Preparation method of high-water-binding-capacity and high-toughness dried bean curd

Abstract

The invention relates to the technical field of bean product processing, and discloses a preparation method of high-water-binding-capacity and high-toughness dried bean curd, and the dried bean curd is prepared from the following raw materials in parts by weight: 1000 parts of conditioned soybean milk; 2.5 to 3.5 parts of a composite slow-release coagulator with protein affinity activity; 0.14 to 0.25 part of glutamine transaminase; the concentration of the conditioned soybean milk is 10-12 degrees Brix, and the pH value of the conditioned soybean milk is 6.8-7.2; the composite slow-release coagulator with protein affinity activity has a core-shell structure and is composed of coagulator core particles accounting for 80-85% of the total mass of the coagulator and a lipid-protein composite shell layer accounting for 15-20% of the total mass of the coagulator. According to the invention, a compact and uniform covalent cross-linked network is constructed through combination of protein affinity modification of a carrier and enzymatic anchoring and gradient heating, and unification of high water binding capacity and high toughness of the dried bean curd is realized.

Description

Technical Field

[0001] This invention relates to the field of soybean product processing technology, specifically to a method for preparing high water-holding and high-toughness dried tofu. Background Technology

[0002] Dried tofu is a traditional soybean product that is popular with consumers due to its unique taste and rich nutritional value. In actual production, the textural characteristics of dried tofu, especially its toughness (chewiness) and water retention (tenderness and yield), are key indicators for measuring product quality.

[0003] In traditional tofu production processes, the water-holding capacity and elasticity of dried tofu are usually inversely related. To achieve a good chewy texture and elasticity, traditional processes often use brine or gypsum for coagulation, combined with a high-intensity pressing and dehydration process. This results in a low moisture content in the finished product, often leading to a dry, hard, and rough texture, and a low yield. Conversely, if the pressing intensity is reduced or a higher moisture content is retained to increase the yield and improve the tenderness, the resulting soybean protein gel network often lacks strength, resulting in a soft and mushy texture, lacking the chewy feel that dried tofu should have, and easily breaking during subsequent processing or cooking, with poor slicing properties.

[0004] To improve the texture and water-holding capacity of dried tofu, existing technologies have attempted to introduce slow-release coagulant technology. This involves encapsulating inorganic coagulants such as calcium and magnesium salts with lipids (e.g., glyceryl monostearate) to slow down the coagulation reaction rate and build a more uniform and delicate gel network. However, this simple physical encapsulation technique suffers from significant interfacial compatibility defects. Because the lipid carrier surface is strongly hydrophobic while the soybean protein gel matrix is ​​hydrophilic, the interfacial binding force between the two is extremely weak. After the gel network is formed, the lipid-encapsulated particles act as structural defects or heterogeneous inclusions within the protein matrix. When the product is subjected to external pressure or chewing shear, stress easily concentrates at the lipid-protein interface, leading to interfacial slippage or even separation. This not only weakens the overall mechanical strength and toughness of the gel network but also makes it easy for water to escape along microscopic interfacial channels, making it difficult to truly achieve high toughness at high water content. Furthermore, existing heating coagulation processes are usually relatively simple and lack the ability to achieve orderly control of protein backbone construction and ion filling, further limiting the improvement of product quality. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing high water-holding and high-toughness dried tofu, which solves the problems of difficulty in achieving both water-holding capacity and toughness in existing dried tofu preparation processes, as well as the poor interfacial compatibility between traditional lipid-release coagulants and hydrophilic protein matrices, resulting in structural defects, insufficient mechanical strength, and poor water-holding stability in the gel network.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a high water-holding capacity and high toughness dried tofu, using the following technical solution:

[0008] A type of tofu with high water retention and high toughness is made from the following ingredients in parts by weight:

[0009] 1000 servings of seasoned soy milk;

[0010] 2.5-3.5 parts of a composite slow-release coagulant with protein affinity activity;

[0011] 0.14-0.25 parts of transglutaminase;

[0012] The concentration of the conditioning soy milk is 10-12°Brix, and the pH value is 6.8-7.2; the composite slow-release coagulant with protein affinity activity has a core-shell structure, consisting of a coagulant core particle accounting for 80%-85% of its total mass and a lipid-protein composite shell accounting for 15%-20% of its total mass.

[0013] The lipid-protein composite shell is wrapped around the surface of the coagulant core particles. The lipid-protein composite shell contains a lipid carrier matrix and an interface modifier dispersed therein. The content of the interface modifier is 5.0%-8.0% of the mass of the lipid carrier matrix.

[0014] By adopting the above technical solution, and through the synergistic effect of a protein affinity-active composite slow-release coagulant with a core-shell structure and transglutaminase, high toughness of dried tofu in a high moisture content state is achieved. The specific mechanism of action and technical effects are as follows:

[0015] Interface compatibility control and defect elimination:

[0016] Traditional lipid-encapsulated coagulants have a hydrophobic lipid shell, which naturally differs from the hydrophilic soybean protein gel matrix, resulting in weak binding between the two. After the gel network forms, lipid microcapsules often become "heterogeneous pores" or stress concentration points within the network, making them prone to interfacial slippage under external forces. This invention introduces 5.0%-8.0% of an interfacial modifier (protein) into the lipid carrier matrix to construct a lipid-protein composite shell. The interfacial modifier, on the one hand, utilizes its amphiphilicity to improve the dispersion stability of the coagulant particles in the aqueous soybean milk; on the other hand, it exposes hydrophilic amino acid residues on the lipid surface, eliminating phase separation at the micro-interface.

[0017] Optimization of sustained-release kinetics:

[0018] The lipid-protein composite shell is controlled at 15%-20% by mass, with 5.0%-8.0% modifier added. This ensures that the coagulant core does not leak during the initial mixing stage while giving the shell appropriate thermoresponsiveness. The addition of the modifier fine-tunes the density of the lipid lattice, allowing the shell to gradually disintegrate and release ions according to a predetermined program during subsequent heating. This avoids coarse protein aggregates caused by excessively high local ion concentrations, forming a uniform and delicate microporous gel structure, and further physically locking in moisture using capillary forces.

[0019] Preferably, the coagulant core particles are selected from anhydrous calcium chloride particles or anhydrous magnesium sulfate particles, with a particle size of 50-100 μm; the lipid carrier matrix is ​​glyceryl monostearate, with a melting point range of 58-65℃; and the interface modifier is selected from sodium caseinate or soy protein isolate.

[0020] By employing the above technical solution, and selecting core particles with a particle size of 50-100μm in combination with glyceryl monostearate within a specific melting point range, the specific surface area of ​​the coagulant is ensured to be moderate. This facilitates both uniform distribution in soy milk and easy encapsulation. Sodium caseinate and soy protein isolate are both rich in TG enzyme activity sites and exhibit good thermal stability, ensuring that they do not undergo denaturation or inactivation during the shell melting process, thus guaranteeing the efficient execution of subsequent enzymatic reactions.

[0021] Preferably, the protein-affinity composite sustained-release coagulant is obtained through the following steps:

[0022] Glyceryl monostearate is heated to 65-75℃ to melt, and an interface modifier is added and stirred to form a lipid-protein composite melt. According to the mass ratio, coagulant core particles are added to the lipid-protein composite melt, and high-shear homogenization is performed at 65-75℃ and 3000-5000 rpm for 1-3 minutes. Subsequently, the homogenized material is rapidly dispersed in an environment of 4-10℃ for cooling and solidification.

[0023] By employing the above-mentioned technical solution, and using a preparation process combining melt blending with high-shear homogenization and low-temperature quenching, the nanoscale or microscale uniform dispersion of the interface modifier in the lipid matrix is ​​ensured, preventing protein powder agglomeration. The high-shear action ensures uniform coating of the melt on the surface of the core particles, while the low-temperature quenching rapidly fixes the shell structure, preventing the inorganic salt core from absorbing moisture or pre-reacting with the protein, thereby obtaining composite microcapsules with uniform particle size, high encapsulation efficiency, and surface activity.

[0024] Secondly, the present invention provides a method for preparing high water-holding and high-toughness dried tofu, using the following technical solution:

[0025] A method for preparing high water-holding and high-toughness dried tofu includes the following steps:

[0026] S1. Mixing, Dispersion and Preliminary Anchoring: Add a complex slow-release coagulant with protein affinity activity to the conditioning soy milk, and add transglutaminase. After mixing evenly, incubate at 45-50℃. Transglutaminase catalyzes the cross-linking of the interface modifier on the surface of the complex slow-release coagulant with the soy milk protein.

[0027] S2, Gradient heating coagulation: A two-stage gradient heating program is performed on the slurry after step S1. In the first stage, the temperature is raised to 55°C and held at a constant temperature. In the second stage, the temperature is raised to 85°C and held at a constant temperature to form a gel structure.

[0028] S3. Compression molding: The gel structure is broken up, put into a mold, and then compressed and cooled to obtain the finished product.

[0029] By employing the above technical solution, this method constructs an ordered protein network structure through stepwise regulation of enzyme-catalyzed reaction kinetics and coagulant slow-release kinetics. Its specific process mechanism and innovative points are as follows:

[0030] Interface anchoring and network pre-construction (S1 phase mechanism):

[0031] In stage S1, the system temperature (45-50℃) is lower than the melting point of the lipid carrier (58-65℃). At this time, the composite slow-release coagulant is in a solid state, and the calcium / magnesium ions inside are locked. The introduced transglutaminase is in a high-activity range, specifically catalyzing the acyl transfer reaction between the glutamine residues of the soy milk protein and the lysine residues of the interface modifier on the surface of the composite slow-release coagulant. Essentially, this step involves covalently fixing the coagulant particles, acting as "cross-linking centers," to the free soy protein molecular chains before the gel network forms. This "anchoring before release" strategy prevents protein aggregation caused by excessively high local ion concentrations during subsequent coagulant release, ensuring a uniform distribution of gel nodes.

[0032] Gradient temperature rise-controlled dual solidification (S2 stage mechanism):

[0033] The two-stage heating program designed in this invention achieves spatiotemporal decoupling between chemical crosslinking and physical solidification:

[0034] Phase 1 (Heating to 55°C and holding at that temperature): At this stage, the temperature is still below the melting point of the lipid carrier, and the coagulant core has not been released in large quantities. This stage primarily involves a full enzymatic cross-linking reaction, further strengthening the covalent network backbone between protein molecules and forming a pre-gel network with a certain degree of elasticity. Simultaneously, a moderate thermal effect causes the protein molecules to stretch appropriately, exposing more hydrophobic groups.

[0035] The second stage (heating to 85°C and holding at that temperature): When the temperature exceeds the lipid melting point (approximately 60°C) and continues to rise, the lipid-protein complex shell undergoes a phase transition and melts, and the encapsulated calcium / magnesium ions are slowly released through the molten lipid channels. The released metal ions rapidly undergo ion bridging with the surrounding protein carboxyl groups that are already covalently anchored.

[0036] Since the protein network backbone has already been constructed through enzymatic catalysis in the first stage, the ions released in the second stage mainly serve to fill and harden the network, rather than being part of the initial network construction phase. This "double coagulation" mechanism avoids the coarse network structure and uneven porosity caused by traditional high-temperature rapid sputtering.

[0037] Microscopic pore structure and physical binding of water:

[0038] The gel structure formed by the above process exhibits a dense and uniform honeycomb network at the microscopic level, with a narrow pore size distribution. Compared to the coarse pores formed by traditional processes, the dense microporous structure generates significantly enhanced capillary forces, enabling the physical locking of a large amount of tap water within the gel pores. Even during subsequent pressing processes, water is not easily migrated or lost, thus achieving high water retention.

[0039] Preferably, in step S1, the static incubation time is 10-15 min; after adding the composite slow-release coagulant with protein affinity activity and before adding glutamine transaminase, the composite slow-release coagulant is stirred at 50-100 rpm for 1-2 min to ensure uniform suspension.

[0040] By adopting the above technical solution, low-speed stirring ensures the uniform dispersion of microcapsules in soy milk while avoiding the premature leakage of ions caused by high shear forces damaging the shell structure of the microcapsules. Strict control of the incubation time ensures that the interface anchoring reaction proceeds fully while avoiding excessive reaction that could lead to excessively high soy milk viscosity and affect subsequent processing operations.

[0041] Preferably, in step S2, the two-stage gradient heating program is as follows: in the first stage, the temperature is increased to 55°C at a heating rate of 1.0°C / min and held at a constant temperature for 30 min; in the second stage, the temperature is increased from 55°C to 85°C at a heating rate of 0.5°C / min and held at a constant temperature for 15-20 min.

[0042] By employing the above technical solution, the rapid heating rate in the first stage allows the system to quickly reach the optimal reaction temperature of the enzyme; the slow heating rate (0.5℃ / min) in the second stage is to match the melting rate of the lipid carrier with the diffusion rate of ions, preventing the lipid carrier from instantly disintegrating and the explosive release of ions due to excessively rapid heating. The gradual ion release process helps to form a more delicate and ordered gel microstructure.

[0043] Preferably, in step S3, the pressing pressure is 0.2-0.4 MPa, and the pressing time is 20-30 min. Before pressing, the gel structure is cut into small pieces of 1 cm × 1 cm × 1 cm and then placed into the mold. By adopting the above technical solution, the appropriate pressing parameters combined with the uniform size of the gel pieces are conducive to the drainage of excess free water and the fusion between the gel pieces. Because the gel network prepared by this invention has high toughness, it can withstand high pressing pressure without network collapse, thus giving the product a firm texture and good chewiness after pressing.

[0044] Preferably, before step S1, the soybeans are washed and soaked, then ground into a slurry at a dry soybean to water ratio of 1:5-1:6, and the soybean residue is separated by filtration. The resulting raw soybean milk is heated to 95-100℃ and maintained at a gentle boil for 5-8 minutes, then naturally cooled to 40-45℃, and the concentration and pH value are adjusted accordingly. By adopting the above technical solution, the specific boiling process (95-100℃ gentle boil) can moderately denature soybean protein, break down the tertiary structure of the protein, and expose the internal sulfhydryl and hydrophobic groups, providing reaction sites for subsequent enzymatic cross-linking and ionic bridging. At the same time, this process inactivates trypsin inhibitors and lipoxygenases, eliminates the beany taste, and improves the food safety of the product.

[0045] Preferably, the enzyme activity of transglutaminase is 100-120 U / g; the concentration of the conditioning soy milk is adjusted by adding deionized water or by evaporation and concentration, and the pH value is adjusted by adding hydrochloric acid or sodium hydroxide solution.

[0046] By adopting the above technical solution and specifying the exact enzyme activity and soy milk conditioning method, the constant ratio of substrate to catalyst in the reaction system is ensured, resulting in good reproducibility and stability of the process. Maintaining the pH value within the neutral range of 6.8-7.2 is not only the optimal pH range for transglutaminase activity but also the best pH environment for soy protein gel formation, which is beneficial for obtaining a product with a fine texture.

[0047] This invention provides a method for preparing high-water-holding and high-toughness dried tofu. It has the following beneficial effects:

[0048] 1. This invention solves the problem of balancing high water retention and high toughness in traditional tofu by constructing a core-shell structured slow-release coagulant with protein affinity and cooperating with the catalytic cross-linking of transglutaminase. Because the coagulant shell contains an interface modifier, it can form stable covalent bonds with the soybean protein matrix, eliminating interfacial defects between the lipid carrier and the hydrophilic matrix. This chemical bonding enhances the overall mechanical strength of the gel network, allowing the product to maintain a shear force exceeding 500g even when the water content is maintained above 67%.

[0049] 2. This invention employs a gradient temperature coagulation process, achieving the spatiotemporal ordered separation of enzymatic covalent crosslinking and ionic physical coagulation. In the first stage of low-temperature isothermal process, the lipid carrier remains intact, and transglutaminase catalyzes the construction of the protein backbone network. In the second stage of temperature increase, the carrier melts and slowly releases metal ions, filling and reinforcing the already formed backbone. This mechanism of first constructing the backbone and then ion filling avoids the localized violent reactions and coarse pores caused by traditional high-temperature sputtering, forming a dense and uniform microporous structure, and significantly improving physical water-holding capacity through capillary forces.

[0050] 3. The composite sustained-release coagulant prepared by this invention exhibits excellent dispersion stability and controllable sustained release. By introducing an amphiphilic protein interface modifier into the lipid carrier, the dispersion uniformity of microcapsules in aqueous soy milk is improved, preventing aggregation and floating due to lipid hydrophobicity. The uniformly dispersed coagulant particles act as cross-linking nodes in the gel network, ensuring the uniformity of the overall gel structure and avoiding the phenomenon of hard cores or soft, uneven textures within the product, thus improving the stability of the production process and the yield. Detailed Implementation

[0051] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0052] The non-GMO soybeans are commercially available food-grade raw materials with a protein content of ≥38% on a dry basis and a moisture content of ≤13%.

[0053] Anhydrous calcium chloride, CAS No.: 10043-52-4.

[0054] Anhydrous magnesium sulfate, CAS No.: 7487-88-9, purity ≥99.0%.

[0055] Glyceryl monostearate, CAS No.: 123-94-4, is a commercially available food additive grade product with a monoester content of ≥40% and a melting point range of 58-65℃.

[0056] Sodium caseinate, CAS No.: 9005-46-3, is a commercially available food-grade product with a protein content of ≥90.0% on a dry basis and good water solubility.

[0057] Soy protein isolate, CAS No.: 9010-10-0, is a commercially available food-grade product with a protein dry basis content of ≥90.0% and a nitrogen solubility index (NSI) of ≥85%.

[0058] Transglutaminase, CAS No.: 80146-85-6, is a commercially available food-grade preparation with an enzyme activity specification of 100 U / g.

[0059] Hydrochloric acid, CAS No.: 7647-01-0.

[0060] Sodium hydroxide, CAS No.: 1310-73-2.

[0061] Preparation Examples 1-5:

[0062] Preparation Example 1:

[0063] This preparation example provides a composite sustained-release coagulant with protein affinity activity, comprising the following steps:

[0064] Preparation of lipid-protein complex melt: Glyceryl monostearate was placed in a constant temperature reactor and heated to 70°C to melt it completely. Dry sodium caseinate powder was added to the molten glyceryl monostearate at a mass of 6.5% of the glyceryl monostearate. The mixture was stirred at 400 rpm for 12 min at a constant temperature of 70°C to uniformly disperse the protein powder in the lipid, thus forming a lipid-protein complex melt.

[0065] Coagulant core encapsulation and homogenization: Anhydrous calcium chloride particles with a particle size of 75 μm were weighed as coagulant cores. The coagulant cores were slowly added to the composite melt at a ratio of 17% of the total mass (i.e., the mass ratio of composite melt to coagulant cores was 17:83). The temperature was kept at 70℃, and a high-shear homogenizer was started and homogenized at a speed of 4000 rpm for 2 min.

[0066] Cooling granulation and curing: The homogenized slurry is rapidly dispersed in a low-temperature environment of 4-10℃, and the cooling time is controlled at 45s to allow the coating layer to solidify rapidly. The particles are collected and sieved to obtain the final product.

[0067] Preparation Example 2:

[0068] This preparation example provides a composite sustained-release coagulant with protein affinity activity, comprising the following steps:

[0069] Preparation of lipid-protein complex melt: Glyceryl monostearate was placed in a constant temperature reaction vessel and heated to 65°C to melt it completely. Dry sodium caseinate powder was added to the molten glyceryl monostearate at a mass of 5.0% of glyceryl monostearate. The mixture was stirred at 300 rpm for 15 min at a constant temperature of 65°C to form a lipid-protein complex melt.

[0070] Coagulation and homogenization of the coagulant core: Anhydrous magnesium sulfate particles with a particle size of 50 μm were weighed as the coagulant core. The coagulant core was added to the composite melt at a ratio of 15% of the total mass of the composite melt (i.e., the mass ratio of the composite melt to the coagulant core was 15:85). The temperature was kept at 65℃ and the mixture was homogenized at 3000 rpm for 3 min.

[0071] Cooling granulation and curing: The homogenized slurry is rapidly dispersed in a low-temperature environment of 4-10℃, and the cooling time is controlled within 30s to allow the coating layer to solidify rapidly. The particles are collected and sieved to obtain the final product.

[0072] Preparation Example 3:

[0073] This preparation example provides a composite sustained-release coagulant with protein affinity activity, comprising the following steps:

[0074] Preparation of lipid-protein complex melt: Glyceryl monostearate was placed in a constant temperature reactor and heated to 75°C to melt it completely. Dry sodium caseinate powder was added to the molten glyceryl monostearate at a mass of 8.0% of the glyceryl monostearate. The mixture was stirred at 500 rpm for 10 min at a constant temperature of 75°C to form a lipid-protein complex melt.

[0075] Coagulation and homogenization of the coagulant core: Weigh anhydrous calcium chloride particles with a particle size of 100μm as the coagulant core. Add the coagulant core to the composite melt at a ratio of 20% of the total mass (i.e., the mass ratio of composite melt to coagulant core is 20:80). Keep the temperature at 75℃ and homogenize at 5000rpm for 1min.

[0076] Cooling granulation and curing: The homogenized slurry is rapidly dispersed in a low-temperature environment of 4-10℃, and the cooling time is controlled at 60s to allow the coating layer to solidify rapidly. The particles are collected and sieved to obtain the final product.

[0077] Preparation Example 4:

[0078] This preparation example provides a composite sustained-release coagulant with protein affinity activity, which differs from Preparation Example 1 only in that it uses soy protein isolate instead of sodium caseinate, and includes the following steps:

[0079] Preparation of lipid-protein complex melt: Glyceryl monostearate was placed in a constant temperature reactor and heated to 70°C to melt it completely. Dry soy protein isolate powder was added to the molten glyceryl monostearate at a mass of 6.5% of the glyceryl monostearate. The mixture was stirred at 400 rpm for 12 min at a constant temperature of 70°C to form a lipid-protein complex melt.

[0080] Coagulant core encapsulation and homogenization: Anhydrous calcium chloride particles with a particle size of 75μm were weighed as coagulant cores. The coagulant cores were added to the composite melt at a ratio of 17% of the total mass of the composite melt. The temperature was kept at 70℃ and the mixture was homogenized at 4000rpm for 2min.

[0081] Cooling granulation and curing: The homogenized slurry is rapidly dispersed in a low-temperature environment of 4-10℃, cooled for 45 seconds, and the particles are collected and sieved to obtain the final product.

[0082] Preparation Example 5:

[0083] This preparation example provides a common sustained-release coagulant without protein affinity activity, which will be used in a subsequent comparative example, and includes the following steps:

[0084] Melting: Glyceryl monostearate is placed in a constant temperature reaction vessel and heated to 70°C to completely melt it without adding any protein modifiers;

[0085] Coagulant core encapsulation and homogenization: Anhydrous calcium chloride particles with a particle size of 75μm were weighed as coagulant cores. The coagulant cores were added to the melt at a ratio of 17% of the total mass of molten glyceryl monostearate. The temperature was kept at 70℃ and the mixture was homogenized at 4000rpm for 2min.

[0086] Cooling granulation and curing: The homogenized slurry is rapidly dispersed in a low-temperature environment of 4-10℃ and cooled for 45 seconds to allow the coating layer to solidify rapidly. The particles are then collected and sieved to obtain the final product.

[0087] Examples 1-4:

[0088] Example 1:

[0089] This embodiment provides a method for preparing high water-holding and high-toughness dried tofu, using the composite slow-release coagulant described in Preparation Example 1, including the following steps:

[0090] S1. Soy milk preparation and standardization: Select soybeans, wash and soak them, grind them into soy milk at a ratio of 1:6 (dry weight of soybeans to water), and filter to obtain raw soy milk; heat the raw soy milk to 100℃ and boil for 5 minutes, then let it cool naturally to 42℃; measure the concentration of soy milk and adjust it to 11°Brix; use dilute hydrochloric acid or sodium hydroxide solution to adjust the pH value of soy milk to 7.0.

[0091] S2. Mixing, Dispersion and Preliminary Anchoring: The composite slow-release coagulant prepared in Preparation Example 1 was added to the conditioned soy milk at a rate of 0.30% of the total mass of the soy milk. The carrier was suspended by stirring at a low speed of 80 rpm for 1.5 min. Then, transglutaminase was added at a rate of 0.40% of the mass of soybean protein in the soy milk. After stirring evenly, the mixture was kept at 48°C for 12 min for interfacial anchoring incubation.

[0092] S3, Gradient temperature rise coagulation: The mixed slurry is placed in a programmable temperature control device. First, the temperature is raised to 55℃ at a rate of 1.0℃ / min and held at this temperature for 30min (first stage); then the temperature is raised to 85℃ at a rate of 0.5℃ / min and held at this temperature for 18min (second stage) to complete the construction of the gel structure.

[0093] S4. Pressing and molding: Cut the solidified tofu into 1cm3 pieces, put them into a mold, press them under a pressure of 0.3MPa for 25 minutes, release the pressure and demold, and let them cool naturally to obtain the product.

[0094] Example 2:

[0095] This embodiment provides a method for preparing high water-holding and high-toughness dried tofu, using the composite slow-release coagulant described in Preparation Example 2, including the following steps:

[0096] S1. Soy milk preparation and standardization: Select soybeans, wash and soak them, grind them into soy milk at a ratio of 1:5 (dry weight of soybeans to water), and filter to obtain raw soy milk; heat the raw soy milk to 95℃ and boil for 8 minutes, then let it cool naturally to 40℃; measure the soy milk concentration and adjust it to 10°Brix, and use dilute hydrochloric acid to adjust the pH value of the soy milk to 6.8.

[0097] S2. Mixing, Dispersion and Preliminary Anchoring: The composite slow-release coagulant prepared in Preparation Example 2 was added to the conditioned soy milk at a rate of 0.25% of the total mass of the soy milk. The carrier was suspended by stirring at a low speed of 50 rpm for 2 minutes. Then, transglutaminase was added at a rate of 0.35% of the mass of soybean protein in the soy milk. After stirring evenly, the mixture was kept at 45°C for 15 minutes for interfacial anchoring incubation.

[0098] S3, Gradient temperature rise coagulation: The mixed slurry is placed in a programmable temperature control device. First, the temperature is raised to 55℃ at a rate of 1.0℃ / min and held at this temperature for 30min (first stage); then the temperature is raised to 85℃ at a rate of 0.5℃ / min and held at this temperature for 15min (second stage) to complete the construction of the gel structure.

[0099] S4. Pressing and molding: Cut and break the solidified tofu into pieces, put them into a mold, press them under a pressure of 0.2MPa for 30 minutes, release the pressure and demold, and let them cool naturally to obtain the product.

[0100] Example 3:

[0101] This embodiment provides a method for preparing high water-holding and high-toughness dried tofu, using the composite slow-release coagulant described in Preparation Example 3, including the following steps:

[0102] S1. Soy milk preparation and standardization: Select soybeans, wash and soak them, grind them into soy milk at a ratio of 1:6 (dry soybean weight to water), and filter to obtain raw soy milk; heat the raw soy milk to 100℃ and boil for 5 minutes, then let it cool naturally to 45℃; measure the soy milk concentration and adjust it to 12°Brix; use sodium hydroxide solution to adjust the pH value of the soy milk to 7.2.

[0103] S2. Mixing, Dispersion and Preliminary Anchoring: The composite slow-release coagulant prepared in Preparation Example 3 was added to the conditioned soy milk at a rate of 0.35% of the total mass of the soy milk. The carrier was suspended by stirring at a low speed of 100 rpm for 1 min. Then, transglutaminase was added at a rate of 0.45% of the mass of soybean protein in the soy milk. After stirring evenly, the mixture was kept at 50°C for 10 min for interfacial anchoring incubation.

[0104] S3, Gradient temperature rise coagulation: The mixed slurry is placed in a programmable temperature control device. First, the temperature is raised to 55℃ at a rate of 1.0℃ / min and held at this temperature for 30min (first stage); then the temperature is raised to 85℃ at a rate of 0.5℃ / min and held at this temperature for 20min (second stage) to complete the construction of the gel structure.

[0105] S4. Pressing and molding: Cut and break the solidified tofu into pieces, put them into a mold, press them under a pressure of 0.4MPa for 20 minutes, release the pressure and demold, and let them cool naturally to obtain the product.

[0106] Example 4:

[0107] This embodiment provides a method for preparing high water-holding and high-toughness dried tofu, using the composite slow-release coagulant modified with soy protein isolate as described in Preparation Example 4, including the following steps:

[0108] S1. Soy milk preparation and standardization: Select soybeans, wash and soak them, grind them into soy milk at a ratio of 1:6 (dry weight of soybeans to water), and filter to obtain raw soy milk; heat the raw soy milk to 100℃ and boil for 5 minutes, then let it cool naturally to 42℃; measure the soy milk concentration and adjust it to 11°Brix, and adjust the pH value to 7.0.

[0109] S2. Mixing, dispersing and initial anchoring: Add the composite slow-release coagulant prepared in Example 4 to the conditioned soy milk, the amount added is 0.30% of the total mass of soy milk, and stir evenly at 80 rpm; then add transglutaminase, the amount added is 0.40% of the mass of soybean protein in soy milk, stir evenly, and then let stand at 48℃ for 12 min.

[0110] S3, Gradient temperature rise and solidification: The mixed slurry is placed in a programmable temperature control device, first heated to 55℃ at a rate of 1.0℃ / min, and held at this temperature for 30min; then heated to 85℃ at a rate of 0.5℃ / min, and held at this temperature for 18min.

[0111] S4. Pressing and molding: Cut and break the solidified tofu into pieces, put them into a mold, press them under a pressure of 0.3MPa for 25 minutes, release the pressure and demold, and let them cool naturally to obtain the product.

[0112] Comparative Examples 1-5:

[0113] Comparative Example 1:

[0114] Compared with Example 1, the difference is that the traditional brine coagulation process is used, without the use of compound slow-release coagulant and transglutaminase; the specific operation is to boil the soy milk and then cool it to 85°C, and directly add the same amount of calcium chloride aqueous solution as in Example 1 for coagulation, let it stand for 15 minutes, break the coagulant, and press it according to the same pressure and time.

[0115] Comparative Example 2:

[0116] Compared with Example 1, the difference is that the coagulant used in step S2 is replaced with the ordinary slow-release coagulant obtained in Preparation Example 5 (i.e., the carrier shell is pure glyceryl monostearate, without protein affinity modification), while the other steps and parameters are the same.

[0117] Comparative Example 3:

[0118] Compared with Example 1, the difference lies in the heating procedure in step S3, which does not use gradient heating. Specifically, after the mixed slurry is kept at 48°C, it is directly heated to 85°C at a rate of 3.0°C / min and held for 20min without going through a 55°C constant temperature stage. The remaining steps and parameters are the same.

[0119] Comparative Example 4:

[0120] Compared with Example 1, the difference is that glutamine transaminase is not added in step S2, while the other steps and parameters are the same.

[0121] Comparative Example 5:

[0122] Compared with Example 1, the difference is that in step S2, a composite slow-release coagulant is not used. Instead, unencapsulated anhydrous calcium chloride particles of the same mass as in Example 1 are directly added. The remaining steps and parameters are the same.

[0123] Test Example 1:

[0124] Experimental objective:

[0125] The physicochemical properties and textural characteristics of the high water-holding and high-toughness tofu prepared in Examples 1-4 were verified, and the feasibility and stability of the "protein affinity modified carrier combined with enzymatic crosslinking and gradient temperature process" proposed in this invention were evaluated in actual preparation.

[0126] Experimental steps:

[0127] Moisture content determination: Direct drying method was used. 3.0 g (accurate to 0.0001 g) of the pulverized tofu sample was placed in a 105℃ forced-air drying oven and dried to constant weight. The percentage of the mass difference before and after drying relative to the original mass was calculated. Each sample was measured in triplicate, and the average value was taken.

[0128] Centrifugal water loss determination: Cut the dried tofu sample into cubes of 1.0cm × 1.0cm × 1.0cm, and accurately weigh and record the weight. Place the sample in a centrifuge tube with a filter and centrifuge at 4000 rpm for 20 minutes using a low-speed refrigerated centrifuge. After centrifugation, remove the sample, blot off any surface oil and moisture with filter paper, and weigh it. The calculation method for centrifugal water loss rate is as follows: ( Divide by Multiply by 100%. The lower the value of this index, the stronger the physical binding ability of the gel network to water.

[0129] Texture properties (TPA and shear force) were measured using a TA-XT Plus texture analyzer.

[0130] Shear force test: The HDP / BS cutter probe was used, the test speed was set to 1.0 mm / s, and the maximum force required to cut the sample was recorded to characterize the toughness of the sample.

[0131] Total Texture Assay (TPA) test: A P / 36R cylindrical probe was used, with a compression ratio set to 50%. The initial compression speed was 2.0 mm / s, the final compression speed was 1.0 mm / s, and the interval between compressions was 5 seconds. Hardness and elasticity parameters were recorded. Ten different locations were selected from each sample for testing, and the average value was taken.

[0132] Experimental data (see Table 1):

[0133] Table 1

[0134] Group Moisture content (%) Centrifugal water loss rate (%) Shear force (g) Hardness (g) Elasticity (mm) Example 1 69.34 2.87 542.6 1256.4 0.94 Example 2 67.82 3.51 518.3 1302.1 0.89 Example 3 68.55 3.14 567.9 1288.7 0.92 Example 4 68.91 3.22 535.8 1243.5 0.93

[0135] in conclusion:

[0136] According to the test data in Table 1, the dried tofu prepared in Examples 1 to 4 all exhibited excellent water retention and mechanical strength.

[0137] The water content of each sample group remained stable above 67%, and the centrifugal water loss rate was controlled below 3.5%. This indicates that the gel network structure constructed in this invention possesses extremely high physical stability. At the microscopic level, this high water retention capacity stems from the uniform microporous structure formed during the gradient heating process. Because the sustained-release carrier did not completely disintegrate during the first stage of heating (55°C), the explosive release of calcium / magnesium ions was limited, avoiding excessive aggregation of protein molecules and the formation of large pores. The capillary forces generated by the fine and uniform pores effectively limited the migration of free water.

[0138] Secondly, the shear force of each group of samples exceeded 500g while maintaining good elasticity. This result confirms that "carrier shell protein affinity modification" and "TG enzyme cross-linking" produced a substantial synergistic effect. By introducing sodium caseinate or soy protein isolate onto the surface of the lipid carrier, transglutaminase can catalyze the formation of covalent cross-links between the coagulant carrier shell and the matrix protein. This chemical bonding eliminates the interfacial compatibility differences between the lipid microcapsules and the protein hydrogel, preventing stress concentration or slippage at the interface under external forces.

[0139] Example 4 uses soy protein isolate instead of sodium caseinate. Its shear force (535.8g) and water content (68.91%) are very close to those of Example 1, indicating that the technical solution has good raw material universality. As long as it is a protein modifier containing glutamine / lysine residues, it can achieve effective anchoring of the carrier and matrix.

[0140] Test Example 2:

[0141] Experimental objective:

[0142] By comparing the physicochemical properties and textural characteristics of the tofu prepared in Example 1 with those of the comparative products, the specific contribution and technical effect of each key technical feature in the present invention (especially the protein affinity modification of the carrier shell, the gradient temperature program, and the enzymatic crosslinking) on ​​the water retention and toughness of the final product are evaluated.

[0143] Experimental steps:

[0144] Sample preparation:

[0145] The tofu products prepared in Example 1 and Comparative Examples 1 to 5 were selected, their edges and corners were removed, and they were cut into samples of uniform size for testing. All samples were placed in a sealed container at 25°C for 2 hours before testing to eliminate the influence of temperature differences on the test results.

[0146] Moisture content and water retention stability test:

[0147] The water content and centrifugal water loss rate of each group of samples were determined using the same method as in Test Example 1. The water content reflects the total amount of water locked in the product, and the centrifugal water loss rate reflects the degree of binding of water in the gel network.

[0148] Experimental data (see Table 2):

[0149] Table 2

[0150] Group Key variable descriptions Moisture content (%) Centrifugal water loss rate (%) Shear force (g) Hardness (g) Chewable (g) Example 1 Complete technical solution (protein modification vector + gradient temperature) 69.34 2.87 542.6 1256.4 985.2 Comparative Example 1 Traditional brine coagulation process 58.21 8.45 485.4 1623.1 845.6 Comparative Example 2 The carrier was not modified with protein affinity (pure lipid shell). 66.85 6.12 415.7 1105.3 723.4 Comparative Example 3 Gradient-free heating (rapid heating to 85°C) 61.43 5.88 462.9 1489.2 812.5 Comparative Example 4 No added transglutaminase 64.12 7.35 312.4 895.6 456.8 Comparative Example 5 Add coagulant directly (without slow-release carrier) 56.90 9.12 588.2 1756.8 910.3

[0151] in conclusion:

[0152] The decisive role of carrier interface modification in toughness (Example 1 vs. Comparative Example 2):

[0153] Comparative Example 2 used the same sustained-release carrier matrix and TG enzyme addition amount as Example 1, except that sodium caseinate was not added to the carrier shell for modification. Data showed that although the water content of Comparative Example 2 (66.85%) was improved compared to the traditional process, its shear force (415.7g) was significantly lower than that of Example 1 (542.6g), a decrease of more than 23%, and the centrifugal water loss rate increased from 2.87% to 6.12%.

[0154] This result confirms the crucial role of "carrier shell protein affinity modification." Without modification, interfacial compatibility defects exist between the lipid carrier surface and the soybean protein hydrogel matrix, creating structural weaknesses. Under external shear or centrifugal forces, water easily seeps out along the lipid interface, and the gel network is prone to slippage and breakage at the interface. Example 1, by introducing protein active sites on the carrier surface and utilizing TG enzymes, achieved covalent anchoring between the carrier and the matrix, eliminating interfacial defects and thus improving network toughness and water-holding stability while maintaining high water content.

[0155] The regulatory effect of gradient heating program on network structure (Example 1 vs. Comparative Example 3):

[0156] Comparative Example 3 eliminated the 55℃ isothermal stage, resulting in a decrease in water content to 61.43% and an increase in hardness. This is because rapid heating shortened the action time of the TG enzyme, leading to insufficient cross-linking density between protein molecules. Simultaneously, excessively rapid heating caused an imbalance between the coagulant release rate and the protein gelation rate, causing the microporous structure to undergo thermal shrinkage or collapse in the early stages of formation, thus failing to effectively lock in moisture through capillary action. The gradient heating program ensured an orderly process of first building the framework and then filling and fixing it.

[0157] Synergistic effect of sustained-release technology and enzyme preparations (Example 1 vs. Comparative Examples 1, 4, and 5):

[0158] Compared with traditional processes (Comparative Example 1): In Example 1, even with an increase of approximately 11 percentage points in moisture content, the shear force was still higher than that of traditional products, thus changing the traditional characteristics of dried tofu, which is "mushy when there is too much moisture and hard when there is too little moisture".

[0159] Compared with the enzyme-free group (Comparative Example 4): When TG enzyme cross-linking is lacking, the product has extremely low shear force (312.4g) and loose texture, which cannot meet the chewiness that dried tofu should have. This shows that calcium and magnesium ions alone cannot support the gel strength under high moisture content.

[0160] Compared with the non-slow-release group (Comparative Example 5): Direct addition of coagulant led to a violent reaction, with extremely high product hardness (1756.8g) but the lowest water content (56.90%) and the highest centrifugal water loss rate, indicating that the formed protein network was coarse and uneven with poor water holding capacity.

Claims

1. A type of tofu with high water retention and high toughness, characterized in that, Made from the following ingredients in parts by weight: Conditioned soy milk: 1000 servings; A composite slow-release coagulant with protein affinity activity: 2.5-3.5 parts; Transglutaminase: 0.14-0.25 parts; The conditioned soy milk has a concentration of 10-12°Brix and a pH value of 6.8-7.2; The composite sustained-release coagulant with protein affinity activity has a core-shell structure, consisting of a core particle of coagulant accounting for 80%-85% of its total mass and a lipid-protein composite shell accounting for 15%-20% of its total mass. The lipid-protein composite shell is wrapped around the surface of the coagulant core particles. The lipid-protein composite shell contains a lipid carrier matrix and an interface modifier dispersed therein. The content of the interface modifier is 5.0%-8.0% of the mass of the lipid carrier matrix.

2. The high water-holding and high-toughness dried tofu according to claim 1, characterized in that, The coagulant core particles are selected from anhydrous calcium chloride particles or anhydrous magnesium sulfate particles, with a particle size of 50-100 μm; the lipid carrier matrix is ​​glyceryl monostearate, with a melting point range of 58-65℃; the interface modifier is selected from sodium caseinate or soy protein isolate.

3. The high water-holding and high-toughness dried tofu according to claim 1, characterized in that, The protein-affinity-based composite sustained-release coagulant is obtained through the following steps: Glyceryl monostearate is heated to 65-75℃ to melt, and an interface modifier is added and stirred to form a lipid-protein composite melt; According to the mass ratio, the coagulant core particles are added to the lipid-protein composite melt, and high-shear homogenization is carried out at 65-75℃ and 3000-5000rpm for 1-3 minutes. The homogenized material was then rapidly dispersed into an environment of 4-10℃ for cooling and solidification.

4. A method for preparing a high water-holding and high-toughness dried tofu as described in any one of claims 1-3, characterized in that, Includes the following steps: S1. Mixing, Dispersion and Preliminary Anchoring: Add the aforementioned composite slow-release coagulant with protein affinity activity to the conditioning soy milk, and add transglutaminase. After mixing evenly, incubate at 45-50℃. Transglutaminase catalyzes the cross-linking of the interface modifier on the surface of the composite slow-release coagulant with the soy milk protein. S2, Gradient heating coagulation: A two-stage gradient heating program is performed on the slurry after step S1. In the first stage, the temperature is raised to 55°C and held at a constant temperature. In the second stage, the temperature is raised to 85°C and held at a constant temperature to form a gel structure. S3. Compression molding: The gel structure is broken, placed into a mold, and then compressed and cooled to obtain the finished product.

5. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, In step S1, the static incubation time is 10-15 min; after adding the composite slow-release coagulant with protein affinity activity and before adding glutamine transaminase, the composite slow-release coagulant is stirred at 50-100 rpm for 1-2 min to ensure uniform suspension.

6. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, In step S2, the two-stage gradient heating procedure is specifically as follows: The first stage involves heating to 55°C at a rate of 1.0°C / min and holding at that temperature for 30 minutes. The second stage involves heating from 55℃ to 85℃ at a rate of 0.5℃ / min and maintaining the temperature at that rate for 15-20 minutes.

7. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, In step S3, the pressing pressure is 0.2-0.4 MPa, and the pressing time is 20-30 min.

8. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, In step S3, before the compression molding, the gel structure is cut into small pieces of 1cm×1cm×1cm and then placed into the mold.

9. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, Before step S1, wash and soak the soybeans, then grind them into a paste at a ratio of 1:5 to 1:6 of dry soybean weight to water, and filter to separate the soybean residue. Heat the resulting raw soybean paste to 95-100℃ and maintain a gentle boil for 5-8 minutes, then let it cool naturally to 40-45℃ before adjusting the concentration and pH value.

10. The method for preparing high water-holding and high-toughness dried tofu according to claim 4, characterized in that, The enzyme activity of the transglutaminase is 100-120 U / g; the concentration of the conditioning soy milk is adjusted by adding deionized water or by evaporation and concentration, and the pH value is adjusted by adding hydrochloric acid or sodium hydroxide solution.