Late fermentation expression of FLO1 in Saccharomyces cerevisiae

A strategy to alter the flocculation properties of nonflocculent yeast in such a way that flocculation occurred toward the end of fermentation was developed. In a double cross-over event, the wild-type FLO1 promoter of the haploid, nonflocculent S. cerevisiae FY23 strain was replaced by a construct consisting of the SMRI-410 marker gene and the HSP30 promoter. In this way, the genomic copy of the wild-type FLO1 open reading frame was brought under transcriptional control of the HSP30 promoter. The transformants showed strong flocculation toward the end of fermentation, resulting in a distinctly clearer beer than the beer obtained with wild-type cells. The other properties of the wild-type strain were conserved. Moreover, it was shown that the transformants were extremely stable and that flocculation could be induced earlier during fermentation by a heat-shock treatment or the addition of ethanol to the medium. These results suggest that the flocculation properties of weakly flocculent brewer's yeast strains can be improved using controlled expression of the FLO1 gene.