Thermal induction preparation method and application of starch-soybean protein isolate composite hydrogel

Abstract

The invention relates to the technical field of starch and protein deep processing, in particular to a thermal induction preparation method and application of starch-protein composite hydrogel. The composite hydrogel is prepared from potato starch (PS), mung bean starch (MBS), corn starch (CS) and cassava starch (TS) respectively with soybean protein isolate (SPI), and the starch-soybean protein isolate composite hydrogel is improved in texture characteristics such as strength, hardness, elasticity and the like and rheological characteristics compared with pure soybean protein isolate hydrogel. The potato starch-soybean protein isolate composite hydrogel has the maximum hardness, elasticity, adhesiveness, chewiness, linear viscoelastic region and storage modulus, the cassava starch-soybean protein isolate composite hydrogel has high viscidity and cohesiveness, and the strain characteristics of the prepared starch-soybean protein isolate composite hydrogel are improved. The composite hydrogel can be better applied to the field of embedding and delivery of some bioactive substances or medicines.

Description

Technical Field

[0001] The invention relates to the technical field of starch and protein processing, and in particular to a thermal induction preparation method and application of a starch-soy protein isolate composite hydrogel. Background Art

[0002] Starch is primarily classified into three categories: cereal starch, legume starch, and potato starch, each with distinct characteristics. Starch is a natural polymer composed of two types of biomacromolecules: amylose and amylopectin. Starch is insoluble in cold water and occurs in irregular, round granules, typically ranging from 2 to 100 μm in size. Besides being edible, starch can be used as a thickener, adhesive, stabilizer, and emulsifier, finding widespread application in food ingredients and finished products. Corn starch, rice starch, and wheat starch are examples of cereal starches. Most cereal starches have relatively small granules, with an amylose content of approximately 20-30%. Starch is the most abundant component in corn, typically accounting for 68-75%. Corn also contains approximately 10% protein, 4-5% fat, and small amounts of various vitamins. Corn starch typically has an average particle size of approximately 13 μm and a typical A-type crystal structure. It is often used in food processing as a stabilizer and thickener. Pea starch and mung bean starch are legume starches. The amylose content in legume starch is generally higher than that in cereal and potato starches. Pea starch has an amylose content of approximately 35%, while mung bean starch has an amylose content of approximately 31.7-41.8%. Mung bean starch accounts for approximately 25-60% of the dry weight of the mung bean. Mung bean starch also has a small and varying particle size, typically around 5-35 μm, and exhibits an A-type or C-type crystalline structure. Potato starch and tapioca starch are tuber starches. Tuber starch particles are generally larger than cereal starches, with an amylose content of approximately 17-20%. Potato starch has an amylose content of approximately 20%, while tapioca starch has an amylose content of approximately 17%. The volume-average particle size of tapioca starch is approximately 12.7 μm. Potatoes are the fourth most important food crop in the world, after rice, wheat, and corn. Compared to corn and wheat starch, potato starch has larger granules, a higher amylopectin content, a higher degree of amylose polymerization, a higher molecular weight, stronger water-binding capacity, and contains natural phosphate groups. This contributes to its high viscosity, ease of gelatinization, and excellent paste stability and transparency. Potatoes contain 70-90% total starch, are relatively white and lustrous, and have granules ranging in size from 5 to 150 μm, larger than typical starch granules, and primarily round or oval in shape. X-ray diffraction patterns indicate that PS is a type B starch. Potato starch is widely used in ready-to-eat foods, sauces, and puffed foods, serving as a good thickener in food processing. In recent years, the effects of certain food gums on the properties of potato starch have garnered increasing research attention.

[0003] Soybean is one of the main oil crops in the world, containing 20% oil, 30% non-starch polysaccharides and 35 - 40% protein. Soybean protein refers to the general term of various proteins in soybeans, rather than a single protein. Among various soybean proteins, the protein content of soy protein isolate is the highest, reaching more than 90%. Soy protein isolate is a complete protein produced from low-temperature defatted soybean meal. The content of essential amino acids is 27%, and the types of amino acids include 20 kinds including essential amino acids for the human body. It is one of the few plant proteins that can replace animal proteins. Soybean 7S and 11S proteins are the main components of soy protein isolate, accounting for more than 70%. Soybean 7S protein, also known as β-conglycinin, accounts for about 34% of soy globulin. Soybean 11S protein, also known as glycinin, accounts for about 42% of the globulin in soybean seeds. Among them, the content of 11S protein reflects the gel properties. The larger the value, the stronger the gel properties. Along with the increase of 7S / 11S, this will lead to a decrease in gel hardness and viscosity. Soybean protein gel can be defined as the phenomenon that protein molecules denature and unfold and then aggregate. During the aggregation process, the attractive force and repulsive force are in a balanced state, so that a highly ordered three-dimensional network structure or matrix that can hold a large amount of water can be formed. Heat treatment is the key factor to promote the unfolding of denatured protein molecules and the rearrangement of subunits. The process of gel formation is first the gradual exposure of functional groups such as sulfhydryl groups and hydrophobic groups, and then the exposed groups form aggregates through hydrophobic, hydrogen bond, electrostatic interaction or disulfide bonds. When the protein concentration is high enough, a gel will be further formed. Therefore, in the food industry, it is common to compound starch and protein. During the formation of protein gel, adding a certain amount of starch can form a polymer with a composite structure with the protein, which helps to form the protein gel network and improve the physicochemical properties of the protein gel, etc. Some bioactive components in food (such as riboflavin, anthocyanins, natural carotenoids, vitamins, flavonoids, probiotics, etc.) are unstable and are easily degraded during processing, storage and digestion. Therefore, it is of great significance to develop a food-based material that can effectively encapsulate riboflavin and is harmless to the human body, improve its bioavailability, and expand its application scope in the food and pharmaceutical fields. [[ID=~1]] Summary of the Invention [[ID=~3]] [[ID=~4]]

[0004] The purpose of the present invention is to provide a method for thermally inducing the preparation of a starch-soy protein isolate composite hydrogel and its application, and through the optimization of starch raw materials, to achieve the improvement of the texture properties such as strength, hardness, and elasticity and rheological properties of the starch-soy protein isolate composite hydrogel, and to use its composite hydrogel to encapsulate riboflavin. [[ID=~6]] [[ID=~7]]

[0005] The purpose of the present invention is achieved by the following technical means: [[ID=~9]] [[ID=~10]]

[0006] A method for thermally inducing the preparation of a starch-soy protein isolate composite hydrogel and its application, comprising the following steps: [[ID=~12]] [[ID=~13]]

[0007] S1. Prepare by adding deionized water to a certain amount of protein powder, and adjust the pH value using NaOH solution;

[0008] S2. First, magnetically stir the soy protein isolate solution and then heat it in a water bath. After taking it out, cool it with cold water to obtain soy protein isolate paste;

[0009] S3. Add deionized water to starch to prepare a starch suspension;

[0010] S4. Mix the starch suspension and the soy protein isolate paste prepared by heating evenly respectively, and heat it in a water bath again. After taking it out and cooling, starch-soy protein isolate composite hydrogel is obtained.

[0011] S5. Select the optimal one of the starch-soy protein isolate composite hydrogels for embedding. After the heating of the starch-soy protein isolate suspension is completed, add riboflavin powder to it and stir quickly and evenly, and then cool it to obtain the starch-soy protein isolate composite hydrogel encapsulating riboflavin.

[0012] As a preferred scheme, in step S1, the proportion of soy protein isolate is 8% - 12% (w / v).

[0013] As a preferred scheme, in step S1, the concentration of the NaOH solution used is 2 mol / L, and the pH value is adjusted to 7.0 - 10.0.

[0014] As a preferred scheme, in step S2, the rotation speed of magnetic stirring is 300 rpm and the time is 10 min.

[0015] As a preferred scheme, in step S2, heat in a water bath at 70 - 100 °C for 5 - 20 min, then take it out and cool it with cold water for 5 min.

[0016] As a preferred scheme, in step S3, the starches are potato starch (PS), mung bean starch (MBS), corn starch (CS) and tapioca starch (TS) respectively, and the proportion of the starch is 1% - 6% (w / v). [[ID=,29]]

[0017] As a preferred scheme, in step S4, heat in a water bath at 70 - 100 °C for 5 - 20 min, then cool it with cold water for 5 min and then cool it overnight at 4 °C.

[0018] As a preferred scheme, in step S5, the addition amount of riboflavin is 5% (calculated based on the dry weight of starch-soy protein isolate), cool it with cold water for 5 min and then cool it overnight at 4 °C. Keep it away from light throughout the process after adding riboflavin to avoid the influence of light on riboflavin.

[0019] The beneficial technical effects of the present invention are reflected in the following aspects:

[0020] 1. The texture properties such as strength, hardness, and elasticity, as well as the rheological properties of the starch-soybean protein isolate composite hydrogel of the present invention are all improved compared to those of the pure soybean protein isolate hydrogel. The hardness, elasticity, adhesiveness, chewiness, linear viscoelastic region, and storage modulus of its potato starch-soybean protein isolate composite hydrogel are the largest, and the tapioca starch-soybean protein isolate composite hydrogel has relatively high viscosity and cohesiveness. The potato starch-soybean protein isolate composite hydrogel has a good encapsulation effect on riboflavin.

[0021] 2. The solvent used in the preparation process of the present invention is water, with low cost. The preparation process involves heating in a 95°C water bath to form a gel. Its process is simple, the process is controllable, safe and environmentally friendly, and is conducive to forming a gel with high strength, good elasticity, and good water retention. Description of the Drawings

[0022] Figure 1 Is the standard curve of riboflavin in simulated intestinal fluid

[0023] Figure 2 Is the starch-SPI composite hydrogel diagram

[0024] Figure 3 (A) Is the strain sweep of the starch-SPI composite hydrogel

[0025] Figure 3 (B) Is the change of the storage modulus (G′) and loss modulus (G″) of the starch-SPI composite hydrogel Detailed Embodiments

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0027] Specific Embodiment Case 1

[0028] Accurately weigh 11.00 g of soy protein isolate powder, add deionized water to prepare a 10% (w / v) soy protein isolate suspension, and then use 2 mol / L NaOH solution to adjust the pH value to about 7; the soy protein isolate suspension is magnetically stirred at 300 rpm at room temperature for 10 min, then heated in a water bath at 85 °C for 10 min, taken out and cooled with cold water for 5 min; then accurately weigh 3.30 g of potato starch (PS), add deionized water to prepare a 3% (w / v) potato starch suspension; mix the potato starch suspension evenly with the soy protein isolate paste prepared by heating, so that the concentration of soy protein isolate is 10% (w / v) and the concentration of potato starch is 3% (w / v), and then heat in a water bath at 85 °C for 15 min again, taken out and cooled with cold water for 5 min, and then cooled overnight at 4 °C to obtain a potato starch-soy protein isolate composite hydrogel. The prepared gel sample is named PS-SPI.

[0029] Specific implementation case 2

[0030] Accurately weigh 11.00 g of soy protein isolate powder, add deionized water to prepare a 10% (w / v) soy protein isolate suspension, and then use 2 mol / L NaOH solution to adjust the pH value to about 7; the soy protein isolate suspension is magnetically stirred at 300 rpm at room temperature for 10 min, then heated in a water bath at 85 °C for 10 min, taken out and cooled with cold water for 5 min; then accurately weigh 3.30 g of mung bean starch (MBS), add deionized water to prepare a 3% (w / v) mung bean starch suspension; mix the mung bean starch suspension evenly with the soy protein isolate paste prepared by heating, so that the concentration of soy protein isolate is 10% (w / v) and the concentration of mung bean starch is 3% (w / v), and then heat in a water bath at 85 °C for 15 min again, taken out and cooled with cold water for 5 min, and then cooled overnight at 4 °C to obtain a mung bean starch-soy protein isolate composite hydrogel. The prepared gel sample is named MBS-SPI.

[0031] The difference between the preparation method of implementation case 2 and that of implementation case 1 is that the raw material potato starch is replaced by mung bean starch, and the other conditions are the same.

[0032] Specific implementation case 3

[0033] S1. Accurately weigh 11.00 g of soy protein isolate powder, add deionized water to prepare a 10% (w / v) soy protein isolate suspension, and then use 2 mol / L NaOH solution to adjust the pH value to about 7; the soy protein isolate suspension is magnetically stirred at 300 rpm at room temperature for 10 min, then heated in a water bath at 85 °C for 10 min, taken out and cooled with cold water for 5 min; then accurately weigh 3.30 g of corn starch (CS), add deionized water to prepare a 3% (w / v) corn starch suspension; mix the corn starch suspension evenly with the soy protein isolate paste prepared by heating, so that the soy protein isolate concentration is 10% (w / v) and the corn starch concentration is 3% (w / v), and then heat in a water bath at 85 °C for 15 min again, taken out and cooled with cold water for 5 min, and then cooled overnight at 4 °C to obtain a corn starch-soy protein isolate composite hydrogel. The prepared gel sample is named CS-SPI.

[0034] The difference in the preparation method between Example 3 and Example 2 is that the raw material potato starch is replaced by corn starch, and the other conditions are the same.

[0035] Specific Example 4

[0036] S1. Accurately weigh 11.00 g of soy protein isolate powder, add deionized water to prepare a 10% (w / v) soy protein isolate suspension, and then use 2 mol / L NaOH solution to adjust the pH value to about 7; the soy protein isolate suspension is magnetically stirred at 300 rpm at room temperature for 10 min, then heated in a water bath at 85 °C for 10 min, taken out and cooled with cold water for 5 min; then accurately weigh 3.30 g of tapioca starch (TS), add deionized water to prepare a 3% (w / v) tapioca starch suspension; mix the tapioca starch suspension evenly with the soy protein isolate paste prepared by heating, so that the soy protein isolate concentration is 10% (w / v) and the tapioca starch concentration is 3% (w / v), and then heat in a water bath at 85 °C for 15 min again, taken out and cooled with cold water for 5 min, and then cooled overnight at 4 °C to obtain a tapioca starch-soy protein isolate composite hydrogel. The prepared gel sample is named TS-SPI.

[0037] The difference in the preparation method between Example 4 and Example 3 is that the raw material corn starch is replaced by tapioca starch, and the other conditions are the same.

[0038] Comparative Example 1

[0039] 11.00 g of soy protein isolate powder was accurately weighed and added to deionized water to prepare a 10% (w / v) soy protein isolate suspension. The pH was then adjusted to approximately 7 using a 2 mol / L NaOH solution. The soy protein isolate suspension was magnetically stirred at 300 rpm at room temperature for 10 minutes, then heated in an 85°C water bath for 10 minutes, removed and cooled with cold water for 5 minutes, and then heated again in an 85°C water bath for 15 minutes. The mixture was removed and cooled with cold water for 5 minutes, and then cooled at 4°C overnight to obtain a pure 10% soy protein isolate gel. The prepared gel sample was designated pure SPI.

[0040] The difference between the preparation methods of Comparative Example 1 and Example 4 is that no starch is added and pure soy protein isolate gel is used, and the other conditions are the same.

[0041] The starch-protein composite hydrogels prepared in the above-mentioned Examples 1-4 and Comparative Example 1 were subjected to relevant property characterization experiments, and the detection method is as follows:

[0042] Determination and analysis of texture properties of starch-protein composite hydrogel

[0043] Gel samples were equilibrated at room temperature for 30 minutes before their texture characteristics were measured in a physical property analyzer using a TA-XT Plus C texture analyzer with a P / 0.5 probe. The following parameters were set: pre-measurement speed of 2.0 mm / s; test speed of 2.0 mm / s; post-measurement speed of 0.8 mm / s; strain of 60%, automatic trigger force of 5 g; and duration of 5 seconds. The probe was pressed twice during each measurement. Six replicates were collected for each sample, and the results were averaged.

[0044] Determination and analysis of starch-protein composite hydrogel strength

[0045] Gel strength was measured using a TA-XT Plus C texture analyzer. A 5mm diameter cylindrical probe (P / 0.5) was used for a triggered downward compression test at a distance of 10.0mm. The trigger force was 3g, and the probe's descending speed was 1mm / s before the test, 1mm / s during the test, and 1mm / s during the return cycle. Gel strength was expressed as the maximum force during the first compression cycle. Six replicates were performed for each sample, and the results were averaged.

[0046] Rheological analysis of starch-protein composite hydrogel

[0047] The rheological properties of the composite hydrogel were measured using a rheometer. The plate gap was set to 1.0 mm, the P35 probe was selected, and the gel sample was placed on the parallel plates and equilibrated for 5 min. First, a strain sweep was performed with a fixed frequency of 1 Hz and the temperature set at 25 °C. The program was set to increase the strain γ logarithmically from 0.01% to 1000% for a preliminary amplitude sweep measurement to determine the range of the linear viscoelastic region (LVR). Then, a frequency sweep analysis was carried out with a strain set at 0.5% and the temperature at 25 °C. A frequency sweep from 0.1 to 20 Hz was performed with 33 sampling points to measure the changes in the storage modulus G′ and the loss modulus G″ with the sweep frequency. Each sample was measured in parallel three times, and the results were averaged.

[0048] Preparation of simulated gastric juice and intestinal juice

[0049] Simulated gastric fluid (SGF) was prepared by dissolving 2 g of NaCl, 7 mL of 37% concentrated hydrochloric acid, and 0.3 mg of pepsin in deionized distilled water to a final volume of 1000 mL, and the pH was adjusted to 1.2. Simulated intestinal fluid (SIF) was prepared by dissolving 6.8 g of potassium dihydrogen phosphate (KH2PO4) and 0.3 mg of trypsin in distilled water, and the pH was adjusted to 6.8 with 0.5 mol / L NaOH, with a final volume of 1000 mL.

[0050] Making standard curves

[0051] Using deionized water, simulated gastric juice, and simulated intestinal juice as solvents respectively, different standard curves were made. A riboflavin solution with a concentration of 100 mg / L (10 mg / 100 mL) was prepared. 2 mL, 4 mL, 6 mL, 8 mL, and 10 mL of the solution were respectively placed in 100 mL volumetric flasks and made up to the mark to obtain standard solutions of 2 μg / mL, 4 μg / mL, 6 μg / mL, 8 μg / mL, and 10 μg / mL. The absorbance of the standard solutions was measured at 445 nm using a UV spectrophotometer. A standard curve was plotted with the riboflavin concentration on the abscissa and the absorbance value on the ordinate.

[0052] The standard curve of riboflavin in simulated intestinal fluid was: y = 0.03787x - 0.00152, R 2 = 0.99966, as Figure 1 shown.

[0053] Determination of the encapsulation efficiency of the starch - protein composite hydrogel

[0054] The freeze-dried hydrogel embedded with riboflavin was ground into powder. 20 mg of the drug-loaded hydrogel powder was weighed and placed in 20 mL of simulated intestinal fluid, and oscillated at 37 °C and 180 rpm for 6 h to fully release riboflavin. Then, it was centrifuged at 8000 r / min for 10 min, and the absorbance of the supernatant was measured at 445 nm. The riboflavin content was calculated according to the standard curve. The calculation formulas for the encapsulation efficiency and loading rate of riboflavin are as follows: Encapsulation efficiency (%) = (riboflavin content in supernatant / total riboflavin content) × 100%

[0055] Examples 1-4 and Comparative Example 1 explored the effect of starch on the texture properties of SPI gels

[0056] The textural properties and gel strength of the starch-SPI composite hydrogels are shown in Table 1 and Table 2, which present the textural properties of the composite hydrogels of mung bean starch (MBS), potato starch (PS), corn starch (CS), tapioca starch (TS) and soy protein isolate (SPI), including hardness, adhesiveness, elasticity, cohesiveness, gumminess, chewiness, resilience and strength. As can be seen from the table, the textural properties of different starch-SPI gels are different. Compared with the pure SPI gel of the control group, the hardness, strength, adhesiveness, gumminess and chewiness of the composite hydrogels after adding starch are significantly enhanced, and the elasticity is also enhanced to varying degrees. Among them, the elasticity of the PS-SPI gel is significantly enhanced, and the cohesiveness and resilience of the MBS-SPI gel are lower than those of the pure SPI gel, but not significantly. First, from the analysis of gel hardness, gel hardness is the force required to simulate the teeth squeezing the composite hydrogel twice to deform it by 30%. There are significant differences in the hardness of the composite hydrogels formed by the four starches and SPI. Among them, the PS-SPI gel has the highest hardness, which is 169.12±7.83 g, the MBS-SPI gel has the second highest hardness, and the TS-SPI gel has the lowest hardness. The increase in hardness may be due to the occurrence of the "filling effect" and the relatively high amylose content. From the analysis of the change in gel adhesiveness, adhesiveness is the negative area of the curve between the first compression curve reaching the zero point of force and the start of the second compression curve. The adhesiveness of the PS-SPI gel is significantly less than that of other starch-SPI gels, while there is no significant difference in the adhesiveness of the MBS-SPI, CS-SPI and TS-SPI gels. From the analysis of the change in gel elasticity, elasticity is the recovery performance of the compressed composite hydrogel without external force. There is no significant difference in the elasticity of the PS-SPI, MBS-SPI and CS-SPI gels. The elasticity of the PS-SPI gel is the largest, which is 0.98±0.01, followed by CS-SPI and MBS-SPI, and the elasticity of the TS-SPI gel is significantly reduced. From the analysis of the change in gel cohesiveness and resilience, cohesiveness is the degree to which the composite hydrogel resists the second compression deformation after the first compression deformation, which can reflect the degree of internal binding of the gel. Compared with other gels, the cohesiveness and resilience of the MBS-SPI composite gel are significantly reduced, and there is no significant difference among the PS-SPI, CS-SPI and TS-SPI gels. From the analysis of the change in gumminess and chewiness, gumminess represents the energy required to chew semi-solid food until it can be swallowed; and chewiness is a supplementary parameter of the gel hardness index. The gumminess and chewiness of the PS-SPI composite gel are both significantly the largest, followed by the MBS-SPI composite gel.

[0057] From the analysis of the gel strength change, the gel strength reflects the physical properties such as the rigidity coefficient of the gel. The gel strengths of PS-SPI and MBS-SPI are significantly greater than those of CS-SPI and TS-SPI gels, and the gel strength of pure SPI gel is significantly the smallest. There is no significant difference between PS-SPI and MBS-SPI, but the gel strength of PS-SPI is relatively large, which is 41.37±4.04. The gel properties can be divided into gel elasticity and gel hardness. The gel strength can be used as an evaluation index for elasticity and hardness. The gel strength characterizes the texture of the gel and the compactness of its structure. The gel properties of the composites formed by starches with different types, addition amounts, and ratios of branched to straight chains and proteins are different. In this study, the enhancement of the hardness, elasticity, and strength of the composite hydrogels after adding starch may be due to the fact that after the starch is heated to the gelatinization temperature, the starch molecules absorb a large amount of water, expand and fill the space structure of the protein gel network, squeezing the gel matrix and thus forming a denser gel network structure, resulting in the "filling effect". The "filling effect" is related to the starch particle size. The "filling effect" of larger particle sizes is more significant. The volume average particle sizes of potato starch, mung bean starch, corn starch, and cassava starch are 38.5μm, 21.1μm, 14.19μm, and 12.7μm respectively. The particle size of potato starch is the largest, making the PS-SPI composite hydrogel have the greatest strength and hardness. In addition, it is also related to the relatively strong gel property of the starch itself. The gel hardness of the starch is related to the amylose content. The amylose contents in mung bean starch, corn starch, potato starch, and cassava starch are 31.7 - 41.8%, 26%, 20%, and 17% respectively. The amylose content in mung bean starch is the highest, which is the reason for the relatively large hardness of MBS-SPI and PS-SPI. Generally, the texture properties of different starch-SPI composite hydrogels are relatively the best for the PS-SPI composite gel, followed by the MBS-SPI composite gel. The relatively best texture properties of the PS-SPI composite gel indicate that the presence of PS enhances the formation of the gel network, making the composite hydrogel have a more stable structure. Table 1 Texture characteristics of starch-SPI composite hydrogel Note: Different letters in the same row represent significant differences (P<0.05) Table 2 Strength of starch-SPI composite hydrogel Note: Different letters in the same column represent significant differences (P<0.05)

[0058] Formation and Appearance Analysis of Starch-Protein Composite Hydrogels Prepared in Examples 1-4 and Comparative Example 1

[0059] The starch-SPI composite hydrogels are as Figure 2 , Figure 2 shown in the physical pictures of SPI, PS-SPI, TS-SPI, MBS-SPI, and CS-SPI composite hydrogels respectively. It can be seen from the figures that all the prepared samples can form gels. Among them, the surface of the SPI gel is not very uniform, and it is a little yellower in color compared with other samples. The gel structure is not stable and is a bit soft and collapsed. Compared with the pure SPI gel in the control group, the PS-SPI composite hydrogel is harder, has a more stable structure, is easy to cut, and presents a better cubic state. The TS-SPI composite hydrogel also significantly collapses, has a softer structure, and cannot maintain a good cubic state. The appearance structure of MBS-SPI is also relatively stable and presents a good quasi-solid state. The hardness and structural stability of CS-SPI are better than those of SPI and TS-SPI, but not as stable as MBS-SPI and PS-SPI, and it is a bit prone to dispersion. This result is also reflected in the texture property analysis.

[0060] Examples 1-4 and Comparative Example 1 Explore the Influence of Starch on the Rheological Properties of SPI Gels

[0061] The strain sweep results of the starch-SPI composite hydrogels are as Figure 3As shown in (A), it can be seen from the figure that the linear viscoelastic region (LVER) and critical strain point of the starch-SPI composite hydrogel. The linear viscoelastic region is a key factor for evaluating the properties of hydrogels. The LVER is the range where the storage modulus G′ and loss modulus G″ remain relatively constant as the shear strength increases. Within the linear viscoelastic region, G' and G” of all composite hydrogels basically remain parallel and do not change with the increase of strain, indicating that the network structure of the gel is maintained. The G' value within the linear viscoelastic region of each starch-SPI composite hydrogel is always greater than the G” value, indicating that the above samples have formed gels and have good solid-like behavior. If the linear viscoelastic region shrinks, it indicates that the internal structure of the gel is easily damaged, indicating that the internal structure of the protein gel added with potato starch is relatively stable and not easily damaged. Compared with the control group of pure SPI, the G' values of the composite hydrogels added with starch are all greater than those of the pure SPI gel. Among them, the G' value of the PS-SPI composite hydrogel is the largest, followed by MBS-SPI, CS-SPI, TS-SPI, and the smallest is pure SPI, which is consistent with the results of the gel texture characteristics. This may be because the particle size of potato starch is relatively the largest, and the “filling effect” is more significant. Mung bean starch contains a relatively high amylose content. Amylose is not easily gelatinized and has good starch retrogradation, so the G' value is also relatively high. Moreover, mung bean starch absorbs a large amount of water during heating, expands and fills the network space structure of the protein gel, and also has a “filling effect”, making the G' value of MBS-SPI relatively large.

[0062] The storage modulus (G′) and loss modulus (G″) of the starch-SPI composite hydrogel vary with frequency as Figure 3 As shown in (B), it can be seen from the figure that G′ and G″ of all composite hydrogels have a high frequency dependence at lower frequencies and a low frequency dependence at higher frequencies. The frequency dependence of G′ and G″ can be used to provide information on the gel structure type. It has been reported in the literature that covalent gels are independent of frequency, while physical gels are frequency-dependent. Therefore, PS-SPI, MBS-SPI, CS-SPI, TS-SPI, and SPI gels are all physical gels, and within the frequency test range, no crossover point is observed between G′ and G″, indicating that the composite hydrogel is mainly a physical type of gel connected by non-covalent interactions between strong gels and weak gels. The G' value of all gels is higher than the G” value, indicating the formation of an elastic gel structure. The G′ and G″ values of the PS-SPI gel are the largest, followed by MBS-SPI, and the smallest is the pure SPI gel, indicating that the PS-SPI composite hydrogel has more significant solid elastic behavior and a stronger gel network, which is also reflected in the texture characteristic analysis of the gel.

[0063] Encapsulation efficiency of riboflavin-loaded starch-SPI composite hydrogel

[0064] The addition of riboflavin does not affect the formation of the composite hydrogel network because the interaction between riboflavin and protein is weak. Only a very small part dissolves in the protein matrix, while most are arranged in a crystalline form uniformly throughout the network. Riboflavin is evenly embedded in the gel network structure. After calculation, the encapsulation efficiency of riboflavin by the potato starch-soybean protein isolate composite hydrogel is 91.35 ± 0.53%. The encapsulation efficiency of the drug is related to the porosity of the gel matrix.

[0065] The above is only used to explain the preferred embodiments of the present invention. In summary, the composite hydrogels added with any one of potato starch, mung bean starch, corn starch or tapioca starch have improved properties in terms of appearance, texture characteristics and rheological characteristics compared with pure SPI gels. The hardness, elasticity, adhesiveness, chewiness, linear viscoelastic region and storage modulus of the potato starch-soybean protein isolate composite hydrogel are the largest. Moreover, the potato starch-soybean protein isolate composite hydrogel has a good encapsulation effect on riboflavin.

[0066] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications that can be made without creative efforts on the basis of the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A preparation method of a starch-soybean protein isolate composite hydrogel induced by heat, characterized in that, It includes the following steps: S1. Add deionized water to a certain amount of protein powder for preparation, and adjust the pH value using NaOH solution; S2. First, magnetically stir the soy protein isolate solution and then heat it in a water bath. After taking it out, cool it with cold water to obtain a soy protein isolate paste; S3. Add deionized water to starch to prepare a starch suspension; S4. Mix the starch suspension and the soy protein isolate paste prepared by heating evenly respectively, and heat it in a water bath again. After taking it out and cooling, a starch-soy protein isolate composite hydrogel is obtained; S5. Select the optimal one of the starch-soy protein isolate composite hydrogels for embedding. After the heating of the starch-soy protein isolate suspension is completed, add riboflavin powder to it and quickly stir evenly, and then cool it to obtain a starch-soy protein isolate composite hydrogel encapsulating riboflavin.

2. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S1, the proportion of soy protein isolate is 8% - 12% (w / v).

3. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S1, the concentration of the used NaOH solution is 2 mol / L, and the pH value is adjusted to 7.0 - 10.

0.

4. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S2, the rotation speed of magnetic stirring is 300 rpm and the time is 10 min.

5. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S2, heat it in a water bath at 70 - 100 °C for 5 - 20 min, then take it out and cool it with cold water for 5 min.

6. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S3, the starches are potato starch (PS), mung bean starch (MBS), corn starch (CS) and tapioca starch (TS) respectively, and the proportion of the starch is 1% - 6% (w / v).

7. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S4, heat it in a water bath at 70 - 100 °C for 5 - 20 min, then cool it with cold water for 5 min and then cool it overnight at 4 °C.

8. The method for thermally inducing the preparation of the starch-soy protein isolate composite hydrogel according to claim 1, wherein: In step S5, the addition amount of riboflavin is 5% (calculated based on the dry weight of the starch-soy protein isolate). After cooling with cold water for 5 min, then cool it overnight at 4 °C. Keep it away from light throughout the process after adding riboflavin to avoid the influence of light on riboflavin.

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