Effect of bambara groundnut flour on the stability and rheological properties of oil-in-water emulsion

Abstract

The effect of bambara groundnut flour (BGNF) on the stability and rheological properties of sunflower oil-in-water (o/w) emulsions was studied. BGNF dispersions were gelatinized and used to stabilize sunflower o/w emulsions. The sunflower oil-in-water emulsions were prepared by homogenizing gelatinized BGNF and sunflower oil using the Ultra Turrax T-25 homogenizer. D-optimal response surface methodology (RSM) was used to investigate the effect of BGNF and sunflower oil (SFO) on emulsion stability. Emulsion stability was studied using Turbiscan MA 2000 by observing changes in the average ∆-backscattering flux (%) at 20°C. Quantification of droplet size and droplet size distribution of emulsions was done by image analysis. Emulsion stability parameters such as microstructure, droplet size (d3,2 and d4,3), initial backscattering (BSAVO) and equilibrium backscattering (BSeq) values were quantified and modeled using RSM. The time-dependent, steady shear and visco-elastic properties were studied using Rheolab MC 1 and Discovery HR-1 rheometers. The rheological behaviours were modeled using rheological equations and predictive equations were developed using RSM. Both BGNF and SFO concentration affected the emulsion stability and rheological properties of emulsions. Increase in oil phase fraction and BGNF concentration improved emulsion stability. The linear and interaction effects of BGNF and SFO were found significant (p < 0.05) on droplet size and BSAVO. BGNF-stabilized emulsions showed multiple destabilization mechanism with flocculation/coalescence more prevalent. Kinetics of destabilization showed that increase in SFO and BGNF concentration has profound effect on emulsion destabilization. Increased BGNF in the emulsion decreased particle size. A quadratic polynomial relationship was found between the emulsion main components and droplet sizes (d3,2 and d4,3) and BSAVO. However, a linear relationship was established between BGNF and SFO for BSeq. Emulsion formulated with 7% (w/w) BGNF and 40% (w/w) SFO possessed highest stability with equilibrium backscattering and desirability values of -0.084% and 89.3% respectively. Regarding the rheological properties, all emulsions were thixotropic, pseudoplastic and viscoelastic fluids. The timedependent behaviour of the emulsions was well modeled by Weltman, Figoni and Shoemaker and Hahn’s models. Both the shear rate and emulsion main components (BGNF and SFO) greatly influenced the time-dependent model parameters. First order stress decay with a zero equilibrium stress value was found not suitable to predict the time-dependent rheological properties of BGNF stabilized emulsions. The time-independent characteristics of the BGNFstabilized emulsions were well predicted by the Power law, Herschel-Bulkley, Bingham and Casson model (high R2 and low RMSE and SE). All emulsions possessed viscous (G’) and elastic (G”) properties. The G’, G” and recoverable strain (Q (t)%) of the emulsion were found to depend on BGNF and SFO concentrations. The linear and quadratic equations were appropriate to describe the relationship between rheological parameters and emulsion components as the case may be. However the emulsion formulated with 7% (w/w) BGNF and 40% (w/w) SFO which possessed maximum storage stability also recorded the highest thixotropic area, pseudo-plasticity, G’, G” and Q (t)%. The result strongly indicated a close relationship between emulsion stability and rheological properties for BGNF-stabilized emulsions. Varietal differences of BGNF was insignificant (p < 0.05) on the stability parameters such as microstructure, oil-droplet size, BS and BSeq and on rheological properties such apparent viscosities, thixotropic area, power law parameters (K and n), Weltman parameters, G’, G” and Q(t)%. Food additives (NaCl, vinegar and citric acid) significantly affected emulsion stability and rheological properties of BGNF stabilized emulsions. Storage stability results indicated that all emulsions were most stable at low temperature of 5oC and least stable at high temperature of 45oC. Destabilization due to oil droplet aggregation was prevalent in all the emulsions. The rheological behaviour during storage was a peculiarity of each of the studied emulsion systems. However, most of the emulsions showed significant (p < 0.05) initial increase in rheological parameters (such as apparent viscosity and consistency coefficient of power law equation) in the first three days of study. There was however no general descriptive trend as a function of storage days at all investigated temperatures. All experimental results demonstrated that BGNF may find applications in food emulsion deliveries if appropriately engineered. All the developed empirical models may find uses during formulation of BGNF stabilized emulsions of predetermined stability and rheological properties. Additives may as well be used to control the rheology and physical stability of such emulsions and the presence of BGNF may present the structures (in terms of the matrix) necessary for rigidity during long term storage stability, particularly if BGNF is used as a stabilizer.