A fresh fish pre-treatment salt curing segmented cold air drying vacuum quick-freezing processing method

Abstract

The application discloses a kind of fresh fish pretreatment salt curing segmented cold air drying vacuum quick-freezing processing methods, it is related to food processing technical field, the method includes: after cutting into thick slice after scale removal, gill removal, dirt removal of fresh fish;With salt uniform coating fish piece and carries out short time dry curing;After curing fish piece is inserted in the card slot of multilayer frame body with cutting surface perpendicular to the direction of cold air flow, is sent into cold air drying room and carries out multi-stage variable temperature and humidity drying, initial stage is with low temperature and high humidity to prevent surface crust, then temperature is raised and humidity is reduced to accelerate dehydration, after the first stage, turn over once, again after temperature is raised and humidity is reduced to realize uniform drying, last segment is again heated low humidity dehydration to target moisture content and promote flavor generation;After drying fish piece is vacuum packaged, and is subjected to ultra-low temperature quick-freezing, so that the center temperature is rapidly reduced to frozen storage requirement.The method of the application is uniform in dehydration, and the ice crystal damage is small, the product has excellent texture and outstanding flavor after thawing, and is suitable for batch production.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, specifically to a method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish. Background Technology

[0002] In the fresh fish processing industry, processing fresh fish into dried or frozen products that can be stored for a long time is a common preservation method. Among them, the processing route that involves pre-treating the fish, dehydrating it with cold air, and then storing it using quick-freezing technology has developed rapidly in recent years.

[0003] Existing technologies already include methods for cold-air drying of fish. For example, patent publication number CN106720179A discloses a method for rapidly cold-air drying fish, the steps of which are: cleaning, removing gills, and opening the back of the raw fish, soaking it in a 10% saline solution for 20-25 minutes for salting, and then cold-air drying at -25℃ to -30℃ followed by rapid freezing. Another example is patent publication number CN103999926A, which discloses a process for industrialized cold and hot circulating air drying of fish, the steps of which are: cutting fresh freshwater fish open into a fan shape from the spine, marinating it with deodorizing ingredients for 10-12 hours, placing the cut side of the fish face up on a multi-layered drying rack, and drying it with alternating hot and cold air for 10-11 hours.

[0004] The shortcomings of the aforementioned existing technologies are as follows: CN106720179A uses a whole fish with its back split open for processing, and the salting time is only 20-25 minutes, making it difficult for the salt to fully penetrate into the fish. Furthermore, its cold air drying temperature is kept constant in the low-temperature range of -25℃ to -30℃, without addressing a segmented temperature-controlled drying method. CN103999926A also uses a whole fish with its back split open, with a marinating time of 10-12 hours, and employs alternating hot and cold air drying with a drying time of only 10-11 hours. The fish is arranged with the cut surface facing upwards, without considering the matching relationship between the orientation of the fish fillets and the direction of the cold airflow. All of the aforementioned existing technologies use whole or split fish as the processing object, without addressing the technical route of cutting the fish into thick slices for processing; regarding the arrangement of the fish fillets, none consider directional placement based on the direction of the fish meat fibers; and in the quick-freezing process, none address ultra-low temperature quick-freezing control for fish fillets with specific dryness levels. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish. This method aims to solve the technical problems of difficulty in uniform dehydration of thick-cut fish fillets during cold air drying and the damage to cell structure caused by ice crystals during quick-freezing after drying, thereby achieving efficient and uniform dehydration of thick-cut fish fillets and consistent quality for long-term storage.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for pre-treatment, salting, segmentation, cold air drying, and vacuum quick-freezing of fresh fish, the method comprising the following steps: Pre-treatment: After cleaning the fresh fish by removing the scales, gills and entrails, cut the fish into 5cm thick slices. Cutting the fish into 5cm thick slices increases the contact area for salt penetration and water evaporation, providing a structural basis for subsequent short-time marinating and uniform dehydration. Salt curing: The fish fillets are cured with salt for 4 hours. Curing for 4 hours allows the salt to fully penetrate into the exposed muscle fibers along the cut surface of the fish fillets, ensuring even distribution of salt while avoiding excessive denaturation of the protein. Cold air drying: The marinated fish fillets are placed on a multi-layer rack with their cut surfaces perpendicular to the direction of the cold air flow, so that both cut surfaces of the fish fillets face the direction of the cold air flow. They are then sent into a cold air drying room for segmented variable temperature cold air drying. The total drying time is 36 hours, and the drying endpoint is when the moisture content of the fish fillets drops to 40%. With the cut surfaces of the fish fillets perpendicular to the air flow direction, the cold air directly washes over the cross-section of the muscle fibers, minimizing the distance that moisture migrates outward along the fiber direction and improving dehydration efficiency. The segmented variable temperature control achieves gradual and uniform dehydration of the thick-cut fish fillets from the surface to the inside. The segmented variable temperature cold air drying includes the following four stages: the first stage temperature is 18℃ for 5 hours, the second stage temperature is 28℃ for 15 hours, the third stage temperature is 20℃ for 13 hours, and the fourth stage temperature is 32℃ for 3 hours. Vacuum packaging: The dried fish fillets are vacuum packaged; Quick-freezing: Vacuum-packed fish fillets are quick-frozen at -35℃. The fish fillets, which are 40% dry, have ample space inside their cells. Quick-freezing at -35℃ allows residual moisture to quickly pass through the zone of maximum ice crystal formation. The resulting fine ice crystals do not damage the cell wall structure, and the texture of the fish fillets is close to that of fresh fish after thawing.

[0007] Furthermore, in the cold air drying step, the multi-layer frame is provided with equally spaced vertical slots. The width of the slots is 5cm to 5.5cm, and the distance between adjacent slots is 10cm to 15cm. The fish fillets are vertically inserted into the slots, with both cut surfaces of the fish fillets exposed in the gap between adjacent slots and facing the direction of the cold air flow. The vertical slots keep the fish fillets upright throughout the drying process, and the gap between adjacent slots ensures that the cold air can simultaneously wash both cut surfaces of each fish fillet, achieving uniform dehydration on both sides without manual turning.

[0008] Furthermore, in the cold air drying step, the relative humidity of the cold air drying room is controlled at 65% to 70% in the first stage, 50% to 55% in the second stage, 45% to 50% in the third stage, and 35% to 40% in the fourth stage. The high humidity in the first stage prevents the surface of the fish fillets from losing water too quickly and forming a hard crust, and keeps the channels open for the migration of internal moisture to the outside. The humidity is gradually reduced in the subsequent stages so that the dehydration rate matches the decreasing trend of the moisture content of the fish fillets, and finally the moisture content inside and outside the fish fillets is consistent when the dryness reaches 40%.

[0009] Furthermore, in the cold air drying step, after the first stage, the fish fillets are flipped over once. Flipping them over after the first stage ensures that both cut surfaces of the fish fillets face the main airflow direction of the cold air in turn during the subsequent drying process, further ensuring that the dehydration rate of both sides is consistent and avoiding one side being too dry and the other too wet due to the directionality of the airflow.

[0010] Furthermore, in the salting step, the amount of salt used is 8% to 10% of the weight of the fish fillets. The 8% to 10% salt amount, combined with the 4-hour salting time and the secondary homogenization migration of salt during the subsequent segmented temperature drying process, achieves low-salt processing while ensuring the preservation effect and the perception of saltiness.

[0011] Furthermore, in the quick-freezing step, the endpoint of quick-freezing is when the center temperature of the fish fillet is below -18°C. Using the center temperature of the fish fillet below -18°C as the endpoint of quick-freezing ensures that the entire fish fillet completely passes through the maximum ice crystal formation zone. During frozen storage, the center temperature of the fish fillet is always maintained below -18°C, ensuring the stability of its quality during long-term storage.

[0012] Furthermore, in the pretreatment step, the thickness deviation of the cut fish slices does not exceed ±0.5cm. Controlling the thickness deviation of the fish slices within ±0.5cm ensures that the dehydration rate of the same batch of fish slices is consistent under the same drying time, avoiding uneven thickness that causes some fish slices to not reach 40% dryness while others are over-dried.

[0013] Furthermore, in the cold air drying step, the cold air flow in the cold air drying room is horizontal, with a wind speed of 2m / s to 4m / s. The horizontal cold air flow of 2m / s to 4m / s ensures effective rinsing of the cut surface of the fish fillets to remove evaporated moisture, while avoiding excessive wind speed that would cause the surface of the fish fillets to dry out too much and form a hard crust, thus maintaining a dynamic balance between the dehydration rate and the internal moisture migration rate.

[0014] Furthermore, in the cold air drying step, the method for determining the drying endpoint is as follows: detect the weight of the fish fillets. When the current weight of the fish fillets drops to 40% of the weight after marinating, it is determined that the drying endpoint has been reached. 40% of the weight of the marinated fish fillets is used as the indicator for determining the drying endpoint. The method is simple and intuitive, does not require complex moisture content detection equipment, and is suitable for rapid determination and batch control in factory mass production.

[0015] Furthermore, the fish fillets are yellow croaker fillets, wherein under the fourth stage temperature rise of 32°C, the surface fat of the yellow croaker skin undergoes a mild oxidation reaction, producing aroma substances unique to yellow croaker, which further enhances the flavor and quality of the product.

[0016] Compared with existing technologies, this method for pre-treating, salting, segmenting, cold-air drying, and vacuum quick-freezing fresh fish has the following advantages: I. This invention involves cutting fish into thick slices and vertically arranging them with the cut surfaces perpendicular to the direction of the cold airflow. Combined with multi-stage variable temperature and humidity cold air drying, the thick-cut fish slices undergo gradual dehydration from the surface inwards during the drying process, avoiding surface hardening and achieving a uniform moisture content throughout. After drying to a suitable moisture content, the slices are rapidly frozen at ultra-low temperatures. This causes the residual moisture inside the fish slices to quickly pass through the maximum ice crystal formation zone, forming fine ice crystals. This significantly reduces damage to the muscle cell structure, thus solving the technical problems of uneven dehydration and ice crystal damage in thick-cut fish slices. The thawed fish slices maintain good muscle texture and elasticity, with a taste close to that of fresh fish.

[0017] Second, by controlling the amount of salt used and the pickling time, combined with the secondary homogenization and migration of salt during the drying process, this invention reduces the salt content of the finished product while achieving the preservation effect, thus meeting the demand for low-salt products. The moderate temperature increase at the end of the segmented temperature-controlled drying process promotes the gentle oxidation of fish fat, producing a pleasant roasted aroma and giving the product a richer flavor. At the same time, the vertical insertion of fish fillets into slots, combined with a single flipping operation, ensures consistent dehydration rates on both sides, simplifies manual intervention, and makes the entire processing flow easy to implement on conventional food processing lines, demonstrating good industrial adaptability.

[0018] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0020] Figure 1 This is a process flow diagram of the processing method of the present invention. Detailed Implementation

[0021] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0022] This embodiment provides a method for pre-treating, salting, segmenting, cold-air drying, and vacuum quick-freezing fresh fish. Freshly caught sea yellow croaker is used as raw material, undergoing thick-slicing and subsequent processing. The overall process route is as follows: raw material acceptance, pre-treatment, salting, cold-air drying, vacuum packaging, quick-freezing, and frozen storage. The specific implementation process of each step is detailed below.

[0023] The raw material acceptance process includes: selecting yellow croakers that have been caught and chilled for no more than 12 hours, with intact bodies, full eyes, bright red gills, and elastic muscles. The weight of a single fish should be controlled between 500g and 750g; fish of this size have dense muscle fibers and a moderate fat content, suitable for thick-cut processing. Upon arrival at the factory, the raw materials are immediately sorted, removing individuals with mechanical damage or abdominal ruptures, and cleaned with clean seawater at 4℃ or 2% saline solution to remove surface mucus and impurities.

[0024] The pretreatment steps include: The pretreatment process is carried out in a processing workshop where the ambient temperature does not exceed 15℃. Stainless steel knives and thickness positioning plates are provided on the worktable. First, use a scaler to remove the scales from the surface of the fish. Then, cut open the abdomen from the anus forward to remove the internal organs and gills, taking care not to puncture the gallbladder. After evisceration, rinse the abdominal cavity with running cold water at 2℃ to 4℃ to ensure no blood residue remains. After draining for 5 minutes, place the fish on the cutting board with the head to the left and the tail to the right, using a 5cm thick positioning plate to assist in the cutting. The cutting direction is perpendicular to the fish's spine. Starting with the first cut behind the head, cut 5cm thick slices towards the tail, with each slice's thickness not exceeding ±0.5cm. The cut slices include the cross-section of the spine and the muscles on both sides, with a smooth cut surface and clearly visible muscle fiber direction. Place the slices in a stainless steel mesh basket and spray again with 2℃ cold water to wash away any oozing tissue fluid from the cut surface. Then drain until there is no obvious dripping water, which takes about 8 minutes.

[0025] It should be noted that cutting fresh fish into 5cm thick slices significantly increases the surface area for salt penetration and moisture loss. When marinating a whole fish, the salt needs to penetrate the skin and intact muscle membrane, resulting in a long penetration path and high resistance. In contrast, the two cut surfaces of thickly sliced ​​fish directly expose the cross-sections of the muscle fibers, allowing both salt and moisture to migrate rapidly along the fiber direction. This provides a structural basis for shortening the marinating time and improving the uniformity of dehydration.

[0026] The salting process includes: Dry salting is used. Take the pre-treated fish fillets, weigh each one, and record the weight. Use food-grade refined sea salt with a grain size of 0.5mm to 1mm, and calculate the amount used as 9% of the fish fillet weight. Evenly coat both cut surfaces and the skin-inclusive surface of the fish fillets with the weighed salt, ensuring that the gaps between the muscle fibers are filled with salt. After coating, lay the fish fillets skin-side down and cut-side up in a single layer in a perforated plastic marinating dish, with a layer of food-grade PE film at the bottom of the dish. After all the fish fillets are arranged on the dish, cover the dish with the film and place it in a refrigerator at 4℃ to 6℃ to marinate for 4 hours.

[0027] During the curing process, salt penetrates into the center of the muscle fibers along the cut surface under the influence of osmotic pressure difference. Simultaneously, some myofibril proteins undergo moderate salt dissolution and swelling, adjusting the muscle's water-holding capacity. A 4-hour curing time allows salt to penetrate essentially to the geometric center of the thick-cut fish fillets; actual measurements show that the salt concentration at the center can reach over 85% of that at the surface, resulting in a uniform overall salt content. Compared to the 20-25 minute short-time immersion mentioned in the background technique, dry curing for 4 hours with a 9% salt content achieves a balanced salt content both inside and out without excessive dehydration. Furthermore, because excessively high salt content or prolonged curing time is avoided, severe protein denaturation and textural hardening are prevented, allowing the fish fillets to maintain good water migration channels during subsequent drying.

[0028] The cold air drying process includes: After marinating, remove the fish fillets from the cold storage and quickly rinse off excess salt with clean 4°C cold water. Each rinse should not exceed 10 seconds, and then spin-dry the surface of any excess water. Spin-drying is performed using a food-grade centrifugal drainer at a speed of 350 rpm for 1 minute. After draining, the surface of the fish fillets should be free of obvious water droplets and retain their original shape.

[0029] Next, the racking process begins. The multi-layer rack used for racking is made of 304 stainless steel, with overall dimensions of 200cm long, 80cm wide, and 180cm high, featuring 12 tray rails. Each layer is equipped with equally spaced vertical slots, made of two parallel stainless steel plates, 5.2cm wide, with a center-to-center distance of 12cm between adjacent slots. The fish fillets are inserted vertically into the slots, with both cut surfaces facing the gap between adjacent slots. The skin side can be facing up or down, but the orientation must be consistent for the same batch. Because the slot width matches the fillet thickness, the fillets can stand stably upright without additional clamping. This racking method fully exposes both cut surfaces of the fillets to the airflow channels within the slot gaps.

[0030] After being mounted, the fish fillets are placed in a cold air drying chamber. The cold air drying chamber is an integrated constant temperature and humidity drying system, with internal dimensions of 6m long, 3m wide, and 2.5m high. It is equipped with an industrial refrigeration unit, electric heater, humidifier, and centrifugal fan. The fan supplies air from one side of the drying chamber and returns air from the opposite side, forming a horizontal main airflow throughout the chamber. The airflow speed is adjusted via a frequency converter; in this embodiment, it is set to 3m / s. The airflow direction is perpendicular to the cut surface of the fish fillets on the rack. The cold air directly impacts the cut surface of the fish fillets and passes through the gaps in the slots, quickly carrying away the water vapor evaporated from the cut surface.

[0031] The drying process uses segmented temperature and humidity control, with a total time of 36 hours, consisting of four stages.

[0032] Phase 1: Temperature 18℃, relative humidity 68%, for 5 hours. This phase is the preparatory period for surface moisture evaporation; the high humidity prevents the cut surface from rapidly losing water and hardening. The 18℃ temperature allows moisture in the fish fillet muscle to gradually migrate to the surface, keeping the surface moist and preventing the formation of a dense dry film. This process essentially provides a smooth migration channel for internal moisture.

[0033] Phase Two: Temperature 28℃, relative humidity 52%, for 15 hours. This phase is a constant-rate drying period. Driven by the higher temperature, free water inside the fish fillets rapidly migrates and evaporates along the muscle fibers towards both sides of the cut surface. As the temperature rises from 18℃ to 28℃, water activity increases, and the drying rate increases; humidity drops to 52%, maintaining the humidity gradient between the surface and the air. 28℃ is below the general protein denaturation temperature, preventing the surface of the fish from cooking. The 15 hours of continuous drying significantly reduces the overall moisture content of the fish fillets to approximately 60%.

[0034] After the first stage, an additional flipping operation is performed. Workers remove the fish fillets one by one, flip them 180 degrees around their center line, and then reinsert them into the slots, maintaining their original positions. The purpose of flipping is to ensure that the side facing the prevailing wind direction in the first stage becomes the leeward or crosswind side in subsequent stages, making the wind path as symmetrical as possible and achieving a consistent dehydration rate on both sides. Practical experience shows that without flipping, the moisture content of the windward side is about 2% to 3% lower than that of the leeward side; after flipping, the difference is reduced to less than 1%.

[0035] Stage 3: Temperature 20℃, relative humidity 48%, lasting 13 hours. After flipping, the fish fillets enter the third stage, with the temperature returning to 20℃ and the humidity slightly decreasing to 48%. At this stage, the moisture content of the fish fillets is at a low to medium level, and the drying process slows down, increasing the resistance to moisture diffusion from the deeper layers to the surface. The lower temperature prevents premature over-drying of the surface, while the relatively mild conditions allow the bound water inside to gradually release. The 48% humidity still provides some mass transfer impetus; although the drying efficiency is lower than in the second stage, the dehydration process is gradual, and the fish fillets are less likely to develop a hard crust or cracks.

[0036] Stage 4: Temperature 32℃, relative humidity 37%, for 3 hours. A short period of heating and dehumidification is performed before the drying endpoint. For yellow croaker fillets, the 32℃ temperature, besides promoting the removal of residual moisture, also triggers a mild oxidation reaction of the surface fat of the fish skin, generating characteristic roasted fish aroma substances. These substances are mainly aldehydes and ketones derived from fat oxidation, at extremely low concentrations, but imparting a pleasant flavor to the product. Reducing the humidity to 37% further decreases the surface water activity of the fillets, ultimately reducing the moisture content to 40%.

[0037] The drying endpoint was determined by weighing. After marinating and draining, and before racking, the initial weight of each fish fillet was measured as a baseline weight. After 36 hours of drying, samples were taken from the drying room for weight testing. If the current weight equals 40% of the corresponding baseline weight, the endpoint was considered reached. To accommodate mass production in the factory, a quick weight lookup table can be prepared in advance, listing the target weights at 40% of different baseline weights, allowing for rapid weighing and comparison on-site. If the target weight is not reached, the fourth stage can be extended by 1 to 2 hours until it is achieved. In this embodiment, the moisture content of the fish fillets remained stable at 40% ± 1.5% after 36 hours of cumulative drying time.

[0038] After being dried using segmented, variable-temperature cold air, the yellow croaker fillets are translucent with a smooth, wrinkle-free cut surface, clear muscle fiber texture, and a soft yet supple consistency that is elastic to the touch. The 40% moisture content (40% dryness) provides an ideal cell state for subsequent quick-freezing.

[0039] The vacuum packaging process includes: after the dried fish fillets are removed from the drying room, they are naturally cooled for 15 minutes in a clean environment not exceeding 20°C, and then individually packaged in food-grade vacuum packaging bags. The vacuum packaging machine is set to a vacuuming time of 20 seconds and a heat-sealing time of 2 seconds, achieving a vacuum level of -0.1 MPa. The bag material is a PA / PE composite film with low oxygen permeability and good barrier properties. The packaged fish fillets are isolated from oxygen and external moisture, preventing oxidation, deterioration, and moisture absorption during storage.

[0040] The quick-freezing process includes: using a tunnel-type liquid nitrogen quick-freezing machine with an operating temperature set at -35℃. Vacuum-packed fish fillets are laid flat in a single layer on the conveyor belt of the quick-freezing machine, with the conveyor belt speed adjusted to 0.3 m / min, and the tunnel travel time is approximately 12 minutes. After entering the quick-freezing tunnel, the surface temperature of the fish fillets drops rapidly. Core temperature tracking and measurement show that it takes approximately 8 minutes to drop from an initial temperature of around 20℃ to -18℃. Within 8 minutes, the core temperature of the fish fillets crosses the maximum ice crystal formation zone (-1℃ to -5℃), taking only about 1.5 minutes. This extremely rapid freezing rate prevents residual water in the intercellular spaces from forming large ice crystals, instead forming a large number of uniformly distributed micro-ice crystals inside and outside the cells. These micro-ice crystals are generally less than 10 μm in diameter, having minimal mechanical impact on the muscle cell walls and cell membranes, thus completely protecting the cell's microstructure.

[0041] The final step of quick-freezing is when the center temperature of the fish fillets reaches below -25℃, which is lower than the standard requirement of -18℃, but with a margin of safety. After quick-freezing, the fish fillets are quickly transferred to a -20℃ finished product cold storage warehouse and stacked to avoid temperature fluctuations. Under these conditions, the product can have a shelf life of more than 12 months. After thawing, the appearance, color, and elasticity of the fish fillets are basically the same as those in their 40% dryness state before freezing. They have no coarse or tough texture when steamed or cooked, and the flavor is normal.

[0042] In this embodiment, fish fillets were cut into 5cm thick slices, dry-cured with 9% salt for 4 hours, then vertically inserted into racks with the cut surfaces perpendicular to the airflow direction, and cold-air dried under four-stage temperature and humidity variations until the moisture content reached 40%. Finally, they were vacuum-packed and quick-frozen at -35℃, resulting in semi-dried quick-frozen fish fillets with good flavor and texture close to fresh fish. The thick-cut design increased the contact area and mass transfer channels, the staged temperature variations prevented surface hardening, the vertical racking and flipping operation ensured uniform dehydration on both sides, and the 40% dryness combined with ultra-low temperature quick-freezing protected cell integrity. The entire process is tightly integrated, the parameters are reliable, and each step can be implemented on general food processing equipment, demonstrating good industrial feasibility.

[0043] The following three specific implementation scenarios will be used to implement and explain this embodiment in detail: In Scenario 1, freshwater grass carp is used as the raw material, and the operation process is the same. The only difference is that the amount of salt used for salting is adjusted to 8% of the weight of the fish fillets, and the humidity and temperature parameters in the four stages of cold air drying are fine-tuned to suit the slightly loose texture and low fat content of freshwater fish. The details are as follows: The raw material is fresh grass carp, with an average weight of 1500g. After slaughtering and bleeding, the scales, gills, and internal organs are removed, and the fish is cut into 5cm thick slices, with a thickness deviation controlled within ±0.5cm. Salt is used for curing, with 8% salt applied evenly and then cured at 4℃ for 4 hours. The fish is also mounted using a vertical insertion slot, with the cut surface perpendicular to the airflow, and the airflow speed set at 2.5m / s.

[0044] The segmented drying process is as follows: Stage 1: 17℃, 70% relative humidity, 4 hours; Stage 2: 27℃, 55% relative humidity, 16 hours; turn over once at the end of Stage 1; Stage 3: 19℃, 50% relative humidity, 12 hours; Stage 4: 30℃, 40% relative humidity, 4 hours. The total time is 36 hours, and the final weight is 40% of the initial baseline weight. In Implementation Scenario 1, the subsequent vacuum packaging, -35℃ quick-freezing, and -20℃ frozen storage are the same as in the previous example.

[0045] The results showed that thick slices of grass carp could also achieve uniform dehydration and good texture under this process. Because grass carp has low fat content, the fourth stage temperature control at 30℃ was sufficient to achieve a slightly dry and fragrant effect, without any off-odors caused by excessive fat oxidation.

[0046] In Scenario 2, small-sized wild-caught mackerel was used as raw material. Mackerel meat is fatty and has a high proportion of red meat. The process adjustments were as follows: the amount of salt used in salting was increased to 10% of the weight of the fish fillets to enhance preservation and adjust the flavor; the cold air drying speed was reduced to 2 m / s to slow down fat oxidation; and the fourth stage temperature was maintained at 32°C for 3 hours to promote the formation of characteristic aromas. All other aspects were the same as in the previous example.

[0047] In the process, mackerel fillets were cut into 5cm thick slices, dry-cured with 10% salt for 4 hours, and then vertically arranged on racks with a fan speed of 2m / s. The first stage was 18℃, 65% salt concentration, and 5 hours; the second stage was 28℃, 50% salt concentration, and 15 hours, with the fillets turned over; the third stage was 20℃, 45% salt concentration, and 13 hours; and the fourth stage was 32℃, 35% salt concentration, and 3 hours. The final weight was 40% of the baseline weight. Mackerel has a high fat content, and after drying, the surface has a slightly oily sheen. After vacuum packaging and quick-freezing, it was thawed and roasted after 12 months of frozen storage. The result was a rich flavor, intact muscle texture, and no oily roasted taste.

[0048] Table 1 summarizes the implementation examples and two implementation scenarios.

[0049] Table 1. Key parameters and results for the examples and two implementation scenarios:

[0050] As shown in Table 1, adjusting the salt content and drying humidity slightly for different fish species can consistently achieve a moisture content of 40% and good quality, indicating that this method has good universality.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for pre-treating, salting, segmenting, cold-air drying, and vacuum quick-freezing fresh fish, characterized in that... The method includes the following steps: Pre-treatment: After removing the scales, gills and entrails from the fresh fish, clean it thoroughly and cut the fish into 5cm thick slices. Salt curing: Marinate the fish fillets with salt for 4 hours; Cold air drying: The marinated fish fillets are placed on a multi-layer rack with their cut surfaces perpendicular to the direction of the cold air flow, so that both cut surfaces of the fish fillets face the direction of the cold air flow. They are then sent into a cold air drying room for segmented variable temperature cold air drying. The total drying time is 36 hours, and the drying endpoint is when the moisture content of the fish fillets drops to 40%. The segmented variable temperature cold air drying includes the following four stages: the first stage temperature is 18℃ for 5 hours, the second stage temperature is 28℃ for 15 hours, the third stage temperature is 20℃ for 13 hours, and the fourth stage temperature is 32℃ for 3 hours. Vacuum packaging: The dried fish fillets are vacuum packaged; Quick-freezing: Vacuum-packed fish fillets are quick-frozen at -35°C.

2. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the cold air drying step, the multi-layer frame is provided with equally spaced vertical slots. The width of the slots is 5cm to 5.5cm, and the distance between adjacent slots is 10cm to 15cm. The fish fillets are vertically inserted into the slots, and the two cut surfaces of the fish fillets are exposed in the gap between adjacent slots and facing the direction of the cold air flow.

3. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the cold air drying step, the relative humidity of the cold air drying room is controlled at 65% to 70% in the first stage, 50% to 55% in the second stage, 45% to 50% in the third stage, and 35% to 40% in the fourth stage.

4. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the cold air drying step, the fish fillets are flipped over once after the first stage.

5. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the salting step, the amount of salt used is 8% to 10% of the weight of the fish fillets.

6. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the quick-freezing step, the endpoint of quick-freezing is when the center temperature of the fish fillets is below -18°C.

7. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the pretreatment step, the thickness deviation of the cut fish fillets shall not exceed ±0.5cm.

8. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the cold air drying step, the cold air flow in the cold air drying room is horizontal, and the wind speed is 2m / s to 4m / s.

9. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, In the cold air drying step, the method for determining the drying endpoint is as follows: detect the weight of the fish fillets, and when the current weight of the fish fillets drops to 40% of the weight after marinating, the drying endpoint is determined to have been reached.

10. The method for pre-treatment, salting, segmented cold air drying, and vacuum quick-freezing of fresh fish according to claim 1, characterized in that, The fish fillets in question are yellow croaker fillets.

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