Plackett and Burman statistical design was applied to screen the nutritional factors affecting β-galactosidase production by Lactobacillus fermentum CM33. Accordingly, lactose, tryptone and Tween80 were found to be the positive effective factors at the significant level above 80%. Central composite design (CCD) and response surface plot predicted that the maximum enzyme activity of 49.81 U/100 ml would be obtained at 3.23 (w/v) lactose, 4.91% (w/v) tryptone and 0.62% (w/v) Tween80 and a trial experiment validated the activity total at 114% at 24 h of cultivation. However, the maximum activity of 63.31 U/100 ml culture was found at 16 h. The initial pH range 6.0 to 6.5 was found to be the most suitable for enzyme production. The final stage of optimization increased the productivity by 14.60 folds over that obtained with the non-optimized medium. The replacing of the expensive ingredients such as yeast extract, peptone and beef extract composed in the optimized medium by skimmed milk, did not significantly alter the enzyme productivity.Keywords: Medium optimization, Lactobacillus sp., β-galactosidase, experimental desig
