Table 1. Values and levels of operating parameters.

Factors	Levels - α (-2) -1 0 +1 +α (+2)
X1: Starch concentration (g l-1)	2	6	10	14	18
X2: Yeast extract concentration (g l-1)	2	3	4	5	6
X3: Peptone concentration (g l-1)	0.5	1.25	2	2.75	3.5
X4: pH	3	5	7	9	11