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Factor levels of the independent variables according to the Box–Behnken experimental design (BBD).
| Independent Variables | Symbol | Coded Levels | ||
|---|---|---|---|---|
| −1 | 0 | +1 | ||
| Treatment time (min) | X1 | 0 | 2.5 | 5 |
| pH | X2 | 4 | 7.5 | 11 |
| Solvent concentration (ethanol) (%) | X3 | 50 | 73 | 96 |
| Solvent-to-seed ratio (X4:1) | X4 | 2 | 6 | 10 |
