Twin Screw Extrusion of Sorghum and Soya Blends: A Response Surface Analysis

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Blends of sorghum and soybean flours were processed in a co-rotating twin screw extruder to prepare expanded product. Response surface methodology (RSM) was used to study the effect of soya level (SL), feed moisture (FM), barrel temperature (BT) and screw speed (SS) on extruder system parameters and physical properties of the extrudate. Response variables were product temperature (PT), motor torque (MT), specific mechanical energy (SME), expansion ratio (ER), bulk density (BD), hardness (H), crispness (C), water absorption index (WAI), and water solubility index (WSI). Second order polynomial models were developed to determine the responses as a function of process variables. FM, BT, and SS had a significant effect on all the responses except BT on WAI, while SL considerably affected ER, BD, H, C, and WAI.  All the models were found to be statistically significant (R2> 0.85; insignificant lack of fit). Sorghum-soya extruded product was found to be feasible and the optimum values of processing variables were: SL: 14 per cent; FM: 14 per cent wb; BT: 129°C; and SS: 422 rpm.