where Yi is the predicted response, xixj are input variables which influence the response variable Y; b0 is the offset term; bi is the ith linear coefficient; bii is the ith quadratic coefficient and bij is the ijth interaction coefficient. The second order polynomial coefficients were calculated and analyzed using Design Expert (Version 8.0, Stat-Ease Inc., Minneapolis, USA) statistical software package. Statistical analysis of the model was performed by using analysis of variance (ANOVA).