Process Optimization For Efficient Production Of L-Lactic Acid Using Lactobacillus Rhamnosus

Lactic acid is one of the three organic acids that has the biggest market and has been used in many fields. The molecular structure of lactic acid has asymmetry carbon atom, so it has three kinds of optical isomer: L-lactic acid, D-lactic acid and DL-lactic acid racemized. In the foodstuff and medicine industry D and DL-lactic acid should be replaced by L-lactic acid because the human body can only metabolize L-lactic acid. Moreover, the potential use of L-lactic acid to produce poly-lactic acid for synthesis of biodegradeing plastic, makes it to have even greater potential market. In this paper a strain of Lactobacillus rhamnosus that can produce L-lactic acid was studied, and fermentation process for production of L-lactic acid with high productivity and efficiency was optimized, and the fermentation dynamic was studied and simulated.Some of the factors that enhance L. rhamnosus in producing L-lactic acid with high efficiently were investigated. When glucose was used as the carbon source, 120 and 146g/L of glucose were converted to 104 and 117.5g/L of L-Lactic acid, respectively. The result was close to the highest level reported. However increasing the inoculation volume could not overcome the growth delay resulted from the substrate inhibition. High concentration of initial L-lactic acid severely inhibited the growth and production of L-lactic acid. When initial L-lactic acid concentration was beyond 70g/L, the growth and L-lactic acid production stopped. Three kinds of neutralization reagents used for pH control had different effects on the growth of L rhamnosus and production of L-lactic acid.Yeast extract enhanced lactic acid production greatly, although it is expensive. The cheaper soybean flour supplemented with a mixture of vitamins can replace the expensive yeast extract. After the optimization, 3.5-4.5% of soybean flour supplemented with optimal amount of the mixture of eight kinds of vitamins produced L-lactic acid of 104g/L from 120 g/L glucose with the yield of 86.7 % using flask cultivation, and L-lactic acid of 128 g/L from 150 g/L of glucose with the yield of 85.3 % using 5 L fermentor cultivation, respectively, which results were comparable to the fermentation results using yeast extract as the nitrogen source and growth factors.In order to produce much L-lactic acid efficiently, the processes of fed-bacth culture and continuous culture were investigated. From the fed-batch experimental results of L. rhamnosus fermentation in flasks and 5L fermentor, the dynamic process of fermentation had no obvious difference between batch and fed-batch fermentation; Fed-batch culture did not effectively promote the growth and lactic acid production. The conditions and means of fed-batch culture need to be further studied.Using continuous culture, the process of the growth and production of L-lactic acid of L. rhamnosus was investigated in 5L fermentor under limitation of nitrogen source. The results were as follows: the dilution rate with D=0.15 1/h had already arrived at critical dilution rate under the condition of restricted nitrogen source. While the dilution rate increased from 0.05 1/h to 0.15 1/h, the biomass,the output of L-lactic acid,and the productivity gradually reduced ,however, residual glucose was increasing gradually and the specific volume productivity was the maximum with D=0.125 1/h. Because the growth was limitted under restricted nitrogen source, continuous culture of L. rhamnosus can only produce L-lactic acid with lower dilution rate.In repeated fed-batch fermentation, the final concentration of L-lactic acid was 93g/L, the product yield was 93% and the residual glucose was 5g/L in the first fermentation process; while the final concentration of L-lactic acid arrived 108g/L, the product yield was 83.8% and the residual glucose was 30g/L in the second fermentation process.Starch is a usual material in industrialized L-lactic acid fermentation. The crude pachyrhizus starch and raw corn flour were selected as materials for L-lactic acid production using simultaneous saccharification and fermentation method. The optimal result for dienzymatic saccharification of starchiness material was: wet sterilized material adding a —amylase of 10 U/g-dry material, glucoamylase of 250 U/g-dry material, and saccharification temperature of 42 °C.The saccharification efficiency of raw corn flour was increased with the increasing of the substrate concentration. High L-lactic acid concentration was obtained by L. rhamnosus fermentation under high raw corn flour concentration. 250 g/L of wet sterilized crude pachyrhizus starch, wet sterilized corn flour and dry sterilized corn flour produced 129 ^ 118 ^ 111 g/L L-lactic acid, with the yield of 54.9%,61.5%,57.9%, respectively. 200 g/L of soluble starch and raw corn flour produced 120 and 115 g/L L-lactic acid with yield of 63.8% and 74.9%, respectively, using SSF. The efficiency of raw corn flour converting to L-lactic acid was higher than soluble starch and crude pachyrhizus starch. Therefore, it is economical to produce L-lactic acid using raw corn flour using SSF method. Raw corn flour of 200g/L using SSF in 5L fermentor could produce 131 g/L of L-lactic acid with the yield of 85.3%. The raw corn flour using repeated fed- batch culture could produce high concentration of L-lactic acid in 5L fermentor using SSF, with the final concentration of L-lactic acid of 121 g/L and the yield of 87.6% of the second fermentation stage.In order to simulate dynamic process of L-lactic acid fermentation, the mathematic models of L-lactic acid fermentation were constructed.