A preparation method of a pH-regulated soy protein emulsion based on metal-polyphenol network modification

Abstract

The application belongs to the technical field of plant protein emulsion food processing, and particularly relates to a preparation method of pH-regulated soy protein emulsion based on metal-polyphenol network modification. The method comprises the following steps: dispersing soybean protein isolate in water and fully hydrating to obtain a soy protein solution; mixing epigallocatechin-3-gallate and a zinc salt solution, and performing coordination assembly under light shielding at a set pH to obtain a metal-polyphenol network solution; adding the metal-polyphenol network solution to the soy protein solution to obtain a metal-polyphenol network modified soy protein system; and adding edible vegetable oil, and performing high-speed homogenization treatment to obtain a soy protein emulsion. By regulating the assembly pH of the metal-polyphenol network, the particle dispersion state and the interfacial stability of the soy protein are improved, so that the emulsification activity and stability of the emulsion are improved. The method is mild in process and simple in operation, and is suitable for the preparation of plant protein beverages, emulsified condiments, plant-based foods and functional emulsion foods.

Description

Technical Field

[0001] This invention belongs to the field of plant protein emulsion food processing technology, specifically relating to a method for preparing pH-controlled soybean protein emulsion based on metal-polyphenol network modification. Background Technology

[0002] Soy protein isolate is an important plant-based food protein with advantages such as wide availability, high protein content, good nutritional value, and relatively low cost. It has been widely used in plant-based protein beverages, emulsified foods, plant-based meat products, sauces, and fortified foods. In addition to its nutritional functions, soy protein isolate also possesses various processing properties such as emulsification, gelling, foaming, and water-holding capacity. Among these, emulsification is a crucial indicator affecting the effectiveness of soy protein in oil-water mixed food systems.

[0003] In oil-in-water emulsion systems, proteins typically need to maintain good dispersion in the aqueous phase and migrate to the oil-water interface to form an adsorption layer, thereby reducing interfacial tension and inhibiting oil droplet aggregation, flocculation, and stratification. The stability of the emulsion depends not only on the interfacial adsorption capacity of the proteins but also on factors such as the protein's conformational state, particle size, surface charge, continuous phase viscosity, and interfacial film strength. Soy protein isolate is mainly composed of components such as β-conglycinin and glycinin, and its molecular structure contains hydrophilic, hydrophobic, and ionizable groups, making it susceptible to influences from factors such as pH, ionic strength, heat treatment, and shear conditions.

[0004] Currently, the application of natural soy protein isolate in emulsified foods still has certain limitations. Especially under acidic or near-isoelectric point conditions, the net surface charge of soy protein decreases, weakening the electrostatic repulsion between particles and making it prone to aggregation or precipitation, thus reducing its solubility, interfacial adsorption capacity, and emulsion stability. Under neutral or weakly alkaline conditions, the dispersibility of soy protein improves somewhat, but the strength of the interfacial film formed may still be insufficient, and the emulsion is still prone to problems such as droplet aggregation, floating, and stratification during storage. Therefore, developing a technology that is mild, has good food applicability, and can effectively improve the stability of soy protein emulsions has significant application value.

[0005] Existing methods for improving the emulsifying properties of soybean protein mainly include physical modification, chemical modification, enzymatic modification, and compound modification. Physical modification methods, such as ultrasonication, high-pressure homogenization, heat treatment, extrusion, and pH shifting, can improve the functional properties of proteins by altering their conformation and aggregation state. However, the intensity of these treatments is difficult to control precisely, and overtreatment may lead to irreversible protein aggregation. Chemical modification methods, such as acylation, phosphorylation, glycosylation, and cross-linking reactions, can significantly alter protein structure, but may involve issues such as reaction byproducts, reagent residues, and label acceptance during food processing. Enzymatic modification is performed under mild conditions, but its effectiveness is greatly affected by the type of enzyme, the degree of hydrolysis, and reaction conditions. Excessive hydrolysis may also affect product flavor and interfacial film stability.

[0006] In recent years, the interaction between polyphenols and proteins has attracted attention. Polyphenol molecules contain multiple phenolic hydroxyl groups and aromatic structures, which can complex with proteins through hydrogen bonds, hydrophobic interactions, and π-π interactions, thereby altering protein conformation and colloidal behavior. Epigallocatechin-3-gallate is an important component of tea polyphenols, possessing numerous phenolic hydroxyl groups and strong protein-binding ability, showing potential for application in food systems. Simultaneously, metal ions can coordinate with the phenolic hydroxyl groups in polyphenol molecules, forming metal-polyphenol networks. These networks exhibit mild assembly conditions, tunable structures, and abundant interaction sites, making them promising interfacial stabilizing structures in food colloidal systems.

[0007] However, existing technologies regarding the application of metal-polyphenol networks in soybean protein emulsion systems remain insufficient. In particular, the assembly pH of the metal-polyphenol network affects the degree of polyphenol hydroxyl dissociation, metal coordination state, and network structure, thereby influencing its complexation with soybean protein and the emulsion's stabilizing effect. Inappropriate assembly conditions may cause soybean protein aggregation, which in turn reduces emulsion homogeneity and storage stability. Therefore, it is necessary to provide a method for preparing soybean protein emulsions by pH-controlled metal-polyphenol network assembly and its application in modifying soybean protein emulsions, thereby improving the emulsifying activity and stability of soybean protein emulsions. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing pH-controlled soybean protein emulsion based on metal-polyphenol network modification, in order to solve the problems of limited emulsifying activity, insufficient storage stability, and the demanding conditions of traditional modification methods in existing soybean protein emulsions.

[0009] The technical problem to be solved by the present invention is achieved through the following technical solution: a method for preparing a pH-controlled soybean protein emulsion based on metal-polyphenol network modification, characterized in that the method includes the following steps: (1) dispersing soybean protein isolate in water and fully hydrating it to obtain a soybean protein solution; (2) mixing the polyphenol solution with a metal salt solution, adjusting the pH of the system to 5.0-9.5, and performing coordination assembly under light-protected conditions to obtain a metal-polyphenol network solution; (3) adding the metal-polyphenol network solution to the soybean protein solution, stirring and compounding under light-protected conditions to obtain a soybean protein system modified by a metal-polyphenol network; (4) adding edible vegetable oil to the soybean protein system modified by a metal-polyphenol network, and homogenizing it at high speed to obtain a soybean protein emulsion modified by a metal-polyphenol network. Preferably, the polyphenol is epigallocatechin-3-gallate, and the metal salt is zinc chloride. Preferably, the assembly pH of the metal-polyphenol network is 7.0-9.0. More preferably, when the assembly pH of the metal-polyphenol network is 7.0, it is beneficial to improve the emulsifying activity of the soybean protein emulsion; when the assembly pH of the metal-polyphenol network is 9.0, it is beneficial to improve the stability of the soybean protein emulsion. Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a metal-polyphenol network to modify soybean protein, and the preparation conditions are mild, avoiding the food application limitations that may be caused by strong chemical modification; (2) The present invention can improve the particle dispersion state and interface stability of the soybean protein composite system by regulating the assembly pH of the metal-polyphenol network; (3) The soybean protein emulsion obtained by the present invention has high emulsifying activity and emulsion stability, and is suitable for plant-based emulsion food processing; (4) The present invention has a simple process, clear raw material sources, and good food industry conversion potential. Attached Figure Description

[0010] Figure 1 This is a flowchart illustrating the process for preparing the metal-polyphenol network modified soybean protein emulsion of this invention.

[0011] Figure 2 The UV absorption spectra of SPI-MPN under different pH conditions are shown.

[0012] Figure 3 The fluorescence spectra of the SPI-MPN composite system under different pH conditions are shown.

[0013] Figure 4 The particle size distribution of the SPI-MPN composite system under different pH conditions is shown.

[0014] Figure 5 Microstructure diagrams of SPI-MPN emulsions under different pH conditions.

[0015] Figure 6The storage stability of soybean protein emulsion modified with metal-polyphenol network under different pH conditions is shown in the figure. Detailed Implementation

[0016] The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Conventional adjustments made by those skilled in the art to the raw material concentration, pH, compounding time, homogenization conditions, and oil phase type without departing from the concept of the present invention are all within the scope of protection of the present invention.

[0017] In this invention, the soy protein isolate can be commercially available food-grade soy protein isolate, or it can be prepared using an alkali-soluble acid-precipitated method. The polyphenol is preferably epigallocatechin-3-gallate. The metal salt is preferably zinc chloride. The edible vegetable oil can be one or more of sunflower seed oil, soybean oil, corn oil, rapeseed oil, peanut oil, and olive oil. Example 1

[0018] Prepare a 48 mmol / L epigallocatechin-3-gallate solution and a 48 mmol / L ZnCl2 solution. Mix the two solutions at a volume ratio of 1:1 and stir for 30 min under light-protected conditions. Adjust the pH of the system to 5.0 with 0.1 mol / L HCl or 0.1 mol / L NaOH to allow epigallocatechin-3-gallate to coordinate with Zn²⁺, resulting in a metal-polyphenol network solution with an assembled pH of 5.0, denoted as MPN-5.

[0019] Soy protein isolate powder was dispersed in deionized water to prepare a 1.0% (w / v) soy protein solution, which was then fully hydrated and set aside. 200 μL of MPN-5 solution was added dropwise to 30 mL of the soy protein solution, and the mixture was stirred for 30 min at room temperature in the dark to allow the metal-polyphenol network to fully interact with the soy protein isolate, resulting in a soy protein system modified with a metal-polyphenol network assembled at pH 5.0, denoted as SPI-MPN-5.

[0020] Take 30 mL of SPI-MPN-5 system, add 3.33 mL of sunflower seed oil, prepare an emulsion at an oil-water volume ratio of 1:9, and homogenize it at 12000 r / min for 10 min using a high-speed homogenizer to obtain a soybean protein emulsion modified by an assembled metal-polyphenol network with pH 5.0.

[0021] The emulsion was measured to have an emulsifying activity index of 13.15 ± 0.17 m² / g, an emulsifying stability index of 939.50 ± 10.16 min, and a turbidity of 1.9208 ± 0.0173. Particle size distribution results showed that the distribution peak of this system shifted significantly towards larger particle sizes, indicating that the system is more prone to forming large particles or aggregates under pH 5.0 conditions. Microscopic observation revealed that the emulsion droplet distribution was relatively uneven, exhibiting some aggregation. Example 2

[0022] Prepare a 48 mmol / L epigallocatechin-3-gallate solution and a 48 mmol / L ZnCl2 solution. Mix the two solutions at a volume ratio of 1:1 and stir for 30 min under light-protected conditions. Adjust the pH of the system to 7.0 with 0.1 mol / L HCl or 0.1 mol / L NaOH to allow epigallocatechin-3-gallate to coordinate with Zn²⁺, resulting in a metal-polyphenol network solution with an assembled pH of 7.0, denoted as MPN-7.

[0023] Soy protein isolate powder was dispersed in deionized water to prepare a 1.0% (w / v) soy protein solution, which was then fully hydrated and set aside. 200 μL of MPN-7 solution was added dropwise to 30 mL of the soy protein solution, and the mixture was stirred for 30 min at room temperature in the dark to allow the metal-polyphenol network to fully interact with the soy protein isolate, resulting in a soy protein system modified with a metal-polyphenol network assembled at pH 7.0, denoted as SPI-MPN-7.

[0024] Take 30 mL of SPI-MPN-7 system, add 3.33 mL of sunflower seed oil, prepare an emulsion at an oil-water volume ratio of 1:9, and homogenize it at 12000 r / min for 10 min using a high-speed homogenizer to obtain a soybean protein emulsion modified by an assembled metal-polyphenol network with pH 7.0.

[0025] The emulsion exhibited an emulsifying activity index of 16.91 ± 0.19 m² / g, an emulsifying stability index of 718.90 ± 12.41 min, and a turbidity of 1.8956 ± 0.0140. The emulsifying activity index of this group was higher than that of the pH 5.0 and pH 9.0 groups, indicating that the metal-polyphenol network modification system formed under pH 7.0 conditions is more conducive to improving the emulsifying activity of soybean protein emulsions. Microscopic observation showed that the emulsion droplet distribution was more uniform, with fewer large droplets and obvious aggregation areas. Example 3

[0026] Prepare a 48 mmol / L epigallocatechin-3-gallate solution and a 48 mmol / L ZnCl2 solution. Mix the two solutions at a volume ratio of 1:1 and stir for 30 min under light-protected conditions. Adjust the pH of the system to 9.0 with 0.1 mol / L HCl or 0.1 mol / L NaOH to allow epigallocatechin-3-gallate to coordinate with Zn²⁺, resulting in a metal-polyphenol network solution with an assembled pH of 9.0, denoted as MPN-9.

[0027] Soy protein isolate powder was dispersed in deionized water to prepare a 1.0% (w / v) soy protein solution, which was then fully hydrated and set aside. 200 μL of MPN-9 solution was added dropwise to 30 mL of the soy protein solution, and the mixture was stirred for 30 min at room temperature in the dark to allow the metal-polyphenol network to fully interact with the soy protein isolate, resulting in a soy protein system modified with a metal-polyphenol network assembled at pH 9.0, denoted as SPI-MPN-9.

[0028] Take 30 mL of SPI-MPN-9 system, add 3.33 mL of sunflower seed oil, prepare an emulsion at an oil-water volume ratio of 1:9, and homogenize it at 12000 r / min for 10 min using a high-speed homogenizer to obtain a soybean protein emulsion modified by an assembled metal-polyphenol network with pH 9.0.

[0029] The emulsion exhibited an emulsifying activity index of 14.18 ± 0.21 m² / g, an emulsifying stability index of 1008.07 ± 13.40 min, and a turbidity of 1.7706 ± 0.0214. The emulsifying stability index of this group was higher than that of the pH 5.0 and pH 7.0 groups, indicating that the metal-polyphenol network modification system formed under pH 9.0 conditions is more conducive to improving the stability of soybean protein emulsions. Microscopic and apparent stability observations showed that the emulsion droplet distribution was relatively uniform, and the degree of stratification during storage was weak.

Claims

1. A method for preparing pH-controlled soybean protein emulsion based on metal-polyphenol network modification, characterized in that, Includes the following steps: (1) Preparation of soybean protein solution: Soybean protein isolate is dispersed in water and fully hydrated to obtain soybean protein solution; (2) Preparation of metal-polyphenol network solution: The polyphenol solution and the metal salt solution were mixed, the pH of the system was adjusted, and coordination assembly was carried out under light-protected conditions to obtain the metal-polyphenol network solution; (3) Preparation of metal-polyphenol network modified soybean protein system: The metal-polyphenol network solution obtained in step (2) is added to the soybean protein solution obtained in step (1), and the mixture is stirred in the dark to obtain the metal-polyphenol network modified soybean protein system. (4) Preparation of soybean protein emulsion: Add edible vegetable oil to the system obtained in step (3), and homogenize it at high speed to obtain a soybean protein emulsion modified by metal-polyphenol network.

2. The preparation method according to claim 1, characterized in that, The soy protein isolate in step (1) has a protein content of not less than 90%, and the mass concentration of soy protein isolate in the soy protein solution is 0.5% to 5.0% (w / v).

3. The preparation method according to claim 1, characterized in that, The polyphenol mentioned in step (2) is epigallocatechin-3-gallate, and the metal salt is zinc chloride.

4. The preparation method according to claim 1 or 3, characterized in that, The concentrations of the polyphenol solution and the metal salt solution in step (2) are 10 to 80 mmol / L, and the molar ratio of polyphenol to metal ion is 1:0.5 to 1:

2.

5. The preparation method according to claim 1, characterized in that, The assembly pH of the metal-polyphenol network in step (2) is 5.0 to 9.5, preferably 5.0, 7.0 or 9.0; the light-protected coordination assembly time is 5 to 60 min, preferably 30 min.

6. The preparation method according to claim 1, characterized in that, In step (3), the amount of metal-polyphenol network solution added is 50-1000 μL per 30 mL of soybean protein solution, preferably 200 μL; the light-protected stirring and compounding time is 10-60 min, preferably 30 min.

7. The preparation method according to claim 1, characterized in that, The edible vegetable oil mentioned in step (4) is one or more of sunflower seed oil, soybean oil, corn oil, rapeseed oil, peanut oil, and olive oil, and the volume fraction of the oil phase in the emulsion is 5% to 30%.

8. The preparation method according to claim 1, characterized in that, The high-speed homogenization conditions in step (4) are 8000-16000 r / min for 3-15 min; preferably 12000 r / min for 10 min, with an oil phase to water phase volume ratio of 1:

9.

9. A soybean protein emulsion modified with a metal-polyphenol network, prepared by the method according to any one of claims 1 to 8.

10. The application of the metal-polyphenol network modified soybean protein emulsion according to claim 9 in plant protein beverages, emulsified condiments, or plant-based emulsion foods.

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