The Study On Fermentation And Purification Of L-Lactic Acid

The fermentation of glucose to L-lactic acid was conducted using Lactobacillus delbrueckii BME5-18M. The effects of temperature, pH, carbon resources and its concentration on cell growth and L-lactate production were investigated during the batch fermentation. The results showed that the optimal carbon resource was glucose, and the optimal operation parameters were 37℃ and pH 6.5.The batch fermentation process was studied in a 5 L system under the optimal conditions. A kinetic model was proposed based on the Logistic equation, Luedeking-Piret equation and simplified substrate consumption equation for cell growth, L-lactate production and glucose consumption. With the evaluated model parameters, the model appears to provide a reasonable description for the L-lactic acid fermentation process.Based on the experiment and kinetics of batch fermentation, the effects of different glucose constant feeding rate were investigated during the fed-batch fermentation. The results indicated that 25mL/h was the optimal strategy for fed-batch fermentation. It was found that fed-batch fermentation was very useful for highly efficient accumulation of biomass and L-lactic acid and the productivity of fermentation was greatly improved in comparison with batch fermentation.Purification of L-lactic acid by molecular evaporator was studied, the effects of wiper agitation speed, feeding flow speed and evaporating temperature on purity and yield of L-lactic acid were investigated, and the effect of feed temperature of L-lactic acid on yield was also investigated. The results showed that molecular distillation was an efficient method for purification of L-lactic acid. The purity of production was higher than 95%.The feasibility was investigated to the application of simplified calcium salt recovery method with molecular distillation technique for the preparation of high purity L-lactic acid at laboratory scale. Fermentation broth was directly taken into sulfuric acid hydrolysis without crystallization, and the conventional active carbon for decolorization and ion exchange were omitted. The crude L-lactic acid produced was concentrated directly by rotary evaporator, and the enriched material was purified in molecular evaporator. The results showed that we still can get high purity L-lactic acid, and we even can get L-lactic acid crystal under the proper condition. The operation units of the calcium salt recovery method were greatly decreased, and the yield of production was increased.