Improving wine fermentation efficiency of Torulaspora delbrueckii by increasing the ploidy of yeast inocula

The life cycle of most non-conventional yeasts, such as Torulaspora delbrueckii (Td), is not as well-understood as that of Saccharomyces cerevisiae (Sc). Td is generally assumed to be haploid, which detracts from some biotechnological properties compared to diploid Sc strains. We analyzed the life cycle of several Td wine strains and found that they were mainly diploid during exponential growth in rich medium. However, most cells became haploid in stationary phase, as observed for Sc haploid heterothallic strains. When transferred and incubated in nutrient-deficient media, these haploid cells became polymorphic, enlarged, and transitioned to diploid or polyploid states. The increased ploidy, that mainly results from supernumerary mitosis without cytokinesis, was followed by sporulation. A similar response was observed in yeasts that remained alive during the second fermentation of base wine for sparkling wine making, or during growth in ethanol-supplemented medium. This response was not observed in the Sc yeast populations under any of the experimental conditions assayed, which suggests that it is a specific adaptation of Td to the stressful fermentation conditions. This response allows Td yeasts to remain alive and metabolically active longer during wine fermentation. Consequently, we designed procedures to increase the cell size and ploidy of haploid Td strains. Td inocula with increased ploidy showed enhanced fermentation efficiency compared to haploid inocula of the same strains.