Table 1 Levels of independent central composite design (CCD) variables
Independent variable | Symbol | Level | ||||
|---|---|---|---|---|---|---|
−α |  − 1 | 0 | 1 |  + α | ||
Ethanol concentration (%) | A | 50 | 60 | 70 | 80 | 90 |
Ultrasonic temperature (°C) | B | 40 | 50 | 60 | 70 | 80 |
Ultrasonic time (min) | C | 30 | 40 | 50 | 60 | 70 |
Liquid–solid ratio (mL/g) | D | 20 | 30 | 40 | 50 | 60 |