Rheological behaviour of Bambara groundnut starch-soluble dietary fibre nanocomposite for delivering active compounds in food systems

Abstract

This aim of this study was to assess the effect of Bambara groundnut (BGN) (Vigna subterranea (L.) Verdc) starch-soluble dietary fibre nanocomposite (STASOL) on the functional, physicochemical and rheological properties of orange oil beverage emulsions. STASOL was composed of 11.5% Bambara groundnut starch (BGNS) and 88.5% Bambara groundnut soluble dietary fibre (BGN-SDF). STASOL had a mean particle size of 74.01 nm and conductivity of -57.3 mV which qualified it as a nanocomposite and a very stable compound, respectively. STASOL and BGN-SDF were amorphous in nature while BGNS was crystalline, showing strong peaks at 15, 17 and 23° (2θ), thus classifying it as type C starch. STASOL, BGN-SDF and BGNS had functional groups in the regions 3600-2900, 1641.71, 1200-900, 1300-800 cm-1 which were attributed to the vibrations of C-H and OH, C=O and OH, C-C and C-H-O as well as C-O and C-C bonds, respectively. BGNS had smooth, oval structures while BGN-SDF and STASOL exhibited irregular, polygonal morphologies. STASOL was the most thermally stable biopolymer suggesting its suitability for high-temperature food applications. STASOL was high in carbohydrates (78.69%) and proteins (6.96%), low in fat (0.84) and had a considerable amount of ash (4.88%). BGNS, BGN-SDF and STASOL showed significant (p = 0.00) differences in solubility with BGNS being insoluble in water. The emulsion activity index (EAI) and emulsion stability index (ESI) of BGNS, BGN-SDF and STASOL were 23.25 and 23.33%, 85.71 and 87.13%, 90.65 and 87.49%, respectively. The significantly (p = 0.000) higher EAI and ESI of STASOL suggested its suitability as a stabiliser in emulsions. The oil binding capacities of BGNS, BGN-SDF and STASOL differed significantly (p = 0.000) and were 1.13, 3.78 and 1.61 g/g, respectively. STASOL had a substantial amount of antioxidant compounds with 1.45 μmol AAE/g ferric reducing antioxidant power and 0.46 mg GAE/g. Colour characteristics described all studied biopolymers as light (L*), reddish (+a*) and yellowish (+b*). The mean initial backscattering (BSAVO) of STASOL stabilised emulsions was in the range 50.73-70.47% for emulsions composed of 14:30:56 and 20:30:50 (STASOL:oil:water), respectively. The turbiscan stability index (TSI) of the emulsions ranged from 0.0005 to 0.1000 for formulation 11 (20:30:50 STASOL:oil:water) and formulation 5 (8:42:50 STASOL:oil:water), respectively. Low TSI values indicate a low probability of phase separation. The hysteresis loop area (HLA) of emulsions ranged from 2.04 Pas-1 [Formulation 10 (12:34:54 STASOL:oil:water)] to 43.09 Pas-1 [Formulation 2 (20:30:50 STASOL:oil:water)]. The first-order stress decay with a zero equilibrium stress value and Herschel-Bulkley models were the best predictors of time-dependent and time-independent rheological flow behaviour, respectively. The most stable emulsion system was characterised by the highest STASOL (20%), lowest orange oil (30%) and lowest water (50%) concentrations and had the highest BSAVO, lowest TSI and highest HLA. All emulsions were non-Newtonian, time-dependent, thixotropic, shear-thinning and possessed yield stress. Both temperature and time largely affected the extent of destabilisation, with emulsions stored at 5 and 45°C showing the least and most destabilisation over time, respectively. The viscosity of emulsions stored at 5°C started significantly (p = 0.000 decreasing after the 9th day while that of emulsions stored at 20 and 45°C significantly (p = 0.000) decreased after the 3rd day. Emulsions stored at 5 and 45°C for 20 days were the most and least stable, respectively. STASOL stabilised emulsions should be refrigerated to prolong their shelf life.