Table 3.

Main effect factors analysis results of Plackett–Burman experimental design

Variable	Effects	Coefficient	Standard error	T value	P value	Significance
Constant		4.761	0.1790	26.59	0.000
Initial pH value	− 2.652	− 1.326	0.1790	− 7.41	0.002a	1
(NH4)2SO4	1.935	0.968	0.1790	5.40	0.006a	2
KH2PO4	− 0.358	− 0.179	0.1790	− 1.00	0.374	5
Inoculation amount	− 0.015	− 0.008	0.1790	− 0.04	0.969	7
Fermentation time	1.185	0.592	0.1790	3.31	0.030b	3
Temperature	0.368	0.184	0.1790	1.03	0.362	4
Rotation speed	0.118	0.059	0.1790	0.33	0.758	6

P < 0.05 was considered as significant and P < 0.01 was considered as very significant

^aSignificant at 1% (P < 0.05)

^bSignificant at 5% (P < 0.05)