Table 1.

Independent variables and their coded and actual levels for central composite design (CCD).

Variables		Coded and Actual Levels
		−2	−1	0	+1	+2
X1	Temperature (°C)	50	60	70	80	90
X2	Ethanol concentration (%)	21.8	40.0	58.3	76.5	94.8
X3	Extraction time (min)	15	30	45	60	75
X4	Ultrasonic power (W)	300	400	500	600	700