Storage quality maintaining technology of tuna product baked at low temperature in vacuum package

Abstract

The invention relates to the technical field of aquatic product processing, and discloses a storage quality maintaining technology for low-temperature baked tuna products under vacuum packaging, and the technology comprises the following steps: pretreating tuna blocks, and performing low-temperature baking at 75-95 DEG C under controlled humidity; then cooling the product, and carrying out high-vacuum-degree vacuum packaging; and finally, carrying out water bath low-temperature sterilization at 80-95 DEG C. According to the invention, through two-stage mild heat treatment of low-temperature baking and low-temperature sterilization, combined with improvement of water binding capacity by pretreatment and obstruction of oxygen by vacuum packaging, comprehensive quality control of microorganisms, texture, flavor and color of the product is synergistically realized; the prepared tuna product has the beneficial effects of fresh, tender and juicy meat, pure flavor, excellent color and luster and long shelf life, and the process aims to solve the technical problems of dry and hard meat, deteriorated flavor and dim color and luster of the tuna product caused by traditional high-temperature hot processing.

Description

Technical Field

[0001] This invention relates to the field of aquatic product processing technology, specifically to a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products. Background Technology

[0002] Tuna, a deep-sea fish with high economic value, is widely favored by consumers for its rich protein, unsaturated fatty acids, and delicious flavor. To extend the shelf life of tuna and expand its market distribution channels, deep processing is the main method used in the industry.

[0003] Currently, commercially available long-shelf-life tuna products are mainly produced using traditional high-temperature, high-pressure (HTHP) sterilization processes. This type of process typically involves prolonged heat treatment at temperatures above 121°C. While this effectively kills microorganisms and ensures commercial sterility, this drastic heat treatment also causes irreversible damage to the tuna's quality. Under high temperatures, the myofibril proteins in tuna undergo severe denaturation and aggregation, leading to the destruction of the protein's three-dimensional network structure and significant moisture loss. Ultimately, this results in a dry, hard, and coarse texture, losing the original tenderness and freshness of the tuna.

[0004] Furthermore, high temperatures accelerate the oxidation of the abundant unsaturated fatty acids in tuna, producing unpleasant flavor compounds that mask the tuna's natural deliciousness. Simultaneously, myoglobin, which determines the characteristic red color of tuna meat, is extremely unstable at high temperatures and easily oxidizes into brown methemoglobin, resulting in a dull product color and reduced commercial value.

[0005] To improve product quality, some tuna products are processed at lower temperatures. However, these products are usually not thoroughly sterilized and require cold chain storage and transportation throughout the entire process. They also have a shorter shelf life and cannot meet the demands of long-term storage at room temperature. Therefore, how to maintain the original fresh texture, characteristic flavor, and color of tuna products to the greatest extent possible while ensuring microbial safety and achieving a long shelf life is a pressing technical challenge in the current tuna processing industry. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a process for preserving the storage quality of vacuum-packed, low-temperature baked tuna products. This process solves the problem of existing tuna processing technologies struggling to balance ensuring the microbial safety of products for long-term storage at room temperature with maximizing the preservation of their original tender texture, flavor, and color.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products, comprising the following steps: S1. Pre-treatment steps for marinating tuna raw materials; S2. The tuna raw material processed in the pretreatment step is subjected to low-temperature baking to obtain the tuna product. In the low-temperature baking step, turbulent circulating air is used to heat the tuna raw material. The turbulent air field is used to break the thermal resistance boundary layer on the surface of the raw material to achieve uniform heating of the fish pieces. S3. Cooling step of cooling the tuna product; S4. Vacuum packaging step of vacuum packaging the cooled tuna products; S5. The sterilization step of subjecting the vacuum-packed tuna products to ultra-high pressure cold sterilization treatment; the ultra-high pressure cold sterilization treatment utilizes a liquid medium to transmit pressure and destroy the microbial cell structure at room temperature or low temperature.

[0008] In one specific implementation, the pretreatment step involves marinating the tuna raw material at a temperature of 2–8°C for 10–24 hours. This long-term marinating at this low temperature allows flavor compounds to slowly and evenly penetrate the fish meat, while also effectively inhibiting the growth and reproduction of microorganisms during the marinating process, providing a lower initial total bacterial count for subsequent processes.

[0009] In one specific implementation, the low-temperature baking step is performed at a baking temperature of 75–95°C for 40–80 minutes. This temperature range allows for the cooking of the tuna raw material and reduces the total bacterial count, while avoiding excessive protein denaturation and moisture loss due to excessively high temperatures, thus maintaining the tenderness of the product.

[0010] In one specific implementation, the cooling step involves cooling the core temperature of the tuna product to below 20°C; the vacuum packaging step results in a vacuum degree of -0.090 to 0.098 MPa after packaging. Rapidly cooling the baked product allows it to quickly pass through a temperature range conducive to microbial growth, reducing the risk of secondary contamination. Subsequent high-vacuum packaging effectively removes oxygen from the packaging, inhibiting the growth of aerobic microorganisms and significantly slowing down the fat oxidation process, which directly contributes to maintaining the flavor and quality of high-fat fish like tuna.

[0011] In one specific implementation, the conditions for the ultra-high pressure cold sterilization treatment are: pressure of 300–600 MPa, holding time of 5–15 minutes, and temperature of the pressure-transmitting medium of 10–25°C. This step is the final heat treatment performed on the vacuum-packed finished product. This combination of temperature and time aims to kill microorganisms that may have contaminated the product during packaging and cooling, as well as any residual nutrients in the fish meat, ensuring the product's shelf-life safety. At the same time, its heat treatment intensity is lower than traditional high-temperature sterilization, avoiding secondary damage to the already formed product structure.

[0012] In one specific implementation, the pretreatment step uses a marinating solution, which, by weight, is made of 3-8 parts salt, 10-20 parts white sugar, 0.2-0.5 parts compound phosphate, 1-3 parts spices, and the balance being water. Salt and white sugar provide basic flavor and regulate osmotic pressure; the compound phosphate acts as a moisture-retaining agent, improving the water-holding capacity of the protein and reducing juice loss during subsequent heat treatment; and the spices impart a unique complex flavor to the product.

[0013] In one specific implementation, during the low-temperature baking step, the relative humidity of the baking environment is controlled at 60%–85%, ensuring that the center temperature of the tuna product reaches 65–75°C at the end of the low-temperature baking process. Maintaining a certain level of ambient humidity reduces moisture evaporation from the surface of the fish pieces, promotes uniform heat transfer throughout, and prevents the surface from drying and hardening too quickly. Ensuring that the center temperature reaches a specific value is a direct indicator of whether the product has reached the desired level of doneness.

[0014] In one specific implementation, before the pretreatment step begins, a color-protecting treatment is included, which involves soaking the tuna raw material in a 3%–5% brine solution for 15–30 minutes. This step utilizes the pretreatment effect of the brine to stabilize the myoglobin in the tuna muscle and slow down its oxidative browning during heat treatment.

[0015] In one specific implementation, the tuna raw material is a piece of yellowfin tuna or bigeye tuna with a thickness of 2-4 cm. Limiting the type and thickness of the raw material helps ensure the stability of process parameters and the consistency of the final product quality.

[0016] In one specific implementation, following the low-temperature sterilization step, the process further includes rapidly cooling the low-temperature sterilized tuna product to a core temperature below 30°C. This rapid cooling step terminates the heat treatment reaction, prevents residual heat from causing excessive cooking of the product, and further fixes the product's texture and flavor.

[0017] This invention provides a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products. It offers the following advantages: 1. This invention replaces the traditional high-temperature treatment with a two-stage mild heat treatment process consisting of a low-temperature baking step and a low-temperature sterilization step. This process combination achieves both cooking and sterilization of the product while avoiding excessive denaturation and aggregation of myofibril proteins caused by high temperatures, thus reducing severe juice loss. Furthermore, the application of compound phosphates in the pretreatment step further enhances the water-holding capacity of fish protein. The combined effect of these two processes ensures that the final product retains a tender and juicy texture.

[0018] 2. This invention ensures product safety without resorting to high-temperature sterilization through a multi-stage microbial control process. Specifically, low-temperature pickling at 2–8°C inhibits the initial bacterial growth of the raw materials; the low-temperature baking step reduces the number of microorganisms inside the product; subsequent rapid cooling and vacuum packaging prevent secondary contamination and microbial growth; and the final low-temperature sterilization step performs heat sterilization on the finished product. These steps work synergistically to achieve effective microbial control.

[0019] 3. By controlling the relative humidity to 60%–85% during the low-temperature baking process, this invention prevents the formation of a dry, hard layer on the surface of the tuna chunks due to excessive evaporation of moisture, ensuring uniform heat transfer to the interior of the chunks. Combined with limiting the thickness of the raw material to 2–4 cm, this invention ensures the uniformity of the center doneness and the consistency of the texture and taste across different batches of products, thereby improving the stability of product quality. Attached Figure Description

[0020] Figure 1 This is a sample texture diagram of the present invention. Detailed Implementation

[0021] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example: Example 1: This embodiment provides a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products, specifically including the following steps: Raw material preparation and color preservation treatment: Select fresh yellowfin tuna, remove the internal organs, skin and dark flesh, and cut into 3cm thick pieces. Soak the fish pieces in a 4% salt solution for 20 minutes to preserve color, then remove and drain the surface water.

[0023] Pre-treatment steps: Mix 5 parts salt, 15 parts white sugar, 0.35 parts compound phosphate, 2 parts spices, and 77.65 parts water by weight to prepare a marinating solution. Place the color-protecting fish pieces in the marinating solution and marinate for 18 hours at a temperature of 5°C.

[0024] Low-temperature baking step: Remove the marinated fish pieces from the marinade, drain them, and place them in a hot air circulating baking equipment. Set the baking temperature to 85℃ and the relative humidity of the baking environment to 75%. Bake for 60 minutes until the center temperature of the fish pieces reaches 70℃, thus obtaining tuna products.

[0025] Cooling step: Place the baked tuna products in a forced cold air chamber at 0°C to cool them down until the core temperature is below 15°C.

[0026] Vacuum packaging steps: After cooling, the tuna products are placed into a polyethylene / nylon seven-layer co-extruded high-barrier composite film packaging bag, and vacuum packaging is performed using a vacuum packaging machine. The vacuum degree after packaging is set to -0.095MPa.

[0027] Low-temperature sterilization step: Place the vacuum-packed tuna products in a water bath at 88°C for 35 minutes for water bath heating and sterilization.

[0028] Final cooling step: After sterilization, immediately immerse the product package in running cold water to cool until its core temperature is below 30°C. Then wipe the surface of the packaging bag dry to obtain the finished product.

[0029] Example 2: This embodiment provides a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products, specifically including the following steps: Raw material preparation and color preservation treatment: Select fresh yellowfin tuna, remove the internal organs, skin and dark flesh, and cut into 2cm thick pieces. Soak the fish pieces in a 3% salt solution for 15 minutes to preserve the color, then remove and drain the surface water.

[0030] Pre-treatment steps: Mix 3 parts salt, 10 parts white sugar, 0.2 parts compound phosphate, 1 part spices, and 85.8 parts water by weight to prepare a marinating solution. Place the color-protecting fish pieces in the marinating solution and marinate for 10 hours at a temperature of 2°C.

[0031] Low-temperature baking step: Remove the marinated fish pieces from the marinade, drain them, and place them in a hot air circulating baking device. Set the baking temperature to 75℃ and the relative humidity of the baking environment to 60% for 80 minutes to obtain tuna products.

[0032] Cooling step: Allow the baked tuna products to cool naturally in a clean environment until their core temperature is below 20°C.

[0033] Vacuum packaging steps: After cooling, the tuna products are placed into packaging bags and vacuum packaged using a vacuum packaging machine. The vacuum level after packaging is set to -0.090MPa.

[0034] Low-temperature sterilization step: Place the vacuum-packed tuna products in an 80°C water bath for sterilization for 45 minutes.

[0035] Final cooling step: After sterilization, immediately immerse the product package in running cold water to cool until its core temperature is below 30°C. Then wipe the surface of the packaging bag dry to obtain the finished product.

[0036] Example 3: This embodiment provides a process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products, specifically including the following steps: Raw material preparation and color preservation treatment: Select frozen and thawed bigeye tuna, remove the internal organs, skin and dark flesh, and cut into 4cm thick pieces. Soak the fish pieces in a 5% salt solution for 30 minutes to preserve the color, then remove and drain the surface water.

[0037] Pre-treatment steps: Mix 8 parts salt, 20 parts white sugar, 0.5 parts compound phosphate, 3 parts spices, and 68.5 parts water by weight to prepare a marinating solution. Place the color-protecting fish pieces in the marinating solution and marinate at 8°C for 24 hours.

[0038] Low-temperature baking step: Remove the marinated fish pieces from the marinade, drain them, and place them in a hot air circulating baking device. Set the baking temperature to 95℃ and the relative humidity of the baking environment to 85%. Bake for 40 minutes to obtain tuna products.

[0039] Cooling step: Place the baked tuna products in a forced air cooling system at -2°C until the core temperature is below 10°C.

[0040] Vacuum packaging steps: After cooling, the tuna products are placed into packaging bags and vacuum packaged using a vacuum packaging machine. The vacuum level after packaging is set to -0.098MPa.

[0041] Low-temperature sterilization step: Place the vacuum-packed tuna products in a 95°C water bath for 25 minutes for sterilization.

[0042] Final cooling step: After sterilization, immediately immerse the product package in running cold water to cool until its core temperature is below 30°C. Then wipe the surface of the packaging bag dry to obtain the finished product.

[0043] Comparative Example 1: Compared with Example 1, the difference is that the low-temperature baking step is replaced with the traditional high-temperature baking step, specifically, the baking temperature is set to 180°C and the baking time is 15 minutes, while the other steps are the same.

[0044] Comparative Example 2: Compared with Example 1, the difference is that the vacuum packaging step is replaced with the conventional heat sealing packaging step, that is, the vacuuming operation is not performed, only the packaging bag is sealed, and the remaining steps are the same.

[0045] Comparative Example 3: Compared with Example 1, the difference is that the low-temperature sterilization step and the final cooling step are omitted after the vacuum packaging step, while the other steps are the same.

[0046] Comparative Example 4: Compared with Example 1, the difference is that the low-temperature sterilization step is replaced with the traditional high-temperature and high-pressure sterilization step. Specifically, the vacuum-packed product is placed in an autoclave and sterilized at 121°C for 15 minutes. All other steps are the same.

[0047] Comparative Example 5: Compared with Example 1, the difference is that the pretreatment step is omitted, that is, the fish pieces after color protection treatment are not marinated in the marinade and are directly subjected to the subsequent low-temperature baking step, while the other steps are the same.

[0048] Test Example 1: Comparison Test of Texture Properties To objectively evaluate the effects of different processes on the final textural properties of tuna products, textural profile analysis was performed on samples prepared in Examples 1-3 and Comparative Examples 1-5.

[0049] Experimental steps: Sample preparation: Equilibrate all samples to be tested at room temperature (25℃) for 30 minutes. Use a scalpel to cut the fish meat in the center of each sample into 20mm×20mm×20mm cubes, ensuring that the sample surface is flat and free of fascia.

[0050] Instrumentation and parameter settings: An analyzer was used for testing. A cylindrical flat-bottomed probe was selected. The test mode was set to texture profile analysis. Specific test parameter settings are as follows: Pre-test speed: 1.0 mm / s; Test rate: 1.0 mm / s; Post-test speed: 1.0 mm / s; Compressive strain: 50%; Interval between two compressions: 5 seconds; Trigger force: 5g; Data Acquisition and Processing: Place the sample block in the center of the instrument stage and activate the probe to perform two cycles of downward pressure and return. The instrument automatically records the force-time curve and calculates the sample's hardness, elasticity, and chewiness. Five parallel samples are taken from each sample group for testing, and the results are averaged.

[0051] Table 1. Test results of textural properties of samples obtained by different processes Results Explanation The data in Table 1 show that the hardness and chewiness values ​​of the samples obtained in Examples 1, 2, and 3 are lower than those of Comparative Examples 1 and 4, while the elasticity values ​​are higher. This is because the two-stage heat treatment process of low-temperature baking and low-temperature sterilization used in this technical solution has a lower heat treatment intensity than the high-temperature baking of Comparative Example 1 and the high-temperature and high-pressure sterilization of Comparative Example 4. The lower temperature treatment slows down the severe thermal denaturation and aggregation of tuna myofibrillar proteins, allowing the water in the three-dimensional network structure of the protein to be retained more effectively, thereby maintaining the initial state of the fish meat tissue.

[0052] Comparing the data from Example 1 and Comparative Example 5, it can be seen that the sample of Comparative Example 5, which omitted the pretreatment step, had higher hardness and chewiness values ​​than the sample of Example 1, while its elasticity value was lower. This result indicates that the application of the marinade in the pretreatment step directly affects the final texture of the product. The complex phosphate components in the marinade can improve the water-holding capacity of the protein, effectively reducing the loss of juices from the fish pieces during subsequent low-temperature baking and low-temperature sterilization heat treatment. Therefore, the sample that underwent the pretreatment step exhibited lower hardness.

[0053] The combined data show that the synergistic effect of the pretreatment, low-temperature baking, and low-temperature sterilization steps can effectively control the textural properties of the final product. The initial textural properties of the samples in Comparative Examples 2 and 3 were not significantly different from those of the sample in Example 1, further illustrating that the product's texture is mainly determined by the mildness of the heat treatment process and the moisture retention effect of the pretreatment step.

[0054] Test Example 2: Comparison Test of Fat Oxidation Degree To quantitatively evaluate the effect of different processes on the control of fat oxidation in tuna products during storage, the thiobarbituric acid reactive substances of the samples prepared in Examples 1-3 and Comparative Examples 1-5 were determined.

[0055] Experimental steps: Sample homogenization: Accurately weigh 5.0 g of the shredded sample and place it in a homogenizing cup. Add 50 mL of 7.5% trichloroacetic acid solution containing 0.1% EDTA and homogenize at 10000 r / min for 1 minute.

[0056] Extraction and filtration: Transfer the homogenized liquid to a centrifuge tube and centrifuge at 4000 rpm for 10 minutes. Collect the supernatant, filter it through double-layer qualitative filter paper, and collect the filtrate for later use.

[0057] Colorimetric reaction: Accurately pipette 2.0 mL of the filtrate into a stoppered test tube, and add 2.0 mL of 0.02 mol / L thiobarbituric acid solution. Place the test tube in a 95°C water bath and heat for 40 minutes. After the reaction is complete, immediately remove the test tube and cool it to room temperature with running tap water.

[0058] Absorbance determination: The absorbance of the reaction solution was measured at a wavelength of 532 nm using a UV-Vis spectrophotometer. A solution prepared by mixing 2.0 mL of trichloroacetic acid solution with 2.0 mL of LTA solution and treating it in the same way was used as a blank reference.

[0059] Results Calculation: Based on the measured absorbance values, the malondialdehyde (MDA) content in the sample was calculated using a standard curve. The results were expressed as milligrams of MDA per kilogram of sample. Three parallel samples were tested for each sample group, and the average result was taken.

[0060] Table 2. TBARS values ​​of samples obtained by different processes after storage Results Explanation Table 2 objectively reflects the TBARS values ​​of the samples obtained in Examples 1, 2, and 3, which are significantly lower than the TBARS values ​​of the sample obtained in Comparative Example 2. The TBARS value is an indicator of the malondialdehyde (MDA) content, a secondary oxidation product of fats. The unsaturated fatty acids abundant in tuna readily undergo a chain-like auto-oxidation reaction under aerobic conditions, producing a rancid flavor. The vacuum packaging step in this technical solution removes most of the oxygen within the packaging bag by setting the vacuum level to -0.090 to -0.098 MPa, providing an oxygen-deficient storage environment for the product. The absence of oxygen directly blocks the conditions for fat oxidation, thus effectively inhibiting the formation of MDA during storage. Comparative Example 2 uses conventional heat sealing, where the residual oxygen within the packaging acts as a reaction substrate, leading to continuous fat oxidation and ultimately a high TBARS value.

[0061] Meanwhile, the TBARS values ​​of Comparative Example 1 and Comparative Example 4 were higher than those of Samples 1, 2, and 3. This is because high heat treatment temperatures act as inducing factors for fat oxidation, accelerating the decomposition of hydroperoxides and forming free radicals, thereby initiating or accelerating an oxidation chain reaction. The combination of low-temperature baking and low-temperature sterilization used in this technical solution provides a relatively mild heat treatment intensity, reducing the thermodynamic driving force of fat oxidation and helping to maintain the stability of fats.

[0062] The TBARS values ​​of Comparative Examples 3 and 5 were also higher than those of the Example samples, but lower than those of Comparative Example 2. This indicates that the synergistic effect of the entire process has an impact on the final result, but the creation of an oxygen-deficient environment through vacuum packaging is the decisive technical feature for controlling the lipid oxidation process during storage. The application of this technical feature directly corresponds to the maintenance of product flavor and quality.

[0063] Test Example 3: Comparative Test of Microbial Safety To evaluate the ability of different processes to control microbial indicators of tuna products after storage, the total bacterial count of samples prepared in Examples 1-3 and Comparative Examples 1-5 was tested.

[0064] Experimental steps: Sample preparation and dilution: In a sterile operating table, weigh 25g of sample and place it in a homogenizing bag containing 225mL of sterile phosphate buffer. Use a tapping homogenizer to tap the sample at a frequency of 8 times / second for 2 minutes to prepare a 1:10 sample homogenate.

[0065] Serial dilution: Using a 1 mL sterile pipette, pipette 1 mL of a 1:10 sample homogenate and inject it into a test tube containing 9 mL of sterile phosphate buffer. Vortex to mix, preparing a 1:100 dilution. Prepare 10-fold serial dilutions using this method.

[0066] Petri dish inoculation: Based on the estimate of sample contamination, select 2 to 3 suitable serial dilutions. For each dilution, take 1 mL of the diluent and inject it into a sterile petri dish. Perform two parallel petri dishes for each dilution.

[0067] Culture medium pouring and incubation: Pour approximately 15–20 mL of plate counting agar medium cooled to about 46°C into a Petri dish, and rotate the dish to thoroughly mix the sample solution with the culture medium. After the agar solidifies, invert the Petri dish and incubate it in a 37°C incubator for 48 ± 2 hours.

[0068] Colony counting and result calculation: Select plates with colony counts between 30 and 300 CFU for counting. Report results in colony-forming units. The calculation formula is: Total colony count = (Average colony count on plate × Dilution factor).

[0069] Table 3. Total bacterial count of samples obtained by different processes after storage Results Explanation The test data in Table 3 show that the total bacterial count of the samples prepared in Examples 1, 2, and 3 was at an extremely low level after accelerated storage, significantly lower than that of the sample in Comparative Example 3. This result directly proves the effectiveness of the low-temperature sterilization step in this technical solution. Comparative Example 3 omitted this step, resulting in a large proliferation of residual microorganisms that might have been introduced during processing during storage, ultimately leading to a total bacterial count far exceeding the microbial limits expected for commercial shelf-life products. The final heat treatment at 80–95°C in this solution is sufficient to inactivate these microorganisms, thereby ensuring the microbiological safety of the product.

[0070] This technical solution achieves systematic control of microorganisms through the synergistic effect of multiple steps. The pretreatment step is carried out at a low temperature of 2–8°C, which inhibits the growth rate of the initial microbial community in the raw materials. The subsequent low-temperature baking step, as the first stage of heat treatment, reduces the initial microbial population inside the fish pieces. The vacuum packaging step creates an anaerobic environment, limiting the growth of aerobic spoilage bacteria, as evidenced by the higher total bacterial count in Comparative Example 2 compared to Example 1. These preliminary steps create conditions for the final low-temperature sterilization step, enabling effective control of the final product's microbial indicators without resorting to excessive heat treatment.

[0071] Comparing Example 1 and Comparative Example 4, it can be seen that although both achieved extremely low total bacterial counts, compared with the results of Test Example 1, Comparative Example 4 came at the cost of product quality. The purpose of the low-temperature sterilization step in this technical solution is to use a mild but sufficient heat treatment intensity, in conjunction with multiple preceding microbial control steps, to ensure the microbial safety of the product while minimizing the negative impact of heat treatment on the sensory quality of the product.

[0072] Test Example 4: Moisture Content Comparison Test To quantitatively analyze the effects of different processes on the final moisture content of tuna products, the moisture content of samples prepared in Examples 1-3 and Comparative Examples 1-5 was determined.

[0073] Experimental steps: Sample preparation and weighing dish preparation: Dry the weighing dishes in a constant temperature drying oven at 105℃ for 1 hour, then remove them and cool them in a desiccator for 30 minutes before accurately weighing. Repeat this operation until constant weight is achieved. Then, homogenize the sample using a tissue homogenizer.

[0074] Sample weighing: Quickly weigh 2-5g of the crushed sample into a pre-weighed weighing dish, accurate to 0.0001g.

[0075] Drying: Cover the weighing dish, but leave it partially open, and place it in a constant temperature drying oven at 105℃. After drying for 4 hours, cover the dish, remove it, and transfer it to a desiccator to cool for 30 minutes.

[0076] Constant weight procedure: After cooling, weigh the weighing dish containing the dried sample. Then, place the weighing dish back into a drying oven at 105°C for 1 hour, remove, cool, and weigh again. Repeat this drying and weighing procedure until the mass difference between two consecutive weighings is no greater than 2 mg, which is considered to have achieved constant weight.

[0077] Results Calculation: The moisture content of the sample was calculated based on the mass loss before and after drying. The calculation formula is: Moisture Content .in For the mass of the weighing dish, This represents the total mass of the sample and weighing dish before drying. This represents the total mass of the dried sample and the weighing dish. Three parallel samples were taken from each sample group for testing, and the results were averaged.

[0078] Table 4. Moisture content of samples obtained by different processes Results Explanation Table 4 shows that the moisture content of the samples obtained in Examples 1, 2, and 3 is higher than that of the samples in Comparative Examples 1 and 4. Moisture in fish meat mainly exists in the form of bound water and non-flowing water within the three-dimensional network structure of myofibrillar proteins. The two-stage heat treatment process of low-temperature baking and low-temperature sterilization used in this technical solution has a relatively low heat treatment temperature. This mild heat treatment condition results in less denaturation of the myofibrillar proteins; the protein molecules only undergo limited stretching and cross-linking, and their spatial structure can maintain a high degree of integrity, thereby effectively binding internal moisture and reducing juice loss during the heat treatment process. In contrast, the high-temperature baking in Comparative Example 1 and the high-temperature, high-pressure sterilization in Comparative Example 4 caused severe denaturation, shrinkage, and aggregation of proteins, destroying the original network structure and resulting in the squeezing out of a large amount of moisture.

[0079] Comparing the data from Example 1 and Comparative Example 5, the moisture content of the sample in Example 1 was higher than that of the sample in Comparative Example 5. This indicates that the pretreatment step has a direct effect on the moisture retention of the final product. In this step, the complex phosphate in the marinade acts as a moisture-retaining agent, raising the pH value of the fish meat and causing it to deviate from the isoelectric point of the protein, thereby increasing the electrostatic repulsion between protein molecules. This repulsion creates more space between protein fiber bundles, thus enhancing the protein system's ability to bind and fix moisture. Comparative Example 5, lacking this step, had a lower ability to retain moisture during subsequent heat treatment compared to Example 1.

[0080] In summary, this technical solution effectively maintains the moisture content of the final product by combining the application of a moisture-retaining agent in the pretreatment step with a two-stage mild heat treatment process. Controlling the ambient humidity during low-temperature baking also reduces moisture evaporation from the surface of the fish pieces, playing a supporting role in maintaining the overall moisture content. A high moisture content is the material basis for the product's juicy texture, a fact corroborated by the textural property test results in Test Example 1.

[0081] Test Example 5: Color Contrast Test To objectively evaluate the impact of different processes on the final color of tuna products, a colorimeter was used to analyze the samples prepared in Examples 1-3 and Comparative Examples 1-5. Color space parameter determination.

[0082] Experimental steps: Instrument calibration: Use a colorimeter. After powering on and warming up, calibrate using the standard white and black boards provided with the instrument to ensure the accuracy of the measurement data.

[0083] Sample preparation: Remove the sample from its packaging and cut it open along its cross-section with a scalpel to expose a clean, fresh cut surface. Place the sample with the fresh cut surface on the test stage.

[0084] Parameter settings and measurement: Set the instrument's measurement conditions to a D65 light source and a 10° standard observer's viewing angle. Ensure the instrument's measuring aperture is tightly fitted to the fresh cut surface of the sample, guaranteeing no external light enters. Trigger the measurement; the instrument will automatically record the measurement point. value, Value and value.

[0085] Data Acquisition: To ensure data representativeness, measurements were taken at five different locations on the cross-section of each sample. After removing outliers, calculations were performed. The average of the three parameters is taken as the final color parameter of the sample.

[0086] Table 5. Color parameters of samples obtained by different processes Results Explanation Table 5 shows that the samples obtained in Examples 1, 2, and 3... The values ​​were all significantly higher than those of Comparative Examples 1 and 4. The red color of tuna meat mainly comes from its myoglobin content. During heat treatment, the heme in myoglobin is easily oxidized to brown methemoglobin, resulting in a product... The color darkens as the myoglobin's value decreases. The two-stage heat treatment process of low-temperature baking and low-temperature sterilization used in this technical solution slows down the oxidation and denaturation rate of myoglobin under gentle thermodynamic conditions. In contrast, the harsh heat treatments in Comparative Examples 1 and 4 lead to rapid degradation of myoglobin, thus... The value decreased significantly. The value decreased. The value increased, showing obvious browning.

[0087] Comparing the data of Example 1 and Comparative Example 5, the sample of Example 1... The color value was higher than that of Comparative Example 5. This indicates that the brine soaking step set before the pretreatment step has a direct positive impact on the color of the final product. The brine solution environment can stabilize the molecular structure of myoglobin to a certain extent, reducing its heat sensitivity during subsequent heat treatment, thereby inhibiting the conversion to metmyoglobin. Comparative Example 5, lacking this step, experienced more significant oxidation of its myoglobin under the same heat treatment conditions, resulting in a lower redness value.

[0088] Comprehensive data show that pre-stabilizing the color-preserving proteins through a color-protecting treatment step, combined with a subsequent two-stage mild heat treatment process, can effectively inhibit the deterioration caused by the oxidative degradation of myoglobin during heat processing, thereby maintaining the high quality of tuna products. The color parameters of the samples in Comparative Examples 2 and 3 were not significantly different from those in Example 1. The technical features that played a decisive role in the color of the final product were the intensity of the heat treatment and the pretreatment of the coloring proteins.

Claims

1. A process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products, characterized in that, Includes the following steps: S1. Pre-treatment steps for marinating tuna raw materials; S2. The tuna raw material processed in the pretreatment step is subjected to low-temperature baking to obtain the tuna product. In the low-temperature baking step, turbulent circulating air is used to heat the tuna raw material. The turbulent air field is used to break the thermal resistance boundary layer on the surface of the raw material to achieve uniform heating of the fish pieces. S3. Cooling step of cooling the tuna product; S4. Vacuum packaging step of vacuum packaging the cooled tuna products; S5. The sterilization step of subjecting the vacuum-packed tuna products to ultra-high pressure cold sterilization treatment; the ultra-high pressure cold sterilization treatment utilizes a liquid medium to transmit pressure and destroy the microbial cell structure at room temperature or low temperature.

2. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, In the pretreatment step, the tuna raw material is marinated for 10 to 24 hours at a temperature of 2 to 8°C.

3. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, The conditions for the low-temperature baking step are: baking temperature of 75-95℃ and baking time of 40-80 minutes.

4. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, The cooling step involves cooling the core temperature of the tuna product to below 20°C. The vacuum degree after the vacuum packaging step is -0.090 to 0.098 MPa.

5. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, The conditions for the ultra-high pressure cold sterilization treatment are: pressure of 300-600 MPa, pressure holding time of 5-15 minutes, and temperature of the pressure transmission medium of 10-25℃.

6. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, The pretreatment step uses a pickling solution, which is made of the following components by mass: 3-8 parts salt, 10-20 parts white sugar, 0.2-0.5 parts compound phosphate, 1-3 parts spices, and the balance water.

7. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 3, characterized in that, In the low-temperature baking step, by controlling the relative humidity of the baking environment to 60% to 85%, the center temperature of the tuna product reaches 65 to 75°C at the end of the low-temperature baking.

8. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, Before the pretreatment step begins, a color-protecting treatment is also included for the tuna raw material, wherein the color-protecting treatment is as follows: The tuna raw material is soaked in a 3% to 5% salt solution for 15 to 30 minutes.

9. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 1, characterized in that, The tuna raw material is a piece of yellowfin tuna or bigeye tuna with a thickness of 2-4 cm.

10. The process for maintaining the storage quality of vacuum-packed, low-temperature baked tuna products according to claim 5, characterized in that, Following the low-temperature sterilization step, the process further includes rapidly cooling the low-temperature sterilized tuna product to a core temperature below 30°C.

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