Independent variables
Levels
-2
-1
0
1
2
Extraction temperature X
1
(°C)
50
60
70
80
90
Extraction time X
2
(s)
360
540
720
900
1080
pH value X
3
5.00
6.00
7.00
8.00
9.00
Ratio of water to raw material X
4
(v/w)
20:1
30:1
40:1
50:1
60:1
Table 1:
Independent variables and their levels used for central composite design.
