Table 1.

Coded and actual values of the variables for the four factor Box–Behnken experimental design.

Variables	Symbol	Coded and Actual Values
		−1	0	+1
pH	X1	4	6.5	9
Temperature (°C)	X2	21	24	27
Agitation speed (rpm)	X3	100	150	200
Fermentation time (h)	X4	24	168	336