Predictive equations and response surface analysis for sorghum grain extrudate

Abstract

The extrusion and extrudate properties measured include expansion indices (or puff indices), bulk density, residence time, solid density, moisture content, maximum stress, water solubility index (WSI), water absorption index (WAI), extruder output, crispness and color. The ranges of the three variables considered were 100 - 160°C for barrel temperature, 100 - 200rpm for screw speed, and 15 - 25% for feed moisture content (w.b.). The density of extrudate measured varied between 176kg/m3 and 1100kg/m3 for the extrusion variables considered. The moisture content of the dry extrudate is between 6% and 7.5% with the mean value of 6.75%. Using the experimental results, predictive equations were developed to relate the influence of processing parameters on the sorghum grain extrudate quality. From the response surface diagram (RSD), it was discovered that the effect of screw speed on density is negligible. Temperature has a significant impact on the density of the extrudate and it has a curvilinear effect on extrudate density and at high temperature, its quadratic effect dominates. The solubility tends to increase linearly with increase in temperature and decreases linearly with increase in moisture content. At temperature below 150°C, the WAI recorded is between 5.1 and 5.3 and it was largely independent of the extrusion moisture content. However, at the temperature higher than 150°C, there is a sharp increase in WAI to a value of six and above. The total specific mechanical energy (SME) varied between 470kJ/kg and 600kJ/kg. SME is directly proportional to screw speed. Comparatively, the screw speed had dominating effects on SME while temperature had a small effect. The sensory evaluation on crispness and color show that the product extruded at the conditioned feed moisture content of 20% and barrel temperature of 150°C gave the best crispness and color quality.