Velvet bean (Mucuna pruriens): A sustainable protein source for tomorrow

Protein is one of the essential major nutrients required for the cell functioning in human body. Complete proteins, which are usually found in animal‐based proteins, despite of having high biovailability also cause adverse effect on the environment and increased the risk of diet‐related chronic diseases. Recent years have witnessed enhanced awareness about the health benefits of substituting animal‐based proteins with plant‐based proteins, especially in developed countries. Nitrogen‐fixing grain legumes are considered important sources of protein in many developing countries as they are generally cheaper than meat or cereals. Extensive research has been conducted on several underutilized legumes with similar nutritional properties to soybean; one of these legumes is velvet beans (Mucuna pruriens). Though it is rich in protein (27%), complementary amino acid pattern to that of cereal grains exerts health beneficial properties, including antiparkinson, antidiabetic, and anticancer properties, it is limited for human consumption due to the presence of antinutrients. This review provides insight into the potential use of this underutilized legume in resolving the global protein crisis and address food insecurity issues. Additionally, the review focuses on various processing conditions necessary to utilize velvet bean for the development of food products and by‐products like protein isolate, concentrate, flour and their functional properties, and toxicological viewpoint. This current might help in driving future research and applications in velvet bean as a sustainable, plant‐based protein source for human foods along with the critical research areas for their improvement.


| INTRODUCTION
Hunger and disease are two of the most serious threats to humanity's existence (Kavitha & Thangamani, 2014). In 2017, 821 million people were estimated to be undernourished where protein energy malnutrition alone contributes to almost 50% of infant and child mortality worldwide, leaving a quarter of children under the age of five being stunted (Awuchi et al., 2020;FAO, 2018). India has been ranked 94 among 107 countries with 14% of its population falling under the undernourishment category by Global Hunger Index 2020 (Singh et al., 2021).
As per the population forecast, it is expected to exceed 9.3 billion by 2050 (Lee, 2011) along with a predicted doubling in demand for animal protein (Agus & Widi, 2018). Increasing public knowledge of food security, vegan diet concepts, environmental impact, use of antibiotics in animal production, and certain ailments (Aiking & de Boer, 2020;Lynch et al., 2018) makes developing countries seek more esoteric plant species as protein alternatives (Cheng et al., 2019).
Legumes have been the world's second most important source of staple food after cereals as "poor man's meat." To date, several legumes have been studied and proposed as protein alternatives for human consumption and among them; soybean is the most prominent in terms of economics (Cheng et al., 2019). As the interest of people shifted from animal-based protein to vegetarian protein, the production-consumption ratio is not met, creating a disproportionate increase in prices (Bhat & Karim, 2009). Underutilized legumes, predominantly the velvet beans (VB) (Mucuna pruriens var. utilis), can be a possible partial solution (Vadivel & Pugalenthi, 2009).
Despite the fact that the beneficial role of VB is known, existing information and research work on its usage as food or feed and its utilization prospects seem to be insufficient. Looking for traditional uses and exploring the possibilities of contemporary processing technologies to detect and minimize toxic substances is one of the ways to discover the full potential of this underutilized legume as food or feed. In this regard, the main objective of this review article is to provide an overview of velvet beans, as an unconventional legume, as a nutrientrich food and feed source.
The VB's total amino acid (TAA) of 75.8 g/100 g protein suggested that it will considerably contribute to the supply of amino acids in the diet. The presence of seven essential amino acids in the seeds of Georgia VB (Stizolobium deeringianum) was regarded as a favorable sign for using it as a supplemental protein source . Amino acid composition of black and white VB with a comparison with other legumes is shown in Table 2. Except sulfurcontaining amino acids and tyrosine, they include all necessary amino acids.
The Oleic/Linoleic acid (O/L) ratio of VB oil (2.31) is higher than peanut oil (1.48), indicating it could be used as frying oil (Balogun & Olatidoye, 2012). Presence of most of the essential amino acids and fatty acids like linoleic, palmitic, and oleic acids makes them comparable to other oil seeds and nuts. By analyzing the fatty acid composition, it was found that linoleic acid content to be 29.41% in white and 28.31% in black seed (Lim, 2012).
White seeds offer more fiber content, whereas both white and black seeds have a moderate range of carbohydrates. The bean has been proven to be a slow-releasing carbohydrate food material based on predicted glycemic index values (Siddhuraju & Becker, 2005). The black seeds have a high energy content of 1,602.14 kJ 100 g À1 DM, whereas white seeds have 1,567.93 kJ 100 g À1 DM (Kalidass & Mahapatra, 2014

| PHYTOCHEMICAL CONSTITUENTS
Total free phenolics, tannins, L-Dopa, and phytic acid were once thought to be antinutrients and their presence in food/feed stuffs was considered undesirable from nutritional perspective. However, a number of researches on the health benefits of such bioactive compounds show to have a wide range of therapeutic qualities in VB (Vadivel & Biesalski, 2012). VB possesses adequate quantities of phytochemicals, for example, alkaloids, glycosides, anthraquinones, tannins, steroids, phenols, saponins, flavonoids, and antioxidants, where phytochemical tests were done in seed extract.
Experimental investigation on the anticholesterol effect of VB was studied on 30 adult male hypercholesterolemic wistar rats and providing 15 mg/kg bodyweight/day diet of ethyl acetate fraction of VB gave the best result. This may be due to high content of flavonoid and antioxidant activity (Ratnawati & Widowati, 2011). The intraperitoneal injections of velvet beans protein hydrolysates induced hypotensive and antihypertensive effects in normotensive and hypertensive rats. These hydrolysates could be used as functional components with antihypertensive efficacy in people due to their angiotensin converting enzyme inhibitory activity in vitro and in vivo (Segura-Campos et al., 2013).
Although L-Dopa is a pharmacologically active ingredient used to treat Parkinson's disease in humans, it can be antinutritional and toxic if consumed in large causing a toxic confusional state, hallucination, and gastrointestinal disturbances such as nausea, vomiting, and anorexia (Pugalenthi & Vadivel, 2007). The presence of L-Dopa in velvet bean has been identified as a major barrier to using it as food or feed (Pugalenthi & Vadivel, 2007). Indian black seed (6.7-7%) had more L-Dopa than white seed accessions (5.9%). The L-Dopa content of dehulled seeds (5.71%) is lower than that of whole seeds (6.98%), indicating the concentration of L-Dopa is higher in seed coats . The content of L-Dopa in the seeds is affected by their maturation and harvesting period (Siddhuraju & Becker, 2001b). More L-Dopa was found in plants grown near the equator (especially at low latitudes) than in plants grown farther away from the equator . This might be due to variations in light intensity and backscattered UV radiation, which is normally high toward the equator (Vadivel & Biesalski, 2012).
Depending on the application, the US Food and Drug Administration classifies L-Dopa as a drug, a medicinal food, or as a nutritional supplement. L-Dopa is categorized as a generally recognized as safe (GRAS) dietary supplement by the FDA (Hinz et al., 2014). L-DOPA has been known to help in ameliorating different types of free radical-mediated diseases like rheumatoid arthritis, aging, male infertility, diabetes, and various other neurological disorders (Zahra et al., 2022). L-Dopa is water soluble and could be removed or reduced by water soaking with boiling  or dry heat treatment, cooking, and autoclaving (Siddhuraju & Becker, 2001a). Boiling seeds in water or germination for 5 or 7 days followed by boiling lowered L-Dopa levels up to 24.9% and 38.5%, respectively. Soaking grits in a 3:1 water:sodium bicarbonate solution for 24 h reduced content by 54%, whereas soaking whole beans in the same quantity of water only reduced it by 5%. Boiling whole beans and grits without soaking in water only reduced L-Dopa by 48.5% and 57%, respectively . These findings show that soaking grits before and after boiling recovered more L-Dopa than using whole beans or a larger water volume (10 times the weight of the dry sample) (Nyirenda et al., 2003).
The reductions in L-Dopa content were 45-66% and 49-70% in the white and black varieties, respectively, when the raw or presoaked seeds were boiled and pressure cooked in distilled water. In the white species, soaking in alkaline solution followed by autoclaving and in the black variety soaking in citric acid solution followed by autoclaving shows to be more effective in removing L-Dopa, and the remaining contents of L-Dopa are considered safe (1.5 g per person per day) for human (Siddhuraju & Becker, 2001a).
The extraction rate of L-Dopa increases as water temperature rises; safe levels are attained in 13 h at 40 C, 3 h at 66 C, and 40 min in boiling water . After 96.5 h of soaking in water at 22 C, 70% of L-Dopa was retained, while retention declined to 51% at 45 C and 27% at 66 C, indicating that water temperature has a substantial role in the reduction. Soaking and changing water (60 C) resulted in a reduction in L-Dopa up to 22-30% in white and mottled velvet seeds within 48 h (Bressani et al., 2003). When soaking for 9 h in distilled water or NaHCO 3 solution, the L-Dopa concentration dropped to 16% and 18%, respectively (Siddhuraju & Becker, 2001c).
Any increase in the seed:water will boost the effectiveness of L-Dopa extraction. According to the Merck Index, L-Dopa has minimal solubility in water (66 mg in 40 ml at 20 C), but it is readily soluble in dilute solutions of HCl . Diallo and Berhe (2003) revealed through their study that cracking the seed and soaking in running water for 36 h can reduce L-Dopa to 0.08%, while leaving them immersed in flowing water for 3 days can reduce the content to 1.6%.
Dry treatment was shown to be the most efficient in lowering L-Dopa due to racemization during roasting. Grilling can be a superior method than cooking (Bhat & Sridhar, 2009). L-Dopa can be extracted completely from velvet beans in less than 5 min by combining 0.1 g powdered seed with 15 ml distilled water and placing it in an ultrasonication bath for 5 min (Gurumoorthi et al., 2008).

| Saponins
Saponins when consumed cause deleterious effects such as hemolysis and intestinal permeabilization . When compared to soybean, the saponin concentration of VB is found to be low (1.15-1.31%). In dehulled seeds, it appears to be lower (10-20%) than in whole beans (Siddhuraju et al., 2000).

| Phytic acid
Phytic acid in legumes has been shown to reduce nutritional value by decreasing the bioavailability of dietary minerals and essential trace elements (e.g., iron, zinc, and calcium) (Humer & Schedle, 2016). The phytic acid of velvet bean has antioxidant, anticarcinogenic, and hypoglycemic properties that are effective even at low doses. Both white and black accessions have around the same quantity of phytates (0.86-0.9%) . The phytate content of dehulled seeds (1.05-1.22%) appears to be greater than that of whole beans (0.86-1.10%) (Siddhuraju et al., 2000). In general, high phytic acid concentration is connected with cultivars that have a high amount of proteins (Vadivel & Biesalski, 2012).
The effects of gamma irradiation on antinutritional components of VB were studied, and phytic acid of seeds was completely eliminated on exposure to doses of 15 and 30 kGy. The cleavage in the structure of phytic acid causes its degradation by radiation .

| Polyphenols
The major concern with phenolic compounds is that they can reduce digestibility by forming complexes with dietary proteins and lowers the activity of various digestive enzymes (CirkovicVelickovic & Stanic-Vucinic, 2018). The black seeds had higher levels of total phenolics and tannins (6.1% and 0.55%, respectively) than the white seeds (5.5% and 0.37%, respectively) (Siddhuraju et al., 2000). Colored beans have more tannin in the form of condensed tannins or proanthocyanidin than yellow or white beans (Vadivel & Biesalski, 2012).

| Hemagglutinins
Lectins have a unique ability to bind carbohydrate-containing molecules and prevent digestion when consumed (De Mejia et al., 2003).
VB seed has a hemagglutinating activity of 164, 82, and 14 HU/mg protein against human erythrocytes A, B, and O, respectively (Deli et al., 2020). In addition, VB seeds from Brazil had no hemagglutinating action against human, rabbit, or pig erythrocytes. In the five acces-  . The globulin protein fraction of VB has been found to strongly agglutinate all types of human erythrocytes without any specificity, whereas the albumin fraction weakly agglutinates with all human blood groups moderate or even fails to agglutinate (Janardhanan, 2000).

| Flatulence factors
The major compound responsible for flatulence is oligosaccharides as monogastric animals lack α-1, 6 galactosidase enzyme in the small intestine; it cannot be digested or absorbed .

| VELVET BEAN AS FOOD
Velvet Bean was extensively cultivated as a green vegetable in the eastern Himalayan foothills, lower highlands, and in Mauritius. (Lampariello et al., 2012). It has also been reported as minor food crop in Ghana, West Africa, Mozambique, and Nigeria (Majekodunmi et al., 2011). The seeds are roasted and consumed in some regions of Asia (Kala & Mohan, 2012), while Kenyan farmers utilize VB seeds in beverage production . In Nigeria, matured velvet beans are used in a variety of recipes including sauce, roasted snack, tempe, condiment, and gel in Enugu state and soup, stew, porridge, fried cake, moi-moi, and fufu in Kogi state. Due to its high viscosity, the starch present in its flour has been utilized as a food thickener in soups, usually blended with or replacing egusi (Lim, 2012). Velvet bean seeds have been roasted and crushed to form a coffee substitute known as Nescafé in Latin America (Huisden, 2008). Biscuits were made by replacing 15% of the wheat flour with defatted velvet bean flour (VBF), which improved the protein level from 7.36 to 10-11.26 g/100 g DM. The general acceptance of these biscuits was quite appreciable (60-62%) (Ezeagu et al., 2002). As the amount of VBF in the formulation increases, the mean score of the sensory attributes drops. The result concluded that up to 30% VBF using xanthan gum as binding agent substitution with Pro-vitamin A cassava flour was acceptable for making bread. The presence of antinutritional compounds identified did not pose harm to human as they fall within the acceptable limit (Olatunde et al., 2020). VBF was used in recipes like nutriflour, nutricafe and nutrichocolate, and people who consumed these had no side effects (Lim, 2012).
VB seeds were used to create four recipes (coffee, porridge, ragout, and tau) and the consumption of such preparations by around 300 trained female volunteers showed no adverse health effects.
Another usage of velvet bean was as gum to stabilize beef burgers which were shown to be efficient

| PROCESSING EFFECTS ON ANTINUTRIENTS
All ANF detected/quantified in VB are heat-labile, so the temperature, duration of heating, particle size, and moisture content will influence its denaturation (Emenalom et al., 2004). Various unit operations/ processes are used to detoxify VB by diminishing the L-Dopa to a safe level as presented in Figure 1.

| Conventional techniques
Fermentation and germination both decrease L-Dopa levels by 73% and 22%, respectively (Mugendi, Njagi, Kuria, Mwasaru, Mureithi, & Apostolides, 2010). Trypsin inhibitors were reduced by 84.5% and 85.4% after 5 and 7 days of germination, respectively ). Furthermore, the maximal level of total oligosaccharides (77-94%) was reduced by sprouting for 48 and 72 h and in the natural fermentation process 62-68% was decreased (Siddhuraju & Becker, 2001b). Most seeds lost a substantial amount of L-Dopa (9-23%) after sprouting and oil-frying. A small level of increase in total free phenolics was noted; this might be due to the mobilization of stored phenolics by polyphenol oxidase during sprouting and the release of free phenolics by the breakdown of cellular constituents and cell walls during the thermal process. The other possible reason might be due to some polymerization/oxidation of phenolic components (Vadivel & Biesalski, 2012).
Processing like boiling and roasting resulted in 0.60% and 0.99% increase in dry matter content and crude protein content by 3.59% and 6.26%, respectively, but the crude fiber content declined (Adepo et al., 2016). According to another research, boiling had no influence on the seeds' proximate composition except for the protein content, which was observed to be lowered according to Nwaoguikpe et al. (2011). During open-pan roasting (24-44%) and dry-heat treatment (48-60%), reduction in total free phenolics was observed. This could be due to the degradation caused by direct heat exposure causing partial oxidation or racemization (Vadivel & Biesalski, 2012 The hemagglutinin activity of Mucuna cochinchinensis seeds was reduced by 75% after 90 min cooking, but this method reduces protein availability and digestibility (Sogwa & Onweluzo, 2010). L-Dopa levels were reduced by over 90% after the beans were boiled for 45 min, dehulled, soaked for 12 or 24 h with removal and replacement of water after 12 h, and then boiled in fresh water for another 45 min (Egounlety, 2003). Dehulling and boiling, followed by soaking with or without sodium bicarbonate, could be used as a starting point for human consumption processing (Nyirenda et al., 2003).
By soaking seeds the flavonoids, alkaloids, saponins, tannins, hydrogen cyanide, phenols, and phytate reduced their concentrations by 26.19%, 21.28%, 71.43%, 48.07%, and 88.65%, respectively. This also increased crude protein content but reduced the total energy content due to the leaching of nutrients into the discarded water.
Soaking in water or changing the water frequently while soaking would result in a greater reduction in phytochemical concentrations (Nwaoguikpe et al., 2011).
L-Dopa, phytic acid, tannins, and total free phenolics were all significantly reduced after soaking in tamarind solution and boiling; this can cause cell wall tissues to soften followed by release of bounded phenolic compounds and tannins into the soaking media. Tannin loss F I G U R E 1 Processing conditions for mucuna seeds might be also due to chemical transformation or decomposition (Vadivel & Biesalski, 2012). It might also be caused by the formation of a tannin-protein complex under an acidic soaking media, as well as by cooking temperature (Ojo et al., 2018). L-Dopa is chemically transformed into melanin pigment in alkaline conditions turning the solution black. The permeability of the seed coat and the presence of L-Dopa in the cotyledons might account for the moderate and variable decrease while soaking in various solutions (Vadivel & Biesalski, 2012).

| Advanced techniques
Overnight soaked and raw seeds of velvet beans (M. pruriens var. utilis) were exposed for different time intervals in a microwave oven and the reduction in contents of total free phenolics, tannins, and phytic acid is significant (87.5%, 69.9-69.6%, and 78.7-82.1%). Microwave causes acute heat stress which destroys the hydrogen cyanide and total oxalate. The reduction in L-Dopa (4.47-6.48%), TIA (89.5-90.8%), and CIA (94.6-96%) due to microwave cooking is significant . Effects of different processing conditions with reference to antinutrients are summarized in Table 3.
Gamma radiation treatment showed a dose-dependent effect on antinutritional components of velvet beans. While the phenolics showed a significant dose dependent increase except at 2.5 kGy, tannin concentration showed a significant increase above 7.5 kGy.
L-DOPA showed a dose-dependent decline, thus making it necessary to optimize the dosage to retain or eliminate anti nutritional components, which have medicinal properties too . Wachisunthon et al. (2021) showed that the shoots developed from velvet bean seeds germinated after treatment with 200 and 300Gy gamma radiation produced 300% and 220% higher L-DOPA levels than nonirradiated group.

| VELVET BEAN FLOUR, CONCENTRATE, AND ISOLATE
Attention is now focused on the use of protein concentrates and isolates rather than seed flour of legume seeds, since they have superior functional properties, low flavor profiles, and relative freedom from toxic factors and indigestible carbohydrates (Adebowale & Lawal, 2003a). The hydrolysates of velvet beans have potential for incorporation into food systems to improve their technological and biological functionality (Segura-Campos et al., 2014). Comparison between velvet bean and soy bean flour, isolate, and concentrate is shown in Table 4.
The moisture mean value of VBF is low, indicating good keeping properties. The ash content fall within the range of 3.6 ± 0.01%, making it suitable for animal feed, as for this purpose the ash level should be between 1.5-2.5%. Fat content is found to be low when compared to soybean (Balogun & Olatidoye, 2012 and content from 77% to 95%. The protein yield was highest in the case of alkaline extraction using NaOH (Adebowale et al., 2007).
The amino acid composition of the protein isolates Momordica cochinchinensis recorded the highest total essential amino acids (TEAA) (557 mg g À1 protein). In comparison with the results of amino acid content of soybean, all the MPI possess a higher TEAA, and with FAO/WHO/ONU reference, it was found that higher values than the recommended level for pre-school and school children except for sulfur amino acids (methionine and cysteine) and tryptophan (Balogun & Olatidoye, 2012). In MBPI to make up for the limiting amino acids, it can be fortified with cereal-based foods that are particularly low in lysine. L-Dopa concentrations in the isolates ranged between 0.12 and 0.17 g 100 g À1 . The levels of antinutrients were either not detectable or reduced to tolerable limits (Adebowale et al., 2007).
VB processing did not change the protein content, but it did improve the protein's digestibility (Siddhuraju & Becker, 2005). The digestibility of the protein isolates ranged between 90.6% and 94.5%.
The VBF starch was examined for morphological index, particle size, and crystalline characteristics and found morphological index and particle size have a significant correlation with starch digestibility.
More research on the usage of VB starches in common foods including starch noodles and snack foods, as well as their structural alteration through chemical processes is needed for making it more beneficial (Marimuthu et al., 2013).

| FUNCTIONAL PROPERTIES
The high protein content combined with outstanding functional properties makes it a viable protein source for food applications (Udensi, 2006). Proteins and starch are the main contributors for changes in functional properties such as foaming, protein solubility, oil, and water absorption and emulsification (Du et al., 2014). During processing, heat denaturation of protein changes their functional characteristics causing reduction in solubility and emulsion characteristics while increase water holding capacity and oil absorption. Controlled cleavage of disulfide bonds improves protein functionality while increased unfolding enhances accessible hydrophobicity of protein . The ability of protein to form gel provide structure for retaining flavors, sugars, water, and food ingredients, which is beneficial in food applications (Wu et al., 2009
Fermentation significantly decreased the WAC of protein isolate while germination significantly increased it. The greater water absorption capacity suggests that velvet bean protein isolate flours could be advantageous in food systems requiring hydration to improve handling qualities, such as bakery products (Udensi, 2006).

| Oil absorption capacity
Maximum lipophilic tendency of 1.67 ml g À1 was observed in velvet bean flour (Adebowale & Lawal, 2004). Poor oil absorption capacity of protein isolate suggests decreased hydrophobic residues on the protein surface compared to other protein isolates . Fermentation slightly decreased oil absorption capacity of the velvet bean protein isolates while for the germination made it remained fairly constant (Udensi, 2006).

| Bulk density
Bulk density is increased by defatting of flours obtaining a range from 0.42 to 0.61 g/cm 3 in full fat flours and 0.72 to 0.88 g/cm 3 in defatted flours. The higher bulk density of the velvet bean species indicates that they could be used as a food thickener (Adebowale et al., 2005).
There were no significant differences in the bulk density of protein isolates from both germinated and fermented velvet bean (Udensi, 2006).

| Foam capacity/foam stability
A progressive increase in foaming capacity was observed as the concentration of the protein concentrate increased (Adebowale & Lawal, 2003a). Moi-moi and akara balls are two possible velvet beanbased cuisines in which foams are significant. Both fermentation and germination significantly increased the foaming capacity of protein isolate (Udensi, 2006). Full fat flours had a foaming capacity of 9.6% in M. veracruz (white), while defatted flours had a foaming capacity of 84.30% (Adebowale et al., 2005). Foam capacity and stability were both affected by pH. At pH 4, the foaming capacity increased by 35%, while at pH 10, the foaming capacity increased by 134% (Adebowale & Lawal, 2003a).

| Emulsifying activity (EA)
Minimum value of 45.5% was observed for velvet bean flour in 1.0 M NaCl solutions. Both EA and stability are pH dependent as maximum EA was observed at pH 10, while the minimum value of 21.9% was recorded for MBF at pH 4 (Adebowale & Lawal, 2004). Fermentation and germination for 48 and 72 h increased EA of the isolate. The high EA of velvet bean protein isolate makes them a potentially useful ingredient in preparing comminuted meat products and analogs (Udensi, 2006).

| Gelation
The values (16-20%) obtained for velvet bean protein isolates are similar to those reported for winged bean isolate and soy isolate (14-20%) (Udensi, 2006). The lower the least gelation concentration endpoint (LGE), the better the gelating ability of the composition. Velvet bean protein concentrate has been shown to have pH-dependent gelation capacity as minimum LGE 8% (w/v) was recorded at pH 4, while the maximum of 16% (w/v), which indicates least tendency to form gel was recorded at pH 2. At pH 10, the LGE increased further to 14% (w/v) suggesting a lower tendency toward gel formation (Adebowale & Lawal, 2003a).

| CONCLUSION
The wild legume, velvet bean is a promising nutraceutically valued natural plant produce that needs further exploration for its improvement and utility in rural development. Cultivar differences are known to occur in the quantity of L-Dopa of velvet bean seeds, and these dif-

CONFLICT OF INTEREST
The authors declare no conflict of interest in this work.