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Independent variables and their levels in Box-Behnken design
| Independent variables | Symbol | Level | ||
|---|---|---|---|---|
| −1 | 0 | 1 | ||
| Extraction time (min) | X1 | 10 | 50 | 90 |
| Extraction temperature (°C) | X2 | 30 | 50 | 70 |
| Cellulase concentration (%) | X3 | 0.5 | 1.5 | 2.5 |
| Ultrasonic power (W) | X4 | 100 | 200 | 300 |
