp meat tissue were mostly in the range of 30-50 micrometers in diameter, which is much smaller than that of traditional single-wind-frozen products (the ice crystal diameter is usually 80-120 micrometers).

[0029] Once the primary freezing is complete, the shrimp enters the cryogenic stabilization phase. The temperature in this phase is set at -45°C, and the wind speed is reduced to 6.5 m / s. The shrimp remains in this phase for 7 minutes. During cryogenic stabilization, the core temperature of the shrimp further decreases from -18°C to -21°C, the overall temperature field becomes more uniform, and the temperature difference between the core and surface shrinks to less than 3°C.

[0030] After cryogenic stabilization, the shrimp are vacuum-packed in a -22°C packaging room. The entire exposure time, from the shrimp leaving the cryogenic tunnel to the completion of vacuum sealing, is strictly controlled to within 2.5 minutes. The packaging uses a 7-layer co-extruded barrier film, achieving a vacuum level of -0.09 MPa or higher. The packaged product is immediately transferred to a -25°C cold storage. After 3 months of refrigeration, random sampling showed that the thawed juice loss rate of the shrimp was 3.2%, and texture analysis showed a hardness retention rate of 92%, far superior to traditional quick-frozen products (which typically have a juice loss rate of 6-8% and a hardness retention rate of approximately 75-80%).

[0031] Example 2 This embodiment uses imported Norwegian salmon as raw material, cut into standard fillets with a thickness of 12-15 mm and a weight of approximately 150-200 grams. The fillets are immediately flash-frozen after cutting to ensure the raw material temperature is maintained at 2-4°C. The pre-cooling stage uses an ice-salt solution at -3°C with a salt concentration of 3.5%. Considering the relatively thick fillets, the pre-cooling time is set to 6 minutes, and the ice-salt solution circulation rate is controlled at 2.5 m / s. During pre-cooling, the center temperature is checked every 2 minutes. Observations show that the center temperature of the fillet drops rapidly to 8°C within the first 3 minutes, then the rate of decrease slows down, stabilizing at around 3.5°C by the 6th minute. Comparative experiments show that while shortening the pre-cooling time to 4 minutes can improve production efficiency, the center temperature of the fillet only drops to around 7°C, resulting in uneven freezing front progression during subsequent freezing and significant fluctuations in the final product quality. Therefore, for fillets with a thickness of 12-15 mm, a pre-cooling time of 5-7 minutes is recommended to ensure the center temperature drops below 4°C. The draining and drying process for fish fillets is basically the same as for whole shrimp. However, due to the larger and flatter surface area of ​​the fish fillets, the drying time can be appropriately extended to 50-60 seconds to ensure that both sides and edges of the fillets are free of visible moisture. During the drying process, care must be taken not to use excessively high wind speeds, otherwise it will cause slight oxidation and discoloration of the fish fillet surface, affecting the product's appearance. The liquid nitrogen spraying section has optimized spraying parameters for the flat shape of the fish fillets. The spraying system has four nozzles on each side of the conveyor belt to ensure that both sides of the fish fillets are sprayed with liquid nitrogen simultaneously. The liquid nitrogen flow rate is controlled at 25-30 liters / minute, and the spraying time is approximately 13 seconds. Because the fish fillets are thicker than shrimp, the surface temperature decreases slightly more slowly, requiring approximately 13 seconds to lower the surface temperature to below -35°C and maintain stability. At this point, a frozen layer of approximately 2-3 mm thickness forms on the surface of the fish fillet. Sectioning observation shows that the internal tissue structure of the frozen layer is intact, the muscle fibers are arranged in an orderly manner, and no obvious ice crystal puncture marks are observed. During the buffering and temperature equalization phase, the fish fillets were kept in an environment of -35℃ and a wind speed of 1.5 m / s for 35 seconds. Due to the thickness of the fish fillets, the temperature difference between the surface and the interior is relatively large; therefore, extending the buffering time appropriately helps to homogenize the temperature field. Monitoring data shows that at the end of the buffering phase, the surface temperature of the fish fillets was approximately -30℃, the temperature 5 mm below the surface was approximately -12℃, and the center temperature was approximately -3℃, showing a gentle temperature gradient. The main freezing phase also employed a two-stage wind speed control. In the first stage, the wind speed was 7.5 m / s, lasting approximately 3.5 minutes, during which the center temperature of the fish fillets dropped from -3℃ to -10℃. In the second stage, the wind speed was increased to 11.5 m / s, lasting approximately 2 minutes, during which the center temperature dropped below -18℃. Because the texture of the fish fillets is denser than shrimp meat and has a slightly lower thermal conductivity, the main freezing time is relatively longer. To ensure freezing quality, it is recommended to appropriately extend the time in the second stage to ensure that the center temperature stably reaches below -18℃ before proceeding to the deep freezing phase. In the deep-cold stabilization phase, the fish fillets are kept in an environment of -45°C for 8 minutes, causing the core temperature to drop to -22°C.After cryogenic stabilization, the fish fillets were individually vacuum-packed in a -22℃ packaging room, with exposure time controlled to within 2 minutes. After packaging, the products were transferred to a -25℃ cold storage. After 6 months of refrigeration, random sampling showed that the fish fillets maintained good color, with no obvious signs of fat oxidation. The juice loss rate after thawing was 4.1%, and texture analysis showed a firmness retention rate of 89%. The sensory evaluation score was 8.5 out of 10, significantly better than traditional air-cooled quick-frozen products (sensory scores are typically 6-7).

[0032] Comparative Example 1 To further verify the rationality of each process parameter, this embodiment uses white shrimp as the subject and designs a series of comparative experiments.

[0033] Precooling was performed using 0℃ ice water, -2℃ ice-salt water, and -4℃ ice-salt water, with a precooling time of 4 minutes for each. Results showed that after precooling with 0℃ ice water, the core temperature only dropped to 6℃, and the subsequent freezing time was extended by approximately 30%; the precooling effect of -2℃ ice-salt water was moderate; although the precooling speed of -4℃ ice-salt water was the fastest, localized micro-freezing occurred on the shrimp surface, affecting the stability of subsequent processes. Considering all factors, -2℃ to -4℃ is the optimal precooling temperature range.

[0034] Comparative Example 2 To further verify the rationality of each process parameter, this embodiment uses white shrimp as the subject and designs a series of comparative experiments.

[0035] Two methods were used: fixed-time control (10 seconds) and temperature feedback control (surface temperature ≤ -35℃ and maintained for 3 seconds). The results showed that under fixed-time control, the surface temperature of shrimp varied greatly among different batches, with some batches only dropping to -28℃ and others dropping to -40℃, resulting in poor consistency in product quality. In contrast, temperature feedback control effectively addressed batch differences, significantly improving product quality stability.

[0036] Comparative Example 3 To further verify the rationality of each process parameter, this embodiment uses white shrimp as the subject and designs a series of comparative experiments.

[0037] Two methods were used: a single wind speed (10 m / s continuously) and a two-stage wind speed (7.5 m / s in the first stage and 11.5 m / s in the second stage). The results showed that although the single wind speed method was simpler to operate, the shell cracking rate of shrimp reached 8%, while the shell cracking rate under the two-stage wind speed control was only 1.5%, which significantly reduced the quality problems caused by thermal stress.

[0038] Comparative Example 4 To further verify the rationality of each process parameter, this embodiment uses white shrimp as the subject and designs a series of comparative experiments.

[0039] A comparison was made between quick-frozen shrimp that had undergone cryogenic stabilization and those that had not, after 6 months of refrigeration. The results showed that the texture of the unstabilized product began to decline significantly after 3 months of storage, and the firmness retention rate was only 68% after 6 months; while the firmness retention rate of the cryogenically stabilized product still reached 92% after 6 months, verifying the important role of cryogenic stabilization in maintaining quality during long-term storage.

[0040] The above embodiments and comparative examples fully verify the technical advantages and practical value of the three-stage gradient quick-freezing process of the present invention. It should be noted that the above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Those skilled in the art can make appropriate adjustments and optimizations to the process parameters without departing from the concept of the present invention, and all such changes and improvements should be included within the scope of protection of the present invention.

[0041] Finally, it should be noted that the described embodiments are merely some, not all, of the embodiments of the present invention. Those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the claims and their equivalents; that is, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A gradient quick-freezing process for aquatic products, characterized in that, Includes the following steps: S1. Immerse the fish and shrimp products to be frozen in ice-salt water with a temperature of -2℃ to -4℃ and a salt concentration of 2% to 4%, and pre-cool them under the condition of ice-salt water circulation until the core temperature of the aquatic products drops below 4℃. S2. After the aquatic products have been pre-cooled, drain and blow dry them so that there is no visible free water on the surface of the aquatic products. S3. Cool the surface-prepared aquatic products with liquid nitrogen spray and monitor the surface temperature of the aquatic products. Stop the liquid nitrogen spray when the surface temperature reaches below -35℃ to form a frozen layer on the surface of the aquatic products. S4. Place the aquatic products that have been sprayed with liquid nitrogen in an environment with a temperature of -30℃ to -40℃ and a constant wind speed for 20 to 60 seconds. S5. Place the buffered and homogenized aquatic products in a freezing environment with a temperature of -38℃ to -42℃ for primary freezing, and control the airflow in the following two stages: The wind speed in the first stage is 7 to 8 m / s, until the core temperature of the aquatic products drops to -8℃ to -12℃. The second stage involves increasing the wind speed to 10–12 m / s until the core temperature of the aquatic products drops below -18°C. S6. Place the fully frozen aquatic products in an environment with a temperature of -42℃ to -48℃ for 5 to 10 minutes, until the core temperature of the aquatic products drops below -20℃.

2. The process method according to claim 1, characterized in that, The ice-salt water described in S1 is driven by a circulating pump to form forced convection relative to the surface of the aquatic products.

3. The process method according to claim 1, characterized in that, The pre-cooling termination criterion described in S1 is that the center temperature reaches below 4°C, and the pre-cooling time is adjusted according to the thickness of the aquatic product.

4. The process method according to claim 1, characterized in that, The draining process described in S2 is carried out using a vibrating conveyor belt, and the drying and preparation are completed within 30 to 60 seconds.

5. The process method according to claim 1, characterized in that, The spraying time for liquid nitrogen spraying described in S3 is 8 to 15 seconds.

6. The process method according to claim 1, characterized in that, The surface temperature of aquatic products described in S3 is monitored using a non-contact infrared thermometer.

7. The process method according to claim 1, characterized in that, The thickness of the frozen layer formed by S3 is 2mm to 3mm.

8. The process method according to claim 1, characterized in that, The S5's wind speed control is achieved through a variable frequency fan, with the center temperature serving as the basis for stage switching.

9. The process method according to claim 1, characterized in that, The buffer temperature equalization of S4 is used to reduce the temperature difference between the surface and the interior before entering S5, so that the surface temperature rises and the near-surface temperature drops further.

10. The process method according to claim 1, characterized in that, After S6 is completed, low-temperature packaging is carried out in an environment of -20℃ to -25℃, and the exposure time from leaving the S6 environment to the completion of packaging is controlled to not exceed 3 minutes.