1. Introduction
Legume is the common name given to the edible seeds of the family Leguminosae. They are the fifth most cultivated vegetable, behind cereals such as rice, wheat or maize and their global production has been estimation in 500 million tons between 2014–2019 [
1]. These crops are largely cultivated for their grains, utilized as valuable ingredients of various products for human consumption and used for animal feed. The significant role of grain legumes species on diet has been studied, especially on nitrogen dynamics in peas and fava beans [
2]. Legumes are highly nutritious crops since they are essential sources of macronutrients, especially protein and polypeptides or amino acids, but also micronutrients, such as vitamins or carotenoids. Furthermore, they have been described to contain several phenolic compounds, suggesting that these crops may also promote health besides their nutritional properties. In fact, legumes have approximately 3 times more protein than cereal grains and are known for being the terrestrial family with the highest protein content.
The health benefits of consuming legumes are fundamentally related to the amount of dietary fiber and polyphenols. The dominant phenolic compounds present in leguminous seeds are flavonoids, phenolic acids and procyanidins, which act as radical scavengers, reducing agents, and chelators of metal ions [
3,
4]. The bioactive compounds, the antioxidant activity and the radical scavenging capacity of various legumes, together with the effect of processing and germination on these processes has been previously reported [
5]. Moreover, several epidemiological studies have correlated the consumption of legumes with high phenolic content to the reduced incidence of diseases such as cancer, ageing, diabetes and cardiovascular disease [
6]. Tannins, phytic acid and saponins, among others have been hypothesized to prevent chronic diseases [
7].
In the present study, the nutritional composition (moisture, ashes, total protein, sugar, and nitrogen content) together with total phenolic content were evaluated in five edible legumes but not suitable for marketing since they do not meet quality standards. The study of these species reveals the convenience of their revaluation in the formulation of nutraceuticals, functional foods, cosmetics or drugs which counteract oxidative stress, also avoiding the generation of large food waste.
2. Materials and Methods
2.1. Sample Preparation
Samples came from different geographical areas, namely: Medicago spp. (France) (1), Phaseolus vulgaris (Spain (3) and Argentina (2)), Cicer arietinum (Spain) (2), Lens culinaris (Spain) (4) and Glycine max (United States (2), France (1) and China (1)). A total of 16 samples of different varieties of these species were evaluated (
Table 1).
2.2. Nutritional Composition
The samples were thoroughly washed with distilled water, air-dried, crushed, and sieved to obtain legumes homogenates, which were stored at −80 °C until use. The nutritional characterization was carried out following previously adapted methodology. The moisture content was determined by drying the homogenate in an oven at 105 °C until a constant weight was obtained. The ash content was measured by incineration in an oven at 550 °C for 24 h. Additional analyses were total nitrogen though the method of Havilah et al. (1977) [
8], proteins were determined according to the method of Lowry et al. (1951) [
9] and total sugars were carried out by phenol–sulphuric reaction [
10], according to the method of Strickland and Parsons (1968) [
11], with glucose as a standard. All determinations were carried out by duplicate, and results were expressed in terms of percentage of composition.
2.3. Total Phenolic Content (TPC)
The TPC was determined using the Folin–Ciocalteu reagent, following an adaptation of the method developed by Singleton and Rossi (1965) [
12]. Deionized water, Folin–Ciocalteu reagent (diluted 1:50) and the sample were prepared in mixture (3:1:1,
v:
v:
v). Then, it was incubated in the dark for 6 min and 100 μL of Na
2CO
3 (75 g/L) were added. After being incubated for 90 min, the absorbance was determined at 765 nm. The results were expressed as mg of gallic acid equivalents (GAE)/g of dw.
3. Results and Discussion
The consumption of legumes has increased in recent years, as they are considered beneficial food ingredients. The protein content in the legumes studied varied between 25.11%—in the case of “Pedrosillano” chickpeas (PCH)—and 50.96%—corresponding to yellow soy (YS). In general terms, the highest protein content was evidenced in legumes belonging to the Glycine max species (soybeans) followed by Lens culinaris (lentils) (
Figure 1). These data agree with Zhao et al. who reported similar amounts of proteins in legumes [
13]. Regarding total sugars, the levels oscillated between 17.46% for white soy (WS) and 57.20% for alfalfa (ALF) whereas for total nitrogen content, values ranged between 2.43% for black beans (BB) and 90.45% in the case of red lentils (RL), showing very variable levels between species. The ash and moisture content were general low. The ash content varied from 4.86% of “Pardina” lentils (PL) to 10.99% of “Granja” beans (GB). For moisture, the values ranged between 2.44% for black soy (BS) and 8.90% for chickpea (CH). Therefore, legumes could be considered as a rich source of proteins and to play a significant role on nitrogen dynamics for their elevated content in total nitrogen.
On the other hand,
Figure 2 shows the total phenolic content (TPC), measured by spectrophotometry of the legumes under study, where the wide variability in the phenolic content between species is evidenced. For example, yellow soy (YS) contained 9.62 mg GAE/g sample, whereas alfalfa (ALF) showed 32.74 mg GAE/g sample. In agreement with Conti et al., phenolic compounds vary considerably according to the species, the geographical area, the climate and the situations of environmental stress, among other aspects, so this could be the main reason for this variability [
3]. However, the high values of some of these samples could be correlated with potential antioxidant activity which will be further studied.
4. Conclusions
There is a growing awareness of how legumes represent an important source of bioactive compounds with important benefits for human health, sharing antioxidant properties, essential to prevent or delay oxidative stress and related diseases. In this study, the rich contribution of phenolic compounds contained in legume by-products that are not suitable for commercialization is demonstrated, as is their nutritional composition. This alternative contributes to waste management and provides useful information on the effective utilization of legumes in food processing for the formulation of functional products.
Author Contributions
Conceptualization, M.C., F.C. and M.A.P.; methodology, M.C., F.C., P.G.-O. and M.B.-M.; validation, M.C., F.C., P.G.-O. and M.B.-M.; formal analysis, M.C., P.G.-P. and L.C.; investigation, M.C., F.C. and M.A.P.; resources, J.S.-G. and M.A.P.; data curation, J.S.-G. and M.A.P.; writing—original draft preparation, M.C. and F.C.; writing—review and editing, M.C., P.G.-P. and L.C.; visualization, M.C., F.C. and M.A.P.; supervision, J.S.-G. and M.A.P.; project administration, J.S.-G. and M.A.P.; funding acquisition, J.S.-G. and M.A.P. All authors have read and agreed to the published version of the manuscript.
Funding
Authors are grateful to Ibero-American Program on Science and Technology (CYTED—AQUA-CIBUS, P317RT0003), to the Bio Based Industries Joint Undertaking (JU) under grant agreement No. 888003 UP4HEALTH Project (H2020-BBI-JTI-2019) that supports the work of M. Barral-Martinez. The JU receives support from the European Union’s Horizon 2020 research and innovation program and the Bio Based Industries Consortium. The project SYSTEMIC Knowledge hub on Nutrition and Food Security, has received funding from national research funding parties in Belgium (FWO), France (INRA), Germany (BLE), Italy (MIPAAF), Latvia (IZM), Norway (RCN), Portugal (FCT), and Spain (AEI) in a joint action of JPI HDHL, JPI-OCEANS and FACCE-JPI launched in 2019 under the ERA-NET ERA-HDHL (No. 696295).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data sharing not applicable.
Acknowledgments
The research leading to these results was supported by MICINN supporting the Ramón y Cajal grant for M.A. Prieto (RYC-2017-22891); by Xunta de Galicia for supporting the program EXCELENCIA-ED431F 2020/12, the post-doctoral grant of L. Cassani (ED481B-2021/152), and the pre-doctoral grants of P. Garcia-Oliveira (ED481A-2019/295), and M. Carpena (ED481A 2021/313). The authors thank the program BENEFICIOS DO CONSUMO DAS ESPECIES TINTORERA-(CO-0019-2021) that supports the work of F. Chamorro. The research leading to these results was supported by the European Union through the “NextGenerationEU” program supporting the “Margarita Salas” grant awarded to P. Garcia-Perez.
Conflicts of Interest
The authors declare no conflict of interest.
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