Abstract: Response surface methodology was used to optimize the MRS-based culture medium of L. delbrueckii subsp bulgaricus LB14. The effects of twelve additives and the initial pH of the media were evaluated by using a Plackett-Burman design. Results showed that tomato juice, carrot juice and the initial pH of the media were the main affecting factors while the other additives had no significant effect on the cell multiplication of LB14. Then, the central composite design and response surface analysis were used to determine the optimal levels of the main factors. The optimized conditions were:MRS supplemented with 52.4ml/L tomato juice and 28.1ml/L carrot juice,and the initial pH of the media was 6.38. It resulted in cell production of LB14 to be increased from 2.4×108cfu/ml to 1.1×109cfu/ml.

Key words: Lactobacillus delbrueckii subsp bulgaricus, enrichment medium, plackett-Burman design, central compos- ite notatable design, response surface methodology