Optimization of Physical and Functional Properties of Sorghum-Bambara Groundnut Extrudates

Blends of sorghum and Bambara groundnut flour at 90:10, 80:20 and 70:30 ratios respectively, were extruded at 20%, 22.5% and 25% moisture levels and 120oC, 140oC and 160oC barrel temperatures using a single-screw extruder. Response surface methodology with central composite face-centered (CCF) design was used to model the viscosity, water absorption index (WAI) water solubility index (WSI), sectional expansion index (SEI), bulk density (BD), apparent specific volume (ASV) and mass flow rate of the extrudates. The viscosity, WAI, WSI, SEI, BD, ASV and MFR of extrudates varied from 8.38 to 18.78 Nsm-2, 5.30 to 6.21 g/g water, 10.5 to 21%, 2.55 to 5.1, 0.223 to 0.499 g/cm3, 4.15 to 6.55 cm3, and 1.19 to 2.3 g/s respectively. The R2 values were 0.83, 0.71, 0.55, 0.61, 0.66, 0.77 and 0.61 respectively with a non-significant lack of fit; except for viscosity. Plots of residuals against fitted values showed that residuals were randomly scattered in all cases, thus validating the empirical models for the responses. The optimum amount of Bambara groundnut flour, feed moisture and extrusion temperature were established for viscosity, WAI, SEI, BD, ASV and MFR of the extrudates. Feed moisture had the most effect on extrudates functional and physical properties followed by extrusion temperature.