Competitive yeast fermentation with selective flocculation and recycle

Continuous fermentations were carried out involving competition between two strains of Saccharomyces cerevisiae. One of the strains has a lower specific growth rate and is very flocculent, whereas the fastergrowing strain is nonflocculent. The product stream from the chemostat was fed into an inclined settler where the flocculent strain was partially separated from the nonflocculent strain as a result of the higher sedimentation rate of the flocculent cells. The underflow from the inclined settler, which was concentrated and enriched with flocculent cells, was recycled to the chemostat. When no recycle was used, the fastergrowing, nonflocculent yeast rapidly overtook the culture. With selective recycle, however, the experiments demonstrated that the slower-growing flocculent yeast could be maintained as the dominant species. A theoretical development is also presented in order to describe the competition between two strains in the bioreactor-settler system. The concept of selective recycle via selective flocculation and sedimentation offers a possible means of maintaining unstable recombinant microorganisms in continuous fermentations.