Impacts of Viticultural conditions and juice composition on the Oxidative and reductive development of Wine

Low molecular weight sulfur compounds (LMWSCs) and volatile aldehyde compounds (VACs) can contribute reductive and oxidative off-flavours to wine, respectively. These are generally regarded as unfavourable wine production artefacts and negative for wine quality. Therefore, it is necessary to understand the prevailing factors that influence the accumulation of these compounds during wine production and maturation.
The measurement of VACs often requires labour intensive methodologies, and/or approximations of the actual analyte concentrations. Indeed, the accurate quantification of the total concentrations of VACs in wine requires determination of both the hydrogen sulfite-bound for (i.e. a-hydroxyalkylsulfonates) and free (unbound) form. This thesis has validated two new techniques to allow improved quantification of total VACs concentrations in wine. Method 1 (Chapter II) utilised p-Benzoquinone to sequester hydrogen sulfite and thereby dissociate the a-hydroxyalkylsulfonates. Once released, the free aldehydes were quantified by gas chromatograpy mass spectromety. In the second method (chapter III), the free aldehyde compounds were converted to their hydrogen sulfite-bound forms via excess SO2 addition to the wine. The resulting a-hydroxyalkylsulfonates were then quantified by liquid chromatograpy mass spectromety. Method 1 had advantages of improved sensitivity, while method 2 had the advantage of minimal sample preparation.
Having optimised the quantification methodologies for VACs in wines, experiments were conducted linking viticultural conditions to wine composition, with ta focus on the accumulation of LMWSCs and VACs in wines.
Initially, Cu(II) and SO2 treatments to Chardonnay and Shiraz juice/must were conducted to assess their impacts on the composition of the respective wines (Chapter IV). The Cu treatment led to differences in the total Cu concentration of the Chardonnay wines after fermentation and then throughout bottle ageing (15 months), while no differences were evident in the Shiraz wines. The CU and SO2 treatments to Chardonnay juice led to long-term effects on the concentrations of hydrogen sulfide, methanethiol, 3-methylbutanal and phenylacetaldegyde in the resulting wines. The same treatments in Shiraz must influence hexanal, 3-methylbutanal and phenylacetaldegyde concentrations immediately after fermentation, however, the differences became negligible by bottling, and new treatment influences appeared after bottle ageing. Overall, Cu and SO2 treatments in juice/must impacted aroma compounds relevant to the reductive-oxidative development during wine production and after bottle ageing.
The final experiment assessed the impacts of viticultural factors (two vineyard locations, two grape varieties and two grape maturities), bottle age )four time points), and oxygen availability (three levels) on Shiraz and Cabernet Sauvignon wine composition (Chapter V). The compositional evolution of the wine samples was tracked over two years. Variables were identified that contributed towards the discrimination of bottle aged wine based on the specific viticultural factors adopted for the corresponding grape production. These may lead to potential markers for specific viticultural conditions. Generally, all of the experimental wine samples were able to be distinguished from each other according to the different viticultural factors or bottle ageing treatments by chemometric analysis of the composition results. However, compared to the decisive influences from viticultural factors, bottle ageing treatments only introduced subtle, but still significant, impacts on red wine composition during evolution.