Redox balance in wine yeast under hyperosmotic stress and the relation to wine quality

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Icewine is the flagship wine of Canada that spearheaded international trade for Canadian wines. It continues to be of critical importance to the Canadian wine industry, with Icewines approved for sale in 2011 worth $100 million. Producing these wines offers many challenges to wine yeast carrying out the fermentations. Solutes in Icewine juice place wine yeast under increased hyperosmotic stress, leading to altered metabolism, fermentation difficulties and ten-times the concentration of acetic acid found in table wines. A high concentration of acetic acid in Icewine is a concern due to its association with wine spoilage. Wine yeast strains display properties that are distinct from well-studied laboratory yeast strains, and these differences can impact on the organoleptic properties of wine. However, the molecular basis of these differences remains largely unknown. The central research goal of this program is to gain a more comprehensive understanding, at the molecular level, of factors influencing wine yeast stress responses during fermentation of concentrated grape juices and how these responses relate to cellular redox balance, glycerol, organic acid production and overall wine quality. We will examine such influences while yeast adapt first to the hyperosmotic stress from concentrated Icewine juice, and then to ethanol and acetic acid stress as fermentation proceeds. Once we understand why certain metabolites are formed during this response, it should be possible to control yeast metabolism to avoid or minimize unwanted compounds such as acetic acid. This research will provide critical information for the development of yeast strains best suited for Icewine and wine styles based on high Brix juice, such as that found in the emerging Canadian appassimento wine style produced from grapes ripened off-vine. This information will be applicable to winemakers, assisting them in maintaining and expanding Canada's global dominance in Icewine quality and supporting their needs as they branch into new wine styles involving high sugar juices.