Characterization of Aroma Volatiles and their Glyosidic Precursors in Grapes and Wines
Summary: The complex aroma of wine is derived from many sources, with grape-derived components being responsible for the varietal character. The ability to monitor grape aroma compounds would allow for better understanding of how vineyard practices and winemaking processes influence the final volatile composition of the wine. Previously we developed a procedure using GC-MS combined with solid-phase microextraction (SPME) for profiling the free volatile compounds in grapes and wines. We also developed a method for monitoring the ‘aroma potential’ of grapes and wines without the need for initial isolation of the glycoside precursor fraction. However, this method still depends on indirect measurement of the glycosides and acid or enzymatic hydrolysis is needed to release the volatile aglycone which can result in artifact formation. In the current project we validated a novel method using UHPLC-qTOF MS/MS for direct analysis of intact aroma glycosides in grapes with minimal artifactual changes in composition. Using this method we tentatively identified 27 monoterpene glycosides including two monoterpene trisaccharide glycosides, tentatively identified for the first time in any plant. We measured the terpene glycosides in six cultivars at three maturity time points and demonstrated differential profiles depending on cultivar and maturity. We also modified the method so that it can be used to monitor monoterpene glycosides in wines and during winemaking. We have analyzed the glycoside content during fermentation for wines made in fall 2016 and 2017 with different varieties (Chardonnay, Merlot, Cabernet Sauvignon) and winemaking/processing methods. Monoterpenyl glycoside profiles differed between the grapes and the first alcoholic fermentation samples. In red wines, malonylated monoterpenol glucosides and monoterpenol hexose-pentoses decreased after the completion of alcoholic fermentation. We also measured the volatile composition of the wines during fermentation and we have started to relate changes in terpene volatiles to changes in the glycoside profiles. This work sheds important insight into possible biochemical changes in glycosylation during grape berry maturation. In addition, this research will allow us to better understand the effects of viticultural and winemaking practices on grape and wine components that affect flavor.