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Concentration ranges of four independent variables used in central composite design
| Variable | Component | Range studied | Levels | ||||
|---|---|---|---|---|---|---|---|
| −α | −1 | 0 | +1 | +α | |||
| X 1 | pH | 4.00–8.00 | 4.00 | 5.00 | 6.00 | 7.00 | 8.00 |
| X 2 | Temperature (°C) | 20.00–40.00 | 20.00 | 25.00 | 30.00 | 35.00 | 40.00 |
| X 3 | Inoculum age (h) | 72.00–196.00 | 72.00 | 96.00 | 120.00 | 144.00 | 168.00 |
| X 4 | Inoculum size (%, v/v) | 1.00–13.00 | 1.00 | 4.00 | 7.00 | 10.00 | 13.00 |
α = 2k/4, where k = 4 represents no. of variables
