Process Optimization by Response Surface Methodology and Characteristics Investigation of Corn Extrudate Fortified with Yam (Dioscorea alata L.)

Response surface methodology was used to investigate the optimum operation conditions of a single screw extruder and to analyze the effects of extrusion processing variables, including yam flour contents (10-30 %), moisture content (10-18 %), and screw speed (250-350 rpm) on characteristics of the corn-yam extrudates. Quadratic polynomial equations were also obtained by multiple regression analysis. The predicted models were adequate based on the lack-of-fit test and coefficient of determination obtained. By superimposing individual contour plots of the different responses, regions meeting the optimum conditions were also derived. The moisture content had critical effect on all responses variables. All the characteristic of the corn-yam extrudates evaluated were significantly affected by three process variables. Feed moisture content was the most significant variable with on most of characteristics. Increasing moisture content caused increase in bulk density, water absorption index and hardness, but decrease in radial expansion ratio. However, graphical optimization studies resulted in 21-23 %, 12-13 %, and 305-320 rpm of yam flour level, moisture content, and screw speed, respectively. The results suggested that corn extrudates fortified with yam produced by using the single-screw extruder are suitable for snack food development.