Several very odourous volatile thiols have been identified as constituents of the particular aroma of Sauvignon blanc but also several other varieties such as Riesling. These thiols, mainly 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexyl acetate (3MHA), 4-mercapto-4-methylpentan-2-ol (4MMPOH), and 3-mercaptohexan-1-ol (3MH), are highly aroma active in very low concentrations (perception threshold 0.8 – 60 ng/L). The compounds are released during alcoholic fermentation by the degradation of their odourless S-cysteine conjugates that were set free from the S-glutathione-conjugates in the grape. The thiol-containing tripeptide glutathione does not only play a role in the release of aroma-active thiols, but is also an important antioxidant in living cells, and as such present in grapes. Aside of its function as antioxidant, glutathione prevents cellular damage by maintaining thiols in a reductive stage. This has been shown in mammal and plant systems alike. Glutathione is also involved in the formation of the “grape-reaction-product” (GRP) which can limit oxidative browning of white grape juice. Both water and nitrogen deficiency in vineyards have been shown to reduce the varietal flavour of Sauvignon blanc but also other white varieties such as Riesling, but the mechanisms are so far unknown. What has been observed in grapes from white varieties under water stress is a strong increase in the flavonoid compound quercetin and its metabolites. Since quercetin also acts as an antioxidant, the observed increases are likely a response to oxidative stress. In mammal systems, and particularly also in the human liver, it has been shown that under certain conditions (oxidative stress), quercetin can be metabolised to either a glutathionyl-quercetin conjugate or a quinone/quinone methide (QQ), the latter being highly reactive with thiols. This mechanism has so far never been investigated in grapes or during the fermentation of grape juice. However, it seems possible that a similar system acts in the fruit during stress and also during fermentation, and that there may be some interactions with thiol-releasing micro-organisms, such as yeasts, which could play a role in the expression of varietal aroma. In addition to the analysis of thiols that are responsible for the varietal flavour also other flavour-active thiols and sulphur compounds which were produced under stress conditions by yeasts will be investigated. Sensorial aspects (sensory analysis, GC-olfactometry) will also be considered in order to search correlations between wine characteristics, vine water and nitrogen status, and wine volatiles composition.

The proposed project will address these questions using 2 varieties (Sauvignon blanc and Riesling) in two locations, the Geisenheim Research Center (GRC) and the Institut de Sciences de la Vigne et du Vin (ISVV) in Bordeaux. The experimental set up will be organised to allow the successful candidate to follow the experiments in the two locations.