Varietal differences between Shiraz and Cabernet sauvignon wines revealed by yeast metabolism

This study investigated if compositional differences between Shiraz and Cabernet Sauvignon grape varieties could influence the production of yeast-derived compounds. This work was based on the analysis of 40 experimental red wines made in triplicate fermentations from grapes harvested from two consecutive vintages in New South Wales (Australia). Grapes were picked at three maturity stages using berry sugar accumulation as physiological indicator, from nine commercial vineyards located in three different climatic regions (temperate, temperate-warm and warm-hot and). A range of 30 yeast-derived wine volatiles including esters and alcohols were quantified by HS/SPME-GC/MS. Ammonia, amino-acids and lipids were analysed in the corresponding grapes. The juice total soluble solids (°Brix) in addition to the wine alcohol and residual sugar levels were also measured.
Most C6-alcohols, known to contribute to green/stalky and fresh fruit character in red wine, were significantly higher in Shiraz wines compared to the Cabernet Sauvignon wines. This was particularly noticeable for cis-3-hexenol, showing concentrations about 6-fold higher in the Shiraz wines. This finding was not observed for trans-3-hexenol. C6-alcohols are principally derived from the degradation of grape lipids during crushing and fermentation. However, the concentration of grape lipids and C6-compounds were poorly correlated with the concentrations of C6-alcohols in wine. Samples taken during fermentation showed that the differences in C6-alcohols between the two varieties were manifested at this stage, suggesting the importance of yeast metabolism to the final C6-alcohols levels. The influence of grape maturity on wine ester composition was also variety dependent, particularly for higher alcohol acetate and ethyl ester of branched acids. This study highlights that varietal differences observed in Shiraz and Cabernet Sauvignon wines involve fermentation-derived compounds, irrespective of the climate or irrespective of the site (soil x climate).