Microwave pretreatment and application of microwave pretreatment and TGase enzyme coupling in quality improvement of dried beancurd stick products

Abstract

The invention relates to microwave pretreatment and application of microwave pretreatment and TGase enzyme coupling in improvement of the quality of a dried beancurd stick product, and particularly provides a method for improving the quality of the dried beancurd stick product. According to the method, the mindset that a microwave technology is applied to a terminal drying link in dried beancurd stick preparation, such as dried beancurd stick drying, and charring and protein denaturation caused by local overheating are avoided is broken through, microwave pretreatment is performed on dried soybeans under specific conditions, and the dried soybeans are in a micro-puffing state through the synergistic effect of a heat effect and a non-heat effect generated by microwaves, so that the dried soybeans are in a micro-puffing state; microwave pretreatment and soaking generate a synergistic effect mechanism, so that the soaking time is effectively shortened, the soybean pre-soaking process and soybean milk components and physical properties are optimized, further coupling with TGase enzyme is carried out, construction of a protein molecule cross-linked network and better embedding of grease in the dried beancurd stick film forming process are effectively promoted, and the dried beancurd stick film forming efficiency and comprehensive quality are remarkably improved; and technical support is provided for mechanical large-scale production, and upgrading of the dried beancurd stick industry is facilitated.

Description

Technical Field

[0001] This application belongs to the field of food processing technology, specifically relating to a method for improving the quality of dried bean curd products, and the dried bean curd products obtained by using this method, specifically relating to microwave pretreatment and its application technology coupled with TGase enzyme in the production of dried bean curd. Background Technology

[0002] With increasing public concern for environmental sustainability and nutritious, healthy foods, plant-based foods are becoming a focus of attention in both the consumer market and scientific research. Tofu skin (also known as bean curd sticks) is a traditional soy product. It is a protein-lipid complex film formed by air convection on the surface of soy milk under heating conditions. Its formation mechanism stems from the endothermic aggregation process of heat-denatured proteins at the gas / liquid interface, and the synergistic effect of intermolecular forces between proteins and lipid molecules. Macroscopically, tofu skin is a film formed by the embedding of some lipids within a protein cross-linking network. The structural characteristics of soy protein are a core factor affecting the quality of soy milk and tofu skin. Its secondary structure (α-helix, β-sheet) and tertiary spatial conformation directly regulate the conformational unfolding kinetics and cross-linking intensity of protein molecules during heat induction, thus determining the film-forming properties and quality of tofu skin.

[0003] In the traditional processing of dried bean curd sticks, soaking is a key pretreatment step. Its process parameters (temperature, time, etc.) have a significant impact on the water absorption rate, hardness, and solid loss of soybeans, which directly affect the quality of soy milk. Existing research mainly focuses on changing the protein aggregation process in soy milk. As a precursor to the formation of dried bean curd sticks, the physical properties of soy milk, such as its composition (protein and oil content), particle size distribution, stability, free sulfhydryl groups, and disulfide bond content, directly determine its film-forming performance. Conventional processing methods are difficult to effectively control these key indicators. Although soybean pulping is currently showing a trend of multi-technology collaborative innovation, problems such as low film-forming efficiency, poor mechanical properties, poor color, unstable quality, complex processes, and high costs still exist, which limit the industrial development and quality improvement of the dried bean curd stick industry.

[0004] Microwave technology, as an efficient physical modification method, can induce high-frequency vibrations of polar molecules through electromagnetic fields, generating thermal and non-thermal effects, inducing protein structure recombination and unfolding, and weakening the intramolecular and intermolecular forces of soybean protein. Theoretically, it has a positive effect on the construction of protein molecular cross-linking networks during the film formation process of dried bean curd sticks. Therefore, some studies have applied microwave technology to the drying process of dried bean curd sticks. However, microwave treatment during the drying process also has drawbacks such as large processing volume, large equipment investment, and long time consumption.

[0005] Enzyme cross-linking technology is highly favored due to its high biodegradability, specificity, rapid catalytic rate, and high yield. Some enzymes used for cross-linking proteins include transglutaminase, lipoxygenase, lysyl oxidase, polyphenol oxidase, and peroxidase. Transglutaminase (TGase), with its highly efficient protein cross-linking ability, is known as the "super adhesive of the 21st century" and has shown unique application value. It has been widely used in the preparation of edible films to improve their mechanical and barrier properties.

[0006] How to apply microwave technology more effectively to the preparation of dried bean curd sticks, thereby improving production efficiency, optimizing the film-forming quality of dried bean curd sticks, and enhancing the quality of dried bean curd stick products, is a challenge faced by technical workers in this field. Summary of the Invention

[0007] Purpose of the invention: One objective of this invention is to provide a method for improving the quality of dried bean curd products based on microwave pretreatment.

[0008] Another objective of this invention is to provide a method for improving the quality of dried bean curd products based on microwave pretreatment.

[0009] Technical issues: This method solves the problems of low film-forming efficiency, poor mechanical properties, unsatisfactory color, and unstable quality in traditional tofu skin processing.

[0010] Technical solution: A method for improving the quality of dried bean curd products involves microwave pretreatment of dried soybeans. The preparation of dried bean curd includes the following steps: (1) microwave treatment of dried soybeans; (2) soaking soybeans; (3) grinding into a paste; (4) boiling the paste; and (5) drying after removing the film.

[0011] Preferably, step (1) involves loading selected soybeans into the feed hopper of a microwave puffing system, with a conveying rate of 90~60m / min, an irradiation time of 30~120s, a working frequency of 2450MHz, a fixed power of 4kw, pretreatment completed in the microwave resonant cavity, and then cooling to room temperature and sealing for storage after discharge.

[0012] Preferably, step (2) involves soaking the soybeans at a water-to-material ratio of 1:5 (w / v) at 25°C for 12 hours, filtering out the unabsorbed water, and then adding water a second time at 8-10 times the weight of the corresponding dry soybeans.

[0013] Preferably, in step (3), the primary crushing is carried out using a soymilk maker, with crushing conditions of 35,000 rpm for 30 seconds; the fine grinding is carried out using a colloid mill, with grinding at 3,000 rpm for 180 seconds, and the soymilk is obtained by passing through a 200-mesh sieve.

[0014] Preferably, step (4) involves adjusting the pH of the soy milk to 7.5 and the solids concentration to 7%, and boiling it at 100°C for 3-5 minutes.

[0015] Preferably, step (5) involves forming a film on the soybean milk at 85-90℃, lifting the film every 20 minutes until it can no longer form a film, and then drying it at 60℃ until the moisture content is about 8.0% to obtain dried bean curd sticks.

[0016] In a further preferred step, step (1) uses 91 soybeans as raw material, performs screening and impurity removal treatment, selects mature and plump soybeans with relatively uniform particle size, the weight of the selected soybeans is 2.0-5.0kg, and adopts XP-4KW microwave puffing system.

[0017] Further preferred, the soy milk obtained in step (3) is preheated to 50°C, TGase enzyme of 60-120 U / kg dry soybeans is added, and after stirring evenly, it is kept at 50°C for 50 min, and then the steps (4) of boiling the soy milk and (5) of removing the film and drying are carried out.

[0018] Further preferred, a process combination is set with material conveying rate gradient parameters of 90, 80, 70, and 60 m / min, and synchronously matched with microwave irradiation time gradients of 30, 60, 90, and 120 s.

[0019] A further preferred process combination is set with a material conveying rate gradient parameter of 70 m / min and a synchronous matching microwave irradiation time gradient of 90 s.

[0020] Compared with existing technologies, the following beneficial effects are achieved: (1) Breaking the conventional thinking of applying microwave technology to the drying of end products (e.g., dried bean curd) in the preparation of dried bean curd to avoid local overheating leading to scorching and protein denaturation, this paper describes the use of microwave technology to treat dried soybeans under specific conditions. Through the synergistic effect of the thermal and non-thermal effects of microwave technology, the dried soybeans exhibit a slightly puffed state, causing the hydrogen bonds and hydrophobic bonds within the protein molecules to break. This induces the reorganization of the secondary structure and the unfolding of the tertiary structure of the protein in the dried soybeans, affecting the content of functional groups. During soaking, the synergistic effect of microwave pretreatment and soaking is observed. Although the hydrogen bonds and hydrophobic bonds within the protein molecules break after microwave treatment of the dried soybeans, the protein molecules are still slightly puffed. When there is little moisture, the soybeans cannot fully expand. Therefore, during soaking, the protein and water molecules combine to promote hydration, full release of proteins and lipids, and protein expansion. This effectively shortens the soaking time and improves the soaking effect, optimizing the soybean pre-soaking process and the composition and properties of soy milk. It effectively promotes the construction of protein molecular cross-linking networks and better oil embedding during the film formation process of dried bean curd sticks. Microwave pretreatment makes the dried bean curd sticks form a denser and more uniform protein network structure, with increased porosity and more uniform distribution, thereby significantly improving the film-forming efficiency, enhancing film strength and toughness, improving rehydration performance, improving color, and improving the overall quality of dried bean curd stick products. Through microwave pretreatment combined with soaking and precise soy milk preparation and dried bean curd stick forming process, the elongation at break of dried bean curd sticks is significantly increased by about 32-81%, the elongation at break is increased by about 75-196%, and the rehydration capacity is increased by about 32-42% (when rehydrated for 30 minutes), making it suitable for the industrial production of traditional soy products. (2) The microwave processing time of this application is only 30 to 120 seconds, that is, it is completed within 2 minutes. The process is simple, the time is short, the raw material processing volume is large, the equipment investment is small, it is not easy to cause pollution, and the production cost is greatly reduced. In addition, the reasonable control of microwave time effectively avoids the phenomenon of holes and uneven thickness that are easy to occur in the production of dried bean curd. It provides technical support for mechanized and large-scale production and promotes the upgrading of the dried bean curd industry. (3) TGase enzyme-assisted microwave pretreatment further improved the quality of dried bean curd sticks. After microwave treatment, the concentration of soluble protein in the soybean milk increased, the concentration of substrate catalyzed by TGase enzyme increased, and the intermolecular and intramolecular interactions during protein cross-linking assembly were rapidly enhanced. The synergistic effect of TGase enzyme and microwave pretreatment optimized the entire process of dried bean curd stick preparation. After TGase-assisted microwave pretreatment, the tensile strength of dried bean curd sticks reached 10.47 g / mm². 2 Compared to the microwave 90s group (5.63±0.23g / mm), 2 The concentration of the substance increased by 87.12%, while the control group only reached 3.08 g / mm. 2 The elongation at break increased to 85.42%; dried bean curd L Value (brightness) and a The redness value was better than that of the control group. TGase enzyme cross-linking promotes the formation of covalent bonds in proteins, which further improves their mechanical properties and color while consuming Maillard reaction precursors; (4) The microwave pretreatment of dried beans in this application also improves the storage stability of dried bean curd sticks. Storage tests show that if the dried bean curd sticks are not microwave treated or microwave treated at other times, the moisture loss is faster, the color changes significantly, and they turn yellow and darken, resulting in a decline in sensory quality; while the dried bean curd sticks pretreated by microwave in this application maintain a relatively stable moisture content, have less color change, and have better sensory quality within the same storage time. Attached Figure Description

[0021] Figure 1 Changes in the stability coefficient and sedimentation rate of soy milk under different microwave pretreatment times.

[0022] Figure 2 The effect of microwave pretreatment time of soybeans before soaking on the yield of dried bean curd sticks.

[0023] Figure 3 Changes in Fourier transform infrared spectra of dried bean curd sticks after different microwave pretreatment times before soaking soybeans.

[0024] Figure 4 Electrophoretic images of dried bean curd sticks after different microwave pretreatment times before soaking soybeans.

[0025] Figure 5 The effect of different microwave pretreatment times before soaking soybeans on the content of free sulfhydryl groups and disulfide bonds in dried bean curd sticks.

[0026] Figure 6 The effect of different microwave pretreatments on the tensile strength of dried bean curd sticks.

[0027] Figure 7 The effect of different microwave pretreatments on the elongation at break of dried bean curd sticks.

[0028] Figure 8 The effect of different microwave pretreatments on the rehydration performance of dried bean curd sticks.

[0029] Figure 9 The effect of different amounts of TGase enzyme added combined with microwave pretreatment on the tensile strength and elongation at break of dried bean curd sticks. Detailed Implementation

[0030] Example 1: The preparation of dried bean curd products includes the following steps: (1) Soybean screening and preparation: Select 91 soybeans, screen to remove impurities, select mature and plump soybeans with uniform particle size, weigh 2.0 kg of high-quality soybean raw materials after screening, and set aside for later use; (2) Microwave treatment of dry soybeans (moisture content 8.20±0.14%): The weighed soybeans were quantitatively loaded into the feed hopper of the XP-4KW microwave puffing system. The material conveying rate was set to 70m / min and the microwave irradiation time was 90s. The material was puffed and modified in the microwave resonant cavity with a working frequency of 2450MHz and a power of 4kw. The material was discharged through a screw conveyor. After discharge, the temperature was recorded by an infrared thermometer. After cooling to room temperature, the material was sealed and stored in a desiccator. (3) Soaking: Weigh 160.0g of raw soybeans after microwave treatment, wash them with deionized water, add deionized water at a material-to-liquid ratio of 1:5 (w / v), and soak at 25℃ for 12h. (4) Grinding: After soaking, filter to remove the free water phase, and then add deionized water a second time at a material-to-liquid ratio of 1:8 (w / v) (the material is the soybeans obtained after soaking for 12 hours and then filtering to remove unabsorbed water). The amount of water added is 8 times the corresponding weight of dry soybeans. The primary crushing process is carried out using a JYL-Y912 wall-breaking soy milk machine at 30s and 35000rpm. The primary ground slurry is then transferred to a JM-L50 colloid mill for nano-level fine grinding at 180s and 3000rpm. Finally, the slurry is filtered through a 200-mesh sieve to obtain soy milk. (5) Preparation of dried bean curd sticks: Adjust the film formation parameters, set the pH to 7.5 and the solids concentration to 7%, boil the soy milk at 100℃ and keep boiling for 3 minutes, then pour it into a constant temperature water bath and form a film at 80℃. Lift the film every 20 minutes until the soy milk can no longer form a film. Place the tofu sticks after lifting the film in an oven and dry them at 60℃ for about 6 hours to obtain a soybean protein film with a moisture content of about 8.0%.

[0031] Furthermore, based on the above, material conveying rate gradient parameters were set to 90, 80, 70, and 60 m / min, and process combinations with microwave irradiation time gradients of 30, 60, 90, and 120 s were synchronously matched to obtain different microwave pretreated soy milk and dried bean curd products.

[0032] Comparative Example 1: Immersion Simultaneous Microwave Treatment Group The preparation of the dried bean curd product differs from that in Example 1 in that it does not undergo treatment (2). After treatments (1) and (3), the soaked soybeans are irradiated for 90 seconds using a MAS-IIPlus high-pressure microwave synthesis reactor (2450MHz, 4kW) after the soaking process (i.e., the filtered and dehydrated soybeans are irradiated after soaking, and the microwave treatment is set approximately synchronously with the soaking stage in the entire dried bean curd product preparation process). Subsequent operations are basically the same as in Example 1 (4) and (5).

[0033] Comparative Example 2: Microwave Processing Unit with Simultaneous Pulping The preparation of the bean curd product differed from that in Example 1 from that in Example 1 in that it did not undergo treatment (2). After treatments (1), (3), and (4), the bean curd was irradiated for 90 seconds using a MAS-II Plus high-pressure microwave synthesis reactor (2450MHz, 4kW). Subsequent operations were basically the same as in Example 1 (5).

[0034] Control group: No microwave pretreatment was performed.

[0035] Example 2: The preparation of dried bean curd products includes the following steps: (1) Soybean screening and preparation: Select 91 soybeans, screen to remove impurities, select mature and plump soybeans with uniform particle size, weigh 2.0 kg of high-quality soybean raw materials after screening, and set aside for later use; (2) Microwave treatment of dry soybeans (moisture content 8.20±0.14%): The weighed soybeans were quantitatively loaded into the feed hopper of the XP-4KW microwave puffing system. The material conveying rate was set to 70m / min and the microwave irradiation time was 90s. The material was puffed and modified in the microwave resonant cavity with a working frequency of 2450MHz and a power of 4kw. The material was discharged through a screw conveyor. After discharge, the temperature was recorded by an infrared thermometer. After cooling to room temperature, the material was sealed and stored in a desiccator. (3) Soaking: Weigh 160.0g of raw soybeans after microwave treatment, wash them with deionized water, add deionized water at a material-to-liquid ratio of 1:5 (w / v), and soak at 25℃ for 12h. (4) Grinding: After soaking, filter to remove free water phase, then add deionized water again at a material-to-liquid ratio of 1:8 (w / v) (the material is soybeans obtained after soaking for 12 hours and then filtering). Use a JYL-Y912 wall-breaking soy milk machine for primary crushing treatment. The crushing conditions are 30s and 35000rpm. Then transfer the primary ground slurry to a JM-L50 colloid mill for nano-level fine grinding. The grinding time is 180s and the speed is 3000rpm. Finally, filter the slurry through a 200-mesh sieve to obtain soy milk. (5) Preheat the soy milk obtained in step (4) to 50°C, add 60-120 U / kg of TGase enzyme from dry soybeans, stir evenly, and keep warm at 50°C for 50 min. (6) Preparation of dried bean curd sticks: Adjust the film formation parameters, set the pH to 7.5 and the solids concentration to 7%, boil the soy milk at 100℃ and keep boiling for 3 minutes, then pour it into a constant temperature water bath and form a film at 80℃. Lift the film every 20 minutes until the soy milk can no longer form a film. Place the toan curd sticks after lifting the film in an oven and dry them at 60℃ for about 6-7 hours to obtain a soybean protein film with a moisture content of about 8.0%.

[0036] In addition, based on the above, TGase enzymes with added 60, 80, and 120 U / kg of dried soybeans were prepared to obtain different tofu skin products.

[0037] Indicator Measurement: (1) Determination of soybean milk particle size Particle size distribution was determined using a Nano-ZS90 laser particle size analyzer.

[0038] (2) Determination of the stability coefficient and centrifugal sedimentation rate of soy milk Take a well-mixed soy milk and dilute it with pure water at a volume ratio of 1:15. Measure the absorbance A0 at a wavelength of 785 nm. Take another 10.0 g of well-mixed soy milk, centrifuge it at 3500 r / min for 15 min, and then take 0.5 mL of the intermediate whey phase. Dilute it using the same method and measure the absorbance A1.

[0039] Stability coefficient =

[0040] In the formula: A0 is the absorbance of the soy milk before centrifugation; A1 is the absorbance of the whey phase after centrifugation.

[0041] Accurately weigh 10.0 g of the mixed soy milk, centrifuge at 3500 r / min for 15 min, discard the supernatant and accurately weigh the mass of the precipitate m1.

[0042] Centrifugal sedimentation rate =

[0043] In the formula: m0 is the mass of the sample before centrifugation, g; m1 is the mass of the precipitate after centrifugation, g.

[0044] (3) Changes in functional groups and chemical bonds: Fourier transform infrared spectroscopy (4) Protein composition analysis: SDS-PAGE electrophoresis (5) Content of free thiol groups and disulfide bonds: acetone pretreatment method is preferred. (6) Determination of mechanical strength of dried bean curd sticks The mechanical strength of dried bean curd sticks is a key indicator for measuring their physical properties, and it is related to the product's toughness and processability.

[0045] The dried bean curd sticks were cut into uniform sizes (2cm × 7cm) and placed in an environment created by saturating barium chloride solution (90% relative humidity, 25℃) for 24 hours to equilibrate moisture. Tensile strength and elongation at break were determined using a physical property analyzer. An A / TG probe was selected, with an initial distance of 4cm, a trigger force of 5g, and a testing speed of 1mm / s before and during the test, and 5mm / s after the test. Each sheet of dried bean curd stick was tested at least 5 times. Tensile strength calculation formula:

[0046] Where TS represents tensile strength, F is the force at fracture, and S is the cross-sectional area of ​​the dried bean curd stick. Formula for calculating elongation at break:

[0047] Where EAB is the elongation at break, L0 is the original length of the dried bean curd stick, and L1 is the length after stretching.

[0048] (7) Determination of the rehydration properties of dried bean curd sticks Rehydration performance reflects the ability of dried bean curd sticks to recover their original shape and physical properties after absorbing water, and is an important factor in evaluating the quality of dried bean curd sticks. Dried bean curd stick products were taken and rehydrated using 30℃ deionized water. After each rehydration, the surface moisture of the dried bean curd sticks was absorbed with filter paper and weighed immediately. Each test was repeated three times, and the average value was taken. Eleven sampling time points were set: 15s, 30s, 45s, 1min, 3min, 5min, 10min, 15min, 20min, 25min, and 30min. The rehydration ratio was calculated using the formula: Ff = Mf / Mg, where Ff is the rehydration ratio, Mf is the mass of the dried bean curd sticks after rehydration, and Mg is the mass of the dried bean curd sticks before rehydration.

[0049] (8) Stability analysis of dried bean curd during storage Storage stability reflects the ability of dried bean curd sticks to maintain quality during storage and is of great significance to the product's shelf life. Dried bean curd sticks of this application and unmicrowaved dried bean curd sticks were placed in the same sealed packaging and stored at room temperature (25°C) and relative humidity 60%. Samples were taken every 7 days to determine the moisture content and color change (L). a b Values) and sensory qualities (including appearance, smell, taste, etc.).

[0050] Measurement results: Hydrated particle size distribution can characterize the degree of protein aggregation and changes in spatial conformation. The results of hydrated particle size distribution are shown in Table 1.

[0051] Table 1. Particle size distribution of soy milk at different microwave pretreatment times

[0052] Table 1 shows that the polydispersity index (PDI) ranged from 0.07 to 0.16, indicating that the particle distribution of the measurement system was uniform. Compared with the control group (516.43 ± 5.63 d.nm), the average particle size of the soybean milk in the microwave pretreatment group was significantly lower than that in the control group. p <0.05 This may be because the structure of soybean protein changes during the microwave process, with the breakage of certain groups and disulfide bonds causing the protein to expand, thereby reducing the particle size of some protein molecules.

[0053] Figure 1 Figures A and B show the changes in the stability coefficient and sedimentation rate of soybeans processed into soy milk with different microwave pretreatment times. Stability is one of the important indicators for evaluating the quality of soy milk. The higher the stability coefficient and the lower the centrifugal sedimentation rate, the more stable the dispersion system of the soy milk. The stability coefficient of soy milk showed a trend of first increasing and then decreasing with microwave time; while the centrifugal sedimentation rate of soy milk showed a trend of decreasing with microwave time. With the extension of microwave pretreatment time, the soluble protein content of soy milk showed a trend of first increasing and then decreasing. Compared with the control group (stability coefficient 57.52 ± 0.25% and centrifugal sedimentation rate 3.30 ± 0.60%), the overall dispersion system stability of the microwave group was improved. p The value <0.05 is likely due to the short-term microwave heating breaking the disulfide bonds within the molecules, shortening the peptide chains, and thus reducing the particle size of the soy milk, thereby enhancing its stability. However, at a microwave time of 120 seconds, the stability coefficient of the soy milk showed an opposite trend to the centrifugal sedimentation rate. This may be because excessive microwave heating leads to a decrease in protein concentration, affecting the centrifugal sedimentation rate of the soy milk.

[0054] Figure 2 Although microwave time showed no significant effect on the yield of dried bean curd sticks (p>0.05), statistical comparison of means revealed that the yield of the microwave pretreatment groups (90, 120 s) was approximately 13% higher than that of the control group (no microwave). This is likely because the strong penetrability and high heating efficiency of microwaves can disrupt cell walls, thereby increasing the exposure of intracellular compounds, improving protein extraction efficiency, and ultimately increasing the yield of dried bean curd sticks. On the other hand, microwaves can promote the depolymerization of some globular proteins, thereby exposing more hydrophobic and polar groups and increasing the interactions on the protein surface.

[0055] like Figure 3 As shown, FTIR spectroscopy confirmed that the molecular chain structure of dried bean curd sticks had the same peaks before and after microwave pretreatment. All dried bean curd stick samples showed peaks in the 3200-3400 cm⁻¹ range. -1 Peaks are observed at various points, which are caused by the bending vibrations of -OH and -NH. Peaks are observed at 2924 and 2850 cm⁻¹. -1 Peaks are observed at each point, representing the stretching vibrations of the CH2 group. 1748 cm⁻¹ -1 1623 cm -1 (Amide I) and 1528 cm -1 The peaks at (amide II) represent the coupling of C=O group stretching, CN stretching vibration, and NH bending vibration in the peptide molecule. Compared to the control group, the peaks at 3200-3400 cm⁻¹... -1 At wavelengths of 2924 and 2850 cm⁻¹, the intensity of microwave pretreatment in all groups of dried bean curd was lower than that in the control group, indicating that microwaves promoted the coiling of protein chains and reduced the exposure of polar groups. -1The presence of these peaks indicates the existence of aliphatic chain structures in the sample. The presence and intensity changes of these peaks may reflect the effect of microwave treatment on the orderliness of hydrophobic regions in the dried bean curd. At the amide I band, the intensity of the microwave pretreated group was also lower than that of the control group, and the peak intensity continued to decrease, which may be related to thermally induced molecular chain breakage or dissociation of hydrogen bond networks. The characteristic peak intensity rebound shown by microwave treatment at 120 s may be due to the formation of new covalent bonds (such as ester bonds or amide bonds recombination) in the microwave overheating crosslinking reaction.

[0056] SDS-PAGE image ( Figure 4 Analysis showed that dried bean curd sticks mainly contain typical 7S globulin α', α and β subunits, and acidic and basic subunits of 11S globulin components. With increasing microwave time, the content of 7S α' and α subunits and the acidic subunit A of 11S globulin components in dried bean curd sticks continuously increased, indicating that microwave treatment promoted the protein components and their cross-linking and aggregation in soy milk. This was manifested in an increase in the acidic subunits of 7S protein components and 11S globulin components, an increase followed by a decrease in the content of free sulfhydryl groups, and an increase in the disulfide bond content from 5.33 μmol / g to 20.84 μmol / g. This promoted the formation of the network structure in dried bean curd sticks and improved their quality.

[0057] Figure 5 The study showed that different microwave pretreatment times affected the content of free thiol groups in dried bean curd sticks. During the formation of dried bean curd sticks, heating the soy milk exposes and accumulates free thiol groups, leading to the formation of disulfide bonds. With prolonged microwave time, the content of free thiol groups in the dried bean curd sticks initially increased and then decreased, while the content of disulfide bonds continuously increased. This is because high microwave power heating causes protein molecules to unfold, exposing more free thiol groups. Simultaneously, due to enhanced hydrophobic interactions, the distance between intramolecular thiol groups shortens. In the presence of oxygen, more free thiol groups and shorter distances can form intramolecular or intermolecular disulfide bonds. Other studies have also found that microwave heating reduces the total thiol content in wheat protein and promotes disulfide bond formation.

[0058] Figure 6 The results showed that the tensile strength (TS) of the dried bean curd sheets significantly increased compared with the control group, the soaking-simultaneous microwave treatment group, and the pulping-simultaneous microwave treatment group. In the soaking-simultaneous microwave treatment group, dynamic soaking and microwave-promoted exposure of protein hydrophilic groups occurred simultaneously, and protein molecules also underwent hydration with water, resulting in enhanced protein water-holding capacity and swelling. This placed higher demands on the intensity and time of microwave treatment. In the pulping-simultaneous microwave treatment group (i.e., microwave treatment of soy milk), the viscosity of the soy milk increased after microwave treatment, the film-forming rate decreased, and to some extent, excessive protein cross-linking occurred, leading to a tendency for the dried bean curd sheet film to be brittle and hard, with reduced flexibility.

[0059] Figure 7The results showed that the microwave pretreatment group of dried beans also showed excellent results in terms of elongation at break compared with the control group, the soaking and microwave treatment group, and the pulping and microwave treatment group.

[0060] Figure 8 The results showed that, compared with the control group, the soaking-simultaneous microwave treatment group, and the pulping-simultaneous microwave treatment group, the dried bean microwave pretreatment group maintained good rehydration performance throughout the rehydration process. This may be because the intermolecular interactions of proteins in the dried bean group were appropriate, the protein lipid membrane structure was more dense and uniform, and the porosity of the bean curd membrane was increased and more uniform, thereby increasing its rehydration capacity.

[0061] The overall weight score of different microwave times on the quality of dried bean curd sticks is shown in Table 2.

[0062] Table 2. Overall weighting of different microwave times on the quality of dried bean curd sticks

[0063] As shown in Table 2, the control group had the worst overall score. The overall score reached its peak after microwaving for 90 seconds, which was 32.44% higher than that of the control group. The dried bean curd in the 90-second microwave group had a bright, light yellow color, a natural oily sheen, excellent mechanical properties, and good rehydration performance.

[0064] Figure 9 The mechanical strength of dried bean curd was determined by combining different amounts of TGase enzyme with microwave pretreatment. Figure 9 The results showed that, as indicated by A, microwave treatment significantly increased the total saturation (TS) of dried bean curd sticks compared to the control group, and further treatment with TGase enzymes significantly increased the TS of dried bean curd sticks. p <0.05). The total sulfide (TS) of dried bean curd sticks was highest at a TGase enzyme addition of 100 U / kg dried beans, at 10.47 ± 0.26 g / mm. 2 Compared to the control group (3.08 ± 0.49 g / mm), 2 The elongation at break increased by 239.98%, which is 87.12% higher than the microwave 90 s group. This may be due to the enhanced film strength, possibly because the covalent cross-linking between protein chains increases the TS value. TGase not only forms strong intermolecular cross-links with protein molecules, but its induced intracellular cross-linking may also lead to an increase in the TS of the bean curd film. As shown in B, the elongation at break of microwave-treated bean curd significantly increases with the addition of TGase enzyme. p <0.05), reaching a peak value of 85.42 ±4.72% when the TGase enzyme addition was 100 U / kg dry beans.

[0065] The color of dried bean curd sticks with different amounts of TGase enzyme added and microwave pretreatment are shown in Table 3.

[0066] Table 3. Effects of different TGase enzyme dosages combined with microwave pretreatment on the color of dried bean curd sticks.

[0067] Table 3 shows the effect of different TGase enzyme addition amounts on the color of microwave-pretreated bean curd sticks. Compared with the control, the L of microwave-treated bean curd sticks with TGase enzyme addition... The value increased significantly ( p <0.05), a value and b The values ​​all decreased significantly ( p< The value of 0.05 indicates that the brightness of the bean curd sticks increased and the reddish-yellow color became lighter after the addition of TGase. Except when the enzyme addition amount was 120 U / kg dried beans, there was no significant difference in color between different enzyme addition amounts. p >0.05), which may be because the ε-(γ-glutamyl)-lysine isopeptide bonds formed by the cross-linking between protein molecules caused by TGase treatment can consume key precursors of the Maillard reaction, thereby reducing non-enzymatic browning during the film formation process of dried bean curd, resulting in increased brightness and a lighter reddish-yellow color in the dried bean curd.

[0068] The above embodiments of the present invention are mainly used to help understand the technical solutions of the present invention, but do not constitute a limitation of the present invention. Any improvements or equivalent substitutions made based on the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the protection scope of the present invention.

Claims

1. The application of microwave pretreatment in improving the quality of dried bean curd products, characterized in that, Microwave pretreatment of dried soybeans and preparation of dried bean curd sticks include the following steps: (1) microwave treatment of dried soybeans; (2) soaking soybeans; (3) grinding into a paste; (4) boiling the paste; (5) drying after removing the film.

2. The application according to claim 1, characterized in that, Step (1) involves loading selected soybeans into the feed hopper of a microwave puffing system, with a conveying speed of 90~60m / min, an irradiation time of 30~120s, a working frequency of 2450MHz, a fixed power of 4kw, pretreatment in the microwave resonant cavity, and cooling to room temperature and sealing for storage after discharge.

3. The application according to claim 1 or 2, characterized in that, Step (2) involves soaking the soybeans at a water-to-material ratio of 1:5 (w / v) at 25°C for 12-13 hours, discarding any unabsorbed water, and then adding water a second time at 8-10 times the weight of the dry soybeans.

4. The application according to claim 3, characterized in that, Step (3) Primary crushing is done using a soymilk maker at 35,000 rpm for 30 seconds; fine grinding is done using a colloid mill at 3,000 rpm for 180 seconds, and the soymilk is obtained by passing it through a 200-mesh sieve.

5. The application according to claim 4, characterized in that, Step (4) involves adjusting the pH of the soy milk to 7.5 and the solids concentration to 7%, and boiling it at 100°C for 3-5 minutes.

6. The application according to claim 5, characterized in that, Step (5) involves forming a film in the soy milk at 85-90℃, lifting the film every 20 minutes until it can no longer form a film, and then drying it at 60℃ until the moisture content is about 8.0% to obtain dried bean curd sticks.

7. The application according to claim 6, characterized in that, Step (1) Using Kedou 91 as raw material, screening and impurity removal are carried out to select mature and plump soybeans with relatively uniform particle size. The weight of the selected soybeans is 2.0-5.0kg. The XP-4KW microwave puffing system is used.

8. The application of microwave pretreatment coupled with TGase enzyme in improving the quality of dried bean curd products, characterized in that, in any one of the applications described in claims 1-7, the soy milk obtained in step (3) is preheated to 50°C, 60-120 U / kg of TGase enzyme from dried soybeans is added, and after stirring evenly, it is kept at 50°C for 50 min, and then the steps (4) of boiling the soy milk and (5) of removing the film and drying are carried out.

9. The application according to any one of claims 1-8, characterized in that, The material conveying rate gradient parameters were set to 90, 80, 70, and 60 m / min, and the process combination was synchronously matched with microwave irradiation time gradients of 30, 60, 90, and 120 s.

10. The application according to any one of claims 1-8, characterized in that, The delivery rate is 70 m / min, the irradiation time is 90 s, and / or 100 U / kg of TGase enzyme is added.

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