After the multifactor analysis of variance and the second-order model prediction determinations, the optimal pre-treatment con-dition was obtained by the desirability function approach and the response surface plots were developed using Design Expert 7 (Stat-
Ease Inc., USA).
Based on the experimental design suggested by Design-Expert®
Software version 7, 16 runs with 2 blocks of experiments were conducted. Experimental results of the 3-factors, 5-levels central composite design are shown in Table 2. The response surface