Industrial yeast strains engineered for controlled flocculation
In many industrial fermentation processes, Saccharomyces cerevisiae yeast should ideally meet two partially conflicting demands. During fermentation a high suspended yeast count is of paramount importance to maintain a rapid fermentation rate, whilst efficient flocculation should ideally be initiated only on completion of the primary alcoholic fermentation, so as to enhance product clarification and recovery. Most commercial wine yeast strains are non-flocculent, probably because this trait was counter-selected to avoid fermentation problems. In this study, we assessed molecular strategies to optimise the flocculation behaviour of non-flocculent laboratory and wine yeast strains. For this purpose, the chromosomal copies of three dominant flocculation genes, FLO1, FLO5 and FLO11, of a non-flocculent S. cerevisiae laboratory strain (FY23) and two commercial wine yeast strains (BM45 and VIN13) were placed under the transcriptional control of the stationary phase-inducible promoters of the S. cerevisiae ADH2 or HSP30 genes. Under standard laboratory media and culture conditions, all six promoter-gene combinations resulted in specific flocculation behaviours in terms of timing and intensity. The data show that the strategy resulted in the expected and stable expression patterns of these genes in both laboratory and industrial wine yeast strains. Most importantly, the data confirm that inducible expression of the native FLO1 and FLO5 open reading frames, albeit to varying degrees, are responsible for a quantifiable cell-cell adhesion phenotype that can be characterized as a Flo1 flocculation phenotype. On the other hand, we found that inducible expression of the native FLO11 ORF under these conditions resulted in flor/biofilm formation and invasive growth phenotypes. However, the specific impact of the expression of individual dominant FLO genes with regard to characteristics such as flocculation efficiency, cell wall hydrophobicity, biofilm formation and substrate adhesion properties showed significant differences between the commercial strains as well as between commercial and laboratory strains. These adhesion phenotype differences may at least in part be attributed to wine yeast FLO gene open reading frames containing significantly smaller intragenic repeat regions than laboratory strains. The data show that the ADH2 regulatory sequences employed in this study were unsuitable for the purpose of driving FLO gene expression under wine-making conditions. However, HSP30p-based FLO1 and FLO5 wine yeast transformants displayed similar flocculent phenotypes under both synthetic and authentic red wine-making conditions, and the intensities of these phenotypes were closely aligned to those observed under nutrient-rich YEPD conditions. The fermentation activities of HSP30p-based transgenic yeast strains were indistinguishable from that of their parental host wine yeast strains. The chemical composition of wines obtained using transgenic yeast strains were similar to those produced by parental strains. The BM45-derived HSP30p-FLO5 transformant in particular was capable of generating compacted or ‘caked’ lees fractions, thereby providing a distinct separation of the fermented wine product and lees fractions. Furthermore, in this study we report a novel FLO11 induced flocculation phenotype that seems to exclusively develop under authentic red wine-making conditions. This strong FLO11 flocculation phenotype was not wine yeast strain dependant, possessed both Ca2+-dependant and Ca2+-independent flocculation characteristics and was insensitive to inhibition by both glucose and mannose. A distinct advantage of this unique FLO11 phenotype was highlighted in its ability to dramatically promote faster lees settling rates. Moreover, wines produced by HSP30p-FLO11 wine yeast transformants were significantly less turbid than those produced by their wild type parental strains. The benefit of this attractive property is it facilitates simpler and faster recovery of wines and also promotes greater volume recovery of the wine product.