Novel Vigna subterranea (L.) Verdc Soluble Dietary Fibre - Starch Nanocomposite: Functional and Antioxidant Characteristics

Research background: Bambara groundnut (Vigna subterranea (L.) Verdc.) is a great source of soluble dietary fibre and starch. Bambara groundnut soluble dietary fibre is rich in bioactive compounds, namely, uronic acids (11.8%) and hydrolysable polyphenols (expressed as gallic acid equivalents (GAE) 20 mg/g), with crucial physiological and functional benefits. The industrial use of native starch is limited because of the inherent undesirable attributes that render it unstable. The aim of this study is to characterise the antioxidant, functional and physicochemical properties of Bambara groundnut starch-soluble dietary fibre nanocomposite (hereafter groundnut nanocomposite).

Experimental approach: The pasting properties by rapid visco analysis, chemical composition, hydration properties, oil-binding capacity, emulsifying activity index (EAI), emulsion stability index (ESI) and antioxidant properties of Bambara groundnut starch, soluble fibre and nanocomposite were studied.

Results and conclusions: The Bambara groundnut soluble dietary fibre and nanocomposite did not exhibit typical pasting properties. The nanocomposite had high mass fraction of carbohydrates (78.7%) and proteins (7.0%), low mass fraction of fat (0.8%) and had a considerable mass fraction of ash (4.9%). The Bambara groundnut starch, soluble dietary fibre and nanocomposite showed significant (p<0.001) differences in solubility. Their EAI values were 23.2, 85.7 and 90.6%, respectively, and the ESI values were 23.3, 87.1 and 87.5%, respectively. The three biopolymers differed significantly (p<0.001) in all colour characteristics: lightness (L*), redness/greenness (a*), yellowness/blueness (b*), chroma and hue angles. Their polyphenolic mass fraction, expressed as GAE, was 0.10, 6.6 and 0.46 mg/g, respectively, and their ferric reducing antioxidant power values, expressed as ascorbic acid equivalents (AAE), were 1.2, 4.8 and 1.4 μmol/g, respectively. The phenolic compounds (in mg/g): chlorogenic acid 18, monocrotaline 20, luteolin 7-O-(6''-malonylglucoside) 4 and casuarine 6-α-d-glucoside 27 were present in the dietary fibre but absent from the starch and nanocomposite. The Bambara groundnut nanocomposite possesses desirable physicochemical and antioxidant properties, making it suitable as an ingredient in various food systems.

Novelty and scientific contribution: Nanocomposites have the potential to revolutionise the food industry but their study as food ingredients is very limited. Furthermore, nothing is known about the physicochemical, functional and antioxidant characteristics of Bambara groundnut nanocomposite, thus investigating these properties will address this knowledge gap.