What Is Mung Bean Protein?

Proteins are one of the main nutrients necessary for the important functions of cells, tissues, organs and systems. Therefore, consuming sufficient amounts of animal and plant proteins on a daily basis has many benefits [1]. Against the backdrop of a rapidly growing global population and drastic climate change, it is crucial to find ways to expand protein sources to meet human nutritional needs. Plant protein is gradually becoming the focus of researchers due to its green, environmentally friendly and sustainable characteristics. Among them, legume protein has attracted much attention due to its low cost, high nutritional value and high bioavailability [2-4].

Mung beans, as one of the edible annual legume crops, are widely cultivated in China due to their short growth cycle, high average yield, drought tolerance and nitrogen fixation. The total annual output is close to 1 million tons, ranking first in the world [5]. At the same time, they are cultivated in more than 90% of the countries in the Asian region, accounting for more than 50% of the global output, and their global planting area accounts for 8.5% of the total legume planting area, with a total output of more than 7.2 million tons [ 6,7].

Mung beans are rich in nutrients, mainly including starch (40.6%–48.9%), protein (14.6%–32.6%), dietary fiber (3.5%–6.5%), fat (1%–1.5%), etc. [8,9]. In addition, mung beans contain a certain amount of functional active ingredients such as vitexin and isovitexin, which have health effects such as protecting the cardiovascular system and regulating blood sugar [10]. At the same time, because of rich nutritional composition and health effects, in recent years there have been new mung bean products such as mung bean milk [11] and plant-based eggs [12].

Studies have shown that mung bean protein has the health effects of regulating abnormal sugar and lipid metabolism, improving obesity, and enhancing mineral utilization [13-15]. Among them, compared to soy and pea proteins, mung bean has a higher total essential amino acid score than pea protein, but lower than soy protein isolate [10,16], which not only attracts the attention of researchers, but is also favored by more vegetarians and vegans [10]. In terms of functional properties, compared to other legume proteins, mung bean protein has similar solubility and oil holding capacity as soybean protein [17,18], and its foaming properties are superior to those of chickpea and lupin proteins but weaker than those of soybean protein [17], and its emulsifying properties are weaker than those of soybean protein [19]. Due to the differences in functional properties from other legume proteins, there are limitations to the application of mung bean protein in food systems [14,20,21].

Therefore, it is beneficial to understand the research information on the correlation between the structure and functional properties of mung bean protein for the in-depth study and application of mung bean protein. Therefore, this paper reviews the structural composition of mung bean protein, the influence of extraction methods on its structure and functional properties in recent years, and comprehensively describes the progress of mung bean protein applications, with the aim of providing some theoretical guidance for the efficient development and application of mung bean protein.

1 Composition and structure of mung bean protein

1.1 Composition of mung bean protein

Osborne was the first to classify proteins based on differences in solubility [22]. According to Osborne's classification, the composition of mung bean protein (as shown in Figure 1) is significantly different from that of soybean protein. Mung bean protein is composed of globulin and albumin, accounting for 60% to 70% and 15% to 20% respectively, while gluten accounts for about 13.3%, and the content of alcohol-soluble protein is the lowest, accounting for about 0.95% [22].

In contrast, soybean protein is mainly composed of globulin, accounting for 70% to 80%, albumin for about 8%, and the rest being gluten and alcohol-soluble protein [23, 24]. The globulins of mung bean protein are mainly composed of 8S and 11S globulins, with molecular weights of 200 and 360 kDa, respectively [25]. 8S globulin is a trimeric protein formed by three subunit proteins [26], and 11S globulin is a hexameric protein formed by six subunits. Each subunit is formed by acidic and basic polypeptides with molecular weights of 40 and 24 kDa peptides linked by disulfide bonds [27]. The main globulins in soy protein are 7S and 11S globulins, with molecular weights of 180–210 kDa and 320–375 kDa [28]. The albumin of mung bean protein mainly consists of 2S albumin with a smaller molecular weight [29]. It is believed that the albumin gives mung bean protein better foaming properties because it can form a strong viscous interfacial layer around the bubbles [30]. Glutenin is only soluble in dilute acid or alkaline solution and can be used as a reducing agent. However, no further research on glutenin in mung beans has been found [31].

1.2 Structure of mung bean protein

The structure of a protein can be divided into primary, secondary, tertiary and quaternary structure levels (as shown in Figure 2) depending on its composition [32]. The primary structure of a protein refers to the linear sequence of amino acid residues [33], which not only determines the nutritional value of the protein, but also affects its functional properties such as solubility and emulsifying properties [34].

Mung bean protein is composed of 20 amino acids, including 8 essential amino acids and 12 non-essential amino acids. Among them, glutamic acid and glutamine have the highest content, followed by aspartic acid and asparagine. Tryptophan and sulfur-containing amino acids have a low content and are limiting amino acids in mung bean protein [32]. In addition, the content of hydrophobic/hydrophilic amino acids in a protein can affect its solubility, surface activity, emulsifying properties, etc. to a certain extent [35].

Kudre et al. [36] analyzed the amino acid composition of mung bean protein and found that compared with Bambara groundnut protein and black soybean protein, mung bean protein has lower solubility, which may be attributed to its low proportion of hydrophilic amino acids. Liu et al. [34] found that the content of acidic amino acids in mung bean protein is much higher than that of basic amino acids, which gives it good emulsifying and foaming properties.

The secondary structure of a protein refers to the sequence of amino acid residues that combine to form a polypeptide chain, which is folded and coiled to form a three-dimensional local fragment. Among these, α-helix, β-fold, β-turn and random coil are the main secondary structures of proteins [33], which can be analyzed using Fourier-Transform Infrared Spectroscopy (FT-IR) [37] and Circular Dichroism Spectrum (CD) [14 (Circular Dichroism Spectrum, CD) to analyze. Fourier transform infrared spectroscopy can be used to analyze the changes in the main characteristic structures of amide I and III bonds (as shown in Table 1), which can help characterize the changes in the main secondary structure of mung bean protein. Although the main secondary structure of the protein can be characterized based on the vibrations at specific wavelengths, other substances such as starch also have repeated vibrations at specific wavelengths, which can interfere with the infrared analysis results [38]. Therefore, the results of infrared analysis are often deconvoluted to further analyze the changes in secondary structure by comparing the changes in the content of characteristic structures [39].

Brishti et al. [37] compared the secondary structure of mung bean proteins with different moisture contents by infrared deconvolution analysis after extrusion treatment. They found that the secondary structure of natural mung bean protein has low α-helix content and high β-sheet content, while the β-sheet and random coil contents of extruded mung bean protein are significantly reduced, and the β-fold content is increased, indicating that the chemical bonds between protein molecules are broken, which stretches the natural structure and forms a dense protein aggregate.

Circular dichroism spectroscopy is based on the fact that when protein molecules absorb polarized light of different degrees, the hydrogen bond conformation changes within a specific wavelength, and the secondary structure is characterized based on the change in the peak within a specific band [40]. Circular dichroism analysis of natural mung bean protein revealed that the contents of α-helix, β-fold, β-curl and random coil in its secondary structure were 19.7%, 26.7%, 21.3% and 32.3%, respectively [37, 41]. Brishti et al. [37] found that after extrusion treatment, the ellipticity of the peak at 194 nm showed a decrease in ellipticity, and the double peaks in the range of 207–224 nm shifted. This may be due to the fact that during the extrusion process, the aromatic amino acid residues of mung bean protein dissociated, the interaction between protein molecules weakened, and the secondary structure of mung bean protein changed.

The tertiary structure of a protein, also known as its three-dimensional structure, is formed by the secondary structure coiling and folding further, relying on the action of chemical bonds such as hydrogen bonds and disulfide bonds between amino acid side chains [33]. Fluorescence spectroscopy mainly uses the endogenous fluorescent groups of tryptophan, tyrosine and phenylalanine residues to analyze the tertiary structure of proteins by observing changes in fluorescence intensity within a specific wavelength after receiving excitation light at a specific wavelength.

Brishti et al. [37] found that after excitation light was emitted at 295 nm, the strongest fluorescence intensity of natural mung bean protein appeared at 39 3 nm. Processing can affect the tertiary structure of mung bean protein. Wang et al. [12] found that after mung bean protein was treated with a pH shift for calcium ion binding, the fluorescence intensity of mung bean protein at 393 nm decreased significantly. This may be due to the fact that the hydrophobic side chains of mung bean protein form larger aggregates in a polar environment and in the presence of calcium ions due to the electrostatic salt bridge effect.

Similarly, analyzing the changes in the content of sulfhydryl groups and disulfide bonds in proteins is also an important indicator for evaluating changes in protein structure and functional properties. The sulfhydryl groups in proteins can be oxidized to disulfide bonds or exchanged with disulfide bonds, which can change the hydrophobic properties of the protein surface and thus change the tertiary structure and functional properties of the protein [45].

Tang et al. [46] found that the total sulfhydryl and free sulfhydryl groups in natural mung bean protein decreased after heat treatment. The sulfhydryl groups were oxidized to form new disulfide bonds, and the protein formed new soluble aggregates through covalent cross-links, thereby improving its solubility. Liu et al. [34] found that the free sulfhydryl group content of mung bean protein changes under different pH environments, which may be attributed to the breaking of chemical bonds such as hydrogen bonds and disulfide bonds in the protein at different pH values, resulting in the stretching of the protein structure and the easier exposure of sulfhydryl groups, which gives mung bean protein better solubility and gel properties.

2 Functional properties of mung bean protein

Changes in the structure of mung bean protein affect its functional properties (as shown in Table 2), thereby affecting the application of mung bean protein. Therefore, analyzing the correlation between the structure and functional properties of mung bean protein can help to explore the potential applications of mung bean protein.

2.1 Solubility

Solubility is the primary prerequisite for proteins to exert their functional properties in various systems, and it is also the key to affecting functional properties such as emulsifying, foaming and gelling properties [51]. The hydrophobic/hydrophilic interactions of proteins in liquid systems affect their hydration effects and solubility [18]. Studies have shown that factors such as the pH value, ionic strength, and protein composition and structure in the system can affect the solubility of proteins [52]. When the pH value is high or below the isoelectric point, the surface charge density of the protein molecule increases, and the hydrophilic and hydration repulsive forces are much greater than the hydrophobic interactions, so the solubility increases [14, 47]. For example, the solubility of mung bean protein shows a “U”-shaped distribution (solubility of 73.5% to 98.3%) at pH 2 to 10, and the solubility is lowest (5.9%) at pH 4 to 5, which is the isoelectric point range of mung bean protein [34].

Ge et al. [18] found that the solubility of soy protein isolate at different pH values followed a similar pattern to that of mung bean protein, but the solubility at pH 5 and pH 9 (16.2% and 98.6%) was higher than that of mung bean protein (14.6% and 89.3%), which may be attributed to the higher surface charge of soy protein isolate than mung bean protein. Kudre et al. [36] found that found that the solubility of mung bean protein varied with the concentration of salt solution in the system, which may be attributed to the fact that the electrostatic repulsion between mung bean protein molecules was weakened in a certain concentration of salt solution, thereby increasing solubility (from 19.0% to 52.7%), while a high concentration of salt solution may reduce the hydration of the protein, enhance hydrophobic interactions, form insoluble protein polymers, and reduce solubility (from 5 2.7% to 41.8%).

2.2 Emulsifying properties

The emulsifying properties of proteins generally refer to their ability to form and stabilize emulsions, which significantly affect their application in food systems such as doughs, ice creams, cakes and mayonnaise. Emulsifying properties generally include emulsifying capacity and emulsifying stability. Emulsifying capacity indicates the ability of proteins to form emulsions, while emulsifying stability is the ability to maintain emulsions for a specified period of time [14]. Factors such as the pH value and ionic concentration of the system in which the protein is located can affect the emulsifying properties of the protein by changing the surface charge, hydrophobic amino acid residues and protein composition and structure of the protein, causing the protein to depolymerize, thereby enhancing its activity and adsorption capacity at the oil-water interface [46].

Liu et al. [34] analyzed the emulsifying properties of mung bean protein at different pH values and found that when the pH value was 10, mung bean protein had the best emulsifying capacity (117.05 m2/g) and relatively good emulsifying stability (20.86 min). As the pH value decreased, the emulsifying capacity (117.05 to 73.48 m2/g) and emulsifying stability (20.86 ~39.6 min) first decreased and then increased, which may be attributed to the fact that mung bean protein will expose more negative charges in an alkaline environment. Negative charges are conducive to the formation of a more stable emulsion at the oil-water interface. However, near the isoelectric point, mung bean protein aggregates, weakening its interfacial adsorption at the oil-water interface, resulting in a decrease in its emulsifying capacity (50.02 m2/g) and emulsifying stability (2.31 min declined.

Ge et al. [18] found that at pH 3, 7, and 9, the emulsifying capacity (8.7, 9.1, and 9.59 m2/g) of mung bean protein was similar to that of soy protein isolate (8.83, 9.23, and 10.1 m2/g), but the emulsion stability (0.86, 1.89, and 2.01 min) was weaker than that of soy protein (2.86 and 6.47 min), which may be due to the fact that the solubility, surface charge and surface hydrophobicity of mung bean protein at different pH values are weaker than those of soybean protein. Brishti et al. [19] compared and analyzed the emulsifying properties of mung bean protein in 3% NaCl solution and pure water and found that the emulsifying capacity (72.03%) and emulsifying stability (66.50%) of mung bean protein in 3% NaCl solution were better than those in pure water (63.18% and 62.75%), which may be attributed to the fact that NaCl can improve the solubility of the protein and thus its utilization rate. At the same time, the 3% NaCl solution can reduce the Columbo interaction force between adjacent droplets, thereby giving mung bean protein better emulsifying properties.

2.3 Foaming properties

The foaming properties of proteins include foaming capacity and foaming stability. Foaming capacity is an indicator of the increase in foam volume after whipping, while foaming stability is the ability of the protein to maintain a stable foam. The advantages and disadvantages of both determine the application properties of the protein in ice cream, baked goods, cakes and other protein products [43]. Studies have found that factors such as the pH of the environment in which mung bean protein is located and the extraction method can all induce denaturation of the mung bean protein structure, thereby affecting the adsorption and extension of the protein at the air-water interface and improving its foaming properties [18].

Liu et al. [34] analyzed the foaming properties of mung bean protein at different pH values and found that when the pH value was 10, its foaming capacity was the best (125%), and its foaming stability was the weakest (58%). As the pH value decreased, the foaming capacity (125%~45%) first decreased and then increased, but the foaming stability (58%~92.7%) first increased and then decreased. This may be because in an alkaline environment, the adsorption and extension of mung bean protein at the adsorption and extension at the air-water interface, while the mutual repulsive force between protein molecules weakens.

However, after the pH value decreases, the surface hydrophobicity and aggregation of mung bean protein increase, which is not conducive to the formation of foam at the air-water interface. Brishti et al. [19] found that the foaming ability (89.66%) of mung bean protein in pure aqueous solution was better than that of soy protein isolate (68.66%), but the foaming stability (50 .40 min) was weaker than that of soy protein isolate (53.66 min), which may be attributed to the different degrees of adsorption and unfolding of the two proteins at the air-water interface. Ratnaningsih et al. [48] found that after mung bean protein was extracted from mung bean by-products using salting-out and alkaline hydrolysis methods, respectively, there were significant differences in the foaming capacity (61.67%, 42.50%) and foaming stability (37. 9%, 29.61%) were significantly different, which may be attributed to the different degrees of protein denaturation induced by different chemical reagents during the extraction process, and the differences in the degree of structural relaxation, which changed the foaming properties of mung bean protein.

2.4 Water-holding and oil-holding capacity

The water-holding/oil-holding capacity of proteins can significantly affect the texture, juiciness and shelf life of protein products. Proteins with high water-holding capacity can better maintain the moisture of the product, better maintain the freshness and taste of the product, while proteins with high oil-holding capacity can help improve the taste and extend the shelf life of the product [18,50].

Due to the differences in the number of polar groups, surface hydrophobicity and structural extension of mung bean protein in different environments, its ability to retain oil or water is affected, which in turn leads to changes in its water/oil holding capacity [14,50]. Brishti et al. [19] found that the water holding capacity of mung bean protein (3.33 g) was better than that of soy protein isolate (3.00 g), but its oil holding capacity (3.00 g) was weaker than that of soy protein isolate protein (3.45 g). This may be because mung bean protein has a higher content of polar groups such as high phosphate than soybean isolate protein.

Hadidi et al. [49] found that after phosphoramide modification, the water-holding capacity (2.12–2.88 g) and oil-holding capacity (4.19–5.11 g) of mung bean protein were significantly improved. This may be because the phosphorylation treatment exposed the hydrophobic groups inside the protein, which improved the oil-holding capacity. At the same time, the increase in polar groups such as phosphoric acid enhanced the interaction between the protein and water molecules, improving its water-holding capacity. In addition, the choice of drying method can also affect the water/oil holding capacity of mung bean protein. Brishti et al. [43] analyzed the oil holding capacity of mung bean protein after different drying treatments and found that the oil holding capacity of freeze-dried mung bean protein (8.38 g) was significantly higher than that of spray-dried (4.00 g) and oven-dried (5.58 g). It is possible that after freeze-drying, a porous structure forms on the surface of mung bean protein, and the fat is encapsulated in the protein network structure , which improves the oil retention capacity of mung bean protein by improving the ability of the protein to trap fat.

2.5 Gel properties

The gel properties of proteins are closely related to the quality of foods such as vegetable meat, yogurt and cheese, and are mainly determined by factors such as the pH of the environment, exogenous ions and the composition of the sub-unit protein [50]. Brishti et al. [19] found that the minimum gel mass concentration of mung bean protein was 12%, while the minimum gel mass concentration of soy protein isolate was 14%.

The minimum gel mass concentration of mung bean protein was slightly better than that of soy protein isolate, which may be because during heating process, the stability of the chemical bonds such as hydrogen bonds and disulfide bonds in mung bean protein is weaker than that in soybean protein, causing the protein structure to stretch more than that in soybean protein and making it easier to form a gel network structure.

Ge et al. [50] analyzed the minimum gel concentration of mung bean protein under different pH environments and found that the minimum gel mass concentration of mung bean protein was the lowest (8%) at pH 3. It is possible that under this pH environment, the 7S and 11S globulins of mung bean protein have high solubility, high surface hydrophobicity, and undergo acid hydrolysis at the same time, producing fibrous polymers, making it easier to form gels during processing . Wang et al. [12] analyzed the strength of mung bean protein gels under pH shifts of calcium ion binding and found that the gel hardness formed by mung bean protein (3.33 N) was close to that formed by diluted egg (3.92 N). It is possible that the electrostatic shielding and ion bridging effects of calcium ions increase the degree and strength of protein aggregation, resulting in a harder gel.

3 Effect of extraction method on the structure and functional properties of mung bean protein

Mung bean protein extracted by different methods can significantly affect the composition and structure of mung bean protein, thereby changing its solubility, emulsifying properties and other functional properties (as shown in Table 3). At present, the extraction methods of mung bean protein are mainly divided into wet or dry methods, as shown in Table 4. Wet extraction methods mainly include alkaline solubilization and acid precipitation, salting-out, acid solubilization and acid precipitation, and aqueous extraction, while dry extraction methods mainly include air classification and electrostatic separation [32].

3.1 Wet extraction

Wet extraction is widely used due to its advantages of high protein extraction efficiency and high purity. However, because a certain proportion of chemical reagents are introduced during the extraction process, and a certain amount of drying treatment is required after extraction, the original protein composition and structure may be destroyed, the surface hydrophobicity of the protein may be changed, and some of the protein's functional properties may be affected [56,57].

3.1.1 Alkali solubilization and acid precipitation

The alkali-solubilization acid precipitation method is a method for extracting and separating proteins that takes advantage of the fact that proteins have high solubility at alkaline pH values and lowest solubility near the isoelectric point. Because of its advantages of being easy to operate and having high purity of protein extraction, it has become the most commonly used method for industrial protein extraction [4].

In 1977, Thompson studied the alkaline solubilization and acid precipitation method for extracting mung bean protein [62] and found that at an alkaline pH, the disulfide bonds in the protein break, while acidic and neutral amino acids ionize [53]. Based on the change in the solubility of the protein in different pH environments, mung bean protein with a purity of 92% (dry basis) was obtained (yield 10%). However, because the alkaline environment damages the hydrogen bonds, acyl amide bonds and disulfide bonds, as well as the amino acid structure, are all destroyed in an alkaline environment, which affects the protein's surface hydrophobicity, adsorption capacity at the air-water interface, etc., resulting in a decrease in its functional properties such as solubility and foaming properties [54].

Du et al. [14] optimized the alkaline solubilization and acid precipitation extraction process of mung bean protein by response surface methodology, and obtained mung bean protein with purity and yield of 86.94% and 77.32%, respectively, and with solubility similar to albumin. In addition, the choice of drying method after wet extraction also affects the purity, structure and functional properties of the extracted mung bean protein. Studies have found that compared with oven drying (77.27%) and spray drying (75.85%), freeze-drying treatment of the protein has the highest protein purity (86.15%) and better solubility, which may be attributed to the high processing temperatures during the oven and spray drying processes, which cause varying degrees of denaturation of mung bean protein, resulting in protein swelling and aggregation, which reduces the purity and solubility of the protein extract [46].

3.1.2 Salting-out method

The salting-out method uses proteins in neutral salt solutions of different concentrations. The salt ions change the surface charge of the protein and its interaction with water molecules, which affects solubility and thus enables protein extraction[4]. Ratnaningsih et al.[48] used three salt (MgSO4, (NH4)2SO4 and CaCl2) solutions to extract protein from peeled mung beans, obtaining yields and purities of 21.09%, 20. 43%, 20.13%, and 78.61%, 50.59%, and 47.22%, respectively. Among them, MgSO4 may have the highest protein extraction rate due to its high affinity with water molecules and the destruction of the protein hydrolytic layer. Penchalaraju et al. [57] obtained mung bean protein with a yield of 11.56% and a purity of 70.76% by salt solubilization and pH adjustment. Compared with the alkaline solubilization and acid precipitation method, salting-out extraction is milder, has the advantage of maintaining the natural structure of the protein and avoiding rapid protein denaturation, and can significantly improve the solubility, emulsifying ability and water retention of the protein [55]. However, due to the introduction of exogenous ions, the protein extracted by salting-out is more prone to aggregate than that extracted by the alkaline solubilization and acid precipitation method [4].

3.1.3 Acid solubilization and acid precipitation method

The acid-soluble acid precipitation method is similar in principle to the alkali-soluble acid precipitation method. It is a method for extracting proteins based on the fact that proteins have a high solubility at a strongly acidic pH (1–3) and precipitate proteins near the isoelectric point [56]. Penchalaraju et al. [57] achieved the dissolution and precipitation of proteins in an acidic environment, and ultimately obtained mung bean protein with a yield of 9.23% and a purity of 74.69%. Although the acid-solubilization and acid precipitation method has high protein purity and is simple to operate, the extraction in a strongly acidic environment consumes too many chemical reagents and can cause the disulfide bonds in the protein to break, as well as cross-linking and hydrolysis of the amino acids, which can increase the degree of protein denaturation and affect the solubility and gel properties of the protein. In addition, the extraction takes a long time and the protein is easily perishable, which makes it less suitable for practical application [4].

3.1.4 Water-soluble extraction method

Water-soluble extraction is a method of extracting proteins with water as the solvent at a relatively low temperature. It is environmentally friendly, gentle, and causes less damage to the natural structure of the protein. However, it is not widely used due to the disadvantages of long extraction times, which cause the protein to aggregate and reduce solubility, and low yields and time-consuming processes. Penchalaraju et al. [57] used a long-time water-soluble extraction combined with spray drying to obtain mung bean proteins with yields and purities of 12. 3%, 83.16% purity mung bean protein.

3.2 Dry separation

Compared to the wet extraction process, which damages the natural structure and functional properties of the protein [54], dry separation is gradually considered to have good application prospects because of its advantages of low energy consumption, sustainability, no sewage production, and maximization of the natural structure and function of the protein. However, due to its the relatively low purity and yield of protein extraction and the high cost of extraction equipment, the process of industrialization is currently slow [64]. At present, the more widely used methods of dry fractionation mainly include two types: air classification and electrostatic separation.

3.2.1 Air classification

Airflow classification refers to a separation method in which the material, whether whole or shelled, is ground into a fine powder. The difference in the particle size and density of the protein, starch and other components in the fine powder causes differences in the settling speed of the components during airflow classification, thereby enriching the protein components [65]. In recent years, air classification has been widely used to enrich the protein content of plant-based raw materials such as grains and beans, but there has been relatively little research on mung bean protein [66]. Zhu et al. [63] used a combination of air impact grinding and an air classifier to enrich mung bean protein, and ultimately obtained mung bean protein with a purity of 63.2% (yield 31.9%). Schlangen et al. [58] used an air classification system enriched the protein in mung beans (purity 58%), and studied the functional properties of mung bean protein. The results showed that mung bean protein enriched by dry air classification has excellent water retention capacity and good gel strength, and can be used as a green method of protein separation and extraction.

3.2.2 Electrostatic separation method

Electrostatic separation is a method that uses the difference in charge between proteins and other components. After applying an electric charge, the proteins and other components are separated according to the principle that unlike charges repel each other, thereby increasing the protein yield [66].

Since the particle sizes of the protein and small particles are similar during the air classification process, the two cannot be completely separated, and electrostatic separation is considered to be one of the effective ways to further increase the protein yield after air classification. Electrostatic separation has long been used in fields such as plastics and waste recycling, and it also shows good application prospects for raw materials such as rice bran and wheat bran. However, it has been less used to enrich mung bean protein [67]. Xing et al. [59] used air classification combined with electrostatic separation to increase the protein purity of mung beans from 56%–58% to 63.4%–67.6%, while maintaining the original structure and functional properties of the protein. Although the electrostatic separation method causes little damage to the natural structure of the protein, giving the protein better emulsifying and foaming properties, the protein composition is relatively intact, and the ability to retain water or oil is weak, making it less water/oil-retaining [59].

3.3 Combined extraction

Due to the long extraction time, high energy consumption and poor environmental friendliness of traditional wet extraction methods, as well as the varying degrees of damage to the protein structure, dry extraction methods have problems such as low protein yield. Therefore, many researchers consider combining multiple extraction methods to improve protein yield while maintaining the original structure and characteristics of the protein and reducing environmental pollution. Yang et al. [61] used air classification combined with the aqueous method to extract mung bean protein. The protein-rich fraction after air classification was separated with the help of an aqueous phase separation to obtain mung bean protein with a yield and purity of 3.59% and 80.92%, respectively. It was found that the extracted mung bean protein fraction had reduced agglomeration and better solubility, and its viscosity was significantly lower than that of commercially extracted mung bean protein.

4 Research progress in the application of mung bean protein

Different protein extraction methods can significantly affect the structural properties of the protein, thereby affecting its functional properties and the application of mung bean protein in foods.

4.1 Application in plant-based products

Due to the high incidence of lactose intolerance, obesity and other problems, as well as environmental and ethical issues, more and more consumers are inclined to choose a vegetarian diet [68]. Mung bean protein is increasingly being used in the development of plant-based products due to its excellent gelation and emulsification properties. Wang et al. [12] found that adding calcium ions to mung bean protein emulsions and adjusting the pH can help the mung bean protein emulsions form gels, and that the gel hardness is similar to that of diluted egg gels, making it a potential substitute for eggs.

Yang et al. [60] used mung bean protein and pea protein to prepare plant-based yogurt and found that compared with pea protein-based yogurt, mung bean protein-based yogurt has higher chewiness, hardness, and water retention, which may be due to the difference in the sub-protein composition of mung bean protein and pea protein, making mung bean protein have better water/oil holding properties and gel properties. Since the natural structure of mung bean protein is relatively intact under dry fractionation, it has better water/oil retention and gelation properties, and can be used as a protein ingredient for plant-based products such as plant yogurt.

4.2 Application of nutritional supplements

Noodles made from cereals are one of the staples of the Asian diet, but they lack lysine in their nutrient composition. Because legume proteins are rich in lysine, many scholars use them as nutritional supplements in the production of noodles, which significantly improves the texture of the noodles while also enhancing their nutritional value [69,70]. Diao et al. [71] found that after adding 6% mung bean protein to the flour, the wheat protein and mung bean protein formed a dense network structure, giving the noodles the best water absorption and cooking properties. Mung bean protein extracted by alkali-soluble acid precipitation method has a more open structure and improved water retention, and can be considered as a source of raw materials for nutritional protein supplements for noodles.

4.3 Application in meat products

As early as 1996, some scholars added mung bean protein to fish sausages and found that 1% to 2% mung bean protein can significantly reduce the effect of thermal denaturation of fish protein on the sausages and significantly improve the hardness and texture of the fish sausages [72]. Kudre et al. [73] found that with the increase of the concentration of mung bean protein added (0% to 1.5%), the protein hydrolysis in the sardine surimi gel was significantly was inhibited, and a stronger gel network was formed with myofibrils, further improving the gel strength and texture. Because the naturally extracted mung bean protein has a relatively intact structure and superior gel properties, it is the preferred extraction method for use as a meat additive extraction method of choice.

5 Outlook

In recent years, mung bean protein has been favored by scholars because of its low price and availability, balanced amino acid composition, and low allergenicity. The sub-unit composition and structure, and functional characteristics of mung bean protein have gradually been elucidated, and the methods of extracting mung bean protein have gradually become more abundant. However, industrialized methods of extracting mung bean protein that are environmentally friendly, low in energy consumption, have a high protein extraction rate, and are low in denaturation still require further research.

At the same time, research on the impact of different mung bean protein extraction methods on the structure and functional properties of mung bean protein also needs to be explored in greater depth. Although mung bean protein has been used in plant-based products, nutritional supplements and meat product additives, there is still ample room for exploration to analyze and rationally address the advantages and disadvantages of different mung bean protein extraction methods, and to strengthen the applicability of extraction methods to food industry applications.

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