Table 4.
Variable levels used in RSM.
| Variables | Coded Variable Levels | ||
|---|---|---|---|
| −1 | 0 | +1 | |
| X1 Extraction time/min | 20 | 25 | 30 |
| X2 Extraction temperature/°C | 40 | 50 | 60 |
| X3 Ultrasonic power/W | 300 | 350 | 400 |
| X4 Liquid to solid ratio/mL·g−1 | 20:1 | 25:1 | 30:1 |
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