Source of materials

Analytical grade chemicals from Sigma-Aldrich, Merck (Johannesburg, South Africa) were used in this study. Bambara groundnut nanocomposite (we named STASOL) is a starch-soluble dietary fibre nanocomposite made from the chemical complexing of Bambara groundnut soluble dietary fibre and its starch. Starch and soluble dietary fibre were extracted from the black-eye Bambara groundnut variety as described by Maphosa et al. (10) and the nanocomposite used in this study was produced following the method described by the same authors. In a 100-mL Schott bottle, 15 g Bambara groundnut starch, 37.5 mL deionised water, 16.5% H₂O₂ (V=120 mL) and 0.1% ascorbic acid were mixed and then incubated at 90 °C for 45 min in a temperature-controlled water bath (Ecobath, LaboTec, Midrand, South Africa). The mixture was cooled to room temperature before precipitating with 40 mL absolute ethanol dropwise with continuous agitation in a sonicator (model 
W-225R; Heat Systems Ultrasonics, now Qsonica, LLC, Newtown, CT, USA) for 10 min at high speed. The mixture was then centrifuged (Jouan MR1812; Thermo Electron Corporation, Thermo Fisher Scientific, Waltham, MA, USA) at 3783×g for 5 min and the supernatant was discarded. To obtain regenerated Bambara groundnut starch particles and to remove H₂O₂ and ascorbic acid, the residue was rinsed three times with 40 mL absolute ethanol. The Bambara groundnut starch (15 g) and its soluble dietary fibre (1.95 g) were dissolved in 40 and 15 mL φ(ethanol,water)=0.5, respectively. The two solutions were mixed to obtain a molecular dispersion of the starch and fibre and left to react for 24 h on a magnetic stirrer (model MS-H-ProT; DLAB Scientific, Beijing, PR China), in a dark fume hood. The resulting solution was freeze-dried (VirTis Genesis 25EL pilot lyophilizer; SP Industries Inc, Warminster, PA, USA).

Pasting properties of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Pasting properties were analysed using a Rapid Visco Analyser (RVA 4500; Perten Instruments, PerkinElmer, Brisbane, Queensland, Australia) following the method of Gulu (1). A 3-gram sample (14% moisture) in 25 g deionised water was subjected to a heating and cooling cycle while continuously under shear stress. The samples were held at 50 °C for 1 min and then heated from 50 to 95 °C at 6 °C/min. Thereafter, the temperature was decreased to 50 °C at 6 °C/min and held for 5 min. The pasting time, peak viscosity, pasting temperature, breakdown viscosity, cold paste viscosity and setback viscosity were obtained from the RVA curves and viscosities were expressed in Pa·s.

Colour characteristics of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Colour characteristics of all Bambara groundnut samples were determined using a spectrophotometer (model CM-5; Konica Minolta Sensing Inc, Sakai, Japan) set at standard observer 10° and D65 following a method of Murevanhema and Jideani (11). The spectrophotometer was calibrated with a black tile (L*=5.49, a*=7.08, b*=4.66) and a white tile (L*=93.41, a*=1.18, b*=0.75) followed by zero calibration (4). Enough samples to completely cover the bottom of the glass sample holders (d=30 mm) were used. Colour parameters were assessed using L*C*h* and CIE-L*a*b* colour space systems. Colour differences (∆E) among the samples were calculated using the colour difference equation:

where L* is lightness, a* is redness/greenness and b* is yellowness/blueness (12).

Chemical analysis of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Chemical profiling was carried out according to AOAC (13) procedures for total fat (gas chromatography), protein (TruSpec® N; Leco Corp., St. Joseph, MI, USA), moisture (moisture analyser IR-30; Denver Instrument Arvada, CO, USA), ash (muffle furnace), minerals (inductively coupled plasma-optical emission spectrometry) and carbohydrates (calculated by difference). Energy (kJ) was calculated using the corresponding values of fat (37 kJ/g), protein (17 kJ/g) and carbohydrates (17 kJ/g) (13).

Hydration properties of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Water absorption capacity (WAC) and solubility index were determined according to the methods described by Lai et al. (14). To determine WAC, 0.2 g samples were weighed into centrifuge tubes and mixed with 9.8 g deionized water. The suspensions were heated at 70 °C for 10 min in a shaking water bath (Ecobath, LaboTec) and then transferred to a boiling water bath for another 10 min. The suspensions were cooled and then centrifuged at 40 000×g and 5 °C for 30 min (Jouan MR1812; Thermo Electron Corporation, Thermo Fisher Scientific). WAC was determined as the ratio of the mass of swollen samples after centrifugation (g) to their dry mass (g).

To determine the solubility index, the supernatant from WAC analysis was decanted into tared evaporating dishes and dried overnight at 110 °C. Solubility index was then calculated as the ratio of the mass of solids after drying (g) to the initial mass of the dry sample (g).

Oil-binding capacity of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

The method of Maphosa and Jideani (4) was applied for the determination of the oil-binding capacities (OBC) of Bambara groundnut starch, its soluble dietary fibre and nanocomposite. Samples (1 g) were mixed with 5 g canola oil in 50-mL centrifuge tubes. The mixtures were vortexed for 30 s at 5 min intervals for 30 min, then centrifuged (Jouan MR1812; Thermo Electron Corporation, Thermo Fisher Scientific) at 1600×g and 23 °C for 25 min. After centrifugation, the free oil was decanted and weighed. The difference between the original mass of oil (5 g) and the mass of the decanted oil was considered as the retained oil. OBC was calculated as the mass of retained oil divided by the original mass of the sample.

Activity and stability of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite emulsions

Emulsifying activity index (EAI) and emulsion stability index (ESI) were determined following the methods of Chove et al. (15). Samples (1 g) were dissolved in 10 mL deionised water and homogenised (Ultra Turrax T-25 homogeniser; IKA, Staufen, Germany) at 4500×g for 2 min. Orange oil (10 mL) was added, the solution was homogenised at 6000×g for 1 min and then centrifuged again at 1200×g for 5 min. To calculate EAI, the emulsion volume was measured in mL of emulsified layer per entire layer in the centrifuge tube and expressed as a percentage (%). To calculate ESI, the emulsion was heated at 80 °C for 30 min. After cooling to ambient temperature, the emulsion was centrifuged at 1200×g for 5 min and the ESI was calculated as the height of the emulsified layer after heating divided by the height of total content before heating.

Total phenolic content and antioxidant activity of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Total phenolic compounds were determined following the method of Maphosa and Jideani (4). Samples (250 mg) were mixed with 10 mL of distilled water and 1 mL of H₂SO₄ in 14-mL centrifuge tubes and then incubated at 80 °C for 20 h before centrifugation (4000×g, 5 min, 21 °C). The supernatant was diluted tenfold and analysed in a 96-well plate using the Folin-Ciocalteu assay by mixing 25 µL sample with 125 µL of 0.2 M Folin-Ciocalteu reagent and 100 µL of 7.5% Na₂CO₃ solution. The mixtures were left to stand for 2 h in the dark. Absorbance was then measured with a spectrophotometer (Multiskan Spectrum; Thermo Fisher Scientific) at a wavelength of 765 nm using a gallic acid standard calibration curve. The results were expressed as mg of gallic acid equivalents (GAE) per g of dry extract.

Ferric reducing antioxidant power (FRAP) assay was conducted following the method of Maphosa and Jideani (4) with vitamin C as a standard. Samples (250 mg) were diluted tenfold and then mixed with 0.3 mL FRAP reagent (30 mL acetate buffer, 3 mL FeCl₃, 3 mL 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) and 6 mL H₂O). The mixture was poured into a 96-well plate, left to stand for 30 min and then read in a spectrophotometer (Multiskan Spectrum; Thermo Fisher Scientific) at a wavelength of 593 nm. The results were expressed as µmol of ascorbic acid equivalents per g of dry extract.

Determination of glucose mass fraction in Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Samples (0.1 g) and 5 mL of 2 M trifluoroacetic acid were accurately weighed into 10-mL headspace vials and hydrolysed at 110 °C for 12 h. The mixtures were then diluted with 0.2% triethylamine and 0.2% NH₄OH in water and with 0.2% triethylamine, 0.1% 2-propanol and 0.2% NH₄OH in acetonitrile. The diluted samples were transferred into glass vials and analysed by ultra-performance liquid chromatography–evaporative light scattering detection (UPLC-ELSD) with a Waters Acquity BEH Amide 2.1 mm×100 mm, 1.7 µm column (Waters, Milford, MA, USA). An injection volume of 5 µL was used with a column temperature of 50 °C, seal wash of 5 min, run time of 10 min and a pressure gradient of 0-1.03421·10⁸ Pa/min.

Phenolic profiling of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Samples (2 g) and 15 mL of 50% methanol and 1% formic acid were accurately weighed into 50-mL centrifuge tubes with screw-caps, then vortexed for 1 min, followed by extraction in an ultrasonic bath for 1 h. Samples (2 mL) were then centrifuged at 8827×g for 5 min and the supernatants were transferred into 1.5-mL glass vials using liquid chromatography-mass spectrometry (LCMS). A Waters Synapt G2 quadrupole time-of-flight (QTOF) mass spectrometer (MS) connected to a Waters Acquity UPLC was used for high-resolution UPLC-MS analysis. The column eluate first passed through a photodiode array detector prior to flowing through the mass spectrometer, allowing concurrent collection of UV and MS spectra. Electrospray ionisation was used in negative mode with a cone voltage of 15 V, desolvation temperature of 275 °C, desolvation gas at 650 L/h, and the rest of the MS settings optimised for best resolution and sensitivity. Data were acquired by scanning from 150 to 1500 m/z in resolution mode as well as in MSE mode. In MSE mode two channels of MS data were acquired; one at low collision energy (4 V) and the other using a collision energy ramp (40-100 V) to obtain fragmentation data. Leucine enkephalin was used as a reference mass and the instrument was calibrated with sodium formate. Separation was achieved on a Waters HSS T3, 2.1 mm×100 mm, 1.7 μm column and the mobile phase was made up of 0.1% formic acid (solvent A) and acetonitrile containing 0.1% formic acid (solvent B). The used parameters were: injection volume 2 μL, flow rate 0.3 mL/min and column temperature 55 °C. The gradient began at 100% solvent A for 1 min, then went to 28% solvent B over 22 min, then changed to 40% solvent B over 50 s and a wash step of 1.5 min at 100% solvent B, and finally re-equilibrated to initial conditions for 4 min. A range of catechin standards were injected from 0.5 to 100 mg/L and used to establish a calibration curve against which compounds were quantified. Data were processed using MS-DIAL and MS-FINDER (RIKEN Center for Sustainable Resource Science, Metabolome Informatics Research Team, Kanagawa, Japan).

Data analysis

For statistical analysis, IBM Statistical Package for the Social Science (SPSS) v. 25 was used (16). All experiments were carried out in triplicate. Data were expressed as mean±standard deviation. The results were subjected to multivariate analysis of variance (MANOVA) to establish differences between treatments. Duncan’s multiple range test was used to separate mean values where a significant (p≤0.05) difference existed.

Bambara groundnut starch, soluble dietary fibre and its nanocomposite pasting properties

The pasting characteristics of Bambara groundnut starch, Bambara groundnut soluble dietary fibre and its nanocomposite are given in Table 1. Pasting is a term used to describe the changes in the structure of starch molecules after gelatinisation (17), such as further swelling of granules, their disruption and leaching of molecular components from them. Several studies have reported the pasting properties of Bambara groundnut starch (1, 2, 5, 17). As expected, its soluble dietary fibre and nanocomposite did not exhibit typical pasting properties since pasting is a characteristic of starch molecules.

Bambara groundnut starch, soluble dietary fibre and its nanocomposite colour characteristics

The lightness (L*), redness/greenness (a*), yellowness/blueness (b*), chroma and hue angles of Bambara groundnut starch were 87.9, 1.3, 12.4, 12.5 and 83.9°, respectively, of the soluble dietary fibre 76.1, 2.2, 18.9, 19.0 and 83.3°, respectively, and of the nanocomposite 89.2, 0.4, 14.6, 14.6 and 88.42°, respectively (Table 2). The three biopolymers differed significantly (p<0.001) in all colour characteristics. The relatively higher degree of lightness observed of the nanocomposite can be attributed to the bleaching properties of H₂O₂ used in the formulation of the nanocomposite. All the polymers had positive a* and b* values, indicating that they were more associated with redness and yellowness, respectively.

The redness and yellowness of the soluble dietary fibre are related to the phenolics and other antioxidant-possessing chemicals such as anthocyanins, cyanidins as well as a wide range of isoflavones and phenolic acids (11). The lightness of biopolymers destined for food use is of importance in the final products as it determines how much the original colour of the food system will be affected (32, 33). As such, the nanocomposite can be used as an ingredient in food systems without negatively impacting their colour. It was observed that lightness increased with increasing hue and this was in agreement with Maphosa and Jideani (4), who studied the soluble dietary fibre of the Bambara groundnut black-eye variety and reported the colour attributes of L*=73.0, a*=1.7, b*=13.8, chroma=13.9 and hue angle=83.1°. These were comparable with the colour attributes reported in this study.

The colour differences among the biopolymers are presented in Table 2. A colour difference (E) of 1 is known as a just-noticeable difference and is the threshold at which trained observers notice the difference between two colours (34). The difference between two colours can be noticeable but still considered acceptable. A colour difference between 4 and 8 is acceptable. Above 8, the colour difference is deemed unacceptable and likely to be rejected by consumers (35). The colour differences between the Bambara groundnut starch and soluble dietary fibre, the starch and nanocomposite, and the soluble dietary fibre and nanocomposite were 13.5, 6.6 and 14.2, respectively. There was a noticeable difference in colour among the three biopolymers. The difference between the starch and nanocomposite (E=6.6) was considered acceptable as it falls in the 4−8 range (4), but other two colour differences were above 8 and unacceptable.

Chemical composition of Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

The chemical composition of Bambara groundnut starch, its soluble dietary fibre and nanocomposite is given in Table 3. Bambara groundnut starch and its soluble dietary fibre did not differ significantly in their moisture (p=0.620) and fat (p=0.116) mass fractions, while both had a significantly (p<0.001) higher moisture mass fraction and a significantly (p<0.001) lower fat mass fraction than the groundnut nanocomposite. All three biopolymers differed significantly (p<0.001) in energy, protein and carbohydrate composition. Bambara groundnut soluble dietary fibre and nanocomposite were not significantly (p=0.217) different in their ash mass fraction and both had a significantly (p<0.001) higher ash mass fraction than the starch.

Ash is an indication of the mineral content of samples and the high values obtained in Bambara groundnut soluble dietary fibre and nanocomposite suggest that they may contribute micro- and macroelements to food systems (17, 34). The moisture mass fraction of all three biopolymers in this study was similar to that of cassava starch (8.21−12.39%) reported by Agyepong and Barimah (34). Lower ash values (0.38-0.98%) were reported for modified cassava starch (34). These were considerably lower than those of the Bambara groundnut nanocomposite and this could be because the modification of the starch in the formation of the nanocomposite involved the introduction of a new biopolymer (Bambara groundnut soluble dietary fibre), which has a relatively high ash mass fraction (4.90%). Other starches are commonly modified using methods that involve the use of enzymes, heat and chemicals without the introduction of a second nutritional compound, hence the ash mass fraction is solely an indication of the minerals present in the starch. As such the nanocomposite makes a better fortifier of minerals in food products. The mineral composition of Bambara groundnut starch, its soluble dietary fibre and nanocomposite determined using inductively coupled plasma-optical emission spectrometry (ICP-OES) is presented in Fig 1.

Fig. 1

Mineral composition of Bambara groundnut starch, its soluble dietary fibre and nanocomposite

Minerals are of importance in the physiological functioning of the human body. Bambara groundnut starch contained the highest mass concentration of potassium (15.69 mg/L) and zinc (0.72 mg/L) but the lowest mass concentration of sodium (1.42 mg/L) and magnesium (4.39 mg/L) among the samples (Fig. 1). Bambara groundnut soluble dietary fibre had the highest mass concentration of sodium (1.67 mg/L), magnesium (14.66 mg/L) and iron (1.41 mg/L). The nanocomposite had the lowest mass concentration of iron (0.98 mg/L) but a high mass concentration of potassium (11.64 mg/L) and magnesium (10.47 mg/L). The total mineral mass concentration of the biopolymers determined using ICP-OES agreed with the nutritional studies, with the soluble dietary fibre having the highest ash mass fraction and the starch the least (Table 3). Bambara groundnut soluble dietary fibre had the highest mass fraction of ash (4.90 g) and total mineral mass concentration (31.55 mg/L), while the starch had the lowest mass fraction of ash (0.07 g) and total mineral mass concentration (23.25 mg/L). This was because the minerals in nanocomposite were from the starch and soluble dietary fibre used in the production. Generally, the values for most of the minerals in this study were lower than those reported for Bambara groundnut seeds (2, 36). This was expected because only the components of Bambara groundnut were analysed in this study, while whole seeds were reported by other researchers.

Hydration and oil-binding properties of Bambara groundnut starch, its soluble dietary fibre and nanocomposite

The solubility index, water absorption capacity (WAC) and oil-binding capacity (OBC) of Bambara groundnut starch, its soluble dietary fibre and nanocomposite are given in Table 4.

Solubility is defined by the IUPAC (37) as the analytical composition of a saturated solution expressed as a proportion of a designated solute in a designated solvent. A high solubility index positively affects the functional properties both in food systems and in the gastrointestinal canal (37). All biopolymers showed significant (p<0.001) differences in the solubility index with native Bambara groundnut starch being practically insoluble in water (0.9%) (Table 4). Native starch is insoluble in cold water because the amylose and amylopectin chains are arranged in a tight interlocking structure that becomes a hydrophobic crystalline-like starch granule (1). This was in agreement with the results of Agyepong and Barimah (34), who reported very low solubility of native cassava starch in the range of 1.29−7.38%. Furthermore, they reported an increase in the solubility of starch after modification. This was also observed for the nanocomposite in this study. The smaller average particle size (74.01 nm) of the nanocomposite (10) means that it has a large surface area, hence increasing its interaction with water molecules. The high solubility of the nanocomposite makes it a useful ingredient in food preparations such as soups, dairy products, beverages, coffee creamers and gravies (38).

WAC is the ability of a sample to retain water, which plays a significant role in food systems where the sample is used as a bulking agent, stabiliser, thickener and anticaking agent (2). The WAC of Bambara groundnut starch was 1.6 g/g, while that of its soluble dietary fibre and nanocomposite could not be determined as they were significantly soluble in water, as shown in Table 4. A similar WAC of the starch was reported by Sirivongpaisal (17) (1.67 g/g), while Gulu (1) reported a lower value (1.17 g/g). Differences could be attributed to the different Bambara groundnut varieties. Gulu (1) used mixed Bambara groundnut seeds, while only the black-eye variety was used here in the production of the nanocomposite. Adebowale et al. (2) reported a higher WAC (2.0 g/g) for the starch, which could be attributed to the different extraction methods. Adebowale et al. (2) employed a wet milling method, while a dry milling method was used in this study. The relatively lower WAC reported in this study suggested that the starch could find use in food where moderate swelling is required (35).

The OBC of Bambara groundnut starch, its soluble dietary fibre and nanocomposite differed significantly (p<0.001) and was 1.1, 3.8 and 1.5 g/g, respectively (Table 4). This is the ability of lipids to form interactions with non-polar side chains of molecules (35) and is affected by factors such as particle size, surface area, charge and hydrophobicity (2). It works on the principle of physically entrapping oil through capillary attraction (1). It indicates the ability and extent to which biopolymers can stabilise high-fat food products and emulsions as well as act as fat binders and replacers in products such as meat (39). The OBC of native starch in this study (1.1 g/g) was lower than its WAC (1.6 g/g) (Table 4), which indicated a higher level of hydrophilicity than of hydrophobicity. The low OBC of the starch than of the nanocomposite and soluble dietary fibre could be attributed to the low levels of proteins (Table 3). The proteins present in the nanocomposite and soluble dietary fibre may be hydrophobic, hence would display superior lipid-binding characteristics (38). The ability of biopolymers and biopolymer nanocomposites like the one used here to bind oil can be applied in the food industry to reduce fat loss during cooking as well as in foodstuffs such as meat, emulsions and baked goods, thus improving palatability, flavour retention and shelf life (40). Physiologically, the OBC of biopolymers allows them to play a significant role in bile acid absorption and consequently cholesterol reduction (35).

Emulsion activity and stability indices of Bambara groundnut starch, its soluble dietary fibre and nanocomposite

Emulsifying activity index (EAI) is the ability of a compound to participate in the formation and stabilisation of a newly created emulsion (41), while emulsion stability index (ESI) measures the ability of an emulsion to resist changes in its structure and maintain its physicochemical properties over time (35). The significantly (p<0.001) lower EAI and ESI of Bambara groundnut starch can be highly attributed to its insolubility and low WAC (Table 4). Biopolymers find use as stabilisers in food emulsions. They accomplish this function by increasing the viscosity of the system, thereby retarding oil droplet migration (42). The EAI and ESI of the nanocomposite and soluble dietary starch suggest that they would effectively carry out these functions. To successfully increase the viscosity of a system, a polysaccharide needs to be adequately soluble in the aqueous phase. The low hydration properties of the starch, therefore, hindered it from being a suitable emulsion stabiliser. As emulsions have a lipid component, a suitable stabiliser needs a suitable degree of hydrophobicity to adequately perform a stabilising function (43). The soluble dietary fibre and nanocomposite both exhibited significantly (p≤0.05) higher hydration and OBC (Table 4), thus making them good potential emulsion stabilisers.

Mass fraction of glucose in Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Table 5 shows the glucose mass fraction in Bambara groundnut starch, its soluble dietary fibre and nanocomposite. In Table 3, the carbohydrate mass fraction in the starch was 86.8%. This was in fair agreement with the results of this section. Starch is made up of repeating units of glucose (44), hence its hydrolysis yielded 87.9% glucose, similar to its carbohydrate mass fraction.

The nanocomposite had a higher carbohydrate mass fraction (78.7%), as shown in Table 3, than its glucose mass fraction (63.4%) (Table 5). Proportionately, its glucose mass fraction should have been higher, however, some may have been lost during production. The nanocomposite is made of Bambara groundnut starch to its soluble dietary fibre ratio 15:1.95, hence the quantified glucose most probably originates from the hydrolysis of the starch and the difference would have been carbohydrates from the soluble dietary fibre, which is not made up of glucose units. Glucose was not detected in the soluble dietary fibre. This was in agreement with Maphosa and Jideani (4), who reported an insignificant amount of glucose (<1%) in the Bambara groundnut soluble dietary fibre.

Antioxidant properties of Bambara groundnut soluble dietary fibre, its soluble dietary fibre and nanocomposite

The content of total polyphenolic compounds (TPC) and ferric reducing antioxidant power (FRAP) value of biopolymers suggest that they have antioxidant properties. The TPC and FRAP values of the tested biopolymers in Table 5 show that they differed significantly (p<0.001). The soluble dietary fibre is rich in antioxidant compounds, while starch is generally low in active compounds (4). This validated the results obtained in this study, whereby the soluble dietary fibre had the highest amount of polyphenolic and ferric reducing compounds, while the starch had significantly (p<0.001) low antioxidant capability. This proved that antioxidant compounds were successfully delivered from the soluble dietary fibre to the starch. However, since a low percentage (11.5%) of the soluble dietary fibre was incorporated into the nanocomposite, its total antioxidant activity was lower than of the soluble dietary fibre.

Jayawardena et al. (45) reported the total phenolic mass fraction of 22 fruit juices, expressed as GAE, in the range of 0.24−0.39 mg/g. These could be a result of the presence of common antioxidants such as vitamin C, especially in the citrus juices. These results were lower than those reported for the nanocomposite (0.46 mg/g). Fruit juices are a reliable source of antioxidants; therefore the superior antioxidant property of the nanocomposite is a positive characteristic.

The significantly (p<0.001) higher phenolic content of the nanocomposite than of the starch could also be attributed to the chemical modification used in the production of the nanocomposite. To expose the reactive starch functional groups, ascorbic acid was oxidised by hydrogen peroxide, resulting in the formation of hydroxyl and ascorbate radical intermediates (46). These intermediates may have interfered with the hydrogen atom transfer mechanism of oxygen radical absorbance capacity (ORAC) assay, thereby influencing the antioxidant mechanism and leading to the higher antioxidant capacity of the nanocomposite (1).

The FRAP, expressed as AAE, of Bambara groundnut starch, it soluble dietary fibre and nanocomposite was 1.2, 4.8 and 1.4 μmol/g, respectively (Table 5). All three biopolymers differed significantly (p<0.001) in their Fe(III) reducing capabilities. The FRAP assay is based on the reduction of Fe(III) ions to Fe(II) ions by the tested compound and is often used to measure the antioxidant capacity of foods and beverages containing polyphenols (47). The polyphenolic content of the nanocomposite suggests that it can be exploited as a novel antioxidant and would be of importance in protecting against superoxide radicals, free radicals and lipid peroxidation (39). It would therefore find use in fatty food products to improve oxidative stability, thereby extending their shelf life (39). As a natural antioxidant, it would be a suitable alternative to artificial antioxidants, which have been reported to be carcinogenic and teratogenic, hence their use in food products is discouraged (48).

Polyphenolic compounds in Bambara groundnut starch, soluble dietary fibre and starch-soluble dietary fibre nanocomposite

Table 6 shows the phenolic composition of the tested biopolymers. All three biopolymers contained appreciable mass fractions of phenolics. The phenolic compounds chlorogenic acid (18 mg/g), monocrotaline (20 mg/g), luteolin 7-O-(6''-malonylglucoside) (4 mg/g) and casuarine 6-α-d-glucoside (27 mg/g) were present in the soluble dietary fibre but absent from the starch and nanocomposite, while blumealactone C (7 mg/g) was present in the starch but absent from the other two biopolymers. Of the three biopolymers, the soluble dietary fibre had the highest mass fraction of phenolics compared to the starch and nanocomposite, with high mass fractions of (+)-sesamin (12 mg/g), 12,13-TriHOME (8602 mg/g), dronabinol (3389 mg/g), 9(S)-HPODE (50 mg/g), α-dimorphecolic acid (273 mg/g), dimethyltryptamine (7281 mg/g) and [1R-(1α,4aβ,6α,8aα)]-1,2,4a,5,6,8a-hexahydro-6-hydroxy-4,7-dimethyl-a-methylene-1-naphthaleneacetic acid methyl ester (7594 mg/g).

The polyphenolic compounds reported in this section were in agreement with the TPC and FRAP results (Table 5), where the soluble dietary fibre had the highest mass fraction of total polyphenols and the strongest ferric acid-reducing power. Starch generally has low antioxidant activity (4) and this was demonstrated by its relatively lower phenolic mass fraction (Table 6). The presence of phenolic compounds in the nanocomposite demonstrated that active compounds were successfully delivered from the soluble dietary fibre to starch. Therefore, it was concluded that the nanocomposite contained appreciable mass fractions of antioxidant compounds. However, the phenolic compounds furcatin, dimethyltryptamine, isatidine, casuarine and 6-α-d-glucoside [1R-(1α,4aβ,6α,8aα)]-1,2,4a,5,6,8a-hexahydro-6-hydroxy-4,7-dimethyl-a-methylene-1-naphthaleneacetic acid methyl ester, luteolin 7-O-(6''-malonylglucoside) were present in high mass fractions in the starch and soluble dietary fibre but were either not delivered at all to the nanocomposite or were delivered in very low mass fractions. This was attributed to the low amount of soluble dietary fibre used in the formulation of nanocomposite. As such, future studies should look into using a higher amount of soluble dietary fibre to increase the antioxidant capacity of the nanocomposite.

The findings of this study highlight the potential of Bambara groundnut as an economic source of natural antioxidants for human consumption and could therefore open horizons to its industrial use in the development of functional food. The antioxidant activity of its seeds and constituents has been reported by several researchers (5, 11). The antioxidant properties of phenolics arise from the ability of their hydroxyl groups to donate hydrogen as well as react with oxygen and nitrogen species, producing radical species in a termination reaction (49). Phenolic compounds such as those identified in these biopolymers have been reported to possess many desirable characteristics such as anti-inflammatory, antimicrobial, antifungal, antiallergic, antioxidant, antiapoptotic, antitumour and estrogenic activity (50).