Table 1.

Two levels of the factors used in Plackett-Burman Design.

Factor	Low level (−1)	High level (+1)
Corn flour concentration (g/L) (X1)	40.000	50.000
Soybean flour concentration (g/L) (X2)	24.000	34.000
CaCO3 concentration (g/L) (X3)	3.000	5.000
K2HPO4 concentration (g/L) (X4)	2.000	4.000
MgSO4·7H2O concentration (g/L) (X5)	0.200	0.400
NaCl concentration (g/L) (X6)	0.200	0.400
Dummy variable (X7)	—	—