where Y is the predicted response; ˇ0 is a constant; ˇ1, ˇ2 are
the linear coefficients; ˇ12 is the cross-coefficients; ˇ11, ˇ22 are
the quadratic coefficients. The response surfaces of the variables
inside the experimental domain were analyzed using Design Expert
software (Stat Ease, 6.0 trial Version, Minneapolis, MN, USA). Subsequently,
additional confirmation experiment was conducted to
verify the validity of the statistical experimental strategies