Development of a continuous electrodialysis fermentation system for production of lactic acid by Lactobacillus rhamnosus
Introduction
In recent years, more attention has been paid to the great potential of l-lactic acid in the manufacture of biodegradable plastics [1], [2], [3]. Generally, l-lactic acid is produced by fermentation. Fermentation, however, is subject to end product inhibition due to the excess of end product in fermentation broth [4]. The electrodialysis (ED) method is desirable since lactic acid produced is collected from fermentation broth during fermentation and there is no need to take out fermentation broth in excess and continuous fermentation can be possible.
Most of the efforts in the past were concentrated on increasing productivity and extending fermentation time. A hollow membrane filter was used to avoid adherence of the bacteria to the anion-exchange membrane and cation-exchange membrane [4], [5]. For the same target, the semi-continuous production of lactic acid by immobilized growing cells entrapped in calcium alginate was discussed [6]. To stimulate the efficiency of electrodialysis, the electrodialytic generation of gluconic acid with bipolar membranes was performed [7].
In previous studies, it was reported that glucose leaked to the recovery solution during fermentation. There is no leakage of glucose theoretically since glucose has no electrical charge. A small amount of glucose, however, goes to the recovery solution with electro-osmosis of water. It was therefore possible that the yield and productivity of lactic acid would be increased by regulation of glucose leakage.
To decrease glucose leakage to the recovery solution, intermittent EDF has been performed by supplying feed medium containing glucose of 242 g/L since the amount of glucose leakage decreased as the amount of water electro-osmosis decreased due to intermittent operation of EDF. In this case, maximum productivity and yield were 4.78 g/L h and 0.85, much higher than those in batch EDF. The volume of fermentation broth, however, increased due to supplying feed medium even if electro-osmosis of water had been caused by ED operation. If it is possible to make water increase by supplying feed medium in balance with the decrease of water due to ED operation, it would not be necessary to take out fermentation broth in excess and, then, long-term continuous EDF can be possible, In this study, the feed medium was controlled by a level meter to keep the volume of fermentation broth constant. In this way, there are two advantages as follows:
- (a)
there is no loss of bacteria nor waste of raw material because the fermentation broth can be kept in a constant volume,
- (b)
there is less glucose leakage and inhibition since the glucose concentration was kept lower in broth.
In addition, EDF was carried out without a hollow fiber filter in this study to simplify this system. To avoid bacterial adherence to membranes, the flow rate was raised to 800 mL/min, at which the efficiency of lactic acid recovery was higher.
Section snippets
Microorganism
Lactobacillus rhamnosus (IFO 3863) was used as lactic acid producer and was stored at −80 °C. The inoculation was carried out for 48 h in an incubator at 37 °C under sterile conditions.
Media
Two fermentation media were used in this study. The medium for batch fermentation contained: glucose of 100 or 200 g/L, yeast extract of 15 g/L, NaCl of 0.1 g/L, K2HPO4 of 0.50 g/L and MgSO4 of 2.0 g/L and the medium for continuous EDF contained: glucose of 50 g/L, yeast extract of 7.0 g/L, NaCl of 0.05 g/L, K2HPO4 of 0.25
Batch fermentation
To make a comparison with EDF, conventional batch fermentation was carried out with initial glucose concentrations set at 100 and 200 g/L. The fermentations came to an end within 50 and 100 h at the initial glucose concentrations of 100 and 200 g/L, respectively (Fig. 2). The total productivity at glucose concentration of 200 g/L was about four times lower than that at glucose concentration of 100 g/L (Table 1). The production rates of lactic acid at the two glucose concentrations were similar
Conclusion
In this study, lactic acid production of high productivity was realized by means of the EDF with the level meter. It was verified that 175 g/L of glucose concentrations in feed medium was the best value. In this case, the continuous EDF lasted for above 350 h and was stable for above 200 h. The maximum productivity, yield and conversion ratio were 8.18 g/L h, 68.8% and 71%, respectively. The glucose, however, began to be excessive after 200 h in fermentation broth, resulting in the decrease of lactic
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