This article is part of the Research Topic Food and Nutrition Security: Underutilized Plant and Animal-Based Foods
Source: Frontiers
March 2, 2023
Photo Source: Unsplash,
Bambara Groundnut as a “Complete Food”
There is a growing trend toward increased consumption of plant-based diets, resulting in a need for more plant-based protein foods. Bambara groundnut is the obvious crop to consider. It serves as an important source of essential nutrients in areas where animal protein is scarce (35). The nutritional composition of Bambara groundnut has earned it the reputation of being a complete food, and this will be explored further in subsequent sections.
Balanced Macronutrient Composition
Carbohydrates
Carbohydrates are the most abundant macronutrient in Bambara groundnut, accounting for up to 64.4% of the total dry weight of the seed (9). The majority of the carbohydrate fraction is complex oligosaccharides and polysaccharides, of which starch accounts for up to 49.5% of the total carbohydrates. The reported starch content of Bambara groundnut seeds varies considerably (22 to 49.5% of dry seed weight), depending on genetic and environmental factors, stage of maturation, and method of analysis (12). Amylose represents 19.6–35.1% of the total starch content, while the rest of the constituents consist primarily of amylopectin and a small quantity (1–2%) of protein, lipid, and ash (12). Raw Bambara groundnut has a higher proportion of slowly digestible starch (SDS) and resistant starch (RS) than rapidly digestible starch (RDS) (36), implicating poor digestibility. Nonetheless, cooking can substantially increase the RDS fraction (37), thereby improving digestibility and carbohydrate availability.
Protein
The protein content of Bambara groundnut ranges from 9.6 to 40% (11), with an average value of 23.6% (9). This variation is also attributed to differences in genetic background, growing conditions, and analytical techniques used for estimation (e.g., nitrogen conversion factor) (10, 35). Storage proteins are the predominant protein fractions in Bambara groundnut, of which vicilin (7S) is reported to be the major constituent, followed by legumin (11S) (38).
High protein content is a desirable trait in foods, but the importance of protein quality, which is determined by both amino acid composition and protein digestibility, should not be overlooked. Variability in amino acid profile between cultivars of Bambara groundnut is evident. In general, most studies report glutamic acid to be the most abundant amino acid in Bambara groundnut, suggesting its potential to be isolated for use as a flavoring agent. Out of the essential amino acids, leucine and lysine are present at a higher concentration while methionine is the lowest (39–41).
Phenylalanine, valine, histidine, and isoleucine were also reported to be present in high concentrations, while tryptophan has been found to be the limiting amino acid (37, 39). Its lysine-rich, methionine-poor composition makes Bambara groundnut a good complementary protein source to cereals, which are often deficient in lysine but rich in methionine (35). The in vitro protein digestibility (IVPD) of raw and cooked Bambara groundnut varies between 70 and 81% and 82 and 87.5%, respectively (37, 42). The increase of IVPD after cooking is attributed to the destruction of heat labile antinutritional factors (ANFs) and fragmentation of native proteins into smaller polypeptides, subsequently improving enzyme accessibility and protein bioavailability.
Lipids
There is considerable variation (1.4 and 9.7%) in the reported values of lipid content in Bambara groundnut (38, 39). The majority of fatty acids in Bambara groundnut are unsaturated, predominated by oleic and linoleic acids (omega-6) (39, 43). Palmitic acid is the third most abundant fatty acid, and linolenic acid (omega-3) is present at a low concentration. While having high unsaturated fatty acid content is appealing from a consumer health perspective, it increases the susceptibility of fats to oxidation and rancidity. Therefore, the end uses should be taken into consideration when selecting the desirable trait of lipid composition.
Rich in Essential Micronutrients
Minerals
The most abundant minerals in Bambara groundnut are potassium, magnesium, phosphorus, zinc, and iron (37, 41, 44). Halimi et al. (9) reported that the levels of these minerals were higher than those found in commonly consumed legumes such as chickpea and mung bean, but they vary by cultivar and growing conditions. The presence of ANFs in the seeds can adversely affect the bioavailability of the minerals. Gwala et al. (45) reported that the concentration and bioaccessibility of calcium, magnesium, iron, and zinc in Bambara groundnut seeds were influenced by factors such as storage period, processing method, location of mineral in the seeds (testa or cotyledons), and the degree and strength of mineral chelation. Despite being a relatively good source of these minerals, it is unlikely that the dietary needs of individuals can be met through consumption of Bambara groundnut alone.
Phytochemicals
Bambara groundnut seeds contain phytochemicals such as flavonoids and tannins. These compounds are usually found in the seed coats and are more abundant in seeds with dark or red-colored seed coats. A positive correlation between darkness of seed coat and total phenolic compounds has been established (46). Mubaiwa et al. (47) reported an abundance of the flavonoids epicatechin and catechin in raw and cooked red seed, respectively.
Catechin and epicatechin can polymerize to form proanthocyanidins, also known as condensed tannins, which have been associated with nutraceutical properties, such as antioxidant, cardioprotective, antitumor, and neuroprotective properties (48). Antioxidant properties have been reported in brown and red Bambara groundnut seeds, levels of which were comparable with commonly consumed legumes, but inferior to the powerful antioxidant ascorbic acid (49, 50). Despite the positive health outcomes associated with consumption of phytochemical compounds, their antinutritional implications should not be overlooked.
Other Important Functional Properties
Dietary Fiber
Bambara groundnut contains appreciable levels of dietary fiber in the form of RS and non-starch polysaccharides. The concentration and composition of dietary fiber are influenced by maturity stage and processing methods (39). Total dietary fiber content of Bambara groundnut ranges from 1.4 to 10.3%, of which insoluble fiber represents a higher fraction than soluble fiber (9). The relatively high proportions of SDS, RS, and dietary fiber in Bambara groundnut reduce the rate of digestion and lower the postprandial glycemic response, rendering Bambara groundnut a low glycemic index (GI) food (36).
From one point of view, it is advantageous to encourage the consumption of low GI foods as these confer numerous health benefits, e.g., lowering postprandial blood glucose and insulin levels, regulating appetite, and reducing the risks of obesity and other non-communicable diseases. Conversely, the increased consumption of flatus-causing non-starch polysaccharides has been associated with irritable bowel (51). More importantly, from a nutritional security point of view, non-digestible dietary fibers can bind to minerals and and form a physical barrier to digestive enzymes, thus reducing the bioavailability of essential minerals (52).
Processing of Bambara Groundnut to Increase Nutritive Value and Utilization
Antinutritional Factors
In common with other legumes, several ANFs have been identified in Bambara groundnut. Their presence can negatively affect the digestion and bioavailability of essential nutrients.
The commonly reported ANFs in Bambara groundnut include condensed tannins, phytic acid, and trypsin inhibitor. Condensed tannins are mainly located in the testa and are more abundant in the darker-colored seeds (53). Despite having an antioxidant capacity, these polyphenolic compounds can form indigestible complexes with dietary minerals, starch, and proteins, thereby reducing their bioavailability (51, 54). Binding with proteins can inhibit the activity of digestive enzymes. Tannin compounds can also impart bitterness and astringency to the food (48), thereby affecting palatability. Phytic acid is more abundant in the seed cotyledon, where it serves as a phosphorus reserve for the plant (52). At physiological pH, the highly charged phosphate groups have a high tendency to chelate to mineral cations and form stable, indigestible complexes (55). Phytic acid can also crosslink with dietary proteins, starch, and digestive enzymes, thus impairing the bioavailability of nutrients (51, 52).
However, it is worth noting that phytic acid has been reported to exhibit antioxidant and anticancer properties, suggesting its potential health-promoting properties (51). The major enzyme inhibitor reported for Bambara groundnut is trypsin inhibitor (55). Inhibition of protease can negatively affect protein digestion and subsequently impede its absorption. Furthermore, low trypsin level can result in increased pancreatic secretory activity, thereby causing pancreatic hypertrophy (43). The reported levels of ANFs among different Bambara groundnut cultivars vary widely (condensed tannins, 0.0011–18.61 mg/g; phytic acid, 1.10–15.11 mg/g; trypsin inhibitor, 0.06–73.40 TI mg/g). These differences are attributed to genetic and environmental factors, as well as extraction and analytical methods (54, 55).
Some forms of dietary fibers are also considered to have antinutritional properties. Pectins can bind to metal cations such as calcium, zinc, and iron, which, not only reduces mineral bioavailability, but affects the cookability of the legume (52). Raffinose and stachyose, the flatus-causing alpha-oligosaccharides, are also present in Bambara groundnut (43, 55). Other ANFs such as oxalate, hydrogen cyanide, and saponins have also been detected in Bambara groundnut (43, 46, 56).
Certain food processing methods are effective at lowering the ANFs, and this will be discussed in the following section. It is possible that the inherent levels of ANFs present in raw beans could be reduced by plant breeding (57), which would be advantageous for improved utilization of the legumes and in their contribution to enhanced nutritional security. However, gains made in improving the nutritional value through reduction of the antinutritional compounds, may be lost through increased susceptibility to pests and diseases during production and subsequent storage of the seeds. This is because these components are plant secondary metabolites that provide some resistance to stress, pests, and pathogens, therefore, reducing the levels may result in a compromised defense system (57).
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