where, “a” the constant coefficient, bi the linear coefficients, bij the interaction coefficients, bii the quadratic coefficients and xi, xj are the coded values of the variables. The statistical significance of the nonlinear regression was determined by Student’s test; the second order model equation was evaluated by Fischer’s test and the proportion of variance explained by the model was given by the coefficient of determination, R2 (Myers and Montgomery, 2002). Experimental runs were performed at random and the results were analyzed using Statistic version 9.1 (Statsoft, USA) software.