3.10. Optimization of the screened variables by Box–Behnken factorial experimental design The experimental design and results are shown in (Table 1). Maximum phytase production was obtained at run No. 4 the parameters included are (0.75 glucose, 0.375 of peptone and 0, 01 tween 80 g/5 g ds). The maximum experimental phytase production by P. purpurogenum GE1 was 444 ± 3.5 U/g ds while the predicted value was 401 U/g ds. The experimental results are approximately 98% of the predicted value. This indicated that the generated model is an adequate prediction of the enzyme productivity. Regression analysis was performed to fit the response function (phytase activity) with the experimental data (Table 2). The analysis of variance for the three variables (glucose, peptone and tween 80) indicated that enzyme productivity could be described well by a polynomial model.