A high F-value (18.4034), which was implied by the model, was statistically significant. Model terms having values of Prob > F (0.0179), which is less than 0.05, were considered significant. The results of the regression equation obtained from ANOVA show that the determination of the coefficient (R2) was calculated as 0.918 for CMCase activity (a value of R2 > 0.75 indicated the aptness of the model), which indicated that the statistical model was suitable to adequately represent the real relationship among the factors determined. The fitness of the model can be checked by the coefficient of determination (R2) and correlation coefficient (R). The R2 value is always between 0 and 1. As R2 reaches close to 1, the model becomes stronger and it predicts the response better [22] and [23]. The value of R2 (0.918) for Eq. (2) being close to 1 indicated a close agreement between the experimental results and the theoretical values predicted by the model equation. Adequate precision is basically a measure of the signal to noise ratio, and a ratio greater than 4 is desirable. In this study, the adequate precision of 12.607 was calculated and it indicated that this model can be used in the design space.
Three-dimensional response surfaces were plotted (Fig. 2) on the basis of the model equation, to investigate the interaction between the variables and to determine the optimum concentration of each factor for maximum production of CMCase. Fig. 2 (A) shows the effects of corn stover and urea and their reciprocal interaction on CMCase production at zero level of ammonium sulfate conc. (4.6 g/L). At the lowest concentration of corn stover (9 g/L), the high concentration of urea caused a low level of CMCase activity. At a low concentration of urea, an increase in the concentration of corn stover did not enhance CMCase production. The CMCase production was strongly inhibited by the high concentration of urea. The effects of corn stover and ammonium sulfate and their reciprocal interaction on CMCase production at zero level of urea concentration (2 g/L) are shown in Fig. 2 (B). At low concentration of corn stover, the activity was stationary affected at wide range of ammonium sulfate. An increase of corn stover concentrations linearly increased the activity at a constant concentration of urea. Fig. 2 (C) presents the effects of urea and ammonium sulfate and their reciprocal interaction on CMCase production at zero level of corn stover concentration (12 g/L). At the highest concentration of urea (2.8 g/L), an increase in the concentration of ammonium sulfate linearly decreased the activity. On the other hand, the activity was slightly affected by ammonium sulfate at the lowest concentration of urea (1.2 g/L). The maximal production was accomplished by a high concentration of ammonium sulfate and a low concentration of urea with a moderate concentration of corn stover. In this investigation, we confirmed that CMCase production was strongly inhibited by more than 2 g/L of urea concentration.