**2.7. Experimental design and statistical analysis**

Response surface methodology (RSM) was used, and the process variables were: temperature of the fourth zone of the extruder (T, 110-130 °C), moisture content of conditioned ground corn (MC, 25-35 % of the corn weight), and concentration of xanthan gum (XG, 0.3-0.7 % of corn weigth). A central composite rotatable design was utilized (Table 1), with three factors and 5 levels (Montgomery 2001; Myers 1971). In order to observe the difference between specific treatments, Tukey's test was used at a level of significance of p= 0.05 . To find the best variables combination of the process to obtain the extruded corn flour, the conventional graphical method was used and maximizing WAI, Tan δ; minimizing Gʹ, Gʹʹ and MF. To obtain contour plots for the visualization and selection of the best combination of T, MC and XG to prepare the extruded nixtamalized corn flour, the contour plots of each of the response variables were utilized, by means of the method of superimposition of surfaces. Data analysis and the elaboration of surface response and contour plots were performed using Design Expert version 6.0.7 software (Design Expert, 2002).
