Breeding Of L-lactic Acid Producing Strain And Study On Lactic Acid Fermentation From Corn Cob Hydrolysate

This paper focused on the screening, identification and mutagenesis of the L-lactic acid producing bacteria, then studying the fermentation condition when using the corncob hydrolysate as fermentation substrate. The detailed contents were as followed:1. Corncob is a cheap and underutilized agricultural residue. Corncob can be used to producing L-lactic acid which is an effective exploitation of renewable resources. In this study, through strain screening method, strain S-8 which can high-efficient convert corncob hydrolysate to L-lactic acid were obtained. The strain was identified as Bacillus coagulans.2. Diethyl sulfate and UV was used to deal with strain S-8. The L-lactic acid producing mutant was selected based on the pathway analysis and metabolic engineering theory. The mutant strain S-8-8-7 showed better yield characters than the start strain。After fermentation for 72 h, the mutant strain S-8-8-7 gave a L-lactic acid output of 69.10 g/L, 54.9% higher than the start strain.3. Using xylose as carbon source, the fermentation medium was optimized for L-lactic acid production by strain S-8-8-7. Through single factor experiment, the optimized fermentation medium determined as followed: peptone 5 g/L, yeast extract 4 g/L, beef extract 2 g/L, K2HPO4 1 g/L, MgSO4 0.3 g/L. Three ingredients (peptone, beef extract, yeast extract) was optimized through Response Surface Analysis. The functional relationship between main factors and response value was established. The regression equation was: Y=82.35+2.18X1+1.16X2+2.45X3-1.21X12-0.93X32 -1.01X1X2-1.37X1X3Through analyzing the regression equation, the optimal concentrations of the main factors were obtained: peptone 3 g/L, beef extract 3.68 g/L, yeast extract 5.68 g/L. Under the optimal conditions, the yield of L-lactic acid reached 86.43 g/L.4. Based on the optimal fermentation medium, corncob hydrolysate was used to producing L-lactic acid. Through single factor experiment, the optimized fermentation conditions determined as followed: concentration factor is 1, inoculum size is 10%, fermentation time is 72 h, fermentation temperature is 45℃, pH 6.5.5. Based on the optimal fermentation conditions, the experiment of fed batch cultivation feeding with concentrated corncob hydrolysate was designed. After fermentation for 72 hours using corncob hydrolysate, the yield of L-lactic acid reached 50.9 g/L, the residual sugar and the conversion rate of sugar were 6.3 g/L and 71.8%.