Table 1. Variables and their levels employed in Plackett-Burman design.

Factor (g/L)	Variables	Levelsa
		−1	1
Glucose	X1	15	25
Yeast extract	X2	3.5	6.5
Tryptone	X3	3	5
K2HPO4	X4	2	4
KH2PO4	X5	2	4
L-Cysteine	X6	0.05	0.15
MgSO4·7H2O	X7	0.0	0.1
FeSO4·7H2O	X8	0.2	0.4

^ax₁ = (X₁ − 20)/5; x₂ = (X₂ − 5)/1.5; x₃ = (X₃ − 4)/1; x₄ = (X₄ − 3)/1; x₅ = (X₅ − 3)/1; x₆ = (X₆ − 0.1)/0.05; x₇ = (X₇ − 0.05)/0.05; x₈ = (X₈ − 0.3)/0.1.