Abstract: The effects of different types of added nitrogen source, carbon source and metal ion on the growth of L. casei LC2W in skim milk were evaluated to identify glucose, soybean polypeptide and MnSO4 as the best nutritional factors for L. casei LC2W. In addition, the logarithmic count of L. casei LC2W was investigated with respect to the concentrations of the selected nutritional factors. Response surface analysis based on a three-variable, three-level Box-Behnken design was employed to optimize these nutritional factors. MnSO4 concentration had the most important influence on the logarithmic count of L. casei LC2W followed by soybean polypeptide concentration; the effect of glucose concentration was the weakest. Moreover, a significant interaction between MnSO4 concentration and soybean polypeptide concentration was observed. The optimal concentrations of glucose, soybean polypeptide and MnSO4 were determined to be 65.90, 7.68 mg/L and 13.88 mg/L, respectively, yielding a L. casei LC2W count of 2.6 × 109 CFU/mL in 24 h of fermentation, which was 11.38 fold higher than before the optimization (2.1 × 108 CFU/mL).

Key words: Lactobacillus casei, skim milk, nutritional factor, response surface methodology