Table 1.

Variables studied and their levels in the 2^8-4 fractional factorial experimental design.

Variables	Low level	Center point	High level
	-1	0	+1
Glycerol (g/L)	10	20	30
Sodium Nitrate (g/L)	1	3.4	5.8
Sodium Chloride (g/L)	0	1	2
Magnesium Sulfate (g/L)	0	0.1	0.2
Ferric Chloride (g/L)	0	0.01	0.02
Calcium Chloride (g/L)	0	0.02	0.04
Yeast Extract (g/L)	0	0.1	0.2
Trace Elements (mL/L)	0	0.5	1