After identification of significant factors, a central composite design (CCD) was selected to evaluate the response pattern and to determine the optimal combination of inoculum age, initial cell
density, initial pH of medium, and temperature for maximizing
butanol yield. The un-coded values for each parameter were as follows
[low star point, low central point, center point, high central
point, high star point]: inoculum age in hour [12.05, 13.5, 17.0,
20.5, 21.95]