The feed, comprising of acha flour and raw full fat soybean flour were mixed in ratios of 0:100, 12.5:87.5, 25:75, 37.5:62.5 and 50:50 of soybean flour to acha flour. The feed moisture content was adjusted to 15, 20, 25, 30 and 35% levels. Extrusion was carried out in a laboratory single screw extruder with variable screws and heaters following a 4K central composite (CCD) rotatable and nearly orthogonal design that required 36 extrusion runs. The extruder screw speed was varied at 90, 120, 150, 180 and 210 rpm while the extruder barrel temperature was adjusted to 100, 125, 150, 175 and 200°C. Effects of feed composition (FC), feed moisture content (FMC), extruder screw speed (SS) and extruder barrel temperature (TP) on extrudate bulk density (BD), browning index (BI) and expansion ratio (ER), were evaluated. Results showed that the BD ranged from 1.39 to 9.48 g/m³ while the BI ranged from 0.60 to 5.72. Extrudate expansion ratio ranged from 2.75 to 6.80. The 2nd order polynomial was adequate in modeling the dependence of BD, ER and BI on the extrusion variables. The cross product effects of the process variables had significant (P ≤ 0.05) effects on BD and BI, while the quadratic effect of feed composition had the greatest (P ≤ 0.00) effect on the expansion ratio. Predicted values were very close to the experimental values, making the model ideal for prediction of the dependence of the extrudate functional properties on the extrusion variables. 

 

Key words: Blends, acha, soybeans, response surface, extrusion.