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

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