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.

Investigating Fruitiness Perception in Red and White Wines

This report details activities that occurred from February 2018 – January 2019. The final date of this project is August 2019 and the next 6 months will include completing the last of the sensory panels and combining all data analysis. A final report will be submitted in January 2020. We are still slightly behind on the timeline due to issues detailed in last year’s report and we also had to renew our IRB (human ethics approval) in June 2018. Once the renewal is submitted it is illegal to run sensory tests on the project until approval is given, which was obtained in September 2018. In January 2019 we completed the last of the Pinot noir sensory panels, although we have not yet done data analysis on the January 2019 panel. Also as stated previously, we have not been able to complete any predictive modeling, some initial reviewer comments said that this objective might have been too ambitious in the timeline and after the 1st year we have to agree and have since removed this objective. We plan on working on predictive modeling in the future but this would be after the current grant is completed.

To date we have investigated 80 different compound combinations and their impact to fruit aroma in Pinot noir wine. We have also completed a panel that shows the influences of phenolic content on fruity aromas in Pinot noir and one panel that shows the impact of ethanol content on fruity aroma in Pinot noir. We have 2 potential marker compounds for red fruit aroma in Pinot noir and 4 red fruit solution sets using fsQCA that show the cause of red fruit aroma in Pinot noir. We have also found 5 solution sets for dark fruit aroma in Pinot noir using fsQCA.

We have also investigated 49 compound combinations for fruitiness in white wine. We are still working on using fsQCA to analyze this data. Preliminary results suggest a combination of low thiols and high esters are responsible for tropical fruit aromas, low to no esters are needed for citrus aromas, and esters and terpenes cause pear, peach and apricot aromas.

We will be running the last 4 Pinot gris sensory panels from February2019-June 2019 and completing the final data analysis. We are in the process of writing the first paper for publication and have done 2 presentations at domestic conferences on the analytical data analysis. Spring /Summer 2019 we will be presenting at 4 different international conferences in Europe and have plans for at least 3 more peer-reviewed publications.

Characterization of Bitter and Astringent Proanthocyanidins During Winemaking

Project Title: Characterization of Bitter and Astringent Proanthocyanidins during


Principal Investigators

  • Dr. Susan E. Ebeler, Department of Viticulture & Enology, University of California, Davis,


  • Dr. Hildegarde Heymann, Department of Viticulture & Enology, University of California,

Davis, 530-752-0380


Polyphenols, including proanthocyanidins (i.e., tannins), are widely distributed in foods and

beverages, including grapes and wines and they are key constituents impacting bitter and

astringent perception. Due, at least in part, to their chemical complexity, the changes in

proanthocyanidin concentration and chemical structure that occur during winemaking and that

impact sensory properties have not been fully evaluated.

During the past year we have developed an ultra-high performance liquid chromatography

quadrupole time-of-flight mass spectrometry (UHPLC-qTOF MS) approach to characterize the

subunit composition and molecular weight/average degree of polymerization of wine

proanthocyanidins. Wines with different maceration treatments were analyzed and we are in the

process of completing the data analysis and comparison of the treatments. This work is

beginning to provide important insight into the impact of maceration treatments on

proanthocyanidin composition of wines.