Yeast species (and even strains within given species like Saccharomyces cerevisiae) differ in their response and ability to cope with the environmental conditions governing the transformation of grape juice to wine. These conditions include the availability of nutrients, such as sugars, nitrogen sources and vitamins, fermentation temperature, oxygen availability, and the presence of other yeast species. Coupled to their genetic backgrounds, their global response to these conditions determines the outcome of alcoholic fermentation and the broader quality of the resulting wine. This response leads to the production of various metabolites of interest, whose nature and quantity is linked to a tightly regulated balance of redox equivalents. Their differing response to environmental conditions is likely linked to a different redox balance management.
This project will monitor the redox status of various yeast species and strains and assess the environmental conditions that can be manipulated by the winemakers to enhance yeasts persistence and contribution. This project seeks to enhance our understanding of metabolic fluxes between yeast species that not only lead to the high/low production of compounds of oenological interest (e.g., C5-C6 polyols, acetic acid, fatty acids, higher alcohols and esters) but also sometimes induce the premature decline of some yeasts. The impact of environmental conditions on these metabolic fluxes must be further investigated so that winemakers can be provided with guidelines to optimize the exploitation of different yeasts species. Indeed, a previous study recently showed that the C5-C6 polyols produced by Torulaspora delbrueckii have a positive impact on mouthfeel, but at this stage, the lack of knowledge on mechanisms governing their production does not allow us to enhance this trait. Certain non-Saccharomyces yeasts produce low amounts of acetic acid and/or high levels of certain esters. Conversely, certain strains of S. cerevisiae, which are commercialised for their production of sought-after aroma compounds, produce excessive amounts of acetic acids. The determinants behind these metabolic fluxes which differ between yeast species and strains are poorly understood. Nevertheless, the production of all these compounds is linked to the intracellular redox status of the cells.
This project aims to confirm this link and propose solutions to improve the persistence and/or impact of different yeast species and strains.
Researchers: Benoit Divol and Evodia Setati
South African Grape and Wine Research Institute (SAGWRI), Department of Viticulture and Oenology, Stellenbosch University