Membrane cell recycle has been a very popular method in achieving a high bioreactor productivity. However, this process has several drawbacks such as difficulties of scale-up, sterilization, and operation. In order to overcome these drawbacks, a new internal filter system which allowed microbial separation inside the fermentor was introduced and its applications for high cell density culture of yeast and high ethanol productivity were investigated.
The filter performance was very important in this system and was highly influenced by agitation speed and yeast concentration. Retention coefficient with the filter of 2 μm pore size was found more than 95%, and the filter was suitable for yeast separation.
Ethanol fermentation of Saccharomyces cerevisiae was carried out and maximum cell concentration up to 208 g/L was obtained. The cell concentration in the fermentor was maintained by proper manipulation of dilution rate and bleed ratio with the growth rate. The ethanol productivity with a feeding glucose concentration of 100 g/L was 41 g/L-h at steady state. The internal filter system was successfully operated for more than 10 days.
The total cell retention culture using molasses as the substrate was successfully carried out. A cell concentration of ca. $4\times10^{12}$ cells/L (100 g/L) was obtained with a sugar concentration of 110 g/L at a dilution rate of 0.26 $h^{-1}$.
With the tapioca hydrolysate only, the yeast concentration obtained in this study was 20∼30 g/L. And then the nitrogen source (urea, yeast extract, and $NH_4OH$) added to the tapioca hydrolysate medium, with no success. However, when tapioca hydrolysate medium treated with activated carbon column, the yeast concentration increased to 40 g/L.
With recombinant yeast RH 51, direct fermentation of starch to ethanol was also investigated. The maximum yeast concentration obtained in this study was ca. 135 g/L. The cell concentration obtained in this study was similar to that in an external cell recycle system with wild type yeast, but operation was simpler. The maximum ethanol concentration and productivity with liquefied and saccharified starch were 57.03 g/L and 10.27 g/L-h. This productivity was 5 times higher than what can be obtained by Hoshino et al. (1990). It was found that the added enzyme could be reduced using RH 51 as compared to a commercially available yeast strain used in ethanol producing industries. Fermentation rate with internal filter at 37℃ was similar to that at 30℃. Operation of more than 7 days employing the internal filter system was found to be possible using soluble starch. Since there were little or no difficulties in the reactor operation, the internal filter system proved to be efficient and stable over the course of its operation.
These studies show that the internal filter system can be used as successfully for high density cell culture and ethanol fermentation, which was comparable with an external cell recycle system. Furthermore, it can be scaled up more easily than the external cell recycle system.
