Table 1.

Variables and levels of Plackett–Burman design

Code	Variables	Unit	Low level (− 1)	High level (+ 1)
A	Initial pH	–	5.5	6.5
B	(NH4)2SO4	g	0.3	0.5
C	KH2PO4	g	0.075	0.1
D	Inoculation amount	%	5	7.5
E	Fermentation time	days	3	4
F	Temperature	°C	26	28
G	Rotation speed	rpm	80	100

Level “− 1” represents the low factor level, level “+ 1” represents the high factor