TABLE 1.

Design and result of central composite design (CCD).

X1: Proportion of material (%)	X2: Concentration of alcohol (%)	X3: Extraction time (min)	X4: Power (kW)	Y: Concentration of ZNC (mg⋅mL–1)
20	0	60	3.0	5.75
20	40	60	3.0	7.27
20	20	60	3.0	7.06
20	20	0	3.0	3.86
10	10	90	4.5	2.42
30	10	90	1.5	8.93
20	20	60	3.0	6.98
30	10	30	4.5	8.02
20	20	60	3.0	5.95
20	20	60	0.0	7.2
40	20	60	3.0	12.78
20	20	60	3.0	7.2
10	30	30	1.5	2.84
30	30	90	1.5	9.68
10	30	30	4.5	3.34
30	30	90	4.5	10.87
10	10	90	1.5	2.48
30	10	30	1.5	9.04
20	20	120	3.0	5.86
30	30	30	4.5	11.43
30	30	30	1.5	9.68
10	10	30	1.5	2.19
10	30	90	1.5	3.36
10	30	90	4.5	4.54
20	20	60	3	6
20	20	60	6	6.47
0	20	60	3	0
10	10	30	4.5	2.3
20	20	60	3	6.04
30	10	90	4.5	7.79