Improvement of filtration performance of stirred ceramic membrane reactor and its application to rapid fermentation of lactic acid by dense cell culture of Lactococcus lactis

Abstract

A ceramic filter was fitted in a stirred ceramic membrane reactor (SCMR) for both extraction of culture supernatant and feeding of distilled water in reverse flow. The dependence of filtration performance on the cell concentration was decreased by about 20% by regularly cleaning the filter using a membrane cleaning system. The improved permeability effected an increase of both the growth rate and viability of Lactococcus lactis by increasing the dilution rate of the culture supernatant. Using the improved SCMR system, a cell concentration of 178 g/ l and viability of 98% were obtained after 198 h of culture, while it took 238 h to obtain a cell concentration of 141 g/ l and 94% viability without the use of the membrane cleaning system. The perfusion culture system was applied to the rapid batch fermentation of lactic acid by retaining cells at a high density in the SCMR. When the cell concentration reached 80 g/ l, the culture supernatant was extracted and replaced with the fermentation medium. Batch fermentation using the retained cells was repeated six times. The concentration of lactic acid increased to more than 30 g/ l within 2 h in each fermentation, while 1.2 h was necessary for replacing the culture supernatant to repeat the batch fermentation. The production rate of lactic acid was increased in proportion to the cell concentration, and a high fermentation activity of the retained cells was maintained via the repeated batch fermentation. These results demonstrate that the improved permeability of the SCMR with use of a membrane cleaning system effected a rapid increase in the concentration and viability of cells, and accordingly, the increased production rate of lactic acid in proportion to the concentration of viable cells.