Nutrient and Antinutrient Composition of Winged Bean (Psophocarpus tetragonolobus (L.) DC.) Seeds and Tubers

Genetic Resources Center, International Institute of Tropical Agriculture (IITA), Oyo Road, Ibadan, Nigeria Food Security and Safety Niche Area, Faculty of Natural and Agricultural Sciences, North-West University, Private Bag X2046, Mmabatho 2735, South Africa Soil Microbiology Laboratory, IITA, Ibadan, Nigeria Food and Nutrition Sciences Laboratory, IITA, Ibadan, Nigeria Biometrics Unit, IITA, Ibadan, Nigeria


Introduction
Legumes are an essential source of oils and proteins [1]. Winged bean, Psophocarpus tetragonolobus (L.) DC., is primarily considered an orphan crop though it is known for its high yield potential and nutritional value when compared to soybean [2]. It is a lesser-known tropical legume grown in Papua New Guinea and Southeast Asia, mainly in Malaysia [1]. Winged bean seeds contain high dietary protein due to their amino-acid content, substantial protein bioavailability, and low levels of antinutritional factors [3]. Winged bean seeds are generating unique research and commercial interest mainly due to their nutritional quality (high proteins and fatty oil content). Plant parts such as flowers, leaves, green pods, and tubers are also suitable for consumption [4].
Knowledge on the nutritional composition of winged bean seeds could help to decipher how the seeds can be used as a substitute for soybean which has similar features. Soybean proteins have been used extensively in food applications [5] and can therefore serve as a reference to evaluate new protein materials [6,7]. In this study, seeds and tubers of selected accessions of winged bean were analyzed to evaluate their nutritional and antinutritional composition and document variability between accessions in response to processing. is study is part of the germplasm prebreeding program at the Genetic Resources Center, International Institute of Tropical Agriculture (IITA), aimed at increasing food security and dietary diversification in tropical agriculture.

Materials and Methods
Standard laboratory methods were used to analyze the seeds and tubers that were processed into flour as described by Alamu et al. [8] for proximate analysis (crude protein, fat, crude fiber, ash, moisture content, and carbohydrate), Adegunwa et al. [9] for tannin determination, and Wheeler and Ferrel [10] for phytic acid analysis. Field-harvested seeds were cleaned and slightly roasted under low heat until they were light brown in color. e roasted grains were coarsemilled and winnowed to remove seed coats. e decorticated grain was milled into fine powder and sieved for processed samples. e unprocessed samples were cleaned and milled until fine flour was obtained. e samples were labelled and stored in airtight containers at 4-6°C for analysis. e harvested tubers were peeled, rinsed with water, and ovendried at 60°C. ey were then milled, labelled, and packaged in airtight containers for analysis. e proximate and antinutrient analyses were conducted at the Food and Nutrition Sciences Laboratory (FNSL), IITA, Ibadan, Nigeria. Statistical Analysis Software (SAS, version 9.4) was used to determine the analysis of variance (ANOVA) of data obtained. Table 1 shows the passport data of the accessions used in this study.

Discussion
In the present study, crude protein content ranged from 40.30% (Tpt17) to 38.88% (Tpt4) for processed seeds and 31.13% in Tpt17 to 28.43% (Tpt125) which are higher than results previously obtained for cowpea (22.5%), pigeon pea (22.4%), and lima beans (23.3%) but similar to the results for soybeans (35%) [11][12][13]. e values were also higher than the 14.70% [14] and 12.86% [15] previously reported for wheat flour. e differences may be linked to the geographical location of the germplasm collected since high nitrogen level in the soil can influence protein level [16]. e protein content of the flours suggests that they may be useful in food formulation systems which can be improved by blending with wheat or cowpea flour and used as composite flours. In the tubers, the result ranged from 19.07% (Tpt42) to 12.26% (Tpt10) which shows that winged bean contains a substantial amount of protein in its tubers. Our results are similar to the findings of Kantha and Erdman [17] who reported the protein content of winged beans to be in the range of 17-19%. We suggest that winged bean seed and tubers are potential sources of protein.
e high protein content positions the crop to play a significant role in improving the nutritional status in tropical agriculture. e values obtained for crude fat were higher (18.91% (Tpt51) to 14.09% (Tpt43)) in processed seeds and 19.01% (Tpt15) to 13.87% (Tpt3-B) in unprocessed seeds. ese figures are higher than that reported by Singh et al. [18], who recorded a crude fat content of 0.47% in the fully mature seeds. ey are however similar to those of previous studies that reported [15][16][17][18][19][20].4% [19] and those of other legumes such as chickpea (5.76-6.87%) as reported by Boye et al. [20]. e result obtained for the tubers (Table 5) ranged from 0.21 (Tpt16) to 4.53 (Tpt33). Due to its high thermal conductivity and oxidative features, winged bean oil is valuable as a frying medium when compared to soybean oil [21]. However, winged bean oil has more saturated fatty acids, thereby making it less preferred. A recent study of the physicochemical properties of winged bean oil found that fatty oil extraction using hexane, which is the most common industrial extraction process, agrees with all edible characteristics and fatty acid compositions [21]. In another study, winged bean oil was superior to soybean oil as a result of its high oxidative strength, solid fat content, and good thermal conductivity, thereby making it suitable for frying food [21].
ese results are similar to those of previous studies that reported between 23 and 40% [4,22] and 28.87 ± 0.45 in a study conducted by Wan Mohtar et al. [3], but low when compared to other studies involving cowpea and wheat flours, where values ranged from 57.35% to 83.60% [23] with wheat flour having the highest carbohydrate content (83.60%). In this study, the carbohydrate content of the flours cannot be compared to that of cowpea (57.17%) and wheat flours (74.22%) reported by Ahmed et al. [24]. Adukpo, Agbemafle [25] recorded a higher range of carbohydrate content (34.97 to 39.86%) for three soybean varieties. High carbohydrate content in legumes suggests that legumes can be used to manage protein-energy malnutrition since they have enough carbohydrate for energy such that the protein can be used for its primary function of body building and repair of worn tissues [26]. Carbohydrates are a good source of energy and are desired in high concentrations in breakfast meals and weaning formulas. e moderate carbohydrate content of winged bean flour can make it a good source of energy in breakfast formulations [26].
ese results are consistent with that of Singh et al. [18] who recorded a crude fiber content of 12.65% in fully mature seeds and 2.76% in tubers. e values obtained for winged bean flour were higher than what Leach et al. [27] reported for brown rice flour (1.23%) and refined wheat flour (0.85%).
e results obtained in this study were higher than the 0.85% reported by Leach et al. [27]. David et al. [23] reported that Asomdwee cowpea flour had the highest crude fiber content (3.21%). Chinma and Gernah [28] reported a crude fiber content of 8.19% for pigeon pea, 9.58% for cowpea, and 4.61% for mungbean flour. ese were all lower and slightly comparable to the crude fiber content obtained for the flours in this study. In human health, crude fiber helps to prevent heart diseases, colon cancer, and diabetes, among others. erefore, it will be useful if winged bean flour is used in food formulations to help relieve constipation. e moisture content of processed and unprocessed seeds differed significantly. Results showed that the moisture content of processed and unprocessed seeds was lower than the 9.20% reported by Olalekan and Bosede [29] for cowpea flours in Nigeria. In the tubers, it ranged between 1.4 (Tpt43) and 7.81% (Tpt3-B). e moisture content was within the acceptable limit of not more than 10% for long-term storage of flour [30]. It is influenced by type, variety, and storage condition of the material stored [31]. e low moisture content of winged bean flour may enhance its storage stability by preventing microbial growth and other biochemical reactions [30]. Sui et al. [32] reported a moisture content of 7.75% for wheat flour, which was within the values obtained in this study. is may explain why winged bean may have a longer shelf life and also confirms its usefulness in bakery products. According to Islam et al. [33] bakery products should have an adequate shelf life without any microbiological deterioration, and therefore the low moisture content of the soft-winged bean flour will in the end extend the shelf life of the final product. e ash content of the flours ranged between 4.98% (Tpt17) and 4.55% (Tpt125) for unprocessed flour and from 4.93% (Tpt126) to 4.45% (Tpt15-4) in the processed flour. In the tubers, it ranged between 1.1% (Tpt43) and 3.31% (Tpt154) ( Table 5). e ash content for winged bean flour in this study was higher than the 2.53% for mung bean flour, 2.53% for chickpea flour, 4.58% for pigeon pea, 4.73% for cowpea, and 3.25% for mucuna bean flour [29]. Ash content is an indication of the mineral content of food; it therefore suggests that winged bean flour could be a more important source of minerals than cowpea, mung bean, pigeon pea, and mucuna flours.
Despite all the positive nutrition benefits offered by winged bean, antinutritive factors (ANFs) also exist such as tannins, lectins, flatulence factors, phytoglutenins, saponins, and cyanogenic glycosides [34]. e use of moist heat or soaking has been shown to safely eliminate these substances without reduction in their nutritional composition. e presence of antinutrients in foods preparations particularly for children could hinder the efficient utilization and digestion of some nutrients and therefore reduce their bioavailability but may have beneficial effects on adults [2]. For example, research effort has yielded important milestone concerning trypsin, which act to hydrolyze proteins as part of the vertebrate digestion, and trypsin inhibitors, proteins that stop the action of trypsin whose action interfere with digestion. It has been suggested that trypsin inhibitors play a significant role in protecting plant tissues against bacterial proteases at the point where the pathogenic bacteria colonizes the host [35]. In addition, studies indicate the involvement of trypsin in defense against insects that suck the phloem sap and against bacteria that invade whenever there is wound [36]. Furthermore, in biomedical research, these modes of action have made trypsin and trypsin inhibitors significant part of molecular cell research, where they are strongly used in cell culture to remove cells from tissue culture plates [37,38].
Tannins have been described to have cross-linked with proteins and caused a reduction in in vitro protein digestion of beans [39][40][41][42].
ey have also been implicated in the inhibition of digestive enzymes, increased excretion of endogenous protein, and effect on digestive tract [39].
Phytate is another important antinutrient factor commonly found in legume seeds. It is an antioxidant that binds to some dietary minerals, interfering with their availability [34]. In this study, the phytate level ranged from 3.78 (Tpt4) to 9.38 (Tpt19) for processed seeds and from 4.09 (Tpt19) to 9.96 (Tpt42) in the unprocessed seeds. Phytate content in winged bean is estimated to be between 6.1 and 7.5 mg of phytate phosphorus per gram of beans, equal to that of soybean. Like many beans, winged bean possesses free phenolics, tannins, phytic acid, flatulence factors, saponins, and hydrogen cyanide. Some of these, especially tannins and phenolic compounds, nonspecifically inhibit enzyme activity and form a complex with food proteins, thus reducing their quality [43]. e tannin level ranges from 1.69 (Tpt30) to 2.57 (Tpt51) for processed seeds and from 1.36 (Tpt30) to 3.43 (Tpt32) in the unprocessed seeds, which is higher than the estimates of another study from 0.03 to 7.5 mg of beans [34]. Notwithstanding, the levels of phytate, etc., are not significant enough to cause adverse effects. Considering that most of these ANFs are destroyed by boiling or autoclaving [34], properly processed winged bean can be safely used as a major plant protein source. Overall, the proximate and antinutritional assessments of winged bean seeds and tubers were similar to those of previous studies of the crop and other similar crops [12,19,21,[44][45][46][47].

Conclusion
is study proved that there are variations in the nutritional and antinutritional values of winged bean. e protein content was very high and compares well with that of other legumes, and it could replace them in meals for protein enrichment.
ese protein levels indicate that winged bean, in particular, could be a replacement in various food formulations where soybean has been used. e crude fiber content of the seeds was higher than that of most other legumes, which indicated that the seeds are positioned as a functional food with health benefits associated with both soluble and insoluble fiber. We also observed that the antinutritional composition was low. e results showed that winged bean flour has the potential to be incorporated into food formulations as a functional ingredient.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request. 6 Journal of Food Quality

Conflicts of Interest
e authors declare no conflicts of interest.