Optimization Of Process Conditions For D-lactic Acid Fermentation And Study On Reuse Of The Condensed Water

At present,the large-scale production of high optical purity D-lactic acid in China mainly uses microbial fermentation.D-lactic acid is a raw material for the synthesis of hydroponically acid herbicides,and with the rise of such herbicides,the market demand for D-lactic acid is also growing.So it is imperative to increase the production efficiency of large-scale D-lactic acid fermentation.According to the related problems in the actual production of D-lactic acid,the fermentation conditions and fermentation feeding mode were optimized with the Escherichia coli engineering strain HBUT-D18 used as the fermentation strain,and the reuses of evaporation condensate from D-lactic acid extraction process were studied.The conclusions are as follows:(1)With optimization experiment carried out on the D-lactic acid fermentation process,the optimal fermentation scheme was obtained:the temperature and pH in the fermentation early period were set to 35°C and 6.4,respectively.Then they were adjusted to 37°C and 6.9 after the OD value of the cells reached the maximum level,as well as 0.15%Tween added into the fermentation medium.The total fermentation time was 22 h,which was shortened by 10h before optimization;while the maximum OD value was reached at the 10^th hour.The yield of D-lactic acid was117.4g/L,and the average acid production intensity was 5.34g/(L·h),which increased by 45.5%by the optimization.(2)Through the study of feeding mode,the optimal feeding scheme was obtained:the fermentation was initiated with the sugar concentration as 90g/L.When the residual glucose reduced to about 40g/L,extra glucose was flowed in.The flow speed was carefully controlled to maintain the sugar concentration in the fermentation tank around 40g/L.Compared to non-feeding mode,this feeding scheme was prolonged by 3h,which increased the fermentation period up to 25h;while the D-lactic acid production reached 144.1g/L,which increased by 23.3%.(3)After studying the effect of reuse of evaporated condensate from later extraction,the following conclusions were drawn:The condensed water contain0.48%D-lactic acid.With condensate reused in the seed culture medium,the seed expansion time was shortened from 20h to 13h;with condensate reused in the fermentation medium,the total fermentation time was 24h,and the D-lactic acid production reached 149.3g/L.The reuse of evaporated condensate,which had left un-extracted D-lactic acid,can reduce the loss of the product.It also help to reduce the effort payed for waste treatment by eliminating the process of decolonization.It can save production costs of about 8 million yuan,taking the production of D-lactic acid as10,000 tons per year in calculation.This study optimized the existing D-lactic acid fermentation conditions,which improved its production efficiency.At the same time,the concentrated condensed water was reused,which reduced the loss of D-lactic acid in the extraction process,further improved the production efficiency.The results of this study are in line with the concept of“reducing consumption and increasing efficiency”,and achieves high-efficiency large-scale fermentation production of D-lactic acid.