Development and applications of novel analytical methods to the identification, formation and fate of two important wine aroma compounds.

Abstract

Wine flavour is complex and encompasses a wide variety of compounds with very different sensorial properties. It is only through detailed investigations, however, that knowledge of aroma compounds is improved. One aroma compound of inconclusive origin and requiring further study was 1,8-cineole, which was especially relevant to the Australian viticultural landscape. Another group of important flavour compounds found in wine that required greater understanding are the sulfur compounds. Varietal thiols, in particular 3-mercaptohexan-1-ol (3-MH), have some of the lowest aroma thresholds of any food or beverage component. Knowledge of thiol precursors is also important for understanding the formation of the corresponding varietal aroma compounds during winemaking, since they are released from odourless precursors in grape juice through fermentation. To enable a better understanding of the formation and fate of the two important aroma compounds chosen, novel analytical methods using stable isotope dilution analysis(SIDA) were developed and thoroughly validated for the quantification of 1,8-cineole and 3-MH in grapes and wine. In addition, a SIDA method was developed and validated for the analysis of the diastereoisomers of the precursors, 3-S-cysteinylhexan-1-ol (Cys-3-MH) and 3-S-glutathionylhexan-1-ol (Glut-3-MH) to complement the studies on 3-MH. 1,8-Cineole was found to be predominantly present in red wines, being extracted during fermentation rather than forming from terpene precursors as previously proposed. Extension of this research revealed that Eucalyptus trees in the vineyard had a strong influence on the concentration of 1,8-cineole in wine. The incorporation of grape leaves and stems, and in particular Eucalyptus leaves into red must fermentations significantly elevated the level of 1,8-cineole concentrations in the wine. Additionally, this study revealed a surprising increase in rotundone concentrations when grape leaves and stems were included during fermentation. An analytical method was developed for 3-MH determination that used conventional electron ionisation GC-MS and eliminated the use of mercuric compounds for thiol isolation. A 3-MH precursor analytical method was also developed which provided the first method where both diastereoisomers of Cys- and Glut-3-MH could be analysed individually in one run. This was improved further by the addition of Cysgly-3-MH into the method following the identification of this compound in grapes for the first time. These methods were applied to wines and grape juices to investigate factors which might affect their concentrations, such as freezing grapes and juice, grape processing and fruit transportation. These studies highlighted the dynamic nature of these precursor compounds. In addition a new conjugated aldehyde which was the obvious missing link between the reaction of (E)-2-hexenal and glutathione in the formation of Glut-3-MH was identified for the first time. The development of the new analytical methods discussed, together with their applications has contributed considerably to our understanding of various aspects of wine flavour. This work has uncovered the origin of 1,8-cineole in red Australian wine. It has also greatly enhanced our knowledge of 3-MH and its precursors so that we can understand how these compounds are formed and what affects their concentrations in juice and wine. Ultimately, this knowledge will enable wine producers to have greater control over the aroma profile of their wines.