Soy protein isolate film and method of making same

Abstract

The application relates to a soybean protein isolate film and a preparation method thereof, which comprises the following steps: S1, taking active dry yeast to activate to obtain a yeast bacterial liquid; S2, mixing the yeast bacterial liquid with gluten to ferment to obtain a fermented WG sample; S3, uniformly mixing the fermented WG sample with soybean protein isolate SPI, water and an alcohol solvent, adjusting the pH of the solution to be greater than 7 by using a brine solution to obtain a protein film-forming liquid; and S4, after defoaming the protein film-forming liquid, pouring the protein film-forming liquid on a mold, and drying to obtain the soybean protein isolate film. The method can improve the mechanical properties, thermal stability and hydrophobic properties of the soybean protein isolate film, improve the comprehensive performance of the soybean protein isolate film, and promote better popularization and application of the soybean protein isolate film.

Description

Technical Field

[0001] This invention relates to the field of edible protein film technology, and in particular to a soybean protein isolate film and its preparation method. Background Technology

[0002] Soy protein isolate (SPI) is a byproduct of soybean oil extraction, obtained from defatted soybean flour through high refining, and contains over 90% soybean protein. β-conglycinin (7S) and glycinin (11S) are the main components of SPI, which can be tightly linked through hydrophobic interactions, hydrogen bonds, and disulfide bonds to form a stable network structure. Therefore, SPI exhibits good film-forming properties, but its barrier properties after film formation are generally poor. To expand the application range of SPI membranes, it is necessary to improve their water vapor barrier properties.

[0003] Currently, improvements in the water vapor barrier performance of SPI membranes are mostly achieved by increasing the content of hydrophobic substances in the membrane matrix. For example, Chinese invention patent CN104262654B discloses a method for preparing a composite membrane of soy protein isolate and modified zein. The specific steps of this method are: a) preparing a protein composite solution using a solution blending method; b) adding a plasticizer and a crosslinking agent; c) adjusting the pH value to make the solution alkaline; d) water bath heating; e) isothermal drying to prepare the protein membrane. The protein composite membrane obtained by this method is pollution-free, biodegradable, and has high mechanical properties and strong barrier performance after disposal. However, this method requires phosphorylation modification of zein to obtain phosphorylated zein, then dissolving the phosphorylated zein in distilled water to prepare a phosphorylated zein solution, and then subjecting it to microwave modification to obtain a microwave-modified phosphorylated zein solution. The operation is relatively complex and highly dependent on zein, which limits its widespread application. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing soy protein isolate films in terms of performance, and to provide a method for preparing soy protein isolate films by using wheat gluten (WG) to improve the soy protein isolate (SPI) film. Wheat gluten is mainly composed of glutenin and wheat gliadin, which are rich in nonpolar amino acids. It is the gluten powder remaining after washing starch and soluble substances from wheat flour, and has good hydrophobicity. Adding appropriately fermented WG can improve some of the physicochemical properties of SPI films and emulsion films.

[0005] Furthermore, this invention also utilizes lye water to treat gluten powder. Lye water is an alkaline salt solution containing a mixture of Na2CO3 and K2CO3, and is a commonly used material in the production of foods such as noodles and Cantonese pastries. Appropriate addition of lye water pretreatment to WG can further improve some physical properties of the SPI membrane.

[0006] The specific plan is as follows:

[0007] A method for preparing a soybean protein isolate membrane includes the following steps:

[0008] S1, Take active dry yeast and activate it to obtain yeast culture liquid;

[0009] S2, the yeast liquid is mixed with gluten powder and fermented to obtain fermented WG sample;

[0010] S3, the fermented WG sample is mixed evenly with soybean protein isolate SPI, water and food-grade plasticizer, and the pH of the solution is adjusted to >7 with lye water to obtain protein film-forming solution;

[0011] S4, after degassing the protein film-forming solution, it is poured onto a template and dried to obtain a soybean protein isolate film.

[0012] Furthermore, the activation described in S1 involves taking active dry yeast and white sugar, dispersing them in distilled water for activation, and obtaining a yeast culture solution with a yeast content of 1-3 wt%.

[0013] Furthermore, the ratio of yeast culture to gluten powder in S2 is (0.5-3 mL): 1 g.

[0014] Furthermore, the fermentation conditions described in S2 are as follows: fermentation at 20-30°C and 60-80% relative humidity for 2-24 hours, preferably 3-5 hours, and more preferably 4 hours.

[0015] Furthermore, the mass ratio of the fermented WG sample to soy protein isolate SPI in S3 is (1-5):(6-10), preferably (1-2):(8-10), and more preferably (1-2):(9-10).

[0016] Furthermore, the food-grade plasticizer mentioned in S3 is any one of glycerol, sorbitol, and polyethylene glycol.

[0017] Furthermore, the lye water mentioned in S3 is an aqueous solution of polyphosphate containing Na2CO3 and K2CO3. Preferably, the concentration of Na2CO3 in the lye water is 0.5-3% (w / v), the concentration of K2CO3 is 0.1-1% (w / v), and the concentration of polyphosphate is 0.1-1% (w / v).

[0018] Furthermore, the pH of the lye water solution in S3 is adjusted to 8-10, preferably pH=9.

[0019] Further, after S3, the protein film-forming liquid is mixed evenly with soybean oil. Preferably, the mass ratio of the protein film-forming liquid to soybean oil is 10:(0.1-0.3) to obtain a film-forming emulsion. Correspondingly, in S4, the film-forming emulsion is defoamed and poured onto a template, and after drying, a soybean protein isolate film is obtained.

[0020] The present invention also protects the soy protein isolate membrane prepared by the method of preparing the soy protein isolate membrane, which has at least one of the following (1) to (5):

[0021] (1) Tensile strength is 4-5 MPa;

[0022] (2) Elongation at break is 90-95%;

[0023] (3) Water vapor transmission rate is (1.3-2.5)×10 -10 g·m -1 ·s -1 ·pa -1 ;

[0024] (4) The water contact angle is 80-105°;

[0025] (5) Solubility is 20-30%.

[0026] Beneficial effects: The method described in this invention effectively improves the mechanical properties, water vapor barrier properties, and hydrophobic properties of soy protein isolate membranes, and can be widely used in traditional pre-prepared food industry practices, promoting the application and promotion of soy protein isolate membranes. Attached Figure Description

[0027] To more clearly illustrate the technical solution of the present invention, the accompanying drawings will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention.

[0028] Figure 1a This is a graph showing the effect of adding WG with different fermentation times on the emulsification performance (EAI) of SPI film-forming emulsion, provided in Embodiment 1 of the present invention.

[0029] Figure 1b This is a graph showing the effect of adding WG with different fermentation times on the ESI emulsification performance of SPI film-forming emulsion, provided in Embodiment 1 of the present invention.

[0030] Figure 2 This is a diagram illustrating the effect of adding WG with different fermentation times on the microstructure of SPI membrane and emulsion membrane, provided in Embodiment 1 of the present invention.

[0031] Figure 3a This is a Fourier transform infrared spectrum of an SPI membrane with WG added for different fermentation times, provided in Embodiment 1 of the present invention.

[0032] Figure 3b This is a Fourier transform infrared spectrum of an emulsion membrane with WG added for different fermentation times, provided in Embodiment 1 of the present invention.

[0033] Figure 4 This is a graph showing the effect of adding WG with different fermentation times on the solubility of SPI membrane and emulsion membrane, provided in Embodiment 1 of the present invention.

[0034] Figure 5 This is a graph showing the effect of adding WG with different fermentation times on the thermal stability of SPI membrane and emulsion membrane, provided in Embodiment 1 of the present invention.

[0035] Figure 6a This is a scanning electron microscope image of the upper surface of a composite film with different WG / SPI ratios provided in Embodiment 2 of the present invention;

[0036] Figure 6b This is a scanning electron microscope image of the lower surface of a composite film with different WG / SPI ratios provided in Embodiment 2 of the present invention;

[0037] Figure 7 This is a Fourier transform infrared spectrum of composite films with different WG / SPI ratios provided in Embodiment 2 of the present invention;

[0038] Figure 8 This is a solubility diagram of composite films with different WG / SPI ratios provided in Embodiment 2 of the present invention;

[0039] Figure 9 This is a DSC spectrum of composite films with different WG / SPI ratios provided in Embodiment 2 of the present invention. Detailed Implementation

[0040] Preferred embodiments of the present invention will now be described in more detail. While preferred embodiments of the present invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products. In the following embodiments, unless otherwise specified, "%" refers to weight percentage.

[0041] The following testing methods are included:

[0042] Emulsifying activity index (EAI) and emulsifying stability index (ESI): A certain amount of film-forming emulsion was taken, diluted with 0.1% (w / v) SDS solution, and mixed thoroughly. Using SDS solution as a blank control, the absorbance was measured at 500 nm using a UV spectrophotometer at 0 and 10 min. The EAI and ESI of the film-forming emulsion were calculated according to formulas (1) and (2), respectively.

[0043]

[0044] In the formula: A0 is the absorbance value at 0 min; DF is the dilution factor; C is the protein concentration (g / mL); is the oil phase volume fraction; L is the optical path length (cm).

[0045]

[0046] Where: A0 is the absorbance value at 0 min; A 10 The absorbance value is the value at 10 min.

[0047] Microstructure: The microstructure of the upper and lower surfaces of the membrane sample was obtained using scanning electron microscopy (SEM). Before measurement, the membrane sample was cut to an appropriate size and thoroughly dehydrated in dry silica gel. After gold sputtering, the sample was photographed at 20000× magnification under an accelerating voltage of 10 kV.

[0048] Fourier transform infrared spectroscopy (FTIR): Use an FTIR spectrometer with the wavelength range set to 4000-800 cm⁻¹. -1 Before testing, the membrane sample was cut into a 10mm × 10mm rectangle and placed in dry silica gel for thorough dehydration. The sample was then heated to 4cm. -1 The FTIR spectrum of the composite film was obtained by scanning 32 times at a resolution of 0.5.

[0049] Mechanical properties: The mechanical properties of the membrane were determined using a texture analyzer. The membrane sample was cut into a rectangle of 20 mm × 45 mm, and the thickness of the membrane sample was measured using a thickness gauge. The initial clamping distance was set to 30 mm, and the testing rate was 60 mm / min. The tensile strength (TS) and elongation at break (EAB) were calculated according to formulas (3) and (4), respectively.

[0050] TS(MPa)=Fm / A (3)

[0051] In the formula: F m A is the maximum tensile force (N) required to break the sample; A is the cross-sectional area (mm²). 2 ).

[0052] EAB(%) = (L / 30) × 100 (4)

[0053] In the formula: L is the length increase (mm) when the membrane breaks; 30 is the initial spacing of the clamps (mm).

[0054] Water vapor transmission rate (WVP): Before measurement, the thickness of the membrane sample was measured using a thickness gauge. The membrane sample was used to seal the mouth of a plastic bottle containing dry silica gel, and the plastic bottle was placed in a sealed environment at 30°C and 100% RH. The weight change of the plastic bottle was measured. The WVP of the membrane sample was calculated according to formula (5).

[0055] WVP(g·m -1 ·s -1 ·Pa -1 )=w×x×A -1 ×t -1 ×Δp -1 (5)

[0056] In the formula: w is the increase in weight of the plastic bottle (g); x is the thickness of the film (m); A is the area of ​​the plastic bottle opening covered by the film (m²). 2 ); t is the storage time (s); Δp is the water vapor pressure difference across the membrane (Pa).

[0057] Contact angle: The surface contact angle of the membrane was measured using an optical contact angle meter. Before measurement, the membrane sample was cut into a rectangle of 10 mm × 50 mm. The membrane sample was laid flat on the stage, and 2 μL of distilled water was deposited on the membrane surface for 2 seconds before being photographed. The surface contact angle of the membrane was calculated using the SDC-200 software.

[0058] Solubility: After weighing the membrane sample, immerse it in distilled water, shake for 24 hours, then remove the undissolved sample and dry it thoroughly at 105℃. Calculate the solubility of the composite membrane according to formula (7).

[0059] Solubility (%) = (m0 - m1) / m0 × 100 (6)

[0060] In the formula: m0 is the initial mass of the membrane (g); m1 is the mass of the membrane after drying (g).

[0061] Differential scanning calorimetry (DSC)

[0062] The membrane sample (approximately 2 mg) was thoroughly dried in dry silica gel. During the measurement, the temperature was increased from 0°C to 100°C at a rate of 5°C / min. After the experiment, the glass transition temperature (T0) of the membrane was obtained using the accompanying software. g ).

[0063] Data analysis: All experiments were repeated at least three times, and results are presented as mean ± standard deviation. Graphs were created using Origin software. One-way ANOVA and Duncan's test were performed using SPSS Statistics 26.0 software to analyze the data for statistical significance (P < 0.05).

[0064] The main reagents used include:

[0065] Soy protein isolate, Linyi Shansong Biological Products Co., Ltd.; gluten powder, Fengqiu Huafeng Flour Industry Co., Ltd.; active dry yeast, Angel Yeast Co., Ltd.; soybean oil, Yihai Kerry Arawana Grain & Oil Food Co., Ltd.

[0066] Example 1

[0067] A method for preparing a soybean protein isolate membrane includes the following steps:

[0068] S1. Accurately weigh 3g of active dry yeast and 10g of white sugar, disperse them in 200mL of 30℃ distilled water and activate for 20min to obtain yeast culture.

[0069] S2, the activated yeast culture (200mL) was mixed evenly with WG (200g) and fermented at 30℃ and 75% relative humidity. After 2, 4, 8 and 24 hours of fermentation, the fermented WG samples were taken out to obtain the fermented WG samples.

[0070] S3. Fermented WG samples (12g) and SPI samples (6g) with different fermentation times were dissolved in 150mL of distilled water, and glycerol (2.4g) was added. The solution was adjusted to pH 9 using lye water containing 0.1% (w / v) sodium tripolyphosphate, 0.9% (w / v) Na2CO3-0.1% (w / v) K2CO3, and then processed in a blender for 10min. After stirring at 75℃ for 30min, the protein film-forming solution was obtained.

[0071] S4. Divide the protein film-forming solution into two portions, one for later use; add soybean oil (0.12g) to the other portion of the protein film-forming solution and process it using a high-speed blender for 5 minutes to obtain a film-forming emulsion. After degassing both the protein film-forming solution and the film-forming emulsion, pour them onto silicone resin plates and dry them at 25℃ and 50% RH for 24 hours. Peel the dried membrane samples from the silicone resin plates to obtain SPI membranes (formed by drying the protein film-forming solution) and emulsion membranes (formed by drying the film-forming emulsion). Equilibrate at 25℃ and 50% RH for 48 hours respectively, and analyze their physicochemical properties. The results are as follows:

[0072] 1.1 EAI and ESI

[0073] EAI and ESI can reflect the protein-lipid interaction in film-forming emulsions and the stability of oil droplets in the emulsion. For example... Figure 1a and Figure 1b As shown, the EAI and ESI of the SPI film-forming emulsion prepared by adding unfermented WG were 31.22 m. 2 g -1 And 52.11 min. With the increase of fermentation time of added WG, the ESI and EAI of SPI film-forming emulsion showed a trend of first increasing and then decreasing.

[0074] 1.2 Microstructure

[0075] Figure 2 The effects of adding WG with different fermentation times on the microstructure of SPI and emulsion membranes are shown. The SPI membrane with unfermented WG has a smooth and dense upper surface, while a rough structure containing numerous glutenin spherical particles is observed on the lower surface. With increasing WG fermentation time, the spherical particles on the lower surface of the SPI membrane gradually disappear, and the roughness of the lower surface initially decreases and then increases. This is because WG undergoes depolymerization after prolonged fermentation, resulting in the disappearance of glutenin spherical protein particles. On the other hand, after appropriate fermentation, the protein structure of WG unfolds, exposing internal hydrophobic groups, which promotes intermolecular interactions in the composite membrane, resulting in a decrease in membrane surface roughness. However, WG degrades after prolonged fermentation, disrupting the protein structure, which may lead to an increase in membrane surface roughness.

[0076] Oil droplets were observed on the upper surface of SPI emulsion films with different fermentation times of WG. This is because the oil droplets migrate upwards with the evaporation of water during the drying process and accumulate on the upper surface of the film. The largest oil droplets were observed on the upper surface of the SPI emulsion film prepared with unfermented WG. With the increase of WG fermentation time, the size of the oil droplets and the surface roughness of the SPI emulsion film showed a trend of first decreasing and then increasing, which may be related to the emulsification stability of the film-forming emulsion. Figure 1a and Figure 1bThe stronger the emulsification stability in the film-forming emulsion, the better it can prevent oil droplet aggregation, resulting in a more uniform distribution of oil droplets in the membrane matrix.

[0077] 1.3 FTIR

[0078] Fourier transform infrared spectra of SPI membranes and emulsion membranes with WG added for different fermentation times are shown below. Figure 3a and Figure 3b As shown. Generally, the wavenumber is 3273 cm⁻¹. -1 The characteristic peaks of amide A are related to -NH and -OH groups, and the intensity of the characteristic peaks is positively correlated with hydrogen bonding interactions; wavenumbers are 1629, 1537, and 1236 cm⁻¹. -1 The characteristic peaks represent the amide I, amide II, and amide III bands, respectively; wavenumber 1743 cm⁻¹ -1 The characteristic peaks represent the characteristic peaks of soybean oil.

[0079] Therefore, from Figure 3a As can be seen, when the fermentation time of added WG increased from 0 h to 4 h, the characteristic peak intensities of the amide A, amide I, and amide Ⅱ I bands of the SPI membrane gradually increased. However, with further increases in fermentation time, the characteristic peak intensities of these bands decreased. This is because the protein structure unfolds after fermentation, promoting intermolecular interactions within the composite membrane, resulting in an increased characteristic peak intensity of the amide A band. However, after prolonged fermentation, WG degrades into small peptides, the disulfide bonds in WG break, and intermolecular interactions weaken, leading to a decrease in the characteristic peak intensity of the amide A band.

[0080] from Figure 3b As can be seen, with the increase of fermentation time of added WG, the characteristic peak intensities of amide A, amide I, and amide ⅠⅠ bands of the SPI emulsion film first increase and then decrease, while the characteristic peak intensity of soybean oil shows a trend of first decreasing and then increasing. This may be because the addition of fermented WG improves the emulsification stability of the SPI film-forming emulsion and inhibits the migration of oil droplets to the upward surface during the drying process of the emulsion film, resulting in a weakening of the characteristic peak intensity of soybean oil on the surface of the SPI emulsion film.

[0081] However, when the fermentation time of the added WG exceeded 4 hours, the emulsification stability of the SPI film-forming emulsion weakened, and a large number of oil droplets accumulated on the surface of the emulsion film, resulting in an enhanced intensity of the characteristic peak of soybean oil in the SPI emulsion film.

[0082] 1.4 Mechanical Properties

[0083] The effects of adding WG with different fermentation times on the mechanical properties of the SPI membrane are shown in Table 1. The TS and EAB of the SPI membrane prepared with unfermented WG were 4.22 MPa and 73.75%, respectively. The TS and EAB of the SPI membrane prepared with WG fermented for 4 hours increased to 4.99 MPa and 89.54%, respectively. Compared with the previously reported mechanical properties of WG-SPI composite membranes (4.01 MPa and 198.15%), the TS was larger and the EAB was smaller. With further increases in the fermentation time of added WG, the TS of the SPI membrane gradually decreased while the EAB remained unchanged.

[0084] However, when the fermentation time of the added WG exceeds 4 hours, the intermolecular interactions in the SPI membrane matrix weaken, which may lead to a decrease in the TS of the membrane.

[0085] Compared to SPI membranes with added fermented WG, SPI emulsion membranes with the same fermentation time showed decreased total pressure (TS) and increased total area (EAB). This is because the addition of lipids plasticizes the membrane matrix, weakening the interactions between proteins, resulting in decreased TS and increased EAB in the SPI emulsion membrane. When the fermentation time with added WG increased to 4 hours, the TS and EAB of the SPI emulsion membrane increased to 2.95 MPa and 121.66%, respectively. Further increases in fermentation time with added WG resulted in decreased TS and increased EAB in the SPI emulsion membrane.

[0086] However, the protein-lipid interaction in the SPI film-forming emulsion with added long-fermented WG was weakened, resulting in a decrease in TS and an increase in EAB in the SPI emulsion film.

[0087] Table 1. Effects of adding WG at different fermentation times on the mechanical properties and water vapor transmission rate of SPI membranes and emulsion membranes.

[0088]

[0089] Note: Different lowercase literals represent significant differences (P<0.05).

[0090] 1.5WVP

[0091] The effects of adding different fermentation times of WG on the WVP of the SPI membrane are shown in Table 1. With increasing fermentation time and WG addition, the WVP of the SPI membrane first decreased and then increased (P<0.05). The WVP of the SPI membrane was lowest (1.34×10⁻⁶) when the fermentation time was 4 h. -10 g·m -1 ·s -1 ·pa -1 This may be related to the fact that the SPI membrane prepared by adding appropriate fermentation treatment to WG has the densest network structure.

[0092] Compared to SPI membranes with added fermented WG, SPI emulsion membranes with added WG for the same fermentation time all showed a decrease in water vapor pressure (WVP). This is because the addition of oil enhances the overall hydrophobicity of the SPI membrane. When the fermentation time of the added WG increased from 0 h to 4 h, the WVP of the SPI emulsion membrane increased from 2.17 × 10⁻⁶. -10 g·m -1 ·s -1 ·pa -1 Reduced to 1.28×10 -10 g·m -1 ·s -1 ·pa -1 .

[0093] However, as the fermentation time of the added WG continued to increase, the WVP of the SPI emulsion membrane gradually increased. The WVP of protein-based emulsion membranes is related to the oil droplet size and the distribution of oil droplets in the membrane matrix. Small oil droplets that are uniformly distributed in the membrane matrix can more effectively hinder the passage of water molecules, thereby reducing the WVP of the emulsion membrane.

[0094] 1.6 contact angle

[0095] The effect of adding WG with different fermentation times on the contact angle of the SPI membrane is shown in Table 2. For SPI membranes with WG added for the same fermentation time, the contact angle of the upper surface was greater than that of the lower surface. When the fermentation time of the added WG increased from 0 h to 4 h, the contact angles of the upper and lower surfaces of the SPI membrane increased from 88.93° and 79.91° to 99.67° and 87.30°, respectively, a result higher than that of the previously studied WG-SPI composite membranes (76.25° and 77.99°). With further increases in the fermentation time of the added WG, the contact angles of both the upper and lower surfaces of the SPI membrane decreased significantly (P<0.05). These results may be related to the surface roughness of the membrane. A smooth surface slows down the diffusion of water droplets on the membrane surface, leading to an increase in the contact angle of the composite membrane. On the other hand, proper hydrolysis of WG exposes internal hydrophobic groups, enhancing the hydrophobicity of WG.

[0096] However, the addition of WG, which has undergone long-term fermentation, roughens the surface structure of the SPI membrane, making it easier for water droplets to spread on the irregular surface, resulting in a decrease in the contact angle of the SPI membrane.

[0097] Compared to SPI membranes with added fermented WG, SPI emulsion membranes with added WG for the same fermentation time all exhibited increased contact angles. This is because the addition of oil significantly improves the surface hydrophobicity of the membrane. When the fermentation time of added WG was 4 hours, the contact angles of the upper and lower surfaces of the SPI emulsion membrane increased to 104.63° and 95.99°, respectively. Further increases in the fermentation time of added WG resulted in a decrease in the contact angles of both the upper and lower surfaces of the SPI emulsion membrane. Therefore, the addition of appropriately fermented WG enhances the protein-oil interaction in the SPI film-forming emulsion, resulting in a lower surface roughness and more uniform oil droplet distribution within the membrane matrix. These combined effects lead to an increase in the contact angle of the SPI emulsion membrane.

[0098] Table 2. Effects of adding WG at different fermentation times on the contact angle of the SPI membrane and emulsion.

[0099]

[0100] Note: Different lowercase literals represent significant differences (P<0.05).

[0101] 1.7 Solubility

[0102] The effect of adding WG with different fermentation times on the solubility of SPI membranes is as follows: Figure 4 As shown, when the fermentation time of the added WG increased from 0 h to 4 h, the solubility of the SPI membrane decreased from 25.48% to 20.48%. With further increases in fermentation time of the added WG, the solubility of the SPI membrane significantly increased (P<0.05), which may be related to the influence of fermentation-treated WG on the intermolecular interactions within the SPI membrane. Enhanced intermolecular interactions in the composite membrane reduce the contact between hydrophilic groups and water molecules, resulting in decreased membrane solubility.

[0103] The solubility of SPI emulsion membranes with added WG for the same fermentation time was greater than that of SPI membranes. With increasing fermentation time of added WG, the solubility of SPI emulsion membranes initially decreased and then increased (P<0.05). The solubility of SPI emulsion membranes was lowest (27.50%) when the fermentation time of added WG increased to 4 hours.

[0104] 1.8 Differential Scanning Calorimetry

[0105] The effect of adding WG with different fermentation times on the T of SPI membrane g The impact such as Figure 5 As shown. The T of the SPI membrane prepared by adding unfermented WG. g The temperature was 53.24℃. When the fermentation time of the added WG was increased to 4 hours, the T of the SPI membrane... g The temperature was increased to 59.75℃, a result consistent with previously reported T values ​​for the WG-SPI composite membrane.g (56.98℃) similar. With continued increase in fermentation time after adding WG, the T of the SPI membrane... g However, it gradually decreases. The T of the composite membrane... g The larger the value, the stronger the interaction between the composite membrane components, and the higher the temperature required to break the molecular chains in the membrane matrix. Therefore, the membrane's T0 value is higher. g Increased. However, the protein-molecule interactions in SPI membranes prepared with WG fermented for a long time were weakened, resulting in a decrease in the membrane's T... g reduce.

[0106] T of SPI emulsion membrane prepared with added unfermented WG g The temperature was 44.34℃. With the addition of WG and increased fermentation time, the T5 of the SPI emulsion membrane increased. g It shows a trend of first increasing and then decreasing. When the added WG fermentation time is 4 hours, the T of the SPI emulsion membrane... g It reached its maximum value (48.11℃). The addition of oil hindered the interaction between proteins, leading to a decrease in the To of the SPI emulsion membrane. g A decrease occurred. The addition of appropriately fermented WG enhanced the protein-lipid interactions in the SPI film-forming emulsion, resulting in smaller, more uniformly distributed oil droplets within the emulsion matrix, forming a dense network structure. This may have led to a decrease in the Tg of the SPI emulsion film. g Increase.

[0107] Example 2

[0108] A method for preparing a soybean protein isolate membrane includes the following steps:

[0109] S1. Accurately weigh 3g of active dry yeast and 10g of white sugar, disperse them in 200mL of 30℃ distilled water and activate for 20min to obtain yeast culture.

[0110] S2, the activated yeast culture (200mL) was mixed evenly with WG (200g), and fermented at 30℃ and 75% relative humidity. After 4 hours of fermentation, the fermented WG sample was taken out to obtain the fermented WG sample.

[0111] S3, the fermented WG sample and SPI were mixed in mass ratios of 0:10, 1:9, 2:8, 3:7 and 4:6 respectively, and then dissolved in 150mL of distilled water. Glycerol (2.4g) was added, and lye water containing 0.1% (w / v) sodium tripolyphosphate, 0.9% (w / v) Na2CO3-0.1% (w / v) K2CO3 was used to adjust the pH of the solution to 9. After processing in a high-speed blender for 10min, the solution was stirred at 75℃ for 30min to obtain the protein film-forming solution.

[0112] S4. After removing air bubbles from the protein film-forming solution, it was cast onto a silicone resin plate and dried at 25°C and 50% RH for 24 hours. The dried membrane sample was then peeled off the silicone resin plate, yielding an SPI composite membrane. After equilibration at 25°C and 50% RH for 48 hours, its physicochemical properties were analyzed, and the results are as follows:

[0113] 2.1 Microstructure

[0114] Figure 6a and Figure 6b The effect of the WG / SPI ratio on the microstructure of the upper and lower surfaces of the composite membrane was demonstrated. When the WG / SPI ratio was 0:10 and 1:9, the upper surface of the composite membrane was smooth and flat. When the WG / SPI ratio reached 2:8, cracks appeared on the upper surface of the composite membrane, and the number of cracks tended to increase with the increase of the WG ratio. This may be due to the hydrophobicity of gliadin leading to rapid water evaporation. On the other hand, the lower and upper surfaces of the SPI membrane were also smooth and dense. However, after adding WG, a large number of spherical particles appeared on the lower surface of the composite membrane, and the particles gradually became larger and cracked with the increase of WG addition. This is mainly because, during the drying process, the more hydrophilic SPI molecules easily migrated to the upper surface of the composite membrane along with the evaporation of water, while the more hydrophobic WG tended to accumulate on the lower surface of the membrane.

[0115] 2.2 FTIR

[0116] To investigate the effect of the WG / SPI ratio on the composite membrane structure, the SPI-WG composite membrane was characterized using FTIR spectroscopy. Figure 7 As can be seen, with the addition of WG, the absorption peak intensities of amide A, amide I, and amide II bands of the WG-SPI composite film gradually decrease, indicating that the addition of WG leads to a weakening of hydrogen bond interactions in the composite film.

[0117] To further investigate the changes in protein secondary structure in the composite membrane, the amide I region was analyzed (Table 4). Table 4 shows that with increasing WG content, the β-sheet structure of the WG-SPI composite membrane gradually decreased, while the contents of α-helices and β-turns gradually increased. The decrease in β-sheet content was associated with a decrease in the tensile strength of the zein-wheat gliadin composite membrane. The increase in β-turn content was associated with an increase in the elongation at break of the cottonseed protein / polyvinyl alcohol composite membrane. Therefore, the results in Table 4 indicate that adding WG may reduce the tensile strength of the composite membrane and increase its elongation at break.

[0118] Table 4. Secondary structure composition of composite films with different WG / SPI ratios

[0119]

[0120] Note: Different lowercase literals represent significant differences (P<0.05).

[0121] 2.3 Mechanical Properties

[0122] The effects of the WG / SPI ratio on the mechanical properties of the composite membrane are shown in Table 5. The TS and EAB of the SPI membrane were 6.60 MPa and 54.91%, respectively. With the increase of WG, the TS of the composite membrane gradually decreased while the EAB gradually increased (P<0.05), which may be due to the non-uniformity of the membrane structure caused by the addition of WG. The results indicate that the mechanical properties of the composite membrane are significantly related to its network structure.

[0123] Table 5 Mechanical properties and water vapor transmission rate of composite membranes with different WG / SPI ratios

[0124]

[0125] Note: Different lowercase literals represent significant differences (P<0.05).

[0126] 2.4 Water vapor transmission rate

[0127] The effect of the WG / SPI ratio on the WVP of the composite membrane is shown in Table 5. The WVP value of the SPI membrane is 2.00 × 10⁻⁶. -10 g·m -1 ·s -1 ·Pa -1 When the WG / SPI ratio increased to 2:8, the WVP of the composite membrane showed a gradual decreasing trend (P<0.05), and further increasing the WG / SPI ratio had no significant effect (P>0.05). This may be because WG contains a large amount of hydrophobic wheat gliadin. Adding hydrophobic substances can enhance the water vapor barrier capacity of starch-based and protein-based composite membranes. However, when the WG / SPI ratio exceeded 2:8, the structure of the composite membrane was somewhat damaged, resulting in the membrane's WVP not decreasing continuously with the increase of WG.

[0128] 2.5 Solubility

[0129] The effect of the WG / SPI ratio on the solubility of the composite membrane is as follows: Figure 8 As shown, the solubility of the SPI membrane was 16.85%, and the solubility of the composite membrane increased significantly when the WG / SPI ratio exceeded 2:8 (P<0.05). This may be because the SPI protein network structure in the composite membrane became loose due to the disruption of the WG.

[0130] 2.6 Differential Scanning Calorimetry

[0131] WG / SPI ratio on composite membrane T g The impact such as Figure 9As shown, the T of the SPI film g The temperature was 53.62℃ when 1 / 10 of the WG composite film was added to T. g The temperature rose to 59.67℃, but further increases in the WG / SPI ratio were necessary. g However, the thermal stability gradually decreases. Therefore, the results show that adding WG improves the thermal stability of the SPI film, but a high proportion of WG will destroy the dense network structure of the SPI film, leading to a decrease in the thermal stability of the composite film.

[0132] 2.7 Contact Angle

[0133] The effect of the WG / SPI ratio on the contact angle of the composite membrane surface is shown in Table 7. When 1 / 10 of WG is added, the contact angle of the upper surface of the SPI membrane increases from 69.80° to 73.00°, while the contact angle of the lower surface decreases from 90.06° to 70.72°. This may be due to the large amount of hydrophobic wheat gliadin in WG and the roughening of the membrane lower surface structure. When the WG / SPI ratio is further increased, the contact angles of both the upper and lower surfaces of the composite membrane show a gradual increasing trend, which is due to the increase in the content of hydrophobic gliadin in the membrane matrix.

[0134] Table 7 Surface contact angles of composite films with different WG / SPI ratios

[0135]

[0136] Note: Different lowercase literals represent significant differences (P<0.05).

[0137] This embodiment reveals the influence of WG (glucose hydrate) treatment on the physicochemical properties of SPI membranes. The added WG mainly aggregates on the lower surface of the composite membrane, hindering cross-linking between SPI molecules. When the WG / SPI ratio exceeds 2:8, it disrupts the network structure of the composite membrane, leading to a decrease in the membrane's total contact angle (TS), solubility, and thermal stability. Adding WG roughens the lower surface of the composite membrane, resulting in a decrease in the lower surface contact angle and transparency. The results show that when the WG / SPI ratio is less than 2:8, the WG / SPI composite membrane exhibits better network structure, effective contact angle (EAB), and thermal stability.

[0138] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0139] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0140] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a soybean protein isolate membrane, characterized in that: Includes the following steps: S1, Active dry yeast is activated to obtain a yeast culture solution, wherein the yeast content of the yeast culture solution is 1-3 wt%; S2, the yeast liquid is mixed with gluten powder in a ratio of (0.5-3 mL): 1 g, and fermentation is carried out under the conditions of 20-30℃ and 60-80% relative humidity for 2-5 h to obtain fermented WG sample; S3, the fermented WG sample is mixed evenly with soy protein isolate SPI, water, and food-grade plasticizer. The mass ratio of the fermented WG sample to soy protein isolate SPI is (1-5):(6-10). The pH of the solution is adjusted to >7 with lye water to obtain a protein film-forming solution. The lye water is an aqueous solution of polyphosphate containing Na2CO3 and K2CO3. The concentration of Na2CO3 in the lye water is 0.5-3% (w / v), the concentration of K2CO3 is 0.1-1% (w / v), and the concentration of polyphosphate is 0.1-1% (w / v). S4, after degassing the protein film-forming solution, it is poured onto a template and dried to obtain a soy protein isolate membrane. The soy protein isolate membrane has an elongation at break of 90-95%, a tensile strength of 4-5 MPa, and a water vapor transmission rate of (1.3-2.5) × 10⁻⁶. 10 g m 1 s 1 pa 1 Water contact angle is 80-105°; solubility is 20-30%.

2. The method for preparing soybean protein isolate membrane according to claim 1, characterized in that: The activation described in S1 involves taking active dry yeast and white sugar, dispersing them in distilled water for activation, and obtaining a yeast culture solution with a yeast content of 1-3 wt%.

3. The method for preparing soybean protein isolate membrane according to claim 1, characterized in that: The fermentation time described in S2 is 3-5 hours.

4. The method for preparing the soybean protein isolate membrane according to claim 3, characterized in that: The fermentation time described in S2 is 4 hours.

5. The method for preparing soybean protein isolate membrane according to any one of claims 1-4, characterized in that: The mass ratio of the fermented WG sample to soybean protein isolate SPI in S3 is (1-2):(8-10).

6. The method for preparing the soybean protein isolate membrane according to claim 5, characterized in that: The mass ratio of the fermented WG sample to soybean protein isolate SPI in S3 is (1-2):(9-10).

7. The method for preparing soybean protein isolate membrane according to any one of claims 1-4, characterized in that: The food-grade plasticizer mentioned in S3 is any one of glycerol, sorbitol, and polyethylene glycol.

8. The method for preparing the soy protein isolate membrane according to any one of claims 1-4, characterized in that: The lye water used in S3 is used to adjust the pH of the solution to 8-10.

9. The method for preparing the soybean protein isolate membrane according to claim 8, characterized in that: The lye water used in S3 is used to adjust the pH of the solution to 9.

10. The method for preparing soybean protein isolate membrane according to any one of claims 1-4, characterized in that: After S3, the protein film-forming solution is mixed evenly with soybean oil to obtain a film-forming emulsion; correspondingly, in S4, the film-forming emulsion is defoamed and poured onto a template, and after drying, a soybean protein isolate film is obtained.

11. The method for preparing the soybean protein isolate membrane according to claim 10, characterized in that: The mass ratio of the protein film-forming solution to soybean oil is 10:(0.1-0.3).

12. The soybean protein isolate membrane prepared by the method described in claims 1-11, characterized in that: It has at least one of the following (1) to (5): (1) Tensile strength is 4-5 MPa; (2) Elongation at break is 90-95%; (3) Water vapor transmission rate is (1.3-2.5) × 10 10 g m 1 s 1 pa 1 ; (4) The water contact angle is 80-105°; (5) Solubility is 20-30%.

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