Abstract:

Corn steep liquor was utilized by Streptococcus thermophilus as the main nitrogen source to produce L-lactic acid in the present study. To improve L-lactic acid production, the culture medium composition was optimized by response surface methodology. The optimal culture medium composition was determined to be composed of glucose 84 g/L, corn steep liquor 41 g/L, ammonium sulfate 1.3 g/L, and pH 6.5. An L-lactic acid yield of 11 g/L was obtained using the optimized medium, approximating to that (9.02 g/L) obtained using beef extract to substitute ammonium sulfate without any changes of three other factors, which resulted in a large reduction of production cost but effectively elevate productivity.

Key words: Streptococcus thermophilus, L-lactic acid, corn steep liquor, response surface analysis