Physical and Functional Properties of Extruded Sorghum-Cowpea Blends: A Response Surface Analysis

Abstract

Blends of sorghum and cowpea flours (90:10, 80:20 and 70:30 respectively) were extruded at 20%, 22.5% and 25% moisture levels and 120^oC, 140^oC and 160^oC 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) and bulk density (BD). Results obtained showed that the viscosity of extrudates varied from 6.90 to 19.58, WAI, 5.10 to 6.69g water/g sample, WSI, 9.50 to 25%, SEI, 2.16 to 4.97 and BD, 0.207 to 0.297 g/cm^-3_. Feed composition had the most effect on the physical and functional properties of the extrudates followed by extrusion temperature and feed moisture. The coefficients of determination (R²) varied from 0.71 to 0.97 with a non-significant (p<0.05) lack-of-fit for the viscosity, WAI and SEI. Correlation coefficients of the observed and predicted values ranged from 0.60 to 0.95 suggesting the adequacy of the second-order polynomial in predicting these functional and physical properties.

Keywords: Extrudate, Optimization, Viscosity, Water absorption, Expansion.