Table 1. The central composite design for optimization of ultrasound extraction conditions of peony root
X
n
Response variables
Levels
−2
−1
0
1
2
X
1
Ethanol concentration (%)
20
40
60
80
100
X
2
Extraction temperature (°C)
30
40
50
60
70
X
3
Extraction time (min)
15
25
35
45
55