Table 2.

Independent variables and levels for extrusion used in the central composite design

Independent variables	Symbol	Coded variables
		−1	0	1
Barrel temperature (third zone) (°C)	X 1	130	150	170
Feed moisture (%)	X 2	14	17	20

The dependent variables were expressed individually as a function of the independent variables (Table 2). Data were fitted to a second‐order approximation model using Equation (1).

$$Y=B_{\mathrm{O}}+{\sum }_{i-1}^k{B}_i{X}_i+{\sum }_{i-1}^k{B}_{ii}{X}_i^2+{\sum }_{\begin{array}{c}i-1\\ i<j\end{array}}^k{B}_{ij}{X}_i{X}_{j+\mathrm{\epsilon }}$$ (1)

where Y is the response function, X _i is the feed moisture content, X _j is the barrel temperature, ε is the random error, B _o the center point of the system, B _i, B _ii, and B _ij represent the coefficients of the linear, quadratic, and interactive effects of the dependent variables, respectively, and X _i, X _i ², and X _i X _j represent the linear, quadratic, and interactive effects, respectively, of the independent variable.