Table 1.

Experimental variables and their coded levels for the central composite design.

Independent variable	Unit	Symbol Code	Levels of coded variables
			- α -2	Low -1	Medium 0	High +1	+ α +2
Temperature	°C	X1	25	30	35	40	45
Glucose	g.dL−1	X2	3	6	9	12	15
Biotin	μg.L−1	X3	0	3	6	9	12
Urea	g.dL−1	X4	0.1	0.3	0.5	0.7	0.9