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Table 1 Independent variables for the performed experimental designs in coded and natural values

From: New keratinolytic bacteria in valorization of chicken feather waste

Variable Unit Designation (− 1) (0) (+ 1)
Plackett–Burman design
 Feathers % (w/v) X1 0.5 2.0
 MgSO4·7H2O % (w/v) X2 0.05 0.20
 KH2PO4 % (w/v) X3 0.005 0.020
 CaCl2 % (w/v) X4 0.005 0.020
 Yeast extract % (w/v) X5 0.02 0.10
 Inoculum size Cells/flask X6 1.2 × 108 1.2 × 109
 Agitation rpm X7 180 200
Box–Behnken design
 Feathers % w/v X1 1 3 5
 MgSO4·7H2O % w/v X2 0.01 0.05 0.09
 KH2PO4 % w/v X3 0.01 0.05 0.09