What Is the Use of Mung Bean Protein?

Native to India, mung beans contain a very balanced nutritional profile, including protein, dietary fiber, minerals, vitamins and a large number of bioactive compounds. They are easily digestible and are one of the richest and cheapest sources of protein [1]. Mung bean protein is rich in essential amino acids such as leucine, lysine, phenylalanine and tyrosine. In particular, the lysine content (64.5 mg/g) is higher than the requirement of the Food and Agriculture Organization of the United Nations (55 mg/g). It can be consumed together with low-lysine cereals such as millet and wheat to obtain an amino acid balance [2-4]. In addition, mung bean protein and its peptides also have many physiological activities, such as antioxidant, anti-proliferation, inhibition of angiotensin converting enzyme activity, and also improve glycolipid metabolism, prevent the occurrence and development of non-alcoholic fatty liver disease and other effects [5]. Compared with other protein sources such as cattle and poultry, mung bean protein has a lower carbon footprint and is a sustainable protein source [6, 7].

Mung bean protein has ideal functional properties, including foaming, emulsifying, water-holding, oil-holding and gelling abilities. Among these, gelling ability is one of its important functional properties. Compared with soybean protein isolate (SPI), mung bean protein has more hydrophobic and/or non-charged amino acids and fewer disulfide bonds, which gives it unique gelling properties [8]. Based on these advantages, mung bean protein can be used to prepare egg substitutes, traditional foods, and plant-based meats [9-12]. However, the poor gelation of natural mung bean protein limits its application in the food industry, so it can be improved by different modification methods [2, 13].

As an emerging plant protein, research on mung bean protein has gradually increased in recent years, but there is less summary on its gel properties. Based on the research of domestic and foreign scholars, this paper reviews the composition and structure, extraction, functional properties and gelation mechanism of mung bean protein, and focuses on the effects of adding different exogenous substances (such as salt ions, polysaccharides, other different types of proteins, etc.) and different processing methods (such as heat treatment, pH shift treatment, ultrasonic treatment, enzyme treatment, etc.) on the structure of mung bean protein and its gelation properties. Finally, it summarizes the application of mung bean protein gel properties in food applications, and it is hoped that it will provide a reference for the further application of mung bean protein in the food industry.

1 The composition, structure, extraction and functional properties of mung bean protein

1.1  The composition and structure of mung bean protein

The protein content in mung beans is about 14.6% to 32.6%, which is mainly divided into albumin, globulin, alcohol-soluble protein and glutenin, among which the globulin content is the highest (60% to 70%) [14]. Globulins can be divided into 7S (135 kDa), 8S (200 kDa) and 11S (360 kDa) globulins according to their sedimentation coefficients [5, 15]. 8S is the main storage protein in mung beans, accounting for about 89% of globulins; 11S and 7S have lower contents, accounting for about 7.6% and 3.4% of globulins, respectively.

The structures of 8S and 1 1S are shown in Figure 1. 8S globulin is a trimeric protein composed of three subunits: α, α′ and β. It has a high degree of structural similarity and homology with 7S globulin from other legumes, with an amino acid sequence similarity of 68% [16, 17]. 11S globulin is composed of acidic and basic subunits with molecular weights of 40 and 24 kDa, which are connected by disulfide bonds [18]. The content of protein components varies among different varieties of mung beans, and there are also significant differences in the functional characteristics and polypeptide composition of different 8S globulins. The differences in composition and structure among genotypes have a significant effect on the functional characteristics of the 8S globulin components of mung beans, but the specific effects still need to be further explored in the future [19].

Amino acids are the basic structural units of proteins, and their composition ratio affects the functional properties of mung bean proteins. The content of hydrophobic amino acids in mung bean proteins (41.98 g/100 g) is much higher than that of hydrophilic amino acids (7.07 g/100 g), which results in mung bean proteins having high surface activity and affecting their emulsifying properties [20, 21]. In addition, the higher the content of hydrophobic amino acids, the better the heat resistance of mung bean protein, which may be attributed to the fact that hydrophobic amino acids are conducive to the formation of a spherical structure with surface activity, while the high enthalpy required to break the hydrogen bonds and hydrophobic interactions in its structure often comes from the high denaturation temperature [20, 22, 23].

1.2 Extraction of mung bean protein

The traditional extraction methods for mung bean protein are chemical methods, such as alkali dissolution and acid precipitation, salt extraction and hot water extraction. Among them, the most commonly used method is alkali dissolution and acid precipitation, which is simple to operate, requires easily available materials and has a high yield. However, it can destroy the amino acid structure and affect the digestibility of the protein [25]. More gentle methods are salt extraction and hot water extraction, but the yield is low. New physical extraction methods such as ultrasonic extraction, high pressure extraction and enzymatic extraction can improve the efficiency of protein extraction. The principles, advantages and disadvantages, and operating conditions of different extraction methods are shown in Table 1.

1.3 Functional properties of mung bean protein

The functional properties of mung bean protein are conducive to its application in the food industry. The solubility of mung bean protein is minimal near the isoelectric point (pH 4.0–5.0). This may be due to the fact that the net charge of mung bean protein decreases at the isoelectric point, reducing the electrostatic repulsion between protein molecules and increasing surface hydrophobicity, which causes the solubility to decrease sharply, leading to protein aggregation and precipitation [20, 37]. Mung bean protein is a macromolecule composed of hydrophilic and hydrophobic amino acids and can be used as an emulsifier [38, 39]. The foam capacity and stability of mung bean protein under different separation methods range from 27.5% to 62.5% and from 21.55% to 49.93%, respectively. Mung bean protein can form a viscous gel-like binding layer with great flexibility, resulting in excellent foam stability [40]. Mung bean protein is comparable to SPI in terms of water-holding capacity (3.33 g/g and 3.00 g/g) and oil-holding capacity (3.00 g/g and 3.45 g/g), and can partially replace SPI [41]. It also compares favorably with other legumes (e.g., pea protein and broad bean protein, each with 1.2 g and 1.6 g fat absorption capacity per gram isolated protein) [42].

Gel properties also play a very important role in food processing. The least gelation concentration (LGC) is one of the common indicators of gelation ability. The lower the LGC, the stronger the gelation ability of the protein. Mung bean protein (12%) has a lower LGC value than other legume proteins (white kidney bean albumin 18%, pea protein 16% and lentil protein 16%), and has gel properties similar to SPI (9–14%). This means that mung bean protein has the potential to replace SPI as a plant-based protein source for meat analogues [20, 41, 43, 44]. As shown in Figure 2, the gel-forming process of mung bean protein involves molecular unfolding, dissociation-association and aggregation, and the main forces involved are hydrophobic interactions and hydrogen bonds, as well as disulfide bonds that help support the gel structure [41, 45-47]. That is, when heated above the minimum unfolding temperature, the original spatial structure of mung bean protein changes, forming a spherical network structure. Aggregation occurs under certain hydrophobic interactions, and disulfide bonds are used to maintain gel gel strength [48, 49].

2 The effect of exogenous additives on the structure and gel properties of mung bean protein

2.1 The effect of salt ions on the structure and gel properties of mung bean protein

Salt ions can change the aggregation behavior of mung bean protein. Commonly used salt ions include Na+ and Ca2+ [50]. With the addition of an appropriate concentration of Ca2+, the content of α-helix structures first decreases and then increases, while the content of β-folded structures shows the opposite trend. Ca2+ pretreatment denatures the protein, exposing the hydrophobic and sulfhydryl groups inside the protein, increasing the content of disulfide bonds in the protein structure, and promoting protein-protein interactions and cross-linking. Compared with Na+, the addition of Ca2+ will also form relatively large aggregates through electrostatic salt bridges, but if excessive salt ions are added, the protein may undergo “salt precipitation” [51]. The mechanism of interaction between mung bean protein and Ca2+ is shown in Figure 3.

An appropriate concentration of salt ions can have a positive effect on the gelling properties of mung bean protein by changing the interactions between mung bean protein and thereby enhancing the gel network structure and increasing the ion concentration of the protein system. However, too high a concentration of salt ions can destroy the gel network structure [51, 53]. In a neutral (pH 7) composite system (7.2% mung bean protein and 10% rapeseed oil), when the amount of added Ca2+ is 0 to 5 mmol/L, the mung bean protein emulsion gel has a loose structure. When the amount of added Ca2+ is 10 to 20 mmol/L, the mung bean protein emulsion gel has a more uniform and dense network structure.

This may be due to the addition of salt ions at the appropriate concentration cross-linking, encapsulating more milk droplets, and making the gel network more uniform and dense [54]. Different concentrations of Na+ and Ca2+ (44 mmol/L and 50 mmol/L, respectively) also significantly improved the gel properties of the mung bean protein/wheat gluten protein (mung bean protein/WG) composite protein gel. At the same salt concentration, Ca2 has a stronger ability to aggregate than Na+ and has a more significant effect on the composite gel. However, high concentrations of salt ions can destroy the gel network [50]. Pretreatment with an appropriate concentration of Ca2+ (5–20 mmol/L) can also improve the gel properties of TG-induced mung bean protein, but too high a Ca2+ concentration (>20 mmol/L) can disrupt the interaction between mung bean protein and TG, forming larger protein aggregates and reducing gel properties [55].

2.2 Effect of polysaccharides on the structure and gel properties of mung bean protein

Proteins and polysaccharides are two important nutrients in foods. The addition of polysaccharides to mung bean protein can change intermolecular and/or intramolecular interactions and form complexes through covalent and non-covalent interactions [56, 57]. Many studies have focused on the analysis of the effect of the addition of different polysaccharides on the structure of mung bean protein. For example, the addition of a new natural gel modifier, Sanzan gum (SANZAN GUM, SAN), at low concentrations (0.5% to 2.5%) to mung bean protein changes the structure of mung bean protein, converting α-helices to β-folds, unfolding the protein molecules and exposing tryptophan residues, and significantly enhancing hydrophobic interactions, hydrogen bonds and electrostatic interactions are significantly enhanced. Since SAN chains are rich in hydroxyl and carboxyl groups, the addition of SAN enhances hydrogen bonding. However, high concentrations of SAN (3%) shield and hide hydrophobic groups and -SH groups, reducing the interaction forces between protein molecules due to their shielding effect [49].

Factors such as the mixing ratio of the protein to different polysaccharides and the characteristics of the protein and polysaccharides all have a different impact on the gel structure and properties [58]. The addition of SAN to mung bean protein improves the gel properties by enhancing the hydrophobic interactions and hydrogen bonds in the mung bean protein gel [49]. After mung bean protein and natural cornstarch (NCS) interact to form a gel [59], due to the high water retention capacity of mung bean protein and the interaction between the amylose of NCS and the carboxyl groups of mung bean protein during gelatinization, the rearrangement of amylose is delayed, which reduces the hardness and structural cohesion, weakens the gel network, reduces the synergistic effect of the gel, and prolongs the shelf life of the product.

High acyl gellan (HAG) and low acyl gellan (LAG) have different effects on mung bean protein-gellan composite gels [52]. When HAG is added, a soft and elastic gel is formed when the mung bean protein concentration is low. As the concentration of mung bean protein increases, the protein network breaks down and the gel becomes weaker. After LAG was added, the gel formed by mung bean protein was less viscous, and there was a synergistic interaction between LAG and mung bean protein, which became more viscous as the protein content increased. The composite gel prepared from mung bean protein and buckwheat starch is a typical pseudoplastic fluid. As the proportion of mung bean protein added increases, the pseudoplastic properties of the composite gel gradually increase, shear thinning becomes more pronounced, a loose gel network structure is formed, and the viscosity decreases [60].

2.3 The influence of animal and plant proteins on the structure and gel properties of mung bean protein

When the system contains multiple proteins, the proteins may interact with each other to overcome the repulsive forces between aggregates and bind together [61]. During the preparation of composite gels, different types of proteins are mixed together evenly before heating begins. After heating, the interactions between the proteins change, forming different types of gel network structures. It is possible that the formation of composite protein gels is promoted by the co-aggregation between proteins. In addition, the filling effect caused by protein incorporation can improve the properties of composite protein gels through thickening and phase separation [62].

Low concentrations of mung bean protein (< 6 wt%) can be used to prepare composite gels with wheat protein (WP). The mechanism of action is that covalent cross-linking occurs through disulfide bonds to aggregate molecules and form a gel network structure. However, when the concentration of mung bean protein is higher (> 6 wt%), it will reduce the interaction between the composite gel and water, thereby destroying the cross-linked structure, hindering gel formation, and destroying the gel structure, reducing the content of disulfide bonds, and reducing the quality of the gel [63]. In addition, mung bean protein has water-binding capacity and can be used as a water-binding agent and TG enzyme substrate. It affects the gel structure of the composite gel of mung bean protein and myosin fibrin and enhances the gelation ability [64, 65]. The high water-holding capacity of mung bean protein can also increase the breaking force and fracture deformation of the composite gel prepared with sardine surimi, delay the protein hydrolysis of sardine surimi associated with endogenous proteases, and improve gel strength [66].

2.4 Effect of other substances on the structure and gel properties of mung bean protein

The interaction between proteins and bioactive substances (such as polyphenols) can enhance the functional properties of proteins and preserve bioactive compounds, and has become a popular research topic in the food industry in recent years. Different concentrations of Vitexin (VT) were added to mung bean protein, and VT and mung bean protein formed new VT-mung bean protein complexes through non-covalent binding. As the VT concentration increased, the particle size of the complex gradually increased, the tertiary structure unfolded, and the fluorescence intensity decreased. The formation of the VT-mung bean protein complex changed the structure and significantly enhanced its functional properties [67]. However, there is currently no research on the effect of polyphenols on the gel properties of mung bean protein, and further exploration is needed in the future.

3 Effect of different processing methods on the structure and gel properties of mung bean protein

3.1 Effect of heat treatment on the structure and gel properties of mung bean protein

Heat treatment affects the degree of protein denaturation and aggregation, causing the structure of mung bean protein to unfold and exposing the sulfhydryl groups that were originally buried in the center of the protein. These sulfhydryl groups are prone to form disulfide bonds between protein molecules, enhancing the intermolecular force [47]. The gelation process of mung bean protein and the gel properties can be adjusted by controlling the degree of protein denaturation and aggregation after heat treatment [68, 69]. The vicilin component (MV) of mung bean protein was heated at 90 °C for 20 min, then cooled in ice water for 15 min, and the process was repeated to provide a double heating cycle. The first heating should not cause the MV to gel, and only the second heating can form the viscoelastic thermal gel of the MV [70].

After heat treatment, the temperature rises, the water retention and hardness of the gel increase, and its network structure becomes denser. However, excessively high heat treatment temperatures and excessively long heat treatment times can cause the gel to deteriorate, with a decrease in water retention and hardness and a looser network structure. Green mung bean protein-HAG emulsion gels were prepared under different heating temperatures and heating times. The results showed that as the heating temperature and time increased, the gel properties gradually improved, and the best results were obtained at 85 °C and 30 min. However, excessive temperatures and long heat treatment times can lead to gel deterioration [71].

The gels formed at different concentrations of mung bean protein at pH 2 and different heating times (1, 3, 6 and 16 h) showed that with an increase in heating time, the hardness, elasticity and yield stress of the mung bean protein gel increased, and a more uniform and dense network structure was exhibited [72]. Mung bean protein powder was uniformly sprayed with different amounts of water (0, 15%, 20%, 25%, 30%, 35%), and then heated at different temperatures (25, 65, 75, 85, 95 and 105 °C) for different times (0, 15, 30, 45, 60 and 75 min). The treated sample powder was diluted to 20% in deionized water, placed in a water bath, heated at 80 °C for 30 minutes to obtain a gel sample, and the gel properties and major structural changes of mung bean protein were analyzed. The gel properties of mung bean protein with an moisture content of 25% were significantly improved when heated at 85 °C for 60 min. Hydrothermal treatment destroys the intramolecular hydrogen bonds, increases the intermolecular forces and the content of β-folding, and promotes the formation of the gel network structure [47].

3.2 Effect of pH shift on the structure and gel properties of mung bean protein

pH shift is an environmentally friendly and simple method of changing the structure of mung bean protein. The pH response of the protein is adjusted by acid-base treatment for a period of time, and then the pH is returned to neutral. The residual trace salt ions are negligible or removed by dialysis. Under extreme pH conditions, protein interactions (including van der Waals forces and hydrophobic interactions between protein molecules) are disrupted by strong electrostatic repulsion. Subsequently, due to the breaking of chemical bonds, the globular protein of mung bean protein becomes a “melted globule” structure, the protein refolds, and the new structure can be maintained at neutral pH [73].

Under alkaline conditions (pH 10 to 12), the flexible structure of mung bean protein increases the solubility of the protein and exposes the amino acid residues on the protein surface, thereby improving their water retention capacity and gel properties. Under acidic conditions, the solubility of mung bean protein (pH 2–4) is lowest at the isoelectric point, while its water-holding capacity and gel properties decrease, as the acid causes aggregation [73]. Among these, the heat-induced gel at pH 12 has higher elasticity and a denser gel structure (compared to the gel formed by native mung bean protein) [13, 73].

In addition, the gel properties of mung bean protein can also be improved by treating it with pH 12 shift and other means in synergy. TG enzyme cross-linking increased the gel hardness of the natural mung bean protein and pH 12 shifted mung bean protein emulsion by 1.3 times and 1.8 times, respectively. It is possible that compared to natural mung bean protein, the pH 12 treated protein exposes more TG enzyme cross-linking sites, so the gel performance is better [74]. As shown in Figure 4, compared with the native mung bean protein, the pH 12 shifted mung bean protein emulsion gel prepared with appropriate salt ions binding has a homogeneous and dense network structure, and its gel hardness, water retention and water distribution are superior to the natural mung bean protein sample [54]. Overall, mung bean protein gels made under alkaline conditions have better gel properties.

3.3 Effect of ultrasonic treatment on the structure and gel properties of mung bean protein

Ultrasonic treatment is a novel physical method that is efficient, economical and easy to operate [75]. Ultrasonic treatment of mung bean protein can reduce the size of the protein and change the molecular structure, reducing the content of α-helix and β-turns and increasing the content of β-folding. It also increases the content of hydrophobic interactions and disulfide bonds in mung bean protein, improves solubility, but excessive ultrasonic power can reduce the content of free sulfhydryl groups and weaken disulfide bonds [76].

Ultrasonic processing can be used to modify the gel properties of proteins in the food industry. It can transform the structure of mung bean protein, increase intermolecular forces, and form a dense and uniform mung bean protein gel network structure, improving gel properties. Among them, the mung bean protein gel after 300 W ultrasonic processing has a maximum of 61.61% disulfide bonds and has the best gel properties. However, excessive ultrasonic power can weaken the disulfide bonds in mung bean protein gels [76]. High-power ultrasound treatment with a power output of 2.2 kW and a working frequency of 20 kHz was applied to mung bean protein at different concentrations, which reduced the gelation temperature of mung bean protein, improved its texture characteristics such as elasticity, cohesiveness, chewiness and resilience, and formed a transparent gel [77]. When mung bean protein was treated by ultrasound in the energy range of 0–3400 J, the gel hardness increased with the increase of ultrasound treatment energy, and the increase of gel hardness was closely related to the ultrasound treatment energy [78].

3.4 Effect of TG enzyme induction on the structure and gel properties of mung bean protein

TG enzyme is an effective green cross-linking agent. The structural changes of mung bean protein are due to the formation of TG enzyme-induced ε-(γ-glutamyl)lysine covalent cross-links, which result in the Gln-Lys isopeptide bond being about 20 times stronger than non-covalent bonds [79]. When the TG enzyme concentration is too high, it will lead to excessive covalent cross-links, hindering intermolecular aggregation.

TG enzyme is one of the most commonly used enzymes to improve protein gelation [80]. After TG enzyme treatment, the solubility of mung bean protein did not change significantly. Covalent cross-linking and disulfide bonds are the main forces that TG enzyme induces mung bean protein gel. Non-covalent interactions are limited, making the gel network structure of TG-treated mung bean protein more compact, with smaller and more uniform pores, especially at 30 U/g [48]. [48]. Microbial transglutaminase (MTG) can enhance the gel strength and water-binding capacity of mung bean protein by increasing the degree of cross-linking, and there is a concentration dependence of MTG enzyme [81]. Compared with untreated mung bean protein, the hardness, gelation, chewiness and adhesion of the MTM 4 and MTM 8 gels were significantly increased after mung bean protein and MTG enzyme (5 U/g protein substrate) were incubated at 45 °C with continuous stirring for 4 h (MTM 4) or 8 h (MTM 8). The hardness of the MTM 8 gel ( 1907.5 ± 20.2) g was higher than that of the MTM 4 gel (1754.6 ± 71.8) g, indicating that an increase in the treatment time promotes the formation of a denser and more uniform gel network structure with protein cross-linking, thereby resulting in a harder gel [82].

3.5 Effect of other techniques on the structure and gel properties of mung bean protein

Different drying methods have different effects on the gel properties of mung bean protein. After treating mung bean protein by freeze drying (FD), spray drying (SD) and oven drying (OD), FD forms a porous protein, while SD and OD form wrinkled and dense crystals, respectively. FD has higher solubility than SD and OD, which may be due to the presence of water-soluble aggregates. FD and SD form elastic gels, while OD forms aggregated gels. The LGC of FD and SD was 12%, while OD required 18% protein to form a gel. The difference in gel structure formation between the different samples may be due to differences in the secondary structure of the protein. Since OD has a relatively large number of β-turn structures compared to FD and SD due to the transformation of β-folding to β-turns after prolonged heating at 50 °C, this structure plays an important role in the formation of protein aggregates, resulting in a relatively aggregated structure [23].

Treatment of mung bean protein with atmospheric cold plasma at 80 kV for 5 min significantly increased the content of α-helix and β-fold, reduced the content of random coil, decreased solubility, and reduced LGC from 16% to 14%, which improved the hardness of the gel and enhanced the gel properties of mung bean protein [77]. Succinylation, acylation and redox modification of mung bean protein resulted in a decrease in gelation compared to untreated mung bean protein. This is due to the unfolding of the protein chains caused by partial denaturation of the protein, while acylation of mung bean protein increases the negative charge of the protein. Succinylation and redox modification of mung bean protein increase the positive charge of the protein during modification, resulting in electrostatic repulsion between the net charges of the protein and poor gelation [83].

In summary, in order to compensate for some of the limitations of the gel properties of mung bean protein, a number of techniques have been used to modify it, including physical modification methods such as heat treatment, ultrasound treatment, plasma technology, and drying treatment, as well as chemical modification methods such as TG enzyme treatment and pH treatment. Physical modification has the advantages of simple operation, short time consumption, and few toxic and side effects, but the modification effect is not very obvious. Compared to physical modification, chemical modification has obvious effects and fast reaction rates, but in practical applications, attention should be paid to the food safety of chemical reagents. At present, there are still some technical means such as ultra-high pressure treatment and glycosylation that have not been studied for their effects on the gel properties of mung bean protein. The research methods are single, and in the future, physical and chemical modification methods can be used in combination to bring their respective advantages into play, thereby better improving the gel properties of mung bean protein.

4. Gel properties of mung bean protein in food applications

4.1 Gel properties of mung bean protein in egg substitutes

Eggs can cause health problems such as high cholesterol and allergens, so finding egg substitutes has become a research hotspot. When pH is 12, the mung bean protein emulsion gel formed after the addition of Ca2+ has excellent mechanical properties and water retention properties, and obtains a texture similar to that of eggs, so it has the potential to be developed into an egg substitute based on mung bean protein [54]. The sensory indicators of the samples of egg tarts prepared with an emulsion based on mung bean protein after treatment with a pH 12 shift instead of 80 and 100 wt% egg liquid were close to those of the egg-based samples in terms of appearance and texture, with good shape, chewy texture, and no fishy smell. has better water retention capacity and a more tender and juicy texture, which may be related to its uniform gel network. Therefore, a substitute egg based on mung bean protein can completely replace real eggs [74].

4.2 Gel properties of mung bean proteins in traditional foods

The lack of gluten in mung bean proteins limits their use in staple foods such as noodles. When mung bean proteins with 25% moisture content were heated at 85 °C for 60 min, the content of their disulfide bonds was slightly increased, significantly improving the gel properties and water absorption capacity of the mung bean proteins. When the modified mung bean protein was added to small noodles at different replacement levels, intermolecular cross-linking occurred between the mung bean protein and the wheat dough at a replacement level of 9%, strengthening the gluten network structure and benefiting the steaming characteristics of the noodles, thereby improving the quality of the mung bean protein-treated noodles [47].

Partial replacement of milk with mung bean protein also allows the development of hybrid cheeses. When mung bean protein replaces 30% of the milk to make cheese, the protein content and moisture content are higher than those of cow's milk cheese [84]. Mung bean protein can be used to prepare plant-based yogurt with good quality. Mung bean protein yogurt has good hardness, chewiness, water retention capacity and energy storage modulus, which proves that yogurt based on mung bean protein has better gel quality. Hydrophobic interactions and disulfide bonds are the main forces that maintain the induced plant protein-based gels [85].

Adding 1% to 2% mung bean protein to fish sausage can reduce the weight loss of the fish sausage due to protein thermal denaturation, reduce the shrinkage rate of the fish sausage, and improve the hardness. Fish sausages containing mung bean protein have higher sensory evaluation scores, which improves the overall acceptance of the fish sausage [86].

4.3 Gel properties of mung bean protein in meat analogues

Mung bean protein has good gelation potential and has been used in meat analogue processing [87]. When the extrusion parameters are 49.33% moisture content, 80.66 r/min screw speed, and 144.57 ℃ barrel temperature, gelatinized mung bean protein with ideal physical properties can be obtained. It has good physical properties and fibrous structure, and has great potential as a meat alternative. Compared with animal protein, it is a healthier and more environmentally friendly choice [11].

3D printing, as an emerging and promising technology, can provide processed foods with customized flavor, color, texture, mouthfeel, and even nutritional characteristics to meet the nutritional dense food expectations of consumers of different ages and lifestyles, opening up the market for new business models [88, 89]. Beet red and xylose were added to mung bean protein to prepare a 3D printable meat analogue with coloring. Glycosylation of mung bean protein with xylose significantly improved the mechanical properties and microstructure of the meat analogue containing coloring agents, resulting in a change in texture. At the same time, the addition of xylose can improve the stability of the meat analogue containing coloring agents before cooking, possibly because xylose increases the shear modulus and changes the structure by altering interactions [90].

In summary, mung bean protein gels have high application potential in the food industry, as shown in Figure 5, and can be further developed for use in other food applications in the future [90, 91].

5 Conclusion

Mung bean is an emerging legume crop that can provide high-quality plant protein. Mung bean protein extracted by the alkali-solubilization and acid precipitation method has a higher protein content, and its gelation can be improved by adding different exogenous substances or using different processing techniques, making it better suited for use in many fields of gel-type foods. The gelation mechanism of mung bean protein involves molecular unfolding, dissociation-binding and aggregation. Different exogenous substances or processing techniques have different effects on the structure of mung bean protein, thereby affecting its gelation properties. For example, adding appropriate salt ions can enhance protein aggregation, and processing mung bean protein at an alkaline pH can enhance the interaction between proteins and improve its gelation properties.

Excess salt ions or high protein concentrations can lead to excessive protein binding and destroy the gel structure. At present, the research on the gel properties of mung bean protein is carried out using a single technical approach. In the future, different combinations of processing techniques can be explored to improve the gel properties of mung bean protein and develop new plant-based foods. In addition, the application of mung bean protein gels is still in the experimental stage. To become a commercial product, it is necessary to optimize processing conditions, texture, formulation, smell, taste, etc., and comprehensively explore the product's functionality, nutritional value and health benefits. Overall, mung bean protein, as an emerging plant protein, has high research value and requires more research to tap its huge potential in the food industry.

Reference:

[1]TANG D, DONG Y, REN H, et al. A review of phytochemistry, metabolite changes,and medicinal uses of the common food mung bean and its sprouts (Vigna radiata) [J]. Chemistry Central Journal, 2014, 8: 4.

[2]DU M, XIE J, GONG B, et al. Extraction, physicochemical characteristics and functional properties of Mung bean protein [J]. Food Hydrocolloids, 2018, 76: 131-140.

[3]GUNDOGAN R, CAN KARACAA. Physicochemical and functional properties of proteins isolated from local beans of Turkey [J].Lwt, 2020, 130: 109609.

[4] Zeng Zhihong, Wang Qiang, Lin Weijing, et al. Analysis of nutritional and functional properties of mung bean protein [J]. Chinese Journal of Cereals, Oils and Foodstuffs, 2012, 27(06): 51-55.

[5]GUPTA N, SRIVASTAVAN, BHAGYAWANT S S. Vicilin—A major storage protein of mungbean exhibits antioxidative potential,antiproliferative effects and ACE inhibitory activity [J]. Plos One, 2018, 13(2):e0191265.

[6]SCHREINEMACHERS  P,  SEQUEROS T, RANI  S, et al. Counting the beans: quantifying the adoption of improved mungbean varieties in South Asia and Myanmar [J]. Food Security, 2019, 11(3): 623-634.

[7]RANDHIR R, LINY-T, SHETTY K. Stimulation of phenolics, antioxidant and antimicrobial activities in dark germinated mung bean sprouts in response to peptide and phytochemical elicitors  [J]. Process Biochemistry, 2004, 39(5): 637-646.

[8]KUDRE T G, BENJAKUL S, KISHIMURA H. Comparative study on chemical compositions and properties of protein isolates from mung  bean,  black  bean  and  bambara  groundnut   [J].  Journal  of  the  Science  of  Food  and  Agriculture,  2013,  93(10):  2429 -2436.

[9]   HOSSAIN BRISHTI F, CHAY S Y, MUHAMMAD K, et al.  Structural and rheological changes of texturized mung bean protein induced by feed moisture during extrusion [J]. Food Chemistry, 2021, 344:128643.

[10] SEETAPAN N, RAKSA P, LIMPARYOON N, et al. High moisture extrusion of meat analogues using mung bean (Vigna radiata L.)protein and flour blends: investigations on morphology, texture andrheology [J]. International Journal of Food Science & Technology,2023, 58(4): 1922-1930.

[11] BRISHTI F H, CHAY S Y, MUHAMMAD K, et al. Texturized mung bean protein as a sustainable food source: Effects of extrusion on its physical, textural and protein quality [J].Innovative Food Science &Emerging Technologies, 2021,67:102591.DOI:10.1016/j.ifset.2020.102591.

[12] GONG K, ZHANG G, JI H. Electrochemical properties and slow release properties of Mung bean protein gel [J]. Journal of Food Measurement and Characterization, 2023, 17(6): 6091-6098.

[13] NIE Y, LIU Y, JIANG J, et al. Rheological, structural, and water-immobilizing properties of mung bean protein-based fermentation-induced  gels:  Effect  of  pH-shifting  and oil   imbedment  [J].  Food Hydrocolloids, 2022,129:107607.

[14] JAIN V, SHARMA S. Protein quality parameters and storage protein profiling of mungbean interspecific lines (Vigna radiata L.Wilczek) [J]. Genetika, 2021, 53(3): 1341-1356.

[15] TECSON MENDOZA E M,ADACHI M,N. BERNARDO AE, et al. Mungbean [Vigna radiata (L.) Wilczek] Globulins: Purification and Characterization [J]. Journal of agricultural and food chemistry, 2001, 49: 1552-1558.

[16] BERNARDO A E N, GARCIA R N, ADACHI M, et al. 8S Globulin of Mungbean  [Vigna radiata (L.) Wilczek]: Cloning and Characterization of Its cDNA Isoforms, Expression in Escherichia coli, Purification, and Crystallization of the Major Recombinant 8S Isoform [J]. Journal of agricultural and food chemistry, 2004, 52: 2552−2560.

[17] ITOH T, GARCIA R N, ADACHI M, et al. Structure of 8Sα globulin, the major seed storage protein of mung bean  [J]. Acta Crystallographica Section D Biological Crystallography, 2006, 62(7): 824-832.

[18] TANG C-H, SUN X. Physicochemical and Structural Properties of 8S and/or  11S Globulins from Mungbean  [Vigna radiata(L.) Wilczek]  with  Various  Polypeptide  Constituents   [J].   Journal   of  Agricultural   and   Food   Chemistry,   2010,   58(10):   6395- 6402.

[19] LIU H, LIU H, YAN L, et al. Functional properties of 8S globulin fractions from 15 mung bean (Vigna radiata (L.) Wilczek) cultivars [J]. International Journal of Food Science & Technology, 2015, 50(5): 1206-1214.

[20] LIU F-F, LI Y-Q, WANG C-Y, et al. Impact of pH on the physicochemical andrheological properties of mung bean (Vigna radiata L.) protein [J]. Process Biochemistry, 2021, 111: 274-284.

[21] LóPEZ-MONTERRUBIO D I, LOBATO-CALLEROS C, ALVAREZ-RAMIREZ J, et al. Huauzontle (Chenopodium nuttalliae Saff.) protein: Composition, structure,physicochemical and functional     properties      [J].     Food      Hydrocolloids, 2020, 108:106043.

[22] ZHOU L, WU F, ZHANG X, et al. Structural and functional properties of Maillard reaction products of protein isolate (mung bean,Vigna radiate(L.)) with dextran [J]. International Journal of Food Properties, 2017: 1-13.

[23] BRISHTIF H, CHAY SY, MUHAMMAD K, et al. Effects of drying techniques on the physicochemical, functional, thermal, structural and  rheological  properties of mung bean(Vigna  radiata)  protein  isolate  powder [J].  Food  Research  International,  2020, 138.

[24] BADLEY RA, ATKINSON D, HAUSER H, et al. The structure, physical and chemical properties of the soy bean protein glycinin.[J]. Biochimica et biophysica acta Proteins and proteomics, 1975, 412(2): 214–228.

[25] KUMAR M, TOMAR M, POTKULE J, et al. Advances in the plant protein extraction: Mechanism and recommendations [J]. Food Hydrocolloids, 2021, 115:106595.

[26] WANG M, JIANG L, LI Y, et al. Optimization of Extraction Process of Protein Isolate from Mung Bean [J]. Procedia Engineering,2011, 15: 5250-5258.

[27] Liu Fangfang, Li Yingqiu. Optimization of mung bean protein extraction process by response surface methodology [J]. Journal of Light Industry, 2020, 35(02): 7-16.

[28] RAHMA E H, DUDEK S, MOTHES R, et al. Physicochemical characterisation of mung bean (Phaseolus aureus) protein isolates [J].Journal  of  the  Science  of  Food  and  Agriculture,  2000,   80(4):  477-483.

[29] WINTERSOHLEC, KRACKEI, IGNATZY LM, et al. Physicochemical and chemical properties of mung bean protein isolate affected by the isolation procedure [J]. Current Research in Food Science, 2023, 7:100582.

[30] Wang Weijian, Pan Yan. Optimization of hot water extraction process of mung bean protein [J]. Food and Machinery, 2013, 29(05): 154-157.

[31] Zhang Yuxia, Yong Guoxin, Li Yuanzhu, et al. Study on the extraction of mung bean separated protein assisted by ultrasound [J]. Food Research and Development, 2014, 35(20): 13-17.

[32] Li Chaoyang, Diao Jingjing, Li Liangyu. Research on the synergistic extraction technology of mung bean protein by ultrasonic and microwave [J]. Journal of Heilongjiang Bayi University of Agriculture and Forestry, 2020, 32(05): 49-55.

[33] Wang Weijian, Pan Yan. Optimization of process parameters for the combined extraction of mung bean protein by homogenization and ultrasound using stoichiometry [J].  Anhui Agricultural Science, 2013, 41(18): 7950-7953.

[34] Tian HJ, Wang WJ, Zhang YN. Study on the extraction of active protein from mung beans by ultra-high pressure assisted physical method. Food Research and Development, 2014, 35(23): 13-16.

[35] ZHU H-G, WANG Y, CHENG Y-Q, et al. Optimization of the powder state to enhance the enrichment of functional mung bean protein concentrates obtained by dry separation [J]. Powder Technology, 2020, 373: 681-688.

[36] Pan Yan, Lv Chunjian, Xie Chuanlei, et al. Preliminary study on the extraction of mung bean protein by enzymatic method and its effects [J]. Food Industry Science and Technology, 2010, 31(09): 238- 241.

[37] Yang Yong, Bi Shuang, Wang Zhongjiang, et al. Effect of ultrasonic treatment on the structure and functional properties of mung bean protein [J]. Food Industry Science and Technology, 2016, 37(09): 69-73.

[38] GULZAR S, NILSUWANK, RAJUN, et al. Whole Wheat Crackers Fortified with Mixed Shrimp Oil and Tea Seed Oil Microcapsules Prepared from Mung Bean Protein Isolate and Sodium Alginate [J]. Foods, 2022, 12):202.

[39] HE S, ZHAO J, CAO X, et al. Low pH-shifting treatment would improve functional properties of black turtle bean (Phaseolus vulgaris L.) protein isolate with immunoreactivity reduction [J]. Food Chemistry, 2020, 07217.

[40] RATNANINGSIH R, SONGSERMPONG S. Protein isolate precipitation using acid and salt on a by-product of mung bean starch extraction [J]. Agriculture and Natural Resources, 2021, 55:882-892.

[41] BRISHTI F H, ZAREI M, MUHAMMAD S K S, et al. Evaluation of the functional properties of mung bean protein isolate for development of textured vegetable protein [J]. International Food Research Journal, 2017, 24(4): 1595-1605.

[42] FERNáNDEZ‐QUINTELA A, MACARULLA M T, BARRIO A S D, et al. Composition and functional properties of protein isolates obtained from commercial legumes grown in northern Spain.[J].Plant foods for human nutrition, 1997,   51:331- 342.

[43] KYRIAKOPOULOU K, KEPPLER J K, VAN DER GOOT A J. Functionality of Ingredients  and Additives in Plant-Based Meat Analogues [J]. Foods, 2021, 10(3):600.

[44] SHRESTHA S, VAN 'T HAG L, HARITOS V S, et al. Lentil and Mungbean protein isolates: Processing, functional properties, and potential food applications [J]. Food Hydrocolloids, 2023, 143:108904.

[45] MCCANN TH, GUYON L, FISCHER P, et al. Rheological properties and microstructure of soy-whey protein [J]. Food Hydrocolloids, 2018, 82: 434-441.

[46] SHRESTHA S, HAG LV T, HARITOS V, et al. Rheological and textural properties of heat-induced gels from pulse protein isolates:Lentil, mungbean and yellow pea [J]. Food Hydrocolloids, 2023, 143:108904.

[47] DIAO J, TAO Y, CHEN H, et al. Hydrothermal-induced changes in the gel properties of Mung bean proteins and their effect on the cooking quality of developed compound noodles [J]. Frontiers in Nutrition, 2022, 957484.

[48] WANG R-X, LI Y-Q, SUN G-J, et al. Effect of Transglutaminase on Structure and Gelation Properties of Mung Bean Protein Gel [J].Food Biophysics, 2023, 18(3): 421-432.

[49] WANG K, WANG J, CHEN L, et al. Effect of sanxan as novel natural gel modifier on the physicochemical and structural properties of microbial  transglutaminase-induced mung bean  protein isolate gels [J].Food  Chemistry, 024,449:139147.

[50] GUO R, LIU L, HUANG Y, et al. Effect of Na+ and Ca2+ on the texture, structure and microstructure of composite protein gel of mung bean protein and wheat gluten [J]. Food Research International, 2023, 172:113124.

[51] YANG Q, WANG Y-R, LI-SHAY-J, et al. Physicochemical, structural and gelation properties of arachin-basil seed gum composite gels: Effects of salt types and concentrations [J]. Food Hydrocolloids, 2021, 3:106545.

[52] ISRAKARN K, BUATHONGJAN C, GAMONPILAS C, et al. Effects of gellan gum and calcium fortification on the rheological properties of mung bean protein and gellan gum mixtures [J]. Journal of Food Science, 2022, 87(11): 5001-5016.

[53] LU X, LU Z, YIN L, et al. Effect of preheating temperature and calcium ions on the properties of cold-set soybean protein gel [J]. Food Research International, 2010, 43(6): 1673-1683.

[54] WANG Y, ZHAO J, ZHANG S, et al. Structural andrheological properties of mung bean protein emulsion as a liquid egg substitute:The effect of pH shifting and calcium [J]. Food Hydrocolloids, 2022, 26:107485.

[55] Wang R, Li Y. Effect of CaCl_2 pretreatment on the structural characteristics of TG-induced mung bean protein gels. Journal of Light Industry, 2023, 38(04): 46-52+60.

[56] LAKEMOND C M M, JONGH H H J D, PAQUES M, et al. Gelation of soy glycinin; influence of pH and ionic strength on network structure in relation to protein conformation [J]. Food Hydrocolloids, 2003, 17: 365-377.

[57] QIAN Z, DONG S, ZHONG L, et al. Effects of carboxymethyl chitosan on the gelling properties, microstructure, and molecular forces of Pleurotus eryngii protein gels [J]. Food Hydrocolloids, 2023, 145:109158.

[58] YANG X, LIA, LID, et al. Applications of mixed polysaccharide-protein systems in fabricating multi-structures of binary food gels— A review [J]. Trends in Food Science & Technology, 2021, 109: 197-210.

[59] TARAHIM, HEDAYATI S, SHAHIDIF. Effects of Mung Bean (Vigna radiata) Protein Isolate on Rheological, Textural, and Structural Properties of Native Corn Starch [J]. Polymers, 2022, 14(15):3012.

[60] Xiu Lin, Zhang Miao, Xu Xiuyin, et al. Effect of mung bean protein on the pasting and rheological properties of buckwheat starch [J]. Food Science, 2020, 41(16): 57-61.

[61] POLYAKOV V, GRINBERG VY, TOLSTOGUZOV VB. Thermodynamic incompatibility of proteins [J]. Food Hydrocolloids, 1997, 11(2): 171-180.

[62] NICOLAI  T.   Gelation  of  food  protein-protein   mixtures  [J].  Advances  in  Colloid  and  Interface   Science,  2019,  270:  147- 164.

[63] Tao Yang, Chen Hongsheng, Wang Changyuan, et al. Effect of interaction between modified mung bean protein and wheat protein on its gel properties [J]. Chinese Journal of Cereals, Oils and Foodstuffs, 2023, 38(06): 60- 69.

[64] LEE H C, CHIN K B. Evaluation of mungbean protein isolates at various levels as a substrate for microbial transglutaminase and water binding agent in pork myofibrillar protein gels [J]. International Journal of Food Science & Technology, 2013, 48(5): 1086 - 1092.

[65] LEE H C, KANG I, CHIN K B. Effect of mungbean [Vigna radiata (L.) Wilczek] protein isolates on the microbial transglutaminase- mediated porcine myofibrillar protein gels at various salt concentrations [J]. International Journal of Food Science and Technology, 2015, 49: 2023-2029.

[66] KUDRE T, BENJAKUL S, KISHIMURA H. Effects of protein isolates from black bean and mungbean on proteolysis and gel properties of surimi from  sardine   (Sardinella   albella) [J].LWT - Food Science and Technology,2013, 50(2): 511-518.

[67] HU J-R, ZHU Y-S, LIU X, et al. Interactions between different concentrations of vitexin and mung bean protein and their effects on the physicochemical and antioxidant properties of the complexes [J]. Lwt, 2023, 186:115282.

[68] LIANG P, CHEN S, FANG X, et al. Recent advance in modification strategies and applications of soy protein gel properties [J].Comprehensive Reviews in Food Science and Food Safety, 2023, 23:e13276.

[69] BANERJEE S, BHATTACHARYA S. Food Gels: Gelling Process and New Applications [J]. Critical Reviews in Food Science and Nutrition, 2012, 52(4): 334-346.

[70] LEE E-J, HONG G-P. Effect of the double heating cycle on the thermal gelling properties of vicilin fractions from soy, mung bean,red bean and their mixture with soyglycinin [J]. Food Hydrocolloids, 2023, 7:108370.

[71] Naiyantu, Liu Shaowei, Yang Qingxin, et al. Effect of heat treatment of protein on gel properties of mung bean protein-gellan gum emulsion [J]. Food Industry Science and Technology, 2022, 43(06):83-90.

[72] LEE M-Y, JO Y-J. Microstructural andrheological properties of heat-induced gels from mung bean protein aggregates [J]. Journal of Food Measurement and Characterization, 2023, 17(4): 3464-3472.

[73] JEONG M-S, CHO S-J. Effect of pH-shifting on the water holding capacity and gelation properties of mung bean protein isolate [J].Food Research International, 2024, 177:113912.

[74] WANG Y, WANG L, ZHANG S, et al. Influence of pH-shifting and transglutaminase on the freeze-thaw stability and thermal gel properties of mung bean protein-based liquid egg substitute prepared with two different oil phases [J]. Food Hydrocolloids, 2024,146:109182.

[75] CHENG Y, OFORI DONKOR P, YEBOAH G B, et al. Modulating the in vitro digestion of heat-set whey protein emulsion gels via gelling properties modification with sequential ultrasound pretreatment [J]. Lwt, 2021, 149:111856.

[76] WANG R-X, LI Y-Q, SUN G-J, et al. The improvement and mechanism of gelation properties of mung bean protein treated by ultrasound [J]. Lwt, 2023, 182:114811.

[77] RAHMAN M M, LAMSALB P. Effects of atmospheric cold plasma and high-power sonication on rheological and gelling properties of mung bean protein dispersions [J]. Food Research International, 2023, 163:112265.

[78] CHAROENSUK D, WILAILUK C, WANLOP C. Effect of high intensity ultrasound on physicochemical and functional properties of whey protein isolate and mung bean protein isolate; proceedings of the The 26th annual meeting of the thai society for biotechnology and international conferenc, F, 2014 [C].

[79] PEI S, WANG Y, ZHANG Y, et al. Structural and textural properties of walnut protein gels induced by ultrasound and transglutaminase: encapsulation and release of tea polyphenols [J].Journal of Food Science and Technology,2023,60(8):2286- 2295.

[80] KUDRE T G, BENJAKUL S. Combining Effect of Microbial Transglutaminase and Bambara Groundnut Protein Isolate on Gel Properties of Surimi from Sardine (Sardinella albella) [J]. Food Biophysics, 2013, 8(4): 240-249.

[81] VIJAYAN P, SONG Z, TOY J Y H, et al. Effect of transglutaminase on gelation and functional proteins of mung bean protein isolate [J]. Food Chemistry, 2024, 454:139590.

[82] MOON S-H, CHO S-J. Effect of Microbial Transglutaminase Treatment on the Techno-Functional Properties of Mung Bean Protein Isolate [J]. Foods, 2023, 12(10):1998.

[83] WONGPRATHEEP N, PUKRUSPAN T, CHAISERI S, et al. Effect of chemical reagents on functional properties of mungbean protein products [J]. Agriculture and Natural Resources, 2005, 39(1): 109-118.

[84] TOJAN S, KAUR L, SINGH J. Hybrid Paneer: Influence of mung bean protein isolate (Vigna radiata L.) on the texture, microstructure, and in vitro gastro-small intestinal digestion [J]. Food Chemistry, 2024, 434:137434.

[85] YANG M, LIN, TONG L, et al. Comparison of physicochemical properties and volatile flavor compounds of pea protein and mung bean protein-based yogurt [J]. Lwt, 2021, 152:112390.

[86] MOHAMED S, BAKAR J, ABD HAMID N. Differences in Functional Properties of Mungbean Protein Concentrate and the Effect of Incorporation into Fish Sausages [J].1996, 19(1): 69-75.

[87] SHAL, XIONG Y L. Plant protein-based alternatives of reconstructed meat: Science, technology, and challenges [J]. Trends in Food Science & Technology, 2020, 102: 51-61.

[88] FENG C, ZHANG M, BHANDARI B. Materials Properties of Printable Edible Inks and Printing Parameters Optimization during 3DPrinting: a review [J].  Critical Reviews  in Food Science and Nutrition, 2018,  59(19): 3074

[89] NACHALN, MOSES JA, KARTHIK P, et al. Applications of 3D Printing in Food Processing [J]. Food Engineering Reviews, 2019,11(3): 123-141.

[90] WEN Y, KIM H W, PARK H J. Effect of xylose on rheological, printing, color, texture, and microstructure characteristics of 3D-printable colorant-containing meat analogs based on mung bean protein [J]. Food Research International,2022,160:111704.

[91] FENG Q, NIU Z, ZHANG S, et al. Mung bean protein as an emerging source of plant protein: a review on production methods ,functional properties, modifications and its potential applications [J]. Journal of the Science of Food and Agriculture, 2023, 104(5):2561-2573.